Free Essay

Facts

In: Science

Submitted By sanjida
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Pages 52
Chapter 01

1.1 Introduction:

A storage device may be used to store energy from renewable energy source in DC form which can be converted to AC power by using power inverter. Pulse Width Modulated (PWM) technique may be used to control output rms voltage of the inverter. As the load is variable, the power consumed by the load (PL) may be smaller than the power generated from the renewable energy source (PR). Therefore a Flexible AC Transmission System (FACTS) Controller may be used to supply the additional power (PR – PL) from the renewable energy source to the grid. On the other hand if the power consumed by the load (PL) is greater than the power generated from the renewable energy source (PR) therefore the same FACTS controller may be used to absorb the additional power (PR – PL) from the grid to the load. In this case the FACTS controller must allow bi-directional power flow. If all the active houses are connected to the grid in the same way (proposed way), the active houses that generate more power than the load can be supplied to the active houses that generate less power than the load. Therefore a suitable FACTS controller should be designed in such a way that it can control the power flow in both directions. The idea is illustrated in the following figure.

1.2 Objectives:

• Study on different FACTS controllers • Study on different renewable energy sources • Study on different energy storage devices • Study on different types of inverters • Implementation of all these resources in a smart grid

Chapter 02

2.1 Introduction:

The operation of an AC power transmission line is generally constrained by limitations of one or more parameters (such as line impedance) and operating variables (such as voltage and current). As a result the power line is unable to direct power flow among generating stations. Therefore, other parallel transmission lines that have an adequate capability of carrying additional amounts of power may not be able to supply the power demand. Flexible Alternating Current Transmission Systems (FACTS) are new emerging technology and its principle role is to enhance controllability and power transfer capability in AC systems. FACTS technology uses switching power electronics to control power flow in the range of a few ten to a few hundreds of megawatts.

FACTS devices that have an integrated control function are known as the FACTS controllers. FACTS controllers are capable of controlling the interrelated line parameters and other operating variables. FACTS devices also control the real and reactive power flow.

In recent years, power demand has increased substantially while the expansion of power generation and transmission has been severely limited due to limited resources and environmental restrictions. As a consequence, some transmission lines are heavily loaded and the system stability becomes a power transfer-limiting factor. Flexible AC transmission systems (FACTS) controllers have been mainly used for solving various power system steady state control problems. However, recent studies reveal that FACTS controllers could be employed to enhance power system stability in addition to their main function of power flow control.

Flexible AC transmission systems (FACTS) have gained a great interest during the last few years, due to recent advances in power electronics. FACTS devices have been mainly used for solving various power system steady state control problems such as voltage regulation, power flow control, and transfer capability enhancement. As supplementary functions, damping the inter-area modes and enhancing power system stability using FACTS controllers have been extensively studied and investigated. Generally, it is not cost-effective to install FACTS devices for the sole purpose of power system stability enhancement.

2.2 Definition of FACTS:

The term FACTS describes a wide range of controllers, many of them which incorporate large power electronic converters, which can increase the flexibility of power systems making them more controllable. Some of these are well established while some are still in the research or development stage.

Flexible Alternating-Current Transmission Systems (FACTS) are defined by the IEEE as “AC transmission systems incorporating power electronics-based and other static controllers to enhance controllability and increase power transfer capability”. Similarly, a FACTS controller is defined as “a power electronics-based system or other static equipment that provides control of one or more AC transmission parameters”.

In general, FACTS devices posses the following technological attributes:

♦ Provide dynamic reactive power support and voltage control. ♦ Reduce the need for construction of new transmission lines, capacitors, reactors, which - Mitigate environmental and regulatory concerns. - Improve aesthetics by reducing the need for construction of new facilities such transmission lines. ♦ Improve system stability. ♦ Control real and reactive power flow. ♦ Mitigate potential sub-synchronous resonance problems.

2.3 Classification of different types of FACTS:

The IEEE groups classified the FACTS controllers into four main categories based on how they are connected to the AC power system:

i) Series FACTS devices ii) Shunt FACTS devices iii) Combined series-series FACTS devices iv) Combined series-shunt FACTS devices

(i) Series FACTS devices:

In series compensation, the FACTS is connected in series with the power system. It works as a controllable voltage source. Series inductance occurs in long transmission lines, and when a large current flow causes a large voltage drop. To compensate, series capacitors are connected.

(ii) Shunt FACTS devices:

In shunt compensation, power system is connected in shunt (parallel) with the FACTS. It works as a controllable current source. Shunt compensation is of two types: (a) Shunt capacitive compensation: This method is used to improve the power factor. Whenever an inductive load is connected to the transmission line, power factor lags because of lagging load current. To compensate, a shunt capacitor is connected which draws current leading the source voltage. The net result is improvement in power factor. (b) Shunt inductive compensation: This method is used either when charging the transmission line, or, when there is very low load at the receiving end. Due to very low, or no load – very low current flows through the transmission line. Shunt capacitance in the transmission line causes voltage amplification (Ferranti Effect). The receiving end voltage may become double the sending end voltage (generally in case of very long transmission lines). To compensate, shunt inductors are connected across the transmission line.

(iii) Combined series-series FACTS devices:

Combined series-series FACTS device is a combination of separate series FACTS devices, which are controlled in a coordinated manner.

(iv) Combined series-shunt FACTS devices:

Combined series-shunt FACTS device is a combination of separate shunt and series devices, which are controlled in a coordinated manner or one device with series and shunt elements.

Figure-2.3: shows the classification of conventional (switched) and FACTS (fast, static) devices. For the FACTS side the taxonomy in terms of 'dynamic' and 'static' needs some explanation. The term 'dynamic' is used to express the fast controllability of FACTS-devices provided by the power electronics. This is one of the main differentiation factors from the conventional devices. The term 'static' means that the devices have no moving parts like mechanical switches to perform the dynamic controllability. Therefore most of the FACTS-devices can equally be static and dynamic.

[pic]

Figure 2.3: Overview of major FACTS devices.

2.4 Configuration and working principle of FACTS-Devices:

2.4.1 Series FACTS devices:

2.4.1.1 TCSC:

Thyristor Controlled Series Capacitors (TCSC) address specific dynamical problems in transmission systems. Firstly it increases damping when large electrical systems are interconnected. Secondly it can overcome the problem of Sub- Synchronous Resonance (SSR), a phenomenon that involves an interaction between large thermal generating units and series compensated transmission systems. The TCSC's high speed switching capability provides a mechanism for controlling line power flow, which permits increased loading of existing transmission lines, and allows for rapid readjustment of line power flow in response to various contingencies. The TCSC also can regulate steady-state power flow within its rating limits.

From a principal technology point of view, the TCSC resembles the conventional series capacitor. All the power equipment is located on an isolated steel platform, including the Thyristor valve that is used to control the behavior of the main capacitor bank. Likewise the control and protection is located on ground potential together with other auxiliary systems. Figure 2.4.1.1 shows the principle setup of a TCSC and its operational diagram. The firing angle and the thermal limits of the Thyristors determine the boundaries of the operational diagram.

[pic]

Figure 2.4.1.1: Principle setup and operational diagram of TCSC

The main principles of the TCSC concept are two; firstly, to provide electromechanical damping between large electrical systems by changing the reactance of a specific interconnecting power line, i.e. the TCSC will provide a variable capacitive reactance. Secondly, the TCSC shall change its apparent impedance (as seen by the line current) for sub-synchronous frequencies, such that a prospective sub-synchronous resonance is avoided. Both objectives are achieved with the TCSC, using control algorithms that work concurrently. The controls will function on the Thyristor circuit in parallel to the main capacitor bank such that controlled charges are added to the main capacitor, making it a variable capacitor at fundamental frequency but a “virtual inductor” at sub-synchronous frequencies. The first TCSC was commissioned in 1996.

2.4.1.2 SSSC:

While the TCSC can be modeled as a series impedance, the SSSC is a series voltage source. The principle configuration is shown in Figure 2.4.1.2, which looks basically the same as the STATCOM. But in reality this device is more complicated because of the platform mounting and the protection. A Thyristor protection is absolutely necessary, because of the low overload capacity of the semiconductors, especially when IGBTs are used.

The voltage source converter plus the Thyristor protection makes the device much more costly, while the better performance cannot be used on transmission level. The picture is quite different if we look into power quality applications. This device is then called Dynamic Voltage Restorer (DVR). The DVR is used to keep the voltage level constant, for example in a factory infeed. Voltage dips and flicker can be mitigated. The duration of the action is limited by the energy stored in the DC capacitor. With a charging mechanism or battery on the DC side, the device could work as an uninterruptible power supply. A picture of a modularized installation with 22 MVA is shown on the right in Figure 2.4.1.2.

[pic]

Figure 2.4.1.2: Principle setup of SSSC and implementation as a DVR for power quality application.

2.4.2 Shunt FACTS devices:

2.4.2.1 SVC:

Electrical loads both generate and absorb reactive power. Since the transmitted load varies considerably from one hour to another, the reactive power balance in a grid varies as well. The result can be unacceptable voltage amplitude variations or even a voltage depression, at the extreme a voltage collapse. A rapidly operating Static VAR Compensator (SVC) can continuously provide the reactive power required to control dynamic voltage oscillations under various system conditions and thereby improve the power system transmission and distribution stability. Installing an SVC at one or more suitable points in the network can increase transfer capability and reduce losses while maintaining a smooth voltage profile under different network conditions. In addition an SVC can mitigate active power oscillations through voltage amplitude modulation.

SVC installations consist of a number of building blocks. The most important is the Thyristor valve, i.e. stack assemblies of series connected anti-parallel Thyristors to provide controllability. Air core reactors and high voltage AC capacitors are the reactive power elements used together with the Thyristor valves. The step-up connection of this equipment to the transmission voltage is achieved through a power transformer. The Thyristor valves together with auxiliary systems are located indoors in an SVC building, while the air core reactors and capacitors, together with the power transformer are located outdoors.

In principle the SVC consists of Thyristor Switched Capacitors (TSC) and Thyristor Switched or Controlled Reactors (TSR / TCR). The co-ordinated control of a combination of these branches varies the reactive power as shown in Figure 2.4.2.1. The first commercial SVC was installed in 1972 for an electric arc furnace. On transmission level the first SVC was used in 1979. Since then it is widely used and the most accepted FACTS-device.

[pic]

Figure 2.4.2.1: SVC building blocks and V-I characteristics.

2.5.2.2 STATCOM:

In 1999 the first SVC with Voltage Source Converter called STATCOM (STATic COMpensator) went into operation. The STATCOM has a characteristic similar to the synchronous condenser, but as an electronic device it has no inertia and is superior to the synchronous condenser in several ways, such as better dynamics, a lower investment cost and lower operating and maintenance costs.

A STATCOM is build with Thyristors with turn-off capability like GTO or today IGCT or with more and more IGBTs. The structure and operational characteristic is shown in Figure 2.4.2.2.

The static line between the current limitations has a certain steepness determining the control characteristic for the voltage. The advantage of a STATCOM is that the reactive power provision is independent from the actual voltage on the connection point. This can be seen in the diagram for the maximum currents being independent of the voltage in comparison to the SVC in Figure 2.4.2.1. This means, that even during most severe contingencies, the STATCOM keeps its full capability.

[pic]

Figure 2.4.2.2: STATCOM structure and V-I characteristics.

2.4.3 Combined shunt and series FACTS devices:

2.4.3.1 DFC

A new device in the area of power flow control is the Dynamic Power Flow Controller (DFC). The DFC is a hybrid device between a Phase Shifting Transformer (PST) and switched series compensation.

A functional single line diagram of the Dynamic Flow Controller is shown in figure 2.4.3.1. The Dynamic Flow Controller consists of the following components:

• a standard phase shifting transformer with tap-changer (PST) • series-connected Thyristor Switched Capacitors and Reactors (TSC / TSR) • A mechanically switched shunt capacitor (MSC). (This is optional depending on the system reactive power requirements)

[pic]

Figure 2.4.3.1: Principle configuration of DFC

Based on the system requirements, a DFC might consist of a number of series TSC or TSR. The mechanically switched shunt capacitor (MSC) will provide voltage support in case of overload and other conditions. Normally the reactances of reactors and the capacitors are selected based on a binary basis to result in a desired stepped reactance variation. If a higher power flow resolution is needed, a reactance equivalent to the half of the smallest one can be added.

The switching of series reactors occurs at zero current to avoid any harmonics. However, in general, the principle of phase-angle control used in TCSC can be applied for a continuous control as well. The operation of a DFC is based on the following rules:

• TSC / TSR are switched when a fast response is required. • The relieve of overload and work in stressed situations is handled by the TSC /TSR. • The switching of the PST tap-changer should be minimized particularly for the currents higher than normal loading. • The total reactive power consumption of the device can be optimized by the operation of the MSC, tap changer and the switched capacities and reactors.

2.4.3.2 UPFC:

The UPFC (Unified Power Flow Controller) is a combination of a static compensator and static series compensation. It acts as a shunt compensating and a phase shifting device simultaneously.

The UPFC consists of a shunt and a series transformer, which are connected via two voltage source converters with a common DC-capacitor. The DC-circuit allows the active power exchange between shunt and series transformer to control the phase shift of the series voltage. This setup, as shown in Figure 2.4.3.2, provides the full controllability for voltage and power flow. The series converter needs to be protected with a Thyristor bridge. Due to the high efforts for the Voltage Source Converters and the protection, an UPFC is getting quite expensive, which limits the practical applications where the voltage and power flow control is required simultaneously.
[pic]
Figure 2.4.3.2: Principle configuration of an UPFC.

2.4.3.3 IPFC:

One of the latest FACTS-devices is named convertible static compensator (CSC) and was recently installed as a pilot by the New York Power Authority (NYPA). The CSC-project shall increase power transfer capability and maximize the use of the existing transmission network. Within the general conceptual framework of the CSC, two multi-converter FACTS devices, the Interline Power Flow Controller (IPFC) and the Generalized Unified Power Flow Controller (GUPFC), are among many possible configurations. The target is to control power flows of multi-lines or a sub-network rather than control the power flow of a single line by for instance DFC or UPFC. The IPFC combines two or more series converters and the GUPFC combines one shunt converter and two or more series converters. The current NYPA's CSC installation is a two converter one and can operate as an IPFC but not as a GUPFC.

When the power flows of two lines starting in one substation need to be controlled, an Interline Power Flow Controller (IPFC) can be used. The IPFC consists of two series VSCs whose DC capacitors are coupled. This allows active power to circulate between the VSCs. Figure 2.4.3.3 shows the principle configuration of an IPFC. With this configuration two lines can be controlled simultaneously to optimize the network utilization. In general, due to its complex setup, specific application cases need to be identified justifying the investment.

[pic]

Figure 2.4.3.3: Principle configuration of an IPFC.

Chapter 03

Static Synchronous Compensator
(STATCOM)

3.1 Introduction:

This shunt connected static compensator was developed as an advanced static VAR compensator where a voltage source converter (VSC) is used instead of the controllable reactors and switched capacitors. Although VSCs require self-commutated power semiconductor devices such as GTO, IGBT, IGCT, MCT, etc (with higher costs and losses) unlike in the case of variable impedance type SVC which use thyristor devices, there are many technical advantages of a STATCOM over a SVC. These are primarily:

(a) Faster response

(b) Requires less space as bulky passive components (such as reactors) are eliminated

(c) Inherently modular and relocatable

(d) It can be interfaced with real power sources such as battery, fuel cell or SMES (superconducting magnetic energy storage)

(e) A STATCOM has superior performance during low voltage condition as the reactive current can be maintained constant (In a SVC, the capacitive reactive current drops linearly with the voltage at the limit (of capacitive susceptance). It is even possible to increase the reactive current in a STATCOM under transient conditions if the devices are rated for the transient overload. In a SVC, the maximum reactive current is determined by the rating of the passive components – reactors and capacitors.

The STATCOM was originally called as advanced SVC and then labeled as STATCON (STATic CONdenser).

3.2 Principle of Operation of STATCOM:

A STATCOM is comparable to a Synchronous Condenser (or Compensator) which can supply variable reactive power and regulate the voltage of the bus where it is connected. The equivalent circuit of a Synchronous Condenser (SC) is shown in Figure: 3.2.1, which shows a variable AC voltage source [pic] whose magnitude is controlled by adjusting the field current. Neglecting losses, the phase angle [pic] difference between the generated voltage [pic] and the bus voltage [pic] can be assumed to be zero. By varying the magnitude of[pic], the reactive current supplied by SC can be varied. When[pic], the reactive current output is zero. When[pic], the SC acts as a capacitor where as when[pic], the SC acts as an inductor. When [pic]= 0, the reactive current drawn [pic] is given by

[pic] ……………………………. (3.1)

[pic]

Figure 3.2.1: A synchronous condenser

A STATCOM has a similar equivalent circuit as that of a SC. The AC voltage is directly proportional to the DC voltage [pic] across the capacitor (see Figure: 3.2.2) which shows the circuit for a single phase STATCOM). If an energy source (a battery or a rectifier) is present on the DC side, the voltage [pic] can be held constant. The self-commutated switches T1 and T2 (based on say GTOs) are switched on and off once in a cycle. The conduction period of each switch is 180˚ and care has to be taken to see that T1 is off when T2 is on and vice versa. The diodes D1 and D2 enable the conduction of the current in the reverse direction. The charge on the capacitors ensures that the diodes are reverse biased. The voltage waveform across VPN is shown in Figure: 3.2.3. The voltage [pic]when T1 is conducting (T2 is off) and [pic]when T2 is conducting (and T1 is off).

[pic]

Figure 3.2.2: A single phase STATCOM

[pic]

Figure 3.2.3: The waveform of [pic]

The switches are synchronized with the supply voltage [pic] which is assumed to be sinusoidal of frequency[pic]. The fundamental component, rms value [pic]is obtained as

[pic] ……………… (3.2)

When[pic], the STATCOM draws a capacitive reactive current, whereas it is inductive if[pic]. Note that, to be compatible with the convention used for SVC, the inductive current drawn is assumed to be positive.

At the instant when [pic] is switched on and [pic] is inductive, the current [pic] flowing through the circuit is negative (as it is a lagging current) and flows through [pic] (as [pic] is negative of[pic]). After 90˚, the current through [pic] becomes zero and as [pic] rises above zero and becomes positive, the diode [pic] takes over conduction. Similar events occur when [pic] turns on and off. Thus, both [pic] and [pic]cease conduction before they are turned off. On the other hand, when[pic]is capacitive, the current [pic] is positive at the instant of turning on [pic] and flows through the diode[pic]. After 90˚, the current reverses its sign and flows through[pic]. At the time of switching off [pic], the current through it is at its peak value. Thus, we need self commutated devices such as GTOs when the STATCOM draws capacitive reactive current. In contrast, [pic] and [pic] carry peak current at turn on when [pic] is inductive.

Note that diode [pic] or [pic] turns off automatically when the parallel device ([pic] or[pic]) turns off. Also, the capacitors can be charged from the source through the diodes.

The steady state control characteristics of a STATCOM are shown in Figure: 3.2.4. The losses in the STATCOM are neglected and [pic] is assumed to be purely reactive. As in the case of a SVC, the negative current indicates capacitive operation while positive current indicates inductive operation. The limits on the capacitive and inductive currents are symmetric[pic]. The positive slope BC is provided for the [pic] characteristic to (i) prevent the STATCOM hitting the limits often and (ii) to allow parallel operation of two or more units. The reference voltage [pic] corresponds to zero current output and generally, the STATCOM is operated close to zero

[pic]

Figure 3.2.4: Control characteristics of a STATCOM

output during normal operating conditions, such that full dynamic range is available during contingencies. This is arranged by controlling the mechanically switched capacitors/reactors connected in parallel with a STATCOM.

3.3 A Simplified Analysis of a Three Phase Six Pulse STATCOM:

The basic building block of a high power GTO based STATCOM is a six pulse circuit shown in Figure: 3.3.1. The circuit consists of six switches, made up of six GTO thyristors with anti-parallel diodes connected as a six pulse Graetz bridge. The analysis of the circuit assumes that each switch is turned on only once in a cycle of supply voltage and conducts for 180 each. Switches (or valves) are numbered in the sequence in which they are turned on (fired). Also, the two switches connected in series in each leg operate in a complementary fashion. Only one of the switches is conducting at any given time to prevent short circuit of the capacitor. Thus, before switch 4 is turned on, the switch 1 must be turned off and vice versa.

[pic]

Figure 3.3.1: A six pulse VSC circuit

To simplify the analysis, to derive the equations describing the steady-state performance, we assume (initially) that i) The capacitor size is infinite (very large) and therefore the DC side voltage is constant. ii) The losses in the circuit are neglected.

The waveform of the voltage [pic] is as shown in Figure: 3.2.3. The waveforms of [pic] and [pic] are also similar except that they are displaced from one another by 120º. ([pic] lags [pic] by 120º and [pic] lags [pic] by 120º.

The voltages [pic], [pic] and [pic] (measured with respect to the source neutral) can be obtained from the following equations

[pic] ……………………………. (3.3) [pic] ……………………………. (3.4) [pic] ……………………………. (3.5)

From the symmetry of the circuit, it can be shown that

[pic] ………………………... (3.6)

Substituting Eq. (3.6) in (3.3) to (3.5), we get

[pic] ………………… (3.7)

And

[pic] ……………….. (3.8) [pic] ………………... (3.9) [pic] ………………... (3.10)

The waveform of [pic] is shown in Figure: 3.3.2 (which also shows the supply voltage [pic]). The fundamental frequency component (rms value) of [pic] is obtained as

[pic] ………………….. (3.11)

[pic] …………………….. (3.12)

[pic]

Figure 3.3.2: Waveforms of [pic] and[pic].
The harmonic component [pic] is obtained as

[pic] , [pic] , [pic] ……………. (3.13)

The rms value of the fundamental component of (reactive) current, [pic] is calculated from

[pic] ………………………….. (3.14)

The harmonic current (rms) is obtained as

[pic] ……………………………… (3.15)

Chapter 04

Static Synchronous Series Compensator
(SSSC)

4.1 Introduction:

The Static Synchronous Series Compensator (SSSC) is a series connected FACTS controller based on VSC and can be viewed as an advanced type of controlled series compensation, just as a STATCOM is an advanced SVC. A SSSC has several advantages over a TCSC such as (a) elimination of bulky passive components capacitors and reactors, (b) improved technical characteristics (c) symmetric capability in both inductive and capacitive operating modes (d) possibility of connecting an energy source on the DC side to exchange real power with the AC network.

4.2 Operation of SSSC and the Control of Power Flow:

4.2.1 Description:

The schematic of a SSSC is shown in Figure: 4.2.1(a). The equivalent circuit of the SSSC is shown in Figure: 4.2.1(b). The magnitude of VC can be controlled to regulate power flow. The winding resistance and leakage reactance of the connecting transformer appear is series with the voltage source VC. If there is no energy source of

[pic]

Figure 4.2.1: Schematic of SSSC

the DC side, neglecting losses in the converter and DC capacitor, the power balance in steady state leads to

[pic] ………………...……… (4.1)

The above equation shows that [pic] is in quadrature with[pic]. If [pic] lags [pic] by 90˚, the operating mode is capacitive and the current (magnitude) in the line is increased with resultant increase in power flow. On the other hand, if [pic] leads I by 90˚, the operating mode is inductive, and the line current is decreased. Note that we are assuming the injected voltage is sinusoidal (neglecting harmonics). Since the losses are always present, the phase shift between [pic] and [pic] is less than 90˚ (in steady state). In general, we can write

[pic] [pic] ……………………………….. (4.2)

where [pic] is the phase angle of the line current, [pic] is the angle by which [pic] lags the current.[pic] and [pic]are the in-phase and quadrature components of the injected voltage (with reference to the line current). We can also term them as active (or real) and reactive components. The real component is required to meet the losses in the converter and the DC capacitor.

We use the convention that the reactive voltage lagging the current by 90˚ as positive. According to this convention, the positive reactive voltage implies capacitive mode of operation while negative reactive voltage implies inductive mode of operation. Since [pic] is close to [pic]˚, we can write

[pic] ………………………....…. (4.3)

[pic]

Figure 4.2.2: Representation of SSSC in a transmission line

where [pic] indicates the signum function whose value is +1 if the argument is positive and [pic] if the argument is negative. Substituting Eq. (4.3) in (4.2) we can write, [pic] , [pic] ………………. (4.4)

Since the losses are expected to be small (typically below 1%) the magnitude of [pic] is very small and may be neglected to simplify the analysis. [pic] will vary during a transient to increase or decrease the voltage across the DC capacitor (particularly in the case of type 2 converter where the ratio between the AC voltage and the DC capacitor voltage is constant, with no modulation).

4.2.2 Power Flow Control Characteristics:

A SSSC controls the power flow in a transmission line by varying the magnitude and polarity of the reactive voltage injected in series with line. In this section, we will study the control characteristics of a SSSC in the [pic] plane where [pic] and [pic] are the power and reactive power at the receiving end. In deriving the control characteristics we will relax the assumptions about losses in the line and the equality of sending end and receiving end voltage magnitudes.

If [pic] represents the series impedance of the line shown in Figure 4.2.2, the complex power at the receiving end [pic] is given by

[pic] ……………… (4.5)

If[pic], then [pic] is defined as

[pic] ……………… (4.6)

Substituting Eq. (3.6) in (3.5), we get

[pic] ………………………………… (4.7)

Assuming [pic] is purely reactive voltage, then

[pic] , [pic] …………………… (4.8)

Substituting Eq. (4.8) in (4.7) and noting that

[pic] ………………………………………. (4.9)

We obtain,

[pic] …………….. (4.10)

From the above, we can solve for [pic] as,

[pic] ………………… (4.11)

[pic]

Figure 4.3.3: Operating region and control characteristics of a SSSC in the [pic] plane.

As [pic] varies from [pic] to [pic], the locus of [pic] in the [pic] plane is a straight line parallel to the [pic] axis and passing through the point [pic]. The locus of the reciprocal of [pic] is a circle with the center [pic] and radius [pic]. From Eq. (4.10), it can be seen that [pic] describes a circle in the [pic] plane with [pic] as center and radius of [pic].

Note that this circle passes through the origin as well as the point [pic] (corresponding to [pic]). The locus of [pic] and [pic] lie on the circumference of the circle which is a function of [pic] and the ratio [pic] . The value of [pic] ([pic] and [pic] in the absence of SSSC) is a function of [pic] and the line impedance [pic] (for specified [pic] and [pic]). However, different combinations of [pic] and [pic] can give a specified value of [pic].

If [pic] and[pic], then the radius of the circle is 5.025 pu. Figure: 4.3.3 shows the control characteristics for [pic], with [pic].

The range of operation of a SSSC is only a part of the circle around the operating point [pic]. This is due to the limitations imposed by the rating of SSSC. Figure: 4.3.3 also shows the range of operation of a SSSC which is bounded by a circle with [pic] as centre and radius [pic].

Chapter 05

Unified Power Flow Controller
(UPFC)

5.1 Introduction:

The Unified Power Flow Controller (UPFC) proposed by Gyugyi is the most versatile FACTS controller for the regulation of voltage and power flow in a transmission line. It consists of two voltage source converters (VSC) one shunt connected and the other series connected. The DC capacitors of the two converters are connected in parallel (see Figure: 5.1). If the switches 1 and 2 are open, the two converters work as STATCOM and SSSC controlling the reactive current and reactive voltage injected in shunt and series respectively in the line. The closing of the switches 1 and 2 enable the two converters to exchange real (active) power flow between the two converters. The active power can be either absorbed or supplied by the series connected converter.

[pic]

Figure 5.1: A schematic diagram of UPFC

The provision of a controllable power source on the DC side of the series connected converter, results in the control of both real and reactive power flow in the line (say, measured at the receiving end of the line). The shunt connected converter not only provides the necessary power required, but also the reactive current injected at the converter bus. Thus, a UPFC has 3 degrees of freedom unlike other FACTS controllers which have only one degree of freedom (control variable).

5.2 Unified Power Flow Controller (UPFC):

“A combination of static synchronous compensator (STATCOM) and a static series compensator (SSSC) which are coupled via a common dc link, to allow bidirectional flow of real power between the series output terminals of the SSSC and the shunt output terminals of the STATCOM, and are controlled to provide concurrent real and reactive series line compensation without an external electric energy source. The UPFC, by means of angularly unconstrained series voltage injection, is able to control, concurrently or selectively, the transmission line voltage, impedance, and angle or, alternatively, the real and reactive power flow in the line. The UPFC may also provide independently controllable shunt reactive compensation”

In UPFC which combines a STATCOM and an SSSC, the active power for the series unit (SSSC) is obtained from the line itself via the shunt unit (STATCOM) as shown in Figure 5.1; the latter is also used for voltage control with control of its reactive power. This is a complete controller for controlling active and reactive power control through the line, as well as line voltage control. Additional storage such as a superconducting magnet connected to the dc link via an electronic interface would provide the means of further enhancing the effectiveness of the UPFC. As mentioned before, the controlled exchange of real power with an external source, such as storage, is much more effective in control of system dynamics than modulation of the power transfer within a system.

5.3 Principle of UPFC:

The UPFC consists of two voltage sources, one connected in series (SSSC) and the other connected in parallel (STATCOM) with the transmission network through transformers as shown in Figure 5.3. In principle, all series controllers inject voltage in series with the line and all shunt controllers inject current into the system at the point of connection. Two VSCs can work independently of each other by separating the dc side. So in that case, the shunt inverter is operating as a STATCOM that generates or absorbs reactive power at the connection point. Instead, the series inverter is operating as SSSC that generates or absorbs reactive power to regulate the current flow, and hence the power flow on the transmission line.

[pic]

Figure 5.3: A UPFC formed by STATCOM and SSSC If the sending end and the receiving end voltages are represented by Vs and Vr respectively and the series injected voltage is Vinj than the phasor diagram of UPFC is shown in Figure 5.4.

Figure 5.4: Phasor diagram of UPFC

5.4 Control attributes of UPFC:

|FACTS controller |Control attributes |
|Unified Power Flow Controller |Active and reactive power control, voltage control, VAR |
|(UPFC) |compensation, damping oscillations, transient and dynamic |
| |stability, voltage stability, fault current limiting. |

5.5 Design of UPFC:

The basic block diagram of the UPFC is illustrated in Figure 5.5. B1, B2, B3, B4, B5 and B6 represent six GTO / Diode double arm bridges. Mux-1, Mux-2, Mux-3, Mux-4, Mux-5 and Mux-6 represent six multiplexers. Conventionally shunt and series controllers are constructed of three phase converters or inverters but it is possible to replace three single phase converters with a three phase converter. T1, T2 and T3 represent the transformer coils of phase A, B and C respectively that form a three phase transformer connected to shunt converter. T4, T5 and T6 form a three phase transformer similarly which is connected to the series converter. A capacitor (C) is used to achieve smooth and constant dc bus and due to store some energy also. The original circuit diagram of a GTO / Diode bridge (B1) is shown in Figure 5.6.

[pic]

Figure 5.5: Block diagram of the UPFC connected to a transmission line

Each bridge consists of four GTO and four diodes where the GTOs and diodes are connected in anti-parallel way. So four different control pulses are required to control each of the bridges. Therefore to apply firing pulses to six different bridges properly total twenty four different pulses are required to control. For each GTO / Diode bridge, a 4 input multiplexer is used.

[pic]

Figure 5.6: A GTO / Diode bridge equivalent circuit

5.6 Capability comparison of different FACTS controllers:

|Controllers |Voltage Control|Transient |Damping |Reactive |Power |SSR |
| | |Stability |Power |Power Compensation |Flow |Migration |
| | | |Oscillation | |Control | |
|SVC | √ | √ | √ | √ | - | - |
|STATCOM | √ | √ | √ | √ | - | - |
|TCSC | √ | √ | √ | - | √ | √ |
|SSSC | √ | √ | √ | √ | √ | √ |
|UPFC | √ | √ | √ | √ | √ | √ |

Chapter 06

Renewable Energy Sources

6.1 Introduction

Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable (naturally replenished).

Such renewables are recognised as vital inputs for sustainability and so encouraging their growth is significant. Subjects will include power from wind, water, biomass, sunshine and other such continuing sources, including waste. Although the scale of local application ranges from tens to many millions of watts, and the totality is a global resource, four questions are asked for practical application:

» How much energy is available in the immediate environment - what are the resources? » For what purposes can this energy be used – what is the end-use? » What is the environmental impact of the technology – is it sustainable? » What is the cost of the energy – is it cost-effective?

6.2 Mainstream forms of renewable energy:

Renewable energy flows involve natural phenomena such as sunlight, wind, tides, biomass, geothermal heat etc. Renewable energy is derived from natural processes that are replenished constantly. In its various forms, it derives directly from the sun, or from heat generated deep within the earth. Included in the definition is electricity and heat generated from solar, wind, ocean, hydropower, biomass, geothermal resources, and bio-fuels and hydrogen derived from renewable resources. Each of these sources has unique characteristics which influence how and where they are used.

6.2.1 Solar energy

Solar energy is the energy derived from the sun through the form of solar radiation. Solar powered electrical generation relies on photovoltaics and heat engines. Solar technologies are broadly characterized as either passive solar or active solar depending on the way they capture, convert and distribute solar energy. Active solar techniques include the use of photovoltaic panels and solar thermal collectors to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air.
6.2.2 Biomass

Biomass, a renewable energy source, is biological material derived from living, or recently living organisms, such as wood, waste, and alcohol fuels. Biomass is commonly plant matter grown to generate electricity or produce heat. Through the process of photosynthesis, plants capture the sun's energy. When the plants are burned, they release the sun's energy they contain. In this way, biomass functions as a sort of natural battery for storing solar energy.

6.2.3 Fossil fuel:

Fossil fuels are fuels formed by natural resources such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years. The fossil fuels include coal, petroleum, and natural gas which contain high percentages of carbon. It is noted that fossil fuels are non-renewable resources because they take millions of years to form, and reserves are being depleted much faster than new ones are being formed but they plays (specially coal) a very important role in the field of generation of electricity in Bangladesh.

6.2.4 Wind power:

Wind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electricity. This could require large amounts of land to be used for wind turbines, particularly in areas of higher wind resources.

6.2.5 Hydropower:

Hydropower, hydraulic power or water power is power that is derived from the force or energy of moving water, which may be harnessed for useful purposes. Hydropower produces essentially no carbon dioxide or other harmful emissions, in contrast to burning fossil fuels, and is not a significant contributor to global warming through CO2. Hydroelectric power can be far less expensive than electricity generated from fossil fuels or nuclear energy. Areas with abundant hydroelectric power attract industry. Environmental concerns about the effects of reservoirs may prohibit development of economic hydropower sources. The chief advantage of hydroelectric dams is their ability to handle seasonal (as well as daily) high peak loads. When the electricity demands drop, the dam simply stores more water (which provides more flow when it releases). Some electricity generators use water dams to store excess energy (often during the night), by using the electricity to pump water up into a basin. Electricity can be generated when demand increases. In practice the utilization of stored water in river dams is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands.

6.2.6 Bio-fuel:

Liquid bio-fuel is usually either bio-alcohol such as bio-ethanol or oil such as bio-diesel. Bio-ethanol is an alcohol made by fermenting the sugar components of plant materials and it is made mostly from sugar and starch crops. With advanced technology being developed, cellulosic biomass, such as trees and grasses, are also used as feedstocks for ethanol production. Ethanol can be used as a fuel for vehicles in its pure form, but it is usually used as a gasoline additive to increase octane and improve vehicle emissions. Bio-diesel is made from vegetable oils, animal fats or recycled greases. Bio-diesel can be used as a fuel for vehicles in its pure form, but it is usually used as a diesel additive to reduce levels of particulates, carbon monoxide, and hydrocarbons from diesel-powered vehicles. Bio-diesel is produced from oils or fats using transesterification.

Chapter 07

Energy Storage Devices (DC)

7.1 Introduction:

Energy storage media are the matter that store some form of energy at a later time to perform some useful operation. A battery is a good example of such devices. In this section we will discuss different types of DC energy storage devices.

7.2 Battery basics:

A battery cell consists of five major components:

(1) Electrodes - anode and cathode; (2) Separators; (3) Terminals; (4) Electrolyte; and (5) A case or enclosure.

Battery cells are grouped together into a single mechanical and electrical unit called a battery module. These modules are electrically connected to form a battery pack, which powers the systems.

There are two terminals per battery, one negative and one positive. The electrolyte can be a liquid, gel, or solid material. Traditional batteries, such as Lead-Acid (Pb-Acid), Nickel-Cadmium (NiCd), and others have used a liquid electrolyte. This electrolyte may either be acidic or alkaline, depending on the type of battery. In many of the advanced batteries under development today for EV applications, the electrolyte is a gel, paste, or resin. Examples of these battery types are advanced sealed Pb-Acid, NiMH, and Li-ion batteries. Lithium-Polymer batteries, presently under development, have a solid electrolyte. In the most basic terms, a battery is an electrochemical cell in which an electric potential (voltage) is generated at the battery terminals by a difference in potential between the positive and negative electrodes.

7.3 The Lead-Acid (Pb-Acid) battery:

A flooded or wet battery is one that requires maintenance by periodic replenishment of distilled water. The water is added into each cell of the battery through the vent cap. Although they have large service lives of up to 20 years, they have been known to be operational for a longer time (up to 40 years). The design of flooded Lead-Acid battery comprises negative plates made of lead (or a Lead alloy) sandwiched between positive plates made of Lead (or a Lead alloy) with calcium or antimony as an additive. The insulator (termed as a separator) is a microporous material that allows the chemical reaction to take place while preventing the electrodes from shorting, owing to contact.

The negative and the positive plates are pasted with an active material lead oxide (PbO2) and sometimes lead sulphate (PbSO4). The active material provides a large surface area for storing electrochemical energy. Each positive plate is welded together and attached to a terminal post (+). Using the same welding each negative plate is welded together and attached to a terminal post (-). The plate assembly is placed into a polypropylene casing. The cover with a vent cap/flame arrestor and hydrometer hole is fitted onto the container assembly. The container assembly and the cover plate are glued to form a leak-proof seal. The container is filled with an electrolyte solution of specific gravity 1.215. The electrolyte solution is a combination of sulphuric acid (H2SO4) and distilled water. Upon charging or application of an electric current, the flooded lead-acid battery undergoes an electrochemical reaction. This creates the cell’s potential or voltage. Based on the principle of electrochemistry, two dissimilar metals (positive and negative plates) have a potential difference (cell voltage). Upon assembly of the plates, a float charge is placed on the battery to maintain a charge or polarization of the plates. During the charge phase, water in the electrolyte solution is broken down by electrolysis. Oxygen evolves at the positive plates and hydrogen evolves at the negative plates. The evolution of hydrogen and oxygen results in up to 30% recombination. A higher battery efficiency means that no watering is required, sharply reducing the maintenance cost compared to the flooded lead-acid battery. It is the recombination factor that improves the VRLA (Valve-Regulated Lead-Acid) battery efficiency. In the VRLA battery, the efficiency is 95 to 99%. Special ventilation and acid containment requirements are minimal with VRLA batteries. This allows batteries to be collocated alongside electronics. The two types of VRLA batteries are the Absorbed Glass Mat (AGM) based battery and the gel technology battery.

7.4 The NiMH battery:

Nickel-metal hydride cell, abbreviated NiMH, is a type of secondary electrochemical cell similar to the nickel-cadmium cell. The NiMH battery uses a hydrogen-absorbing alloy for the negative electrode instead of cadmium. The specific energy density for NiMH material is approximately 70 Wh/kg (250 kJ/kg), compared to 40–60 Wh/kg for the more common nickel-cadmium, or 100-160 Wh/kg for Li-ion. NiMH has a volumetric energy density of about 300 Wh/L (1080 MJ/m³), significantly better than nickel-cadmium at 50-150 Wh/L, and about the same as Li-ion at 250-360 Wh/L.

7.5 The Li-ion battery:

Li-ion batteries are the third type most likely to be commercialized for EV applications. Because lithium is the metal with the highest negative potential and lowest atomic weight, batteries using lithium have the greatest potential for attaining the technological breakthrough that will provide EVs with the greatest performance characteristics in terms of acceleration and range. Unfortunately, lithium metal, on its own, is highly reactive with air and with most liquid electrolytes. To avoid the problems associated with metal lithium, lithium intercalated graphitic carbons (LixC) are used and show good potential for high performance, while maintaining cell safety.

During a Li-ion battery’s discharge, lithium ions (Li+) are released from the anode and travel through an organic electrolyte toward the cathode. Organic electrolytes (i.e., non-aqueous) are stable against the reduction by lithium. Oxidation at the cathode is required as lithium reacts chemically with the water of aqueous electrolytes. When the lithium ions reach the cathode, they are quickly incorporated into the cathode material. This process is easily reversible. Because of the quick reversibility of the lithium ions, lithium-ion batteries can charge and discharge faster than Pb-acid and NiMH batteries. In addition, Li-ion batteries produce the same amount of energy as NiMH cells, but they are typically 40% smaller and weigh half as much.

Solid-state Li-ion batteries offer several advantages over their liquid electrolyte counterparts. Although the liquid Li-ion batteries have been around for several years, the solid-state Li-ion battery introduced in 1995 into the commercial market is substantially superior. Energy densities exceed 100Whr/kg and 200Whr/L. The operating temperature of these batteries is also wide, from -20°C to 60°C.

During the charging process, the Li-ion cell anode equation is represented as:

LixC6 + xLi+ + xe- → LiC6

And the Li-ion cell cathode equation is represented as,

LiCoO2 → xLi + xe- + Li(1 - x)CoO2

During the discharging process, the Li-ion cell anode equation is represented as,

LiC6 → LixC6 + xLi+ + xe-

And the Li-ion cell cathode equation is represented as,

xLi+ + xe- + Li(1 - x)CoO2 → LiCoO2

7.6 The Li-Polymer battery:

Lithium-polymer batteries are the fourth most likely type of battery to be commercialized for EV applications. The discovery of nonmetallic solids capable of conducting ions has allowed for the development of these batteries. Lithium-polymer batteries have anodes made of either lithium or carbon intercalated with lithium. One candidate cathode under evaluation contains vanadium oxide (V6O13). This particular battery chemistry has one of the greatest potentials for the highest specific energy and power. Unfortunately, design challenges associated with kinetics of the battery electrodes, the ability of the cathode and anode to absorb and release lithium ions, has resulted in lower specific power and limited cycle life for lithium-polymer batteries.

The current collector for lithium-polymer batteries is typically made of either copper or aluminum foil surrounded by a low thermal conductivity material such as polyurethane. The battery case is made of polypropylene, reinforced polypropylene, or polystyrene. Lithium-polymer batteries are considered solid-state batteries since their electrolyte is a solid. The most common polymer electrolyte is polyethylene oxide complexed with an appropriate electrolyte salt.

The polymers can conduct ions at temperatures above about 60°C (140°F), allowing for the replacement of flammable liquid electrolytes by polymers of high molecular weight. Since the conductivity of these polymers is low, the batteries must be constructed in thin films ranging from 50 to 200 mm thick. There is, however, a great safety advantage to this type of battery construction. Because the battery is solid-state by design, the materials will not flow together and electrolyte will not leak out in case there is a rupture in the battery case during an EV accident. Because the lithium is intercalated into carbon anodes, the lithium is in ionic form and is less reactive than pure lithium metal. Another major advantage of this type of battery construction is that a lithium-polymer battery can be formed in any size or shape.

7.8 Comparison between electrolytes fuel-cell electrolytes:

|Type |Electrolyte |Temperature (oC) |Efficiency |
|Alkaline |KOH (OH-) |60-120 |High efficiency |
|PEMFC |Polymer electrolyte (H+) |20-120 |High power density |
|PAFC |Phosphoric acid (H+) |160-220 |Limited efficiency |
|MCFC |Molten Carbonate (CO3-) |550-650 |Complex control |
|SOFC |Solid doped Zr-Oxide (O-) |850-1000 |Stationary |

Chapter 08

Inverter

8.1 Introduction:

An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. An inverter is essentially the opposite of a rectifier.

Static inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high-voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries.

8.2 Classification of inverters:

An inverter circuit is used to convert DC power to AC power. This conversion can be achieved either by transistors or by SCR's. For low and medium power inputs, transistorized inverters are suitable but for high power outputs, SCR's should be used. Depending on inversion technologies, inverters can be classified on the basis of following number of factors:

a) Classification according to the nature of input source: i) Voltage Source inverter (VSI) ii) Current Source Inverter (CSI) b) Classification according to the wave-shape of the output voltage: i) Square-wave inverter ii) Quasi-square wave inverter iii) Pulse-width modulated inverter c) Classification according to the use of Thyristor switches: i) According to the method of commutation 1. Line commutated inverter 2. Forced commutated inverter ii) According to the connections 1. Series inverter 2. Parallel inverter 3. Bridge inverters: Bridge inverters are further classified as » Half-bridge inverter » Full-bridge inverter
8.3 Single-phase voltage source inverter:

Single-phase VSI can be found as half-bridge and full-bridge topologies. Although, the power range they cover is the low one, they are widely used in power supplies, single-phase UPSs, and currently to form high-power static power topologies, such as the multicell configurations.

8.3.1 Half-bridge VSI

Figure 8.1 shows the power topology of a half-bridge VSI, where two large capacitors are required to provide a neutral point N, such that each capacitor maintains a constant voltage vi /2. Because the current harmonics injected by the operation of the inverter are low-order harmonics, a set of large capacitors (C+ and C−) is required. It is clear that both switches S+ and S− cannot be on simultaneously because a short circuit across the dc link voltage source vi would be produced. There are two defined (states 1 and 2) and one undefined (state 3) switch state as shown in Table 8.1. In order to avoid the short circuit across the dc bus and the undefined ac output-voltage condition, the modulating technique should always ensure that at any instant either the top or the bottom switch of the inverter leg is on.

[pic]

Figure 8.1: Single-phase half-bridge VSI

Table 8.1: Switch states for a half-bridge single-phase VSI

| State |State # | v0 |Components conducting |
|S+ is ON and S- is OFF |1 |vi/2 |S+ if i0 > 0 |
| | | |D+ if i0 < 0 |
|S- is ON and S+ is OFF |2 |-vi/2 |D- if i0 > 0 |
| | | |S- if i0 < 0 |
|S+ and S- are all OFF |3 |-vi/2 |D- if i0 > 0 |
| | |vi/2 |D+ if i0 < 0 |

Figure 8.2 shows the ideal waveforms associated with the half-bridge inverter shown in Fig. 8.1. The states for the switches S+ and S− are defined by the modulating technique, which in this case is a carrier-based PWM.

[pic]

Figure 8.2: The half-bridge VSI. Ideal waveforms for the SPWM (ma = 0.8, mf = 9); (a) carrier and modulating signals; (b) switch S+ state; (c) switch S− state; (d) ac output voltage; (e) ac output voltage spectrum; (f) ac output current; (g) dc current; (h) dc current spectrum; (i) switch S+ current; and (j) diode D+ current.

8.3.2 Full-bridge VSI:

Figure 8.3 shows the power topology of a full-bridge VSI. This inverter is similar to the half-bridge inverter; however, a second leg provides the neutral point to the load. As expected, both switches S1+ and S1− (or S2+ and S2−) cannot be on simultaneously because a short circuit across the dc link voltage source vi would be produced. There are four defined (states 1, 2, 3, and 4) and one undefined (state 5) switch state as shown in Table 8.2.

The undefined condition should be avoided so as to be always capable of defining the ac output voltage always. In order to avoid the short circuit across the dc bus and the undefined ac output voltage condition, the modulating technique should ensure that either the top or the bottom switch of each leg is on at any instant. It can be observed that the ac output voltage can take values up to the dc link value vi , which is twice that obtained with half-bridge VSI topologies.

[pic]

Figure 8.3: Single-phase full-bridge VSI.

Table 8.2: Switch state for a full-bridge single-phase VSI.

|State |State # |vaN |vbN |vo |Components |
| | | | | |Conducting |
|S1+ & S2- ON |1 |vi/2 |-vi/2 |vi |S1+ & S2- if io > 0 |
|S1- & S2+ OFF | | | | |D1+ & D2- if io < 0 |
|S1- & S2+ ON |2 |-vi/2 |vi/2 |-vi |D1- & D2+ if io > 0 |
|S1+ & S2- OFF | | | | |S1- & S1+ if io < 0 |
|S1+ & S2+ ON |3 |vi/2 |vi/2 |0 |S1+ & D2+ if io > 0 |
|S1- & S2- OFF | | | | |D1+ & S2+ if io < 0 |
|S1- & S2- ON |4 |-vi/2 |-vi/2 |0 |D1- & S2- if io > 0 |
|S1+ & S2+ OFF | | | | |S1- & D2- if io < 0 |
|S1-, S2-, S1+ & S2+ all are OFF|5 |-vi/2 |vi/2 |vi |D1- & D2+ if io > 0 |
| | |vi/2 |-vi/2 |-vi |D1+ & D2- if io < 0 |

[pic]

Figure 8.4: The full-bridge VSI. Ideal waveforms for unipolar SPWM (ma = 0.8, mf = 8); (a) carrier and modulating signals; (b) switch S1+ state; (c) switch S2+ state; (d) ac output voltage; (e) ac output voltage spectrum; (f) ac output current; (g) dc current; (h) dc current spectrum; (i) switch S1+ current; and (j) diode D1+ current.

8.4 Three-phase voltage source inverters:

Single-phase VSIs cover low-range power applications and three-phase VSIs cover medium to high power applications. The main purpose of these topologies is to provide a three-phase voltage source, where the amplitude, phase, and frequency of the voltages should always be controllable. Although most of the applications require sinusoidal voltage waveforms (e.g. ASDs, UPSs, FACTS, VAR compensators), arbitrary voltages are also required in some emerging applications (e.g. active filters, voltage compensators).

[pic]

Figure 8.5: Three-phase VSI topology.

The standard three-phase VSI topology is shown in Fig. 8.5 and the eight valid switch states are given in Table 8.3. As in single-phase VSIs, the switches of any leg of the inverter (S1 and S4, S3 and S6, or S5 and S2) cannot be switched on simultaneously because this would result in a short circuit across the dc link voltage supply. Similarly, in order to avoid undefined states in the VSI, and thus undefined ac output line voltages, the switches of any leg of the inverter cannot be switched off simultaneously as this will result in voltages that will depend upon the respective line current polarity. Of the eight valid states, two of them (7 and 8 in Table 8.3) produce zero ac line voltages. In this case, the ac line currents freewheel through either the upper or lower components. The remaining states (1 to 6 in Table 8.3) produce non-zero ac output voltages. In order to generate a given voltage waveform, the inverter moves from one state to another. Thus the resulting ac output line voltages consist of discrete values of voltages that are vi, 0, and −vi for the topology shown in Fig. 8.5. The selection of the states in order to generate the given waveform is done by the modulating technique that should ensure the use of only the valid states.

Table 8.3: Valid switch states for three-phase VSI.

|State |State # |vab |vbc |vca |
|S1, S2 & S6 are ON |1 |vi |0 |-vi |
|S4, S5 & S3 are OFF | | | | |
|S2, S3 & S1 are ON |2 |0 |vi |-vi |
|S5, S6 & S4 are OFF | | | | |
|S3, S4 & S2 are ON |3 |-vi |vi |0 |
|S6, S1 & S5 are OFF | | | | |
|S4, S5 & S3 are ON |4 |-vi |0 |vi |
|S1, S2 & S6 are OFF | | | | |
|S5, S6 & S4 are ON |5 |0 |-vi |vi |
|S2, S3 & S1 are OFF | | | | |
|S6, S1 & S5are ON |6 |vi |-vi |0 |
|S3, S4 & S2 are OFF | | | | |
|S1, S3 & S5 are ON |7 |0 |0 |0 |
|S4, S6 & S2 are OFF | | | | |
|S4, S6 & S2 are ON |8 |0 |0 |0 |
|S1, S3 & S5 are OFF | | | | |

[pic]

Figure 8.6: The three-phase VSI. Ideal waveforms for SPWM (ma = 0.8, mf = 9); (a) carrier and modulating signals; (b) switch S1 state; (c) switch S3 state; (d) ac output voltage; (e) ac output voltage spectrum; (f) ac output current; (g) dc current; (h) dc current spectrum; (i) switch S1 current; and (j) diode D1 current.

8.5 Application of inverters:

Inverters are used in the following systems: » Variable speed AC motor drives. » Aircraft power supplies. » HVDC Transmission. » Uninterruptible power supply. » Regenerative DC/AC drives. » Induction heating etc.

Chapter 09

Smart Grid

9.1 Introduction:

A smart grid delivers electricity from suppliers to consumers using two-way digital technology to control appliances at consumers' homes to save energy, reduce cost and increase reliability and transparency. It overlays the electricity distribution grid with an information and net metering system. Such a modernized electricity network is being promoted by many governments as a way of addressing energy independence, global warming and emergency resilience issues. Smart meters may be part of a smart grid, but alone do not constitute a smart grid. A smart grid includes an intelligent monitoring system that keeps track of all electricity flowing in the system. It also incorporates the use of superconductive transmission lines for less power loss, as well as the capability of integrating renewable electricity such as solar and wind. When power is least expensive the user can allow the smart grid to turn on selected home appliances such as washing machines or factory processes that can run at arbitrary hours. At peak times it could turn off selected appliances to reduce demand. The smart power grid distributed energy system would provide the platform for the use of renewable sources and adequate emergency power for major metropolitan load centers and would safeguard in preventing the complete blackout of the interconnected power systems due to man-made events and environmental calamity and would provide the ability to break up the interconnected power systems into the cluster smaller regions.

9.2 What is a smart grid?

The function of an electrical grid is not a single entity but an aggregate of multiple networks and multiple power generation companies with multiple operators employing varying levels of communication and coordination, most of which is manually controlled. Smart grids increase the connectivity, automation and coordination between these suppliers, consumers and networks that perform either long distance transmission or local distribution tasks. Transmission networks move electricity in bulk over medium to long distances, are actively managed, and generally operate from 345kV to 800kV over AC and DC lines. Local networks traditionally moved power in one direction, "distributing" the bulk power to consumers and businesses via lines operating at 132kV and lower. This paradigm is changing as businesses and homes begin generating more wind and solar electricity, enabling them to sell surplus energy back to their utilities.

9.3 Smart grid functions:

Before examining particular technologies, a proposal can be understood in terms of what it is being required to do. The governments and utilities funding development of grid modernization have defined the functions required for smart grids. According to the United States Department of Energy's Modern Grid Initiative report, a modern smart grid must:

1. Be able to heal itself
2. Motivate consumers to actively participate in operations of the grid
3. Resist attack
4. Provide higher quality power that will save money wasted from outages
5. Accommodate all generation and storage options
6. Enable electricity markets to flourish
7. Run more efficiently
8. Enable higher penetration of intermittent power generation sources

Self-healing:

Using real-time information from embedded sensors and automated controls to anticipate, detect, and respond to system problems, a smart grid can automatically avoid or mitigate power outages, power quality problems, and service disruptions. As applied to distribution networks, there is no such thing as a "self healing" network. If there is a failure of an overhead power line, given that these tend to operate on a radial basis (for the most part) there is an inevitable loss of power. In the case of urban/city networks that for the most part are fed using underground cables, networks can be designed (through the use of interconnected topologies) such that failure of one part of the network will result in no loss of supply to end users.

Consumer participation:

A smart grid is, in essence, an attempt to require consumers to change their behavior around variable electric rates or to pay vastly increased rates for the privilege of reliable electrical service during high-demand conditions. Smart grid incorporates consumer equipment and behavior in grid design, operation, and communication. This enables consumers to better control (or be controlled by) “smart appliances” and “intelligent equipment” in homes and businesses, interconnecting energy management systems in “smart buildings” and enabling consumers to better manage energy use and reduce energy costs. Advanced communications capabilities equip customers with tools to exploit real-time electricity pricing, incentive-based load reduction signals, or emergency load reduction signals.

Resist attack:

Smart grid technologies better identify and respond to man-made or natural disruptions. Real-time information enables grid operators to isolate affected areas and redirect power flows around damaged facilities. One of the most important issues of resist attack is the smart monitoring of power grids, which is the basis of control and management of smart grids to avoid or mitigate the system-wide disruptions like blackouts. The traditional monitoring is based on weighted least square (WLS) which is very weak and prone to fail when gross errors (including topology errors, measurement errors or parameter errors) are present. New technology of state monitor is needed to achieve the goals of the smart grids.

High quality power:

Assuring more stable power provided by smart grid technologies will reduce downtime and prevent high losses.

Accommodate generation options:

As smart grids continue to support traditional power loads they also seamlessly interconnect fuel cells, renewables, microturbines, and other distributed generation technologies at local and regional levels. Integration of small-scale, localized, or on-site power generation allows residential, commercial, and industrial customers to self-generate and sell excess power to the grid with minimal technical or regulatory barriers. This also improves reliability and power quality, reduces electricity costs, and offers more customer choice.

Enable electricity market:

Significant increases in bulk transmission capacity will require improvements in transmission grid management. Such improvements are aimed at creating an open marketplace where alternative energy sources from geographically distant locations can easily be sold to customers wherever they are located. Intelligence in distribution grids will enable small producers to generate and sell electricity at the local level using alternative sources such as rooftop-mounted photo voltaic panels, small-scale wind turbines, and micro hydro generators. Without the additional intelligence provided by sensors and software designed to react instantaneously to imbalances caused by intermittent sources, such distributed generation can degrade system quality.

Optimize assets:

A smart grid can optimize capital assets while minimizing operations and maintenance costs. Optimized power flows reduce waste and maximize use of lowest-cost generation resources. Harmonizing local distribution with interregional energy flows and transmission traffic improves use of existing grid assets and reduces grid congestion and bottlenecks, which can ultimately produce consumer savings.

Enable high penetration of intermittent generation sources:

Climate change and environmental concerns will increase the amount of renewable energy resources. These are for the most part intermittent in nature. Smart Grid technologies will enable power systems to operate with larger amounts of such energy resources since they enable both the suppliers and consumers to compensate for such intermittency.

9.4 Features of smart grid:

Existing and planned implementations of smart grids provide a wide range of features to perform the required functions.

Load adjustment

The total load connected to the power grid can vary significantly over time. Although the total load is the sum of many individual choices of the clients, the overall load is not a stable, slow varying, average power consumption. Imagine the increment of the load if a popular television program starts and millions of televisions will draw current instantly. Traditionally, to respond to a rapid increase in power consumption, faster than the start-up time of a large generator, some spare generators are put on a dissipative standby mode. A smart grid may warn all individual television sets, or another larger customer, to reduce the load temporarily (to allow time to start up a larger generator) or continuously (in the case of limited resources). Using mathematical prediction algorithms it is possible to predict how many standby generators need to be used, to reach a certain failure rate. In the traditional grid, the failure rate can only be reduced at the cost of more standby generators. In a smart grid, the load reduction by even a small portion of the clients may eliminate the problem.

Demand response support:

Demand response support allows generators and loads to interact in an automated fashion in real time, coordinating demand to flatten spikes. Eliminating the fraction of demand that occurs in these spikes eliminates the cost of adding reserve generators, cuts wear and tear and extends the life of equipment, and allows users to cut their energy bills by telling low priority devices to use energy only when it is cheapest.

Decentralization of power generation:

Another element of fault tolerance of smart grids is decentralized power generation. Distributed generation allows individual consumers to generate power onsite, using whatever generation method they find appropriate. This allows individual loads to tailor their generation directly to their load, making them independent from grid power failures. Classic grids were designed for one-way flow of electricity, but if a local sub-network generates more power than it is consuming, the reverse flow can raise safety and reliability issues. A smart grid can manage these situations.

Price signaling to consumers:

In many countries the electric utilities have installed double tariff electricity meters in many homes to encourage people to use their electric power during night time or weekends, when the overall demand from industry is very low. During off-peak time the price is reduced significantly, primarily for heating storage radiators or heat pumps with a high thermal mass, but also for domestic appliances. This idea will be further explored in a smart grid, where the price could be changing in seconds and electric equipment is given methods to react on that. Also, personal preferences of customers, for example to use only green energy, can be incorporated in such a power grid.

9.5 Smart grid technologies:

The bulk of smart grid technologies are already used in other applications such as manufacturing and telecommunications and are being adapted for use in grid operations. In general, smart grid technology can be grouped into five key areas.

Integrated communication:

Some communications are up to date, but are not uniform because they have been developed in an incremental fashion and not fully integrated. In most cases, data is being collected via modem rather than direct network connection. Areas for improvement include: substation automation, demand response, distribution automation, Supervisory Control And Data Acquisition (SCADA), energy management systems, wireless mesh networks and other technologies, power-line carrier communications, and fiber-optics. Integrated communications will allow for real-time control, information and data exchange to optimize system reliability, asset utilization, and security.

Sensing and measurement:

Core duties are evaluating congestion and grid stability, monitoring equipment health, energy theft prevention, and control strategies support. Technologies include: advanced microprocessor meters (smart meter) and meter reading equipment, wide-area monitoring systems, dynamic line rating (typically based on online readings by Distributed temperature sensing combined with Real Time Thermal Rating (RTTR) systems), electromagnetic signature measurement/analysis, time-of-use and real-time pricing tools, advanced switches and cables, backscatter radio technology, and Digital protective relays.

» Smart meters: A smart grid replaces analog mechanical meters with digital meters that record usage in real time. Smart meters are similar to Advanced Metering Infrastructure meters and provide a communication path extending from generation plants to electrical outlets (smart socket) and other smart grid-enabled devices. By customer option, such devices can shut down during times of peak demand.

» Phasor measurement units: High speed sensors called PMUs distributed throughout their network can be used to monitor power quality and in some cases respond automatically to them. Phasors are representations of the waveforms of alternating current, which ideally in real-time, are identical everywhere on the network and conform to the most desirable shape.

Advanced components:

Innovations in superconductivity, fault tolerance, storage, power electronics, and diagnostics components are changing fundamental abilities and characteristics of grids. Technologies within these broad R&D categories include: flexible alternating current transmission system devices, high voltage direct current, first and second generation superconducting wire, high temperature superconducting cable, distributed energy generation and storage devices, composite conductors, and “intelligent” appliances.

Advanced control:

Power system automation enables rapid diagnosis of and precise solutions to specific grid disruptions or outages. These technologies rely on and contribute to each of the other four key areas. Three technology categories for advanced control methods are: distributed intelligent agents (control systems), analytical tools (software algorithms and high-speed computers), and operational applications (SCADA, substation automation, demand response, etc). The Voltage Stability Monitoring & Control (VSMC) software uses a sensitivity-based successive linear programming method to reliably determine the optimal control solution.

Improved interfaces and decision support:

Information systems that reduce complexity so that operators and managers have tools to effectively and efficiently operate a grid with an increasing number of variables. Technologies include visualization techniques that reduce large quantities of data into easily understood visual formats, software systems that provide multiple options when systems operator actions are required, and simulators for operational training and “what-if” analysis.

Conclusion:

Through the presentation of this paper we have done a study based work where different types of FACTS controllers, energy storage devices, inverters and several components have been discussed and the comparison among the devices has been reviewed. This comparison shows which FACTS controller and the sub devices are economical if they are used in a smart grid. If a simulation is performed by simulator software, such as MATLAB, a more accurate result is get and we could find the exact FACTS device which would be economical. We hope we will extend our proposal through simulation in the future.

References:

1. http://en.wikipedia.org/wiki/Flexible_AC_transmission_system 2. Xiao-Ping Zhang, Christian Rehtanz, Bikash Pal - Flexible AC Transmission Systems: Modelling and Control – Springer, 2006. ISBN (13): 978-3-540-30606-1 3. R. Mohan Mathur, Rajiv K. Varma - Thyristor-Based FACTS Controllers for Electrical Transmission Systems – IEEE Press & JOHN WILEY, 2002. ISBN: 0-471-20643-1 4. K.R.Padiyar - FACTS Controllers in Power Transmission and Distribution - NEW AGE, 2007. ISBN (13): 978-81-224-2541-3 5. Vijay K. Sood– HVDC and FACTS Controllers: Applications of Static Converters in Power systems – KLUWER, 2004. ISBN: 1-4020-7891-9 6. Sandeep Dhameja – Electric Vehicle Battery Systems –Newnes, 2002 ISBN: 0-7506-9916-7 7. http://en.wikipedia.org/wiki/Renewable_energy 8. http://en.wikipedia.org/wiki/Smart_grid 9. http://en.wikipedia.org/wiki/Power_grid

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