CHAPTER 4 POWER QUALITY AND VAR COMPENSATION IN DISTRIBUTION SYSTEMS

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1 84 CHAPTER 4 POWER QUALITY AND VAR COMPENSATION IN DISTRIBUTION SYSTEMS 4.1 INTRODUCTION Now a days, the growth of digital economy implies a widespread use of electronic equipment not only in the industrial and commercial sector, but in the domestic environment too. Studies undertaken in different countries on the contribution of information and communication technology to the consumption of electricity conclude that office and telecommunication equipment used in the non-residential sector represents about 3% or 4% of the annual consumption of electricity. This will not only bring about a greater demand for power, but in addition a higher level of power quality and reliability, in quantities and time frames that have not been experienced before. It has been estimated that more than 30% of the power, currently being drawn from utility companies is now heading for sensitive equipment and this trend is increasing day by day. Further, with the deregulation of the power industry, competitive pressures, force the electric utilities to cut the costs, which sometimes affect power quality and reliability. Hence, it must be ensured by suitable regulations that customer do not suffer from reduced power quality and reliability. This chapter intends to address the issue of Importance of power quality, modeling and analysis of Distribution Static Var Compensator

2 85 (D-SVC), the most widely used power electronic device, using an industry standard power system package, namely PSCAD/EMTDC, meant for studying the transient behavior of electrical networks Importance of Power Quality Before the widespread use of power electronic equipment, microprocessors for industrial control and automation in factories & offices, the minor variations in power did not seriously affect the operation of conventional equipment such as lights and induction motors. If the supply voltage dipped because of a fault (i.e., occurrence of voltage sag), the lights just dimmed and the induction motor produced a lower output. These days the effects of power interruptions are rather costly. Reference (Moreno- Munoz 2007) lists the following cases to illustrate the cost of short-duration power interruptions: a. One glass plant estimates that a momentary interruption less than a tenth of second can cost about One crore rupees. b. A major computer center reports that a 2-s interruption can cost some Three crore rupees. c. In some factories, following voltage sag, the restarting of assembly lines may require clearing the lines of damaged work, restarting of boilers and reprogramming automatic controls at a typical cost of rupees 25 lakhs, per incident. d. One automaker estimated that total losses from momentary power interruptions at all its plants run to about 50 crore rupees a year.

3 Common Disturbances in Distribution System Table 4.1 Categories and typical characteristics of power system electromagnetic phenomena Categories 1.0 Transients 1.1 Impulsive Nanosecond Microsecond Millisecond 1.2 Oscillatory Low frequency Medium frequency High frequency 2.0 Short-duration variations 2.1 Instantaneous Sag Swell 2.2 Momentary Interruption Sag Swell 2.3 Temporary Interruption Sag Swell 3.0 Long-duration variations 3.1 Interruption, sustained 3.2 Under voltages 3.3 Over voltages Typical Spectral Content 5 ns rise 1 µs rise 0.1 ms rise <5 khz khz MHz Typical Duration <50 ns 50 ns - 1 ms >1 ms ms 20 µs 5 µs cycles cycles cycles 30 cycles-3 s 30 cycles-3 s 3 s-1 min 3 s-1 min 3 s-1 min >1 min >1 min >1 min Typical voltage Magnitude 0-4 pu 0-8 pu 0 4 pu pu pu <0.1 pu pu pu <0.1 pu pu pu 0.0 pu 0.8 pu pu 4.0 Voltage imbalance Steady state 0.5-2%

4 87 The common disturbances in a power system are as follows: a. Voltage sag b. Voltage swell c. Momentary interruptions d. Transients e. Voltage unbalance f. Harmonics Table 4.1 describes the characteristics of these electromagnetic disturbances, the recommended practice for power quality (IEEE (2003) STANDARD) Short-Duration Voltage Variation and its Definitions A voltage sag (dip) is defined as a decrease in the root-meansquare (rms) voltage at the power frequency for periods ranging from a half cycle to a minute. It is caused by voltage drops due to fault currents or starting of large motors. Sag may trigger shutdown of process controllers or crashing of computer systems. A voltage swell is defined as an increase of rms voltage at the power frequency up to a level between 1.1 and 1.8 p.u. for periods ranging from a half cycle to a minute. An interruption occurs when the supply voltage decreases to less than 0.1 p.u. for a period of time not exceeding 1 min. Interruptions can be caused by faults, control malfunctions or equipment failures. All these types of disturbances, such as voltage sags, voltage swells and interruptions can be classified into three types, depending on their duration.

5 88 a. Instantaneous: cycles b. Momentary: 30 cycles-3 sec c. Temporary:3 sec -1 min It is helpful to distinguish the term outage used in reliability terminology from sustained interruption when the supply voltage is zero for longer than 1 min. Outage refers to the state of a component in a system that has failed to function as expected and is used to quantify reliability statistics regarding continuity of service, whereas sustained interruption as used in monitoring power quality to indicate the absence of voltage for long periods of time Solution to Power Quality Problems Various power electronics devices and their combinations are addressed for a particular range of voltage disturbance. A particular solution is applied taking into account demands for voltage quality and network configuration. Among the shunt connected device, the Distribution Static Var Compensator (D-SVC) and Distributed Static Synchronous Compensator (D- STATCOM) are the most important controllers for distribution network. Similarly, among the series connected device, Dynamic Voltage Regulator (DVR) is the most popular one. Static shunt power electronics voltage-quality controllers protect the utility electrical system from the unfavorable impact of customer loads. Shunt controllers are recommended mainly for mitigation of the causes of disturbance and not their effects in distanced nodes of a powerelectronics system. In the case when reduction of disturbances effects is required, which leads to protection of sensitive loads from the deterioration in the supply side voltage, we should rather prefer DVR.

6 DISTRIBUTION STATIC VAR COMPENSATOR (D-SVC) The Static Var Compensators (SVC) has been widely used by utilities since mid-1970s. SVC is based on conventional capacitors and inductors combined with fast semiconductor switches without turn off capability (i.e. SCR thyristor). The purpose, topologies and control principle of D-SVC is similar to SVC. They mimic the working principles of a variable shunt susceptance and use fast thyristor controllers with settling times of only a few fundamental frequency periods. From the operational point of view, the D-SVC adjusts its value automatically in response to changes in the operating conditions of the network. By suitably controlling its equivalent reactance, it is possible to regulate the voltage magnitude, thus enhancing significantly the performance of the power system (Acha et al 2002) Operating Principle From the operational point of view, the D-SVC behaves like a shunt-connected variable reactance, which either generates or absorbs reactive power in order to regulate the voltage magnitude at the point of connection to the AC network. In its simplest form, D-SVC consists of a Thyristor Control Reactor (TCR) in parallel with a bank of capacitors. Figure 4.1 shows the schematic diagram of the most basic FC/TCR arrangement of the D-SVC. Its equivalent variable susceptance representation is shown in Figure An ideal variable shunt compensator is assumed to contain no resistive components, i.e. G SVC =0. Accordingly, it draws no active power from the network. On the other hand, its reactive power is a function of nodal voltage magnitude at the connection point, say node j and the D-SVC equivalent susceptance, B SVC. Thus the active power (P j ) and reactive power (Q j ) in SVC are given by equations (4.1) and (4.2): P j = 0 (4.1) Q j = - V j 2 B SVC (4.2)

7 90 Figure 4.1 Schematic representation of an FC-TCR arrangement of D-SVC Figure 4.2 Variable susceptance representation of D-SVC Modeling of D-SVC for Transient Studies In order to illustrate the design and implementation of the D-SVC control system, a single phase circuit where the D-SVC is connected between the source and the load is selected as shown in Figure 4.3. The aim of the D- SVC in this application is to provide voltage regulation at the point of connection, following load variations. Initially the D-SVC is operated in open loop mode where the power exchange between the D-SVC and the AC system should be zero.

8 91 Figure 4.3 D-SVC Model used in transient studies When the breaker is closed, the load is increased and the voltage at the load point experiences voltage sag. The D-SVC controller operation changes to closed loop mode in order to restore the voltage back to the target value. The D-SVC parameters have been determined according to the compensation requirements for the case when the second load is connected. Based on the reactive power (Q SVC > 800 MVAR) required by Load 2, the D- SVC is sized with enough capacity to supply at least this reactive power in order to drive the voltage V bus back to the reference. Thus the Q SVC chosen in the present study is 840 MVAR. The values for the capacitance and the TCR inductance are then calculated based on this setting. The capacitive reactance is given by X C = (V bus ) 2 /Q SVC = (13.8 kv) 2 / 840 MVAR = (4.3) Let X L = X C /2 (4.4) => C = 14.04mF (4.5) => L = 0.361mH (4.6)

9 92 Once the capacitance and the inductance have been sized, it is necessary to determine the initial operating condition of the D-SVC. The selection of initial firing angle should be such that under this operating condition the D-SVC does not exchange any power with the AC system. It can be determined from the firing angle - reactive power characteristic of the SVC, which is a function of inductive and capacitive reactance. Firstly, it is necessary to obtain the effective reactance X SVC as a function of firing angle as below: X TCR = X L / ( sin ) (4.7) = 2 ( ) (4.8) where is the conduction angle of the thyristor. At = 90, the TCR conducts fully and the equivalent reactance X TCR = X L. At = 180, the TCR is blocked and its equivalent reactance becomes infinite. The total effective reactance of the D-SVC, including the TCR and capacitive reactance, is determined by the parallel combination of both components, as given in equations (4.9) and (4.10). X SVC = X C X TCR / (X C + X TCR ) (4.9) X SVC = X C X L / (X C [ 2( ) + sin2 ] X L ) (4.10) But, Q SVC = (V bus ) 2 / X SVC (4.11) From the equation (4.11), it is clear that Q SVC takes a value of zero when the effective reactance is extremely large i.e. X C [2( ) + sin2 ] X L => 0 (4.12)

10 93 With the D-SVC parameters used in this example, the value of firing angle that satisfies the equation 4.12 is found to be 115. This angle is used as the initial condition for in the open loop control of D-SVC. In order to illustrate the D-SVC s ability to provide voltage regulation at the point of connection, a simplified control scheme has been implemented for the single phase D-SVC circuit shown in Figure 4.3. The block diagram of the control scheme is shown in Figure 4.4. Figure 4.4 Basic control scheme designed for the D-SVC topology The scheme works as follows: The amplitude of the bus voltage V bus is measured and filtered. It is then compared against the voltage reference V ref. The error between these two signals is processed by a PI controller which causes a corresponding change in the firing angle. The value provided by the PI controller is used as input to the TCR firing control unit. The zero crossing of the bus voltage V bus is taken as the reference for the firing angle. 4.3 ELECTROMAGNETIC TRANSIENT MODELING OF D-SVC USING PSCAD SOFTWARE Experimental studies provide valuable data to evaluate models, test proposed control algorithms and analyze dynamic performance. In order to fully understand the dynamic performance of the modeled systems, simulation

11 94 study of the same is done. The simulator for evaluating the transient performance of the FACTS and Custom Power devices is implemented with PSCAD/EMTDC software. PSCAD/EMTDC is an industry standard simulation tool for studying the transient behavior of electrical networks. It provides a flexible user interface, enabling an integrated visual environment that supports all aspects associated with the simulation, including circuit assembly, run-time control, analysis and reporting. Its comprehensive library of models support most ac and dc components and controls of power plant, in such a way that FACTS, Custom power and HVDC systems can be modeled with speed and precision. It s modeling capabilities and highly complex algorithms are transparent to the user, leaving him free to concentrate his efforts on the analysis of results rather than on mathematical modeling Modeling of D-SVC Test System in PSCAD The simulation circuit for D-SVC developed in PSCAD using the topology described in section is shown in Figure 4.5. The associated control scheme is shown in Figure 4.6. The aim of the D-SVC in this application is to provide voltage regulation following load variations. Initially, the D-SVC is operated in open-loop mode where the power exchange between the D-SVC and the ac system is zero. When the breaker BRK1 is closed, the load is increased and the voltage at the load point experiences voltage sag. When the load is increased, the D-SVC controller operation changes to closed-loop mode in order to adjust its effective impedance X svc so that it injects capacitive current into the system to restore the voltage to the original value.

12 95 Figure 4.5 D-SVC Test system implemented in PSCAD Figure 4.6 D-SVC Control scheme implemented in PSCAD

13 Test Cases and Simulation Results with D-SVC The simulation studies carried out are as follows: 1) In the first case, D-SVC is operated in open-loop mode with =115. The voltage V rms at the load point is close to 0.96p.u. At time t = 0.6s, Load2 is switched on by closing BRK1. Under this condition, the voltage at the load point drops by as much as 16%, giving a V rms value equal to 0.8p.u. as shown in Figure ) In the second case, the closed-loop mode of SVC is enabled. At t=0.6s, the D-SVC begins to control the firing angle of the thyristors to =162 so that the voltage is regulated and driven back to the original value as shown in the Figure 4.8. Figure 4.7 Voltages V rms and V load with D-SVC in open-loop mode

14 97 Figure 4.8 Voltages V rms and V load with D-SVC in closed-loop mode From Figure 4.8, it is seen that the AC system incorporating D-SVC undergoes a voltage dip at 0.6 seconds when the second load is switched on, but precisely returns to the voltage close to the reference value of 0.96 p.u. within a period of less than 0.02 seconds. It is clear from these results that the D-SVC is an effective system controller which may be used to provide voltage regulation at the point of connection and to improve substantially the voltage quality in power systems. Tables 4.2 and 4.3 give the typical values of parameters namely real, reactive and apparent power, power factor and voltage both in the open and the closed loop mode. Table 4.2 Values in open loop mode Parameter Real Power Reactive Power Apparent Power Power Factor Voltage Value KW 445.4KVAR 464.1KVA 0.283(lag) 0.8 p.u.

15 98 Table 4.3 Values in closed loop mode Parameter Real Power Reactive Power Apparent Power Power Factor Voltage Value KW 638KVAR 681.3KVA 0.351(lag) 0.96 p.u. From the Table 4.2 and 4.3, it is clear that the voltage profile increases in the closed loop mode when compared to the open loop mode. The D-SVC injects reactive power in the closed loop mode with the increase in the load current. Overall there is an increase in the voltage profile and the power factor when the D-SVC is operating in the closed loop mode. 4.4 SUMMARY Introduction to Power Quality and Reliability, IEEE standard on Power quality and definition on short term voltage variation have been outlined. The chapter also discusses about VAR compensation in power system through D-SVC, its modeling and control strategy. The next chapter deals with Electromagnetic Transient modeling of Custom power devices, namely D-STATCOM, DVR and SSTS in PSCAD/ EMTDC environment.