Thyristor switched capacitor

Thyristor switched capacitorThyristor Switched Capacitor for reactive power management in

electrical systems

A seminar report submitted in partial fulfillment for the award of the degree of Bachelor of Engineering in Electrical & Electronics Engineering of the Visvesvaraya Technological University,

Belagavi

Submitted byName: RAJEEV RANJAN

USN: 2GI12EE037

Staff Counselor Staff Counselor

Department of Electrical & Electronics Engineering

Karnataka Law Society’s

GOGTE INSTITUTE OF TECHNOLOGYUDYAMBAG, BELAGAVI-59008

Visvesvaraya Technological University2015-2016

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Thyristor switched capacitorABSTRACT

In the modern power system the reactive power compensation is one of the main issues, the transmission of active power requires a difference in angular phase between voltages at the sending and receiving points (which is feasible within wide limits), whereas the transmission of reactive power requires a difference in magnitude of these same voltages (which is feasible only within very narrow limits). The reactive power is consumed not only by most of the network elements, but also by most of the consumer loads, so it must be supplied somewhere. If we can't transmit it very easily, then it ought to be generated where it is needed." (Reference Edited by T. J. E. Miller, Forward Page ix).Thus we need to work on the efficient methods by which VAR compensation can be applied easily and we can optimize the modern power system. VAR control technique can provides appropriate placement of compensation devices by which a desirable voltage profile can be achieved and at the same time minimizing the power losses in the system. This report discusses the transmission line requirements for reactive power compensation. In this report thyristor switched capacitor is explained which is a static VAR compensator used for reactive power management in electrical systems.

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Thyristor switched capacitor

CONTENTS Page No.

1. Certificate 2

2. Abstract 3

3. Contents with page number 4

4. Introduction 5

Reactive Power

Need For Reactive Power Compensation

5. Compensation Techniques 6-9

6. The thyristorswitched capacitor (TSC) 9-19

Principles of operation

Switching transients

7. Voltage/Current Characteristics 19-20

8. Advantages and Disadvantages 20

9. Result 21

10. Conclusion 21

11. Future Developments And Requirements 21

12. References 22

13. Hard copy of published paper related to this topic

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Thyristor switched capacitor4. INTRODUCTION

Reactive Power

Reactive power is the power that supplies the stored energy in reactive elements. Power, as we know, consists of two components, active and reactive power. The total sum of active and reactive power is called as apparent power.

In AC circuits, energy is stored temporarily in inductive and capacitive elements, which results in the periodic reversal of the direction of flow of energy between the source and the load. The average power after the completion of one whole cycle of the AC waveform is the real power, and this is the usable energy of the system and is used to do work, whereas the portion of power flow which is temporarily stored in the form of magnetic or electric fields and flows back and forth in the transmission line due to inductive and capacitive network elements is known as reactive power. This is the unused power which the system has to incur in order to transmit power.

Inductors (reactors) are said to store or absorb reactive power, because they store energy in the form of a magnetic field. Therefore, when a voltage is initially applied across a coil, a magnetic field builds up, and the current reaches the full value after a certain period of time. This in turn causes the current to lag the voltage in phase.

Need for Reactive power compensation.

The main reason for reactive power compensation in a system is: 1) the voltage regulation;2) increased system stability; 3) better utilization of machines connected to the system; 4) reducing losses associated with the system; and 5) to prevent voltage collapse as well as voltage sag. The impedance of transmission lines and the need for lagging VAR by most machines in a generating system results in the consumption of reactive power, thus affecting the stability limits of the system as well as transmission lines. Unnecessary voltage drops lead to increased losses which needs to be supplied by the source and in turn leading to outages in the line due to increased stress on the system to carry this imaginary power. Thus we can infer that the compensation of reactive power not only mitigates all these effects but also helps in better transient response to faults and disturbances. In recent times there has been an increased focus on the techniques used for the compensation and with better devices included in the technology, the compensation is made more effective. It is very much required that the lines be relieved of the obligation to carry the reactive power, which is better provided near the generators or the loads. Shunt compensation can be installed near the load, in a distribution substation or transmission substation.

5. C O M P E N SATION T EC HN I Q U E S

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Thyristor switched capacitorThe principles of both shunt and series reactive power compensation techniques are described below:

Shunt compensation

Fig 1.1

Fig 1.2

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Thyristor switched capacitorThe figure 1.1 comprises of a source V1, a power line and an inductive load. The figure1.1 shows the system without any type of compensation. The phasor diagram of these is also shown above. The active current Ip is in phase with the load voltage V2. Here, the load is inductive and hence it requires reactive power for its proper operation and this has to be supplied by the source, thus increasing the current from the generator and through the power lines. Instead of the lines carrying this, if the reactive power can be supplied near the load, the line current can be minimized, reducing the power losses and improving the voltage regulation at the load terminals. This can be done in three ways: 1) A voltage source. 2) A current source. 3) A capacitor. In this case, a current source device is used to compensate Iq, which is the reactive component of the load current. In turn the voltage regulation of the system is improved and the reactive current component from the source is reduced or almost eliminated. This is in case of lagging compensation. For leading compensation, we require an inductor.

Therefore we can see that, a current source or a voltage source can be used for both leading and lagging shunt compensation, the main advantages being the reactive power generated is independent of the voltage at the point of connection.

Series compensation

Fig 1.3

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Fig 1.4

Series compensation can be implemented like shunt compensation, i.e. with a current or a voltage source as shown in figure 1.4. We can see the results which are obtained by series compensation through a voltage source and it is adjusted to have unity power factor at V2. However series compensation techniques are different from shunt compensation techniques, as capacitors are used mostly for series compensation techniques. In this case, the voltage Vcomp has been added between the line and the load to change the angle V2’. Now, this is the voltage at the load side. With proper adjustment of the magnitude of Vcomp, unity power factor can be reached at V2

FACTS devices used

Flexible AC transmission system or FACTS devices used are:

1) VAR generators.

a) Fixed or mechanically switched capacitors. b) Synchronous condensers.c) Thyristorized VAR compensators.

1. Thyristors switched capacitors (TSCs).2. Thyristor controlled reactor (TCRs). 3. Combined TSC and TCR.4. Thyristor controlled series capacitor (TCSC).

2) Self Commutated VAR compensators.

a) Static synchronous compensators (STATCOMs).b) Static synchronous series compensators (SSSCs).c) Unified power flow controllers (UPFCs).d) Dynamic voltage restorers (DVRs

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6. THE THYRISTOR SWITCHED CAPACITOR ( T S C ) Thyristor switched capacitor is defined as 'a shunt connected, thyristor-switched capacitor whose effective reactance is varied in a stepwise manner by full or zero conduction operation of the thyristor valve'.

Principles of operationThe principle of the TSC is shown in Figures 6.12 and 6.13. The susceptance is adjusted by controlling the number of parallel capacitors in conduction. Each capacitor always conducts for an integral number of half cycles. With k capacitors in parallel, each controlled by a switch as in Figure 6.13, the total susceptance can be equal to that of any combination of the k individual susceptances taken 0, 1, 2 . . . . or k at a time. The total susceptance thus varies in a stepwise manner. In principle the steps can be made as small and as numerous as desired, by having a sufficient number of individually switched capacitors. For a given number k the maximum number of steps will be obtained when no two combinations are equal, which requires at least that all the individual susceptances be different. This degree of flexibility is not usually sought in power system compensators because of the consequent complexity of the controls, and because it is generally more economical to make most of the susceptances equal. One compromise is the so-called binary system in which there are (k 1) equal susceptances B and one susceptance B/2. The halfsusceptance increases the number of combinations from k to 2k.

The relation between the compensator current and the number of capacitors conducting is shown in Figure 6.14 (for constant terminal voltage). Ignoring switching transients, the current is sinusoidal, that is, it contains no harmonics

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Fig. 6.12 Alternative arrangements of threephase thyristorswitchet capacitor. (a) deltaconnected secondary, Deltaconnected TSC; ant (b) wyeconnected secondary, wyeconnected TSC (fourwire system).

Fig. 6.13 Principles of operation of TSC. Each phase of Figure 6.12 comprises of parallel combinations of switched capacitors of this type

Fig. 6.14 Relationship between current and number of capacitors conducting in the TSC.

Switching transients and the concept of transientfree switchingWhen the c u r r e n t i n a n i n d i v i d u a l c a p a c i t o r r e a c h e s a natural zero-crossing, the thyristors can be left unbated and no further current will flow. The reactive power supplied to the power system ceases abruptly. The capacitor, however, is left with a trapped charge (Figure 6.15(a)). Because of this charge, the

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Thyristor switched capacitorvoltage across the thyristors subsequently alternates between zero and twice the peak phase voltage. The only instant when the thyristors can be gated again without transients is when the voltage across them is zero (Figure 6.15(b)). This coincides with peak phase voltage.

Fig. 6.15 Ideal transientfree switching waveforms. (a) Switching on; And (b) switching off.

Ideal transientfree switching

The simple case of a switched capacitor, with no other circuit elements than the voltage supply, is used first to describe the im po r ta n t c o n c e p t o f t rans ient -f ree s w i t c h i n g . Figure 6.16 shows the circuit.

With sinusoidal AC supply voltage v = v sin (ω0t + α), the thyristors can be gated into conduction only at a peak value of voltage, that is, when

Fig. 6.16 Circuit for analysis of transientfree switching.

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(6.8)

Gating at any other instant would require the current i = Cdv/dt to have a discontinuous step change at t = 0+. Such a step is impossible in practice because of inductance, which is considered in the next section. To permit analysis of Figure 6.16, the gating must occur at a voltage peak, and with this restriction the current is given by

(6.9)

Where α = ±π/2. Now ω0C = BC is the fundamental frequency susceptance of the capacitor, and XC = 1/BC its reactance, so that with α = ±π/2

(6.10)

Where îAC is the peak value of the AC current, îAC=vBC= v/XC .

In the absence of other circuit elements, we must also specify that the capacitor be precharged to the voltage VC0 = ±v, that is, it must hold the prior charge ±v/C. This is because any prior DC voltage on the capacitor cannot be accounted for in the simple circuit of Figure 6.16. In practice this voltage would appear distributed across series inductance and resistance with a portion across the thyristor switch.

With these restrictions, that is, dv/dt = 0 and VC0 = ±v at t = 0, we have the ideal case of transient free switching, as illustrated in Figure 6.15. This concept is the basis for switching control in the TSC. In principle, once each capacitor is charged to either the positive or the negative system peak voltage, it is possible to switch any or all of the capacitors on or off for any integral number of half cycles without transients

Switching transients in the general caseUnder practical conditions, it is necessary to consider inductance and resistance. First consider the addition of series inductance in Figure 6.16. In any practical TSC circuit, there must always be at least enough series inductance to keep di/dt within the capability of the thyristors. In some circuits there may be more than this minimum inductance. In the following, resistance will be neglected because it is generally small and its omission makes no significant difference to the calculation of the first few peaks of voltage and current.

The presence of inductance and capacitance together makes the transients oscillatory. The natural frequency of the transients will be shown to be a key factor in the magnitudes of the voltages and currents after switching, yet it is not entirely under the designer's control because the total series inductance includes the supply system inductance which, if known at all, may be known only approximately. It also includes the inductance of the stepdown transformer (if used), which is subject to other constraints and cannot be chosen freely.

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Thyristor switched capacitorIt may not always be possible to connect the capacitor at a crest value of the supply voltage. It is necessary to ask what other events in the supply voltage cycle can be detected and used to initiate the gating of the thyristors, and what will be the resulting transients.

Fig. 6.17 Circuit for analysis of practical capacitor switching.

The circuit is that of Figure 6.17. The voltage equation in terms of the Laplace transform is

(6.11)

The supply voltage is given by v = v sin (ω0t + α). Time is measured from the first instant when a thyristor is gated, corresponding to the angle α on the voltage wave form. By straightforward transform manipulation and inverse transformation we get the instantaneous current expressed as

(6.12)

Where ωn is the natural frequency of the circuit

(6.13)

and

(6.14)

n is the per unit natural frequency.

The current has a fundamental frequency component iAC which leads the supply voltage by π/2 radians. Its amplitude îAC is given by

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Thyristor switched capacitor (6.15)

and is naturally proportional to the fundamentalfrequency susceptance of the capacitance and inductance in series, that is, Bcn

2/ (n21). The term n2/ (n21) is a magnification factor, which accounts for the partial seriestuning of the LC circuit. If there is appreciable inductance, n can be as low as 2.5, or even lower, and the magnification factor can reach l .2 or higher. It is plotted in Figure 6.18. The last two terms on the right-hand side of equation (6.12) represent the expected oscillatory components of current having the frequency ωn. In practice, resistance causes these terms to decay.

The next section considers the behavior of the oscillatory components under important practical conditions.

Fig. 6.18 Voltage and current magnification factor n2/ (n2 1).

1. Necessary condition for transient free switching.

For transientfree switching, the oscillatory components of current in equation (6.12) must be zero. This can happen only when the following two conditions are simultaneously satisfied:

(6.16)

(6.17)

The first of these equations means that the thyristors must be gated at a positive or negative crest of the supply voltage sinewave. The second one means that the

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Thyristor switched capacitorcapacitors must also be precharged to the voltage vn2/ (n2 1) with the same polarity. The presence of inductance means that for transient free switching the capacitor must be 'overcharged' beyond v by the magnification factor n2/(n2 1). With low values of n, this factor can be appreciable (Figure 6.18).

Of the two conditions necessary for transientfree switching, the precharging condition expressed by equation (6.17) is strictly outside the control of the gatingcontrol circuits because VC0, n, and v can all vary during the period of nonconduction before the thyristors are gated. The capacitor will be slowly discharging, reducing VC0; While the supply system voltage and effective inductance may change in an unknown way, changing n. In general, therefore, it will be impossible to guarantee perfect transientfree reconnection.

In practice the control strategy should cause the thyristors to be gated in such a way as to keep the oscillatory transients within acceptable limits. Of the two conditions given by equations (6.16) and (6.17), the first one can in principle always be satisfied. The second one can be approximately satisfied under normal conditions. For a range of system voltages near 1 p.u., equation (6.17) will be nearly satisfied if the capacitor does not discharge (during a nonconducting period) to a very low voltage: or if it is kept precharged or 'topped up' to a voltage near ±vn2/(n+2 1).

2. Switching transients under nonideal conditions.

There are some circumstances in which equations (6.16) and (6.17) are far from being satisfied. One is when the capacitor is completely discharged, as for example when the compensator has been switched off for a while. Then VC0 = 0. There is then no point on the voltage wave when both conditions are simultaneously satisfied.

In the most general case Vco can have any value, depending on the condition under which conduction last ceased and the time since it did so. The question then arises, how does the amplitude of the oscillatory component depend on VC0? How can the gating instants be chosen to minimize the oscillatory component?

Two practical choices of gating are: (a) at the instant when v =VC0, giving sin α = VC0/v; and (b) when dv/dt = 0, giving cos α = 0. The first of these may never occur if the capacitor is overcharged beyond v. The amplitude îosc of the oscillatory component of current can be determined from equation (6.12) for the two alternative gating angles. In Figures 6.19 and

6.20 The resulting value of îosc relative to îAC is shown as a function of VC0 and n, for each of the two gating angles.

From these two figures it is apparent that if VC0 is exactly equal to v, the oscillatory component of current is nonzero and has the same amplitude for both gating angles, whatever the value of the natural frequency n. For any value of Vc0 less than v, gating with v = VC0 always gives the smaller oscillatory component whatever the value of n.

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Fig. 6.19 Amplitude of oscillatory current component. Thyristors gated when v = Vco

Fig. 6.20 Amplitude of oscillatory current component. Thyristors gated when dv/dt = 0.

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The conditions for transientfree switching appear in Figure 6.20 in terms of the precharge voltage required for two particular natural frequencies corresponding to n = 2.3 and n = 3.6.

Switching a discharged capacitor

In this case VC0 = 0. The two gating angles discussed were: (a) when v = VC0 = 0; and (b) when dv/dt = 0 (cos α = 0). In the former case only equation (6.17) is satisfied. From equation (6.12) it can be seen that in the second case (gating when dv/dt = 0) the oscillatory component of current is greater than in the first case (gating when v = VC0 = 0). An example is shown in Figure 6.21 and Figure 6.22.

Fig. 6.21 switching a discharge capacitor; Circuit diagram.

The reactances are chosen such at îAC = 1 p.u. and the natural frequency is given by n = XC / (Xt + Xs) = 3.6 p.u. In case (a), the amplitude of the oscillatory component of current is exactly equal to îac

.In case (b), the oscillatory component has the amplitude nîAC and much higher current peaks a r e experienced. The capacitor experiences higher voltage peaks and the supply voltage distortion is greater

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Fig. 6.22 switching transients with discharge capacitor. (a) Gating when V = VC0 = 0; (b) Gating when dv/dt = 0.

7. VOLTAGE / CURRENT CHARACTERISTICSIn a TSC for transmission system application the cost of the thyristor switches and other complications make it desirable to minimize the number of parallel capacitor units. A figure of 3 or 4 is typical of existing or presently planned installations. With so few capacitors a smooth voltage/current characteristic is unobtainable, and a stepped characteristic is obtained (Figure 35). The capacitor characteristics 1, 2, and 3 intersect the system voltage/current characteristic at discrete points, and operation can be at any of these points depending on the number of capacitors conducting. With two capacitors conducting, operation would be at point A.

In order to obtain a smoother voltage/current characteristic it is usual to have a parallel-connected TCR which "interpolates" between the capacitor characteristics. If the TCR characteristic has a small positive slop, the resultant characteristic is shown by the heavy-line segments in figure 35. This construction shows that the TCR current rating must be little larger than that of one capacitor bank at rated voltage, otherwise deadbands arise as shown by the shading in Figure 35. The increase in rating enables the heavy segments to be extended to the left through the deadband.

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Fig: Effect of paralleling a TCS and TCR before control systems are properly coordinated.

With fixed TCR controls (i.e., fixed knee voltage and slope) the voltage/current characteristic of this hybrid TSC/TCR is still stepped, giving rise to the possibility of bistable operation. It is therefore necessary adjust the TCR knee voltage and slope by a small amount every time a switch is in or out. This can in principle be done either by open-loop or closed-loop (current feedback) modification of the control system giving a continuous V/I characteristic as in Figure 14. A further sophistication in the control system is to incorporate a hysteresis effect so the capacitors are switched in at a lower voltage than that at which are switched out. This helps to prevent a "hunting" instability which can arise if the system characteristic intersects the compensator characteristic near the junction of two segments.

8. ADVANTAGES AND DISADVANTAGES

In the chapter 2 of the book Edited by T. J. E. Miller, the following are listed as advantages of using reactive compensation:

1. Limit Rapid Voltage Increase or Decline.2. Limit Slow Voltage Increase or Decline.3. Reactive Power Support at DC Converter Terminals.4. Increase Short Circuit Levels.5. Decrease Short Circuit Levels.6. Improving Steady-state Stability.7. Improve Dynamic Stability 8. Improve Transient Stability9. Limit Fast Wave-front Overvoltages due to lighting, switching etc.

A TSC is usually a three phase assembly, connected either in a delta or a star arrangement. Unlike the TCR, a TSC generates no harmonics and so requires no filtering. For this reason, some SVCs have been built with only TSCs .This can lead to a relatively costeffective solution where the SVC only requires capacitive reactive power, although a disadvantage is that the reactive power output can only be varied in steps. Continuously variable reactive power output is only possible where the SVC contains a TCR or another variable element such as a STATCOM.

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9. RESULT Improved voltage regulation Reduced power losses Increased utilization of equipment Improved power factor

10. CONCLUSIONThe results shows that, when we use the compensation device, it can balance the voltage and

current to normal levels, as we know that there is reactive power due to capacitive and inductive elements in the grid which can make the current and voltage phase difference,

due to which the real power in the system reduces from the ideal level, but after we introduce the compensation device, the current and voltage waveforms have same phase, and due

to this compensation effect, power in the system achieves to the desired value with stable voltage.

11. FUTURE DEVELOPMENTS AND REQUIREMENTSAlthough the demand for electric power in world is increasing only slowly, the trends in worldwide generation and transmission suggest that reactive power compensation will become increasingly important. This follows from a wide commitment to ac transmission, together with economic factors which necessitate the maximum utilization of generation and transmission facilities. In addition, there is no evidence that large irregular loads like arc furnaces will become less common.

AS far as bulk transmission is concerned, the continued development of remote hydro resources worldwide should continue to provide a demand for EHV compensators. Among the competitive issues in this field are power losses and harmonics, and innovations that improve performance in these areas are greatly to be desired. Overvoltage performance is another area where improvements are desirable. Most static compensators, including the polyphase saturated reactor, are limited in their ability to hold down the system voltage under emergency. Developments in HVDC transmission also generate reactive power control requirements which compensators can help to fulfill.


12. REFERENCES

1. Transmission system reactive power compensation , B. F. Wollenberg Minnesota Univ., Duluth, MN, USA, Date of Conference: 2002 Page(s): 507 - 508 vol.1 Print ISBN:0-7803-7322-7, INSPEC Accession Number: 7386352, DOI:10.1109/PESW.2002.985054 , Publisher: IEEE

2. T.J.E. Miller, “Reactive power control in electric systems” (New York: John Wiley & Sons, 1982).

3. Akwukwaegbu I. O, Okwe Gerald Ibe “Concepts of Reactive Power Control and Voltage Stability Methods in Power System Network” IOSR Journal of Computer Engineering (IOSR-JCE) e-ISSN: 2278-0661, p- ISSN: 2278-8727Volume 11, Issue 2 (May. - Jun. 2013), PP 15-25

4. J.F. Aldrich & H.H. Happ, “Benefits of voltage scheduling in power systems”, IEEE Transactions on Power Apparatus and Systems, PAS, 99 (5), 1980, 1701 - 1712.

5. “Final Report on the August 14, 2003 Blackout in the United States and Canada: Causes and Recommendations,” U.S.-Canada Power System Outage Task Force, April 5, 2004. a

6. L.L. Lai, Intelligent System applications in power engineering (London: John Wiley & Sons, 1998).

7. P. Sakis Meliopoulos, George Christoforidis, “Effects of DC Ground Elect rode on Converter Transformers” IEEE Transactions on Power Delivery, 1989, pp. 995 – 10021.

8. Juan dixon, senior member, IEEE, “Reactive Power Compensation Technologies: State-of-the-Art Review” Proceedings of the IEEE, VOL.93, NO.12, december2005

9. Baodong Bai, Zhijia Zhang, Bo Kang, Dezhi Chen, “The Research on Reactive Power and DC Bias Compensation”, IEEE Electrical Machines and Systems (ICEMS), 2011 International Conference Beijing, Aug 2011.

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http://dx.doi.org/10.1109/PESW.2002.985054

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