Design And Operating Characteristics Of A Series Static Voltage Controller

IEEE Transactions on Power Delivery, Vol. 6, No. 4, October 1991

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Design and Operating Characteristics of a Series Static Voltage Controller THOMAS H. ORTMEYER, MEMBER MOHAMED ALI NOMAN Electrical & Computer Engineering Department Clarkson University Potsdam, New York 13676

Abstract- The problem of waveform distortion of the power system terminal voltage is becomin a critical problem especially for computers and otfey sensitive loads. Elimination of this distortion requires the ability to generate the compensating pulses of energy in times much less than the source period. This paper presents the theory and the experimental implementation of a new device called the Static Voltage Controller (SVC) which can eliminate waveform distortion and provide a sinusoidal volt age. Keywords: Power Quality, Power Electronics, Load Management. 1. Introduction

The quality of the power supplied to sensitive electronic equipment is an important issue. The power disturbances that affect these sensitive electronic loads have a variety of sources. Harmonic and aperiodic currents in power lines generated by electronic switching circuits in electrical appliances and by power system devices such as solid state motor drives and lar e controlled rectifiers cause voltage drops across t e power system impedance. These voltage drops will cause waveform distortions and produce amplitude modulation of the power system t ermi n a1 volt age [ 1- 31. Most of the industrial and commercial electrical equipment is relatively insensitive to these distortions on the power line. However, many electronic devices such as computers and process controls can be adversely affected by high distortion[4]. Consequently, the objective of this work is to obtain a sinusoidal volta.ge from a source which may have harmonics, spikes, surges or any other irregularities in shape due to the above mentioned reasons. This

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91 WM 105-7 PWRD A paper recommended and approved by the IEEE Transmission and Distribution Committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1991 Winter Meeting, New York, New York, February 3-7, 1991. Manuscript submitted August 27, 1990; made available for printing January 3, 1991.

work is based on the idea of the hysteretical switching of a parallel capacitor-switched inductor circuit presented in [ 5 ] , t o absorb or generate the compensating pulses of energy required to shape the output volt age waveform. 2. Theory of the SVC Elimination of the harmonics, spikes, sags, and sur es from the source voltage and obtaining a sinusoifal voltage at the SVC output terminal requires the ability t o generate or absorb the compensating pulses of energy in times shorter than a half-cycle. Basically, the idea behind the compensation for the input voltage irregularities and distortions consists in storing energy in the power module and by making use of the capabilities of high speed switchin devices t o give this energy back when it is neede t o make up the distortions in the input voltage or t o absorb any pulses of energy coming from the line which will cause output voltage distortions. In this manner, the output voltage can be controlled up t o the limit of the energy stored in the SVC. The general block diagram of the SVC is shown in Figure 1. This device consists of three basic elements, the voltage sensor across the load, the operating logic, and the power module. The power module employed t o perform the above mentioned task is shown in Figure 2. It consists of a capacitor in parallel with a switched inductor and four switching elements. The switching elements must be selfcommutating switches, such as bipolar transistors, FET’s or gate-turn-off thyristors (GTO’s). Losses in the capacitor-inductor circuit and in the switching elements are neglected for the time being and can be included later. For the proper operation of the device, the inductor current needs t o be greater than the peak load current. The SVC operates without distorting the supply lines, as the current drawn from the source equals the load current, which will be sinusoidal for linear loads. With nonlinear loads, the line current &stortion will not be affected by the device apart from the effect of feeding the load from a stiff sinusoidal source.

0885-8977~1$01.~ 1991 IEEE

%

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3. Operation of the Power Module

If we assume that the inductor current if is always positive and greater than the load current, then the power module can be operated in two modes. Op:1 eration in mode 1 will be when the transistors T2 and T 3 are on. This will cause the inductor current il and the load current i t o be added causing ( the ca acitor volta e t o increase. While o eration I in moBe 2 will be wgen T1 and T4 are on wgich will II cause these two currents to be subtracted causing the capacitor voltage to decrease. The switching between modes is determined by comparison of the actual output voltage with the sinusoidal reference voltage. Referring to Figure 3, the switching occurs when the output voltage v, deviates from the reference command voltage U,* by where VH is the width of the hysteresis winv,*- vo dow. Therefore, the circuit lends itself to hysteretical switching with the actual output voltage being of the reference voltage v,*. controlled within In order t o determine the voltages and the currents as functions of time (which is needed for controlling the circuit), computer simulation of the problem was carried out using state space approach. Two sets of state space equations (one set for each mode) were derived which will describe the behavFigure 1: General Block Diagram of the Static Volt- ior of the system in both modes of operation. These equations are: age Controller

. i ' O-uo

fy,

&%

d i = -(L R +RI U, )i- t- u= dt L , t Ll L, + L, L , + Ll

(1)

Operation in mode 1 is described by:

dil- --U ,

ii

+I

I I

dt

Lind

d v, - - ir+ - i dt C C And the operation in mode 2 is described by: dil- - v, -

dt

(4)

Lind

dv, = -if -

(3)

i

(5) dt C+C In these equations the supply source, which might be a transformer, is represented by vs in series with the windin resistance R, and the leakage inductance L,. T%e load is represented by a series LI and RI as shown in Figure 1. Note that the current Figure 2: Power Module of the Sta.tic Voltage Con- drawn from the source is identical to the current drawn by the load, so that the SVC will pull a sinutroller soidal current from the source when linear loads are being supplied. An initial simulation was conducted with the reference voltage given a fixed amplitude and a fixed

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?SDI

A

T

g I:

- IAi

W de,&

COMPARATOR 3 0

-!i -30

Figure 5 : Inductor Current under Unregulated v,* Figure 3: Hysteresis Voltage Controller

1

M

than the load current, the output voltage will not follow the reference voltage. After the inductor current has reached a sufficient value, the SVC will operate satisfactorily to regulate the output voltage. However, the inductor current will continue to increase indefinitely and it is desired to control this current. 4. SVC Analysis

Assuming the time required to increase or decrease the capacitor voltage to the upper or lower limits is much smaller than the source period, the time required t o increase the capacitor voltage to the upper limit can be approximated from Equation 3 as follows : Figure 4: Output Voltage Following the Reference Volt age. Similarly, from Equation 5 , the time required t o decrease the capacitor voltage t o the lower limit is : (7) The change in the inductor current during the switching times A t , and At2 is found from Equations 2 and 4 as follows:

Ail1 = - - AVCt , Lind VC

Ail2 = -At:! Lind LInd

C

L,

R,

Load

mH

pF 2000

pu 0.03

pu 0.02

0.55

2

pf

V,

v

v

0.9

115.5

110

R

V,

(8)

(9)

The total inductor current change due to both modes of operation is:

bind

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5 . Dynamic Performance of the

v o

"...

,'

.,)(P

+ 'p)

Under sinusoidal source conditions, Equation 13 describes the steady state operating conditions. Referring to Figure 6, if we choose the case when v, is leading i by go", this phasor diagram is valid. For a given laggin power factor angle 'p, the capacitor volta e angle is set. Therefore, in the steady state in or er to keep V, constant for the increases in the input voltage Vi.,,the capacitor voltage and the angle 6 both have to increase. The acceleration of the output voltage phasor from Figure 7 will be:

f

Figure 6: Vector Diagram of the Power Module Substitutin the values of 4 t I and At2 from 6 and 7 respective y, we find:

5

Ail =

SVC

d24 dt2 -

2 C v ~ v,i -

dil

-sf dt

(14)

(12) Therefore, referring to Figure 6, there can be three cases: Expressing the capacitor voltage and the load cur1. Case 1. ( p + (r?) = In this case the net rent phase angles as in Figure 6, the total change Of average energy input to the device is equal to the inductor current over 2x is equal to: zero, i.e. il is constant, and the Equation 13 is

Ltnd i; - i 2

4.

c 4 i 4 w t ) = -(-)(Lind

.m

4KvcvH

1

1,,,(p

d'4

$- (r?) (13)

Consequently, in order to limit the indefinite inductor current increase as in Figure 5 , we must insure that the active power input t o the device equal to zero over the source cycle. This means that we must have the angle (p+cp) between the load current and the capacitor voltage equal t o * t f . In order t o regulate the an le of the controller operation, a frequency control oop is added to the circuit. By using a proportional controller on the inductor current driving a voltage controlled oscillator, a frequency control circuit is obtained which will keep the steady state angle at 90" as described in Equation 13. The resulting controller is shown in Figure 7. The output of the angle controller U,' = f i V * s i n 4 is fed to the hysteresis controller shown in Figure 3. The controller includes supplementary magnitude control to increase circuit stability. The controllers parameters are: The magnitude and the frequency of the reference voltage Inductor current and its reference value. RMS output voltage. Reference frequency. Magnitude and frequency control gains of the reference volt age respectively.

P

I

satisfied, therefore = 0 and the circuit is in dt2 the steady state. 2. Case 2. Suppose that V,, is increased in magnitude by AT.:.,, in this case energy stored will 'p) causing il increase at a rate V,Icos(P to increase and consequently, this will make d2 4 - < 0. In this case the output voltage phasor

+

dt2 will slow down, 6 will increase causing ( p 4-'p) to increase towards which will decrease the energy stored and by that keeping i; constant.

3. Case 3. Suppose the angle 6 has increased making VJcos(p (r?) < 0 which will cause the energy t o be taken out from the inductor. In d24 > 0 which means that the output this case dt2 voltage phasor will accelerate causing the angle ( p 'p) t o decrease and therefore more energy will go into the inductor. This will continue

+

+

d24 = 0 and the steady state operation is until dt2 restored. The effectiveness of this proposed control strategy was studied under the same conditions as in Table 1 with the frequency control gain gf=1.2 (Rad/Sec/A) and the magnitude control gain g,=0.5 VIA. The output voltage and the inductor current waveforms with this angle controller are shown in Figures 8-9. It can be seen that the controller angle adjusts to 90" so that the inductor current remains constant. The presence of losses in the inductor (either on switching or conduction), can be easily accounted

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IO

TWO

Figure 9: Inductor Cnhent under Angle Regulation

Figure 7: Block Diagram of the AngIe Regulator

Figure 10: Output Voltage Following the R,eference Voltage together with the Inductor Current. for in Equation 2. In this case the equilibrium will exist at an angle ( p cp) slightly less than 2 t o cover the losses in the inductor. 8. Experimental Results

+

A prototype of this device was built and made to operate according to SVC principle of operation explained earlier. The output voltage obtained is shown in Figure 10. The command voltage and the inductor current are also shown in this figure. The voltages are scaled by a factor of 20, and the scale for the inductor current is one volt corresponds to Figure 8: Output and Reference Voltages with Angle one ampere. The center line for the trace is the zero point for all these signals. In this figure, the Regulation output volta e clearly follows the command voltage throughout t8e cycle. The inductor current experiences a relatively small 120 Hz ripple current as expected. Figure 11 shows the output voltage switchings on an expanded time scale. The demonstration of the ability of the Static Voltage Controller to provide an output voltage independent of the source voltage is shown in Figure

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Figure 13: Experimental Current Ripple Corresponding to Table 3 Figure 11: Output Voltage Switching around the Reference Voltage.

1

IO,

i. a io

Figure 14: Simulation Current Ripple Cor responding to Table 3 Figure 12: Output Voltage of the SVC under Nonsinusiodal Command Voltage 12. In this figure a trapezoidal command voltage is used, and the SVC output voltage follows this command within the the usual hysteresis window. This figure shows that the output voltage is independent of the source voltage. From this we can conclude that if the source voltage has harmonics, sags and surges, the output voltage of this device will not be affected by these disturbances and will follow the sinusoidal reference command as long as there is enough energy stored in the inductor and the rise time of the transients is longer than the SVC switching period. In order to get a close look at the performance of this circuit, the experimental system was modeled in the computer. The data to the esperimental circuit are shown in the following table.

I

Table 2 Data To The Experimental System ( 0 ) Ltnd (mH) c ( p F ) I LS (mH) R, (0) 1.357 6.1 1.0 1.0 11.0 pf gf(R/S/A) gm(V/A) i; (A) 740) 45.31 0.0 5.18 16.18 0.9192

Rtnd

I

I

type

Instab.

Exp. Sim.

R/S/A 20.7 32.0

gj

I

I

Table 3 ifmat

I A 6.8 6.81

I ilmtn 1 IA

3.2 3.13

L

g

IA 4.85 4.97

I I 1

I

fsw

kHz 19.255 19.53

.

The inductor current ripple for the experimental and simulation models are shown in Figures 13 and 14. This circuit exhibits a discrete range over which the desired operation is realized. At low gain, the in-

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7 E. F. Fuchs, D. J. Roesler, F. S. Alashab, ”Sensistivity of Electrical Appliances To Harmonics And Fractional Harmonics of The Power System’s Voltage. Part I: Transformers And Induction Machine” IEEE Transactions on Power Delivery, Vol. $WD-2, April 1987, pp. 437-444. E. F. Fuchs, D. J. Roesler, K. P. Kovacs, ”Sensistivity of Electrical Appliances To Harmonics And Fractional Harmonics of The Power System’s Voltage. Part 11: Television Sets, Induction Watthour Meters and Universal Machines.”, IEEE Transactions on Power Delivery, Vol. PWD-2, April 1987, pp. 445-451.

Rdsrsnra Inductor Cumnt

Figure 15: Effects of gm on the Range of gf ductor current drops below load current, and waveform error results. At high gains, an instability occurs, where the VCO frequency no longer tracks the system frequency. It was shown in the laboratory that voltage magnitude feedback (gm in Figure 7) will increase the stable region for this circuit as shown in Figure 15.

Francois D. Martzloff And Thomas M. Gruzs, ”Power Quality Site Surveys: Facts, Fictions, and Fallacies”, IEEE Transactions on Industry Applications, Vol. 24, No. 6, November/December 1988, pp. 1005-10018.

T. H. Ortmeyer, ”Static Line Drop Compensator”, Patent Disclosure To Research Corporation, Sep. 25, 1986, pp. 1-17. 8 . Biography

7. Conclusions This paper presents the theory and the experimental verification of a laboratory prototype of a new device capable of providing clean power t o sensitive loads. The device operates by using energy stored in a switched inductor t o shape the output volta e. This stored energy is used to compensate for t e distortion in the supply voltage whenever the output voltage deviates from its reference value by a predetermined level. The principle of operation of this device is t o compare the instantaneous value of the output voltage with the instantaneous value of the sinusoidal reference voltage. The error is given to the hysteresis comparator t o switch the power transistors in one mode or in the other, so that the output voltage is made t o follow the reference voltage. The laboratory prototype of this device was built and made to operate according to the principle of operation of this device. The experimental results presented are encouraging. It was shown that the SVC follows the output voltage command independent of the source voltage. A good match between experimental and computer simulation results was found. Furthermore, a stabilizing signal based on voltage magnitude was introduced.

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References [l] David C. Griffith, ”Working With Waveform Distortion In Digital Systems”, Powertechnics Magazine, sep. 1986, pp. 31-34.

Mohamed Ali Noman was born in Taiz, Yemen Arab Republic in 1952. He received his B.S.E.E. in 1980 from Kharkov Polytechnic Institute, USSR and the Master of Engineering in Electric Power Engineering in 1986 from Rensselaer Polytechnic Institute, Troy, New York. In 1986 he joined the Electrical En ineering department at Clarkson University wheree! received the Ph.D. degree in Electrical Engineering in 1990.

Thomas H. Ortmeyer received the B.S.E.E. degree in 1972, the M.S.E.E.in 1977, and the Ph.D. in 1980, all from Iowa State University. From 1972 t o 1976 he worked in the Operational Analysis Department, Commonwealth Edison Company, Chicago, Illinois. Since 1979, he has been at Clarkson University, Potsdam, New York, where he is currently an Associate Professor. His current interests include harmonic performance, power electronics and machine control, and power system protection. He is a member of IEEE, as well as Eta Kappa Nu, Phi Kappa Phi, and Sigma Xi.