Development of a SMES system as a fluctuating load compensator

H . C. Tay M . F. Co n I on

Indexing ternis: SMES systeni, Fluctuuting loud, Cornpensation

Abstract: The development of a superconducting magnetic energy storage (SMES) device and associated experimental system is described. The IGBT-based converter bridge is controlled under a pulse width modulation (PWM) scheme, to achieve instantaneous and independent control of active and reactive power flow, to and from the superconducting coil. The SMES device is applied to the problem of fluctuating loads at a distribution level. Two control strategies are described: a direct strategy based on equal and opposite compensation of the active and reactive power components of the fluctuating load, and an optimised strategy where the best use is made of the limited capacity of the SMES device. Both strategies are directed at the minimisation of the voltage fluctuation at the load point. The application of these strategies is demonstrated on the experimental system and their performances compared. The effectiveness of the optimised strategy in reducing the voltage fluctuation when compared with reactive power only compensation is quantified.

List of symbols

ZIine Pst = Short-term flicker severity PLoud = Active power load QLoud = Reactive power load PLO = Constant component of active power load QLo = Constant component of reactive power load APL,,, = Fluctuating component of active power load AQLoud = Fluctuating component of reactive power

AP = Fluctuating component of line active power AQ = Fluctuating component of line reactive

PsMEs = Active power flow to SMES QsMEs = Reactive power flow to SMES APSMES = Fluctuating component of active power flow

= R + j X = Line impedance, Q

load

power

to SMES

OIEE, 1998 IEE Proceedings online no. 19982370 Paper first received 3rd May 1996 and in revised form 27th April 1998 The authors are with the Centre for Electrical Power Engineering, Monash University, Wellington Road, Clayton 3 168, Melbourne, Australia

AQsMEs = Fluctuating component of reactive power

ASsMEs = Fluctuating component of apparent

V L 6 ELO" Z L @ J

H = J-l = [

flow to SMES

power flow to SMES = Voltage phasor at PCC = Voltage phasor at source = Line current phasor = Jacobian matrix of network

= Inverse of Jacobian matrix of network = Fluctuation in voltage magnitude at PCC A V

A6 = Fluctuation in voltage phase angle at

VAreuctive = V A required for reactive power only com-

VAoptimised = VA required for optimised compensation

PCC

pensation

= Transfer function of coil current controller = Combination of scaled fluctuating compo-

nents of load active and reactive power = Coil inductance = Reference current for coil current control-

= DC coil voltage = Offset voltage in linearised model of

= Series resistance in linearised model of

= Coil current = Error current as difference between coil

= Proportional gain constant of coil current

= Integral gain constant of coil current con-

= Accumulated current error = Modulation index of PWM pattern = Phase delay angle of PWM pattern = Line-to-line voltage magnitude = Offset active power = Offset reactive power = Active and reactive demanded power

flows from SMES = Active and reactive outputs of load com-

pensation controller = Proportional gain and zero of coil current

controller

ler

bridge voltage drop

bridge voltage drop

and reference currents

controller

troller

IEE Pior.-Gener. Trunsm. Distrih., Vol. 145, Nil. 6, November 1998 700

Coefficients of the load compensa- tion controller Maximum allowable coil current Maximum apparent power Maximum reactive power

In electrical distribution networks, voltage fluctuations are usually caused by large varying loads. If the fault level at the point of the fluctuating load is inadequate, the voltage fluctuations will affect other customers con- nected to the same supply. Voltage fluctuations can be minimised by reducing the disturbing loads, by strengthening the supply, or by utilising compensating equipment [l]. Strengthening the supply is usually the most expensive remedy, but it is often adopted if future expansion is anticipated. Compensating equipment can be used if the other methods are not satisfactory. Com- monly used compensating equipment include reactive power compensators, such as saturated reactor and thyristor-controlled var compensators, which work by supplying reactive power in such a way as to maintain the voltage at the point of common coupling (PCC). The success of these compensators is due to the closed coupling relationship between the reactive power and the voltage [2].

The instantaneous reactive compensator, also known as the active power filter, is also used to suppress load disturbances [3]. The compensator works by injecting all the components of the load current, except the in- phase fundamental network frequency component, which represents the useful power of the load. The compensator does not only correct the load power fac- tor to unity, it also removes the low-order harmonic currents of the load. The main disadvantage of the active power filter is its relatively high switching fre- quency; this makes it technologically difficult to imple- ment a high power compensator. Thus, small power applications of the active power filter, such as har- monic compensation, are more attractive [4, 51.

Superconducting magnetic energy storage (SMES) allows electrical energy to be directly stored and

released with practically no loss. With an appropriate power conditioning system which interfaces the SMES to the AC power network, the SMES unit can be con- trolled so that it has independent real and reactive power flows [6]. Unlike reactive power compensators, the SMES unit has an additional benefit of real power flow, and this enhances the control flexibility of a SMES-based compensator. This feature motivated the present investigation of the performance of a SMES unit as an alternative compensating device.

SMES has been proposed for and/or used in many applications. These include power system stability enhancement [7], diurnal load demand levelling [SI, fre- quency control [9], and uninterruptible power supplies [lo]. Small-scale SMES in a fluctuating load applica- tion has been recognised to have greater potential [ 1 I]. This is because the problems of fluctuating loads are widely spread in the distribution system, and, with the current technology, small-scale SMES units are more practical and economically viable.

Battery system energy storage (BESS) has similar functions to SMES. Both systems can provide active and reactive power. The energy discharge rate of a bat- tery system is limited by chemical reaction rates, which results in its low power density as compared to SMES. In general, BESS is more suitable for high energy applications, while SMES is suited to high power appli- cations [12].

This paper describes the development of a SMES sys- tem carried out at Monash University, and the control strategies of a SMES system for suppressing fluctuating load disturbances. Two control strategies are proposed in this paper. The fundamental aim of each strategy is to suppress voltage fluctuation caused by disturbing loads. Their implementation and some experimental results are also presented.

2 System description

An experimental SMES system has been developed for the study of fluctuating load compensation. Fig. 1 shows the schematic representation of the overall experimental system. The supply is a 415V source, in

Fig. 1

IEE Proc

N

coil protection system

copocitors Luborutory network niodel wirh SMES sysretn

.-Gener. Truiisni. Distrib., Vol. 145, No. 6. Novenibrr 1998 70 1

series with an impedance of ZLine = 1.31 + j6.07Q per phase, to represent a radial distribution network. The disturbing load is star-connected and each phase of the load consists of an inductor in series with a bank of binary-weighted resistors, which can be individually switched by solid-state relays. The SMES system is connected in parallel with the load at the point of com- mon coupling (PCC).

2. I SMES system The SMES system comprises a 4.5H superconducting coil acting as an energy storage element, a cryogenic system to maintain the temperature of the coil at 4.2"K, a 15kVA IGBT-based AC-DC converter to interface the DC coil to the AC power network, a 80386-based microprocessor controller to regulate the power flow between the SMES and the AC network, and a transformer for matching the system and SMES voltages.

The coil conductor is multifilamental NbTi/Cu wire, and the coil capacity is 16kJ when carrying a maximum current of 86A. The auxiliary circuits connected in par- allel with the superconducting coil are the coil protec- tion system and the converter protection circuit. The coil protection system protects the coil during a quench, when the superconductor reverts to its normal resistive state, by rapid discharge of coil energy. The converter protection circuit protects the converter and the coil in the event that the converter unintentionally becomes open circuit, due to a fault in the converter switching or power failure in the switching device gate drivers. The converter PWM switching is based on an unbalanced switching scheme and will be described in a future publication.

2.2 Digital flicke rm eter A digital flickermeter has also been developed to meas- ure the severity of voltage fluctuation in terms of lamp flicker. The meter simulates the process of human physiological visual perception, and gives a reliable indication of the reaction of an observer to any type of flicker, independently of the source of the disturbance. The meter is based on the specifications of an UIE- flickermeter published in IEC-868 which is an interna- tionally agreed meter for evaluating the voltage fluctua- tion caused by fluctuating loads [13].

0 1 2 3 1 5 Fig. 2 Flickermeter display Weighted voltage fluctuation

The flicker value from the meter is expressed in units of perceptibility with the value one corresponding to the visual perceptibility threshold of a flicker occur-

702

rence. Higher flicker values mean that the flicker inten- sity is more than perceptible and can become annoying or intolerable. The meter is implemented on a 80486- based PC, together with a data acquisition system. The software simulates the desired characteristics of a flick- ermeter in real time and performs data acquisition simultaneously for the experimental system. Figs. 2-5 show the display of the meter, which displays the weighted voltage fluctuation, percentage voltage fluctu- ation, instantaneous flicker value, cumulative probabil- ity function of the flicker values, and short-term flicker severity, P,,.

I

51 I I

Y I I I

0 1 2 3 L 5 flicker

Fig. 4 Flickermeter display Flicker perceptibility

C 0

C 3

.- I

v

L

x I - .- ._ n n

a

0

L

classes ( 6 1 ) Fig. 5 Flickermeter display Cumulative probability function

3 SMES control strategy

The main objective of the compensation is to reduce voltage disturbances, and thus flicker, at PCC. For reactive power compensators, compensation is achieved by injecting reactive power in such a way as to reduce voltage fluctuation. For the SMES system, which has two controllable outputs of active and reactive power, the compensation method is more flexible. Depending

IEE Proc-Gener. Transm. Distrib.. Vol. 145, No. 6 , November 1998

on the control strategy of SMES power flow, the SMES system can behave as a reactive power compen- sator, an active power compensator, or a combined active-reactive power compensator. In this paper, two control strategies of assigning the injection of SMES active and reactive power for the compensation are proposed: direct and optimised. These strategies assume that the SMES system is connected in parallel with the disturbing loads, as illustrated in Fig. 6, which is a normal practice for load compensation.

PCC

I SMES I device U

Fig. 6 Distribution network with fluctuating load and SMES system

3. I Direct strategy The direct strategy is the simpler method proposed. It requires the SMES system to inject an equal amount of active and reactive power, as demanded by the fluctu- ating components of the loads, shown in eqn. 1:

PLoad = P L O + A P L o a d

&Load = Q L O + AQLoad

PSMES = -AP Q S M E S = -AQ (1)

where PLO and Q L O are the constant components of the load active and reactive power, respectively, and APLoad and AQLoad are the load fluctuating components. In this way, the supply only needs to provide the constant components of the load, thus eliminating the voltage disturbance at the PCC. The SMES controller of this strategy is basically a unity gain controller. If the volt- age disturbance is mainly due to reactive power fluctu- ation, PSM, may not be required, and the strategy is reduced to the compensating scheme of a thyristor-con- trolled reactive power compensator [14, 151.

3.2 Optimised strategy The cost of compensation is usually proportional to the capacity of compensator. If the power transfer can be reduced to achieve similar reduction in load distur- bance, then a smaller compensator would be more ben- eficial. The objective of the optimised strategy is to minimise the apparent power flow in the SMES system, while mitigating the voltage disturbance at the PCC:

VL6 = ELO" - I L ~ Z L ~ , , VIL(S - 4) = ( P L o a d + P S M E S )

+ J ' ( & i o a d + Q S M E S )

(2) where ZLd is the phase current of the line.

This strategy is based on the linearised model of the distribution network. The linear behaviour of the net- work in Fig. 7 can be described as follows:

[Z?]=J[Z] (3)

where J is the Jacobian matrix of the network, AP is the sum of changes in and PsMEs, AQ is the sum

IEE ProcGener. Transin. Distrib., Vol. 145, No. 6, November 1995

of changes in QLoad and QSMES, and A V and A d are the changes in voltage magnitude and angle at the PCC, respectively.

t PSMES QSMES

Fig. 7 pensator

Distribution network with fluctuating toad and SMES-based com-

Inverting J, the changes in voltage amplitude and angle can be expressed in terms of the changes in active and reactive powers at the PCC as follows:

where H = J-'. The derivation of H is presented in the Appendix, Section 9. Since the aim of the SMES sys- tem is to maintain the voltage at the PCC, A V = 0 and the following equation is obtained:

hi1 A P s M E s h i a A Q s ~ ~ s = - h l l A P L o a d - h 1 2 a Q ~ o a d

= A D (5)

where h l l and hI2 are the elements of matrix H. For reactive power only compensation, hPsMEs = 0 since there is no active power flow. The control function of the SMES system is then given by

A D ~ Q S M E S = -

hl2 For both active and reactive power compensation, the control function is obtained by minimising the appar- ent power flow

Substituting for A P S M E S from eqn. 5, say, in eqn. 7, and minimising ASsMEs with respect to AQsMES, gives the following optimised control functions of the SMES system:

h12 A D h?l + h22

~ Q S M E S = (9)

If the apparent power flow, which reflects the capacity of compensators for reactive power compensation and optimised active-reactive power compensation, are compared as shown in eqn. 10, the resultant ratio is always greater than 1, which implies that the optimised method requires less effort to rectify the disturbance for any network impedance. This is intuitively expected because the optimised method has an additional degree of freedom, namely the active power.

where h121hll which is the ratio of active and reactive power components of the combined active-reactive

703

power compensation. For this radial distribution net- work, the ratio hl,lhll is found to be

hl2/h.1l = tan(S + e ) (11) where 6 is the voltage phase angle at the PCC, and 0 is related to the resistancelreactance ratio of the line impedance ZLblL' as follows:

e = tan-l(X/R) (12) Fig. 8 shows the benefit of optimised active-reactive

power compensation over reactive power compensation vs. the RIX ratio of line impedance under different loading conditions. The loading on the network has the effect of shifting the benefit curve horizontally. As the supply impedance increases or the fault level decreases, as it would for a weak distribution network, the magni- tude of 6 will increase, which will therefore increase the benefit of optimised method for the same RIX ratio. In practice, the typical RIX ratio of the transmission line is less than 20% [16]. From this analysis, the bene- fit of the optimised method is insignificant for this case. However, at the distribution network level, the RIX ratio can be much higher and the optimised method will be more beneficial in terms of capacity.

R/X rot io , % Fig. 8 conipensution (i) Z = 10%. load = 1.0 +jO.5 (ii) Z = S ' X , load = 1 .O + j0.5 (iii) Z = 5%, load = 0.5 + j0.25 ( IV) no load

Cupucity rutio of reactive and optiini.sed uctive-reactive power

3.3 Coil current controller The SMES control strategies described earlier aim at the suppression of load disturbances, and have no reg- ulation function as regards the superconducting coil current. It is important to maintain the coil current at its desired operating level. If the current is allowed to drift to a lower level, the available power transfer of the SMES system is reduced. On the other hand, if the current drifts to a higher level, the coil may operate too close to an upper limit, and a quench may be pro- voked. Therefore, it is necessary to include a coil cur- rent controller as part of the system. In addition to maintaining the coil current, another requirement for this controller is that its response must be sufficiently slow, so that it does not significantly affect the response of the load compensation controller, which implements one of the strategies discussed earlier.

Vo f f s e t I

'd 1 LS *RI

G ( s ) = -

Fig.9 Lineuri.sed model of SMES unit with coil current controller

Fig. 9 shows the block diagram of a feed forward controller for the coil current control, and a linearised

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model of the SMES unit. The SMES model only repre- sents the DC side of the SMES unit, which is a control- lable DC voltage source in series with a nonlinear resistance and a superconducting coil. In the plant model G(s), L is the inductance of the coil and VcljJset and RI are the linearised constants.

The controller is required to have zero steady state error, and so the system must be of type 1. Using a root locus design technique [17], and designing the closed loop poles to be well away from the minimum load fluctuating frequency that is to be compensated, the final form of the controller obtained is

which is in fact a proportional-integra1 (PI) controller. The design parameters of the controller are k = 1.1 and z1 = 0.065rad/s, which result in two closed-loop poles of -0.109radls and -0.144radIs.

4 Implementation of SMES control strategy

The digital control system of the SMES is made up of two separate controllers: the load compensation con- troller and the coil current controller. The sampling rate of these controllers is 300Hz. The load compensa- tion controller implements one of the strategies described in Section 3. During the stand-by mode in which the coil is charged and maintained at an operat- ing level, only the coil current controller is active and is implemented by the following algorithm, which is an implementation of a PI controller of active power:

v d

J 1 5 K i n e m =

a=O Sew = S e r r + IeTr

(14) where Z,.. is the reference signal of the coil current, K,, is the proportional gain, K, the integral gain, m is the required modulation index of the PWM generation which ranges between 0 and 1.0, a is the phase delay angle of the modulation and is preset to zero to give a unity power factor, VI,,, is the measured AC line-to-line voltage of the converter, and Se,, accumulates error in the current which functions as an integrator.

When the SMES system is in compensation mode, both controllers operate simultaneously. Because the SMES system has only one converter, the outputs of the controllers are combined to produce a single set of switching patterns for the converter. The process of combining the outputs is achieved by translating the voltage output Vd of the coil current controller to an equivalent power requirement, which can be directly added to the power output of the load compensation controller. The following algorithm illustrates how the combined controller is implemented for the direct and optimised strategies:

I e w = ITef - I d

Po = Id (Icp IeTT + se,,) Se,, = S e r r + A P L o a d = PLoad - P L O

IEE pro^ -Genet Trunsni Di>rrrb, Vol 145, N o 6 November 1998

A Q L o a d = QLoad - Q L O

Pc = lcll .AP+ l c z z . AQ Q~ = kal . ap i- ~c~~ . AQ Pd = PC PO Q d = Qc + Qo

(15) PLO and QLo are average quantities of load active and reactive power, respectively. These two quantities can be predetermined or evaluated on-line. In practice, they vary with the network conditions and the load opera- tion, and so they should be evaluated on-line. For this study, the load is preset to have constant PLO and QLo, and hence on-line evaluation is not required, and both constant components are predetermined by simulation. An offset reactive power Qo is included in the SMES reactive power injection, for correcting the system volt- age level if required. k, , , k,,, k2, and k22 are the gain constants of the load compensation controller, and their values are tabulated in Table 1 for both strategies. Pc and Qc are the outputs of the load compensator controller. Pd and Q,[ are the active and reactive load values which are required to meet the variable compo- nent of the load, as well as the requirement of main- taining the coil current (Po) and offset reactive power (eo).

Table 1: Controller gains of direct and optimised strate- gies

Optimised strategy Controller Direct gains strategy

kl 1 -1 -hfll(h:l+ h7$)

kl2 0 -hl lhl2/(hf i + h i$)

k2 1 0 -hl lh l* / (hf l + h i$)

k22 -1 -h&/(h:~+ hf2)

After the required SMES active and reactive power (PLi and Qd) are evaluated, the modulation index m and phase delay angle 01 for the PWM generation can then be determined, as shown in the flow chart of Fig. 10. Fig. 11 shows the block diagram of the SMES control system.

With reference to Fig. 10, the active power is set to a minimum value if the coil current exceeds its safe max- imum value. If the coil current is within limits, the cal- culated value of Pd is used. The maximum available apparent power is S,,,, and clearly the active power demanded must be less than the available apparent power. Likewise, the reactive power demanded must be less than the remaining reactive power. Finally, the modulation index m and phase delay angle a are calcu- lated to achieve the required active and reactive power values.

5 Experimental results

5.1 Coil current controller The closed-loop step response of the coil current con- troller was simulated and experimentally verified. The disturbing load was disconnected, and a 50A step refer- ence was applied to the controller during its stand-by mode. Fig. 12 shows the response of the coil current. The corresponding average voltage across the coil is shown in Fig. 13. The solid curves are the experimental

IEE Proc.-Gener. Transin. Disfrib.. Vol. 145. No. 6, November 1998

results, and the dotted curves are the simulated results. The overshoot of the current was 6%~, and the system reached a zero steady state error within 55s . The coil voltage initially stepped up to about 50V, and decayed exponentially to zero as the controller regulated the current to its reference level. The designed closed-loop response is relatively slow, so that it does not interfere with the response of the load compensation controller. In the simulation, only the DC circuit of the SMES system was simulated. The converter output voltage was approximated by a linear relationship with the PWM index m. The simulation results were obtained through much testing and modelling of the system. Both experimental and simulated results are essentially identical, which shows that the superconducting coil and the converter were accurately modelled.

(5)

- Fig. 10 Evaluation of m and afrom Pd and Q,,fOr SMES systan

5.2 Direct strategy The disturbing load was reconnected to the system, and generated a sinusoidal 2Hz disturbance. Direct strategy was used for the SMES load compensation controller to suppress the fluctuation. Fig. 14 shows the experi- mental results. The compensation was applied after 2s. In the transition between stand-by mode and compen- sation mode, the load compensation controller went through a transition mode, in which it gradually increases its gain of SMES power injection in order to avoid a step disturbance to the system. Without this mode, a step injection of SMES power had been observed to cause overvoltage in the filtering capacitor. Therefore, the transition mode is important for this experimental system where many tests are being carried out.

The measured voltage fluctuation was reduced from 9.7%J to 2.30/0. The compensation was not perfect, which was mainly due to the delay in the feedback measurement of load active and reactive power. The load fluctuation shown in Fig. 14b increased from 1400W and 2900var to 2300W and 3800var. The

705

SMES converter ’

L

:a so- -

Fig. 1 1 Block diagram of loud compensution using SMES system

--- -

o u 0 10 20 30 1 0 50 60

time.s Fig. 12 ured und simulated coil currents ure shown

~ experimental -~~~ simulated

Closed-loop step response of coil current controller. The meas-

-101 I 0 10 20 30 LO 50 60

time,s Fig.13 ured and simulated coil DC vokzges are shown

~ experimental ~~~~ simulated

Closed-loop step res onse of coil current controller. The meus-

b

C

5 52r

time,s d

Fig. 14 a Measured line voltage at PCC b Measured load power flow c Measured SMES power flow d Measured coil current (i) active, (ii) reactive

706

Experimental results of direct strategy

increase in load fluctuation was expected because the supply was strengthened by the compensator. In Fig. 14c, the SMES system injected amounts of active and reactive power, equal to the load fluctuation, to oppose the disturbance. The variation in the coil cur- rent in Fig. 14d reflects the energy transfer between the SMES system and the network. The instantaneous flicker value of the sinusoidal disturbance is shown in Fig. 15. The flicker was also correspondingly reduced after compensation.

lot 5 1 \ 0 0

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 L.0 time,s

Fig. 15 Measured flicker at PCC using digitul flickermeter

5.3 Optimised strategy To highlight the benefit of optimised strategy, a larger RIX ratio of supply impedance was selected, because the analysis carried out in Section 3.2 shows that opti- mised strategy would only have significant advantage for a compensator in a network with a large RIX ratio. ZLine was then modified to 3.37 + j6.82Q2, which gives an RIX ratio = 50% . Experiments were performed for both reactive power only, and optimised active-reactive power compensations under the same conditions. Fig. 16 shows the results for the reactive power only compensation, with the elements of controller gains shown in case 1 of Table 2. k l l and kI2 were zero, because the active power component is zero for reactive power compensation. Fig. 17 shows the results for the optimised methods with the elements of controller gains listed in case 2 of Table 2. Note that k I 2 and kzl do not have the same value, which is different from the definitions shown in Table 1. This is because in the actual implementation, the leakage impedances associ- ated with the variac and the transformer were also included in the evaluation of controller gains.

Compensation was again applied after 2s. Both methods reduced the voltage fluctuations from 1 1.1% to 2.7%. In the reactive power only compensation, there was no SMES active power injected during com- pensation, as shown in Fig. 16c. Therefore, there were no changes in the coil current either. The component of injected reactive power to suppress the disturbance was

IEE Proc.-Gener. Transm. DDrrib.. Vol. I45, No. 6, November 1998

about 4970var. In the optimised active-reactive power compensation, the components of injected active and reactive power, as shown in Fig. 17c, were 2424W and 3212var, which gives an approximate reduction of 20% in VA flow to rectify the same disturbance. Therefore, a SMES-based compensator with an optimised strategy can maximise the usage of the compensator.

P 2 < 50’

‘5 L8 -

a

I

b

C

P -E - ;< ::p7-----;1: ‘5 1 8

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 1.0 time,s

d Experimental results of reactive power compensation for line Fig. 16

impedance of 50% R/x ratio a Measured line voltage at PCC b Measured load power flow c Measured SMES power flow d Measured coil current (i) active, (ii) reactive

Q

b

Table 2: Controller gains of reactive power and opti- mised active-reactive power compensation

1 0.0, 0.0, -0.747, -1.020

2 -0.358, -0.490, -0.476, -0.65 1

IEE Proc.-Gener. Transm. Distrib.. Vol. 145. No. 6, November 1998

6 Conclusions

A SMES system has been developed for the study of load compensation using a SMES-based compensator. The system has been successfully tested and operated to verify the results of the analysis. The developed control system of the SMES system is made up of two control- lers: a coil current controller and a load compensation controller. The coil current controller regulates the superconducting coil current at its desired operating level. The load compensation controller is responsible for the suppression of load disturbances. Two control strategies were proposed for this controller: direct and optimised. The direct strategy is the simpler technique. The optimised strategy maximises the usage of a com- pensator. Both control strategies have been demon- strated by extensive experimentation on the actual system, and have been confirmed to be effective in sup- pressing fluctuating load disturbances.

This paper covers the compensation of fluctuating balanced loads. Studies have also been carried out for the compensation of fluctuating unbalanced loads using the same SMES system. These will be presented in another paper in the near future.

7 Acknowledgments

This work was supported by the Australian Electricity Supply Industry Research Board (AESIRB) and the Energy Research and Development Corporation (ERDC). The authors would also like to thank the other members of the SMES Research Group at Monash University and the group leader, Prof. W.J. Bonwick.

8 References

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5

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13 ‘Flickermeter - functional and design specifications’. IEC 868, International Electrotechnical Commission (IEC) Report, Publi- cation 868, 1986

14 FRANK, H., and IVNER, S.: ‘TYCAP, power-factor correction equipment using thyristor-controlled capacitors for arc furnaces’, ASEA J . , 1973, 46, (6), pp. 147-152

15 WANNER, E., MATHYS, R., and HAUSLER, M.: ‘Compensa- tion system for industry’, Brown Boveri Rev., 1983, (9-lo), pp. 330-340

I6 Westinghouse Electric Corporation: ‘Electrical transmission and distribution - reference book’ (Westinghouse Electric Corpora- tion, East Pittsburgh, Pennsylvania, 1950, 4th edn.), Chapter 3

17 KUO, B.C.: ‘Automatic control systems’ (Prentice-Hall Interna- tional, 1991, 6th edn.)

9 Appendix

Derivation of H used in Section 3.2:

and

708 IEE Proc.-Gene?. Transm. Distrib., Vol. 145, No. 6, Novtwibrr 1998