GB2167582A

GB2167582A - Reactive power compensating circuit

Info

Publication number: GB2167582A
Application number: GB08520171A
Authority: GB
Inventors: Eric Anthony Lewis
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1984-11-23
Filing date: 1985-08-12
Publication date: 1986-05-29
Also published as: GB8429683D0; ZA856502B; GB8520171D0; GB2167582B

Abstract

A reactive power compensating circuit is particularly for use with mine winders and similar heavy industrial motor loads (1). The reactive power generated by such loads commonly varies widely in operation and consequently makes compensation difficult. The invention provides a capacitor bank (11) of basically fixed value in parallel with a variable inductive load, provided by a fixed reactor (15) supplied by phase controlled thyristor convertors (17). The net compensating load can then be adjusted to match the motor load reactive power sensed by transformers (35) and compared with a manually set reference (41) or with a reference derived from the current in the capacitor bank (11). <IMAGE>

Description

SPECIFICATION Reactive power compensating circuit This invention relates to so-called reactive power compensating circuits commonly employed for balancing the reactive-power, orVARs, generated by typical industrial loads. For large machines, for example mine winder motors, the generated VARs can be very substantial and have a significant lagging effect on the power factor of the installation and on the supply voltage level.

It has been proposed to compensate for this effect in various ways. According to one method the supply is loaded by a rotating machine having a leading power factor. A disadvantage of this scheme is the inertia of the machine and the resultant inability to follow load changes rapidly. In another method a bank of capacitors is loaded on to the supply and is switched in steps by tap changers.

Again the control is rather slow and in view of the step changes can only approximate to the desired compensation.

According to the present invention, a reactive power compensating circuit for a variable inductive load, comprises A.C. supply terminals, an inductive reactance connected to said A.C. supply terminals by way of a phase-controlled semiconductor convertor circuit, means for controlling said convertor in dependence upon the reactive power generated at the load terminals in such manner that the variation in reactance at said supply terminals with variation of said load, is substantially reduced.

There may be included capacitance means of reactance magnitude comparable to that of the inductive reactance and also connected to the A.C.

supply terminals, the convertor being controlled in such manner that the negative reactive power of the capacitance means tends to balance the positive reactive power of the load and of the convertorl inductive reactance combination despite variations of load.

The means for controlling the convertor preferably comprises a thyristor firing circuit and means for controlling the firing angle in dependence upon a control signal which is derived from the difference between a reference signal and a signal representative of the reactive power generated by the load. The reference signal may be derived from a manually controllable source, or, alternatively from current drawn by the capacitance means. The reference signal may include a component responsive to variations of voltage at the A.C. supply terminals.

This component may include means for determining a relatively long term average magnitude of the A.C.

supply voltage and relatively short term deviations, beyond a predetermined extent, of the supply voltage from this average magnitude.

The reference signal may be responsive to the powerfactor attheA.C. supply term ina Is in such manner as to tend to decrease the effective reactance of the inductive reactance in response to a fall in power factor.

The circuit may comprise means providing a reference signal from the capacitance means by way of amplifying means in dependence upon a current average value of the power factor.

One embodiment of a reactive power compensating circuit will now be described, by way of example, with reference to the accompanying drawings, of which: Figure lisa circuit diagram of a mine winder drive system including the compensating circuit; Figures 2, 3, 4 & 5 show various modifications of a compensating convertor circuit and its connections; Figure 6shows a circuit for obtaining a convertor reference signal from a capacitor bank; Figure 7 shows a circuit for making a convertor reference signal responsive to overall power factor; and Figure 8 shows a circuit for making a convertor reference signal responsive to the supply voltage level.

Referring to Figure 1, a mine winder D.C. motor 1 is driven from a 3-phase A.C. supply system 7 by way of a transformer 9 and winder convertor 3. The motor is connected to the convertor by way of a D.C.

circuit breaker 5. The convertor3 is a standard phase-controlled thyristor convertor and may be a 12 pulse, 12/24 pulse or 6/12 pulse system for example. Since this convertor unit is standard, its firing control circuit is not shown although it operates to raise and lower the winder cage at specified speeds and travel. The motor 1 constitutes an inductive load and consequently generates a substantial reactive 'power' or VARs (volt-amps reactive) which, without compensation would have to be carried by the supply two cause a lowering of the power factor and poor supply regulation. The effect would be easily remedied if the reactive power generated were constant but in fact it varies with the winder load. In a typical case it might vary from zero at no load to 3 MVAR at full load.A compensating capacitor bank, shown as a single capacitor 11 is connected to the supply line 7 by an A.C. circuit breaker 13. This capacitor bank is designed to provide a capacitive, i.e., leading, load of 3 MVAR and thus will only balance the winder reactive power at full winder load.

A further inductive reactance load is therefore provided by a reactor 15 which is supplied from two series-connected phase-controlled semiconductor (and in fact, thyristor) convertor units 17. Each convertor unit comprises a three-phase thyristor bridge which, in known manner, produces six current pulses per supply period if the thyristors are fired at 60 intervals in phase sequence. Six current pulses per cycle can produce substantial current harmonics which have to be suppressed, or rather, bypassed from the supply, by correspondingly expensive filters. To reduce the size of the filters necessary, two convertor units 17 are employed, each giving six current pulses but interlaced uniformly. A 12-pulse system is thus produced. To achieve this the convertor units 17 are fed from phase displaced supplies.Transformer 19 has primary and secondary windings of, say, delta-start formation while transformer 21 has delta-delta formation. The transformer primaries are connected to the supply system 7 by an A.C. circuit breaker 23.

Harmonic currents, although greatly reduced by the provision of a 12-pulse system, are still present.

A fixed (inductive) reactor 37 is therefore connected in series with the capacitor bank 11 to provide a short circuit for the harmonics and keep them out of the supply system.

By using two convertor units in series as described, the voltage rating of the thyristors in each may also be reduced.

The convertor units 17 are controlled by firing circuits 25 which produce a firing pulse at a point in the forward half cycle of each thyristor determined by the level of a signal derived from an amplifier 27.

The amplifier input signal is derived from a differencing circuit 29 which obtains the difference between a control signal from amplifier 31 and a current feedback signal derived from a current transducer 33. This negative feedback signal increases with the magnitude of current drawn by the convertor units 17 and tends to stabilize the operation. Amplifier 31 is provided with a control signal which in turn is derived as the difference between a reference signal and a signal representative ofthe reactive power generated by the winder motor. This latter signal is derived from a VARs transducer circuit 36 which gives a D.C. output proportional to the reactive power present at the input of the transformer 9, this reactive power being derived from current and voltage transformers indicated diagrammatically by a winding 35.

In Figure 1,the reference signal input to difference circuit 39 is derived from a potentiometer 41 which is manually set to provide full firing of the convertor units 17 in the absence of any reactive power feedback from transducer 36.

In operation, as the winder load increases, so the winder lagging reactive power increases. The control signal input to amplifier 31 therefore falls, and the compensating lagging reactive power of reactor 15 falls correspondingly. The total lagging reactive power thus remains substantially constant and is balanced by the leading reactive power of the capacitor bank 11. For example, at no load the capacitor bank provides 3 MVAR leading, the reference signal setting provides 3 MVAR lagging, and the winder reactive power feedback signal is zero. At two-thirds full load, the capacitor bank remains at 3 MVAR leading, the reference setting remains at 3 MVAR lagging but the feedback signal reduces this by 2 MVAR so that the compensating reactive power of reactor 15 is 1 MVAR.This together with the winder reactive power still balances the capacitor bank to give zero reactive power in the supply system.

The convertor circuit of Figure 1 comprises, as explained above, a single reactor 15 in series with two convertor units. Several alternative configurations are possible. Figure 2 shows a parallel arrangement of two convertor units 17 (for connection to the A.C. circuit breaker 23) having delta-star and deltadelta transformer supplies to provide a 12 pulse system. However, each convertor has it own reactor load 43, the two reactors having a common connection.

Figure 3 shows a similar arrangement but in which the reacors 43 have no common connection.

A single convertor unit 17 and reactor 43 could, of course, be employed if a 6-pulse system can be tolerated, i.e., if the harmonic filter reactor 37 is sufficiently highly rated. This system can be duplicated, ortriplicated as in Figure 4 to obtain greater overall compensation rating. The benefit of a 12pulse system has been sacrified to the benefit of standardised transformer components in this arrangement.

Figure 5 shows a further alternative in which two isolated compensating convertor units 17 are employed, each having its own reactor 43. These convertor units are however, coupled to the outputs of winder drive transformers 45 corresponding to the transformer 9 in Figure 1. Two main drive convertors 47 are used in this arrangement. Thus no compensating convertor transformers are used at all. The main transformers would, however, have to be rated for full VARs at all winder loads, since the compensating convertor VARs would always 'topup' the drive convertor VARs to the full-load value.

It has been assumed above that winder lagging VARs must be fully compensated by the capacitor bank and therefore that the compensating convertor must be rated for continuous operation at the winder maximum VARs (for long periods of winder noload). There are, however, alternative rating schemes.

If, for example, the winder produces a 7% fall in the supply voltage (due to generation of reactive power) in the absence of any compensation, i.e. no capacitor bank and no compensating convertor, and if in fact a 3% fall would be tolerable, then a capacitor bank rated at only 4/7 of peak generated VARs would be adequate. Consequently, a compensating convertor similarly down-rated would also suffice. In this case, when the winder generated VARs rose above the capacitor bank rating there would be a net surplus of lagging reactive power producing the 3% supply voltage fall. An improvement on this situation could be obtained by increasing the capacitor bank rating without increasing the compensating convertor rating.Thus instead of obtaining zero to 3% voltage fall forthe upper load condition of the windertherewould be, say, a zero to 11/2% fall forthe upper load condition and a 11/2% to zero rise corresponding to the surplus, unbalanced, capacitor VARs in the low load condition.

A power factor close to unity can thus be obtained, on an average basis, over a complete winding operation.

In the arrangement of Figure 1,the reference signal input to differencing circuit 39 is derived from a potentiometer, manually set to designate the no-load value of compensating VARs. This reference signal can be derived in other ways.

Figure 6 shows one arrangement for deriving the reference signal in dependence upon the capacitor bank reactive power. The level of reactive power is derived from a transformer 49 and converted to a suiable D.C.-value by transducer 51. The output of this transducer is applied to a potentiometer which replaces the potentiometer 41 in Figure 1. Thus, when the capacitor bank is switched in or out the compensating convertor load will vary in step. This is important for avoiding surges otherwise occurring on switching the capacitor bank.

Figure 7 shows an arrangement which is a development of that of Figure 6. The reference signal is derived as before, in dependence upon the capacitor bank current. However, a variable gain amplifier 55 controls the resulting reference signal in dependence upon the power factor at the A.C. supply terminals, as determined by transformers 59 and a transducer 57 having a time constant of one or more 'wind' durations. The gain of amplifier 55 is controlled so as to decrease the compensating reactance, i.e., to increase the compensating reactive power in response to a fall in power factor. This arrangement enables the average power factor to be held close to unity even for changes of winder duty.

The supply voltage can be more directly controlled than by merely balancing the reactive power to zero.

Figure 8 shows an arrangement in which the control signal to the compensating convertor is derived from three components via differencing circuit 39; the drive convertor VAR demand, the reference signal from the capacitor bank, both as before, and in addition, a signal derived from the supply voltage.

This latter signal is derived from an operational amplifier 61 which has two inputs from the A.C.

supply line. First, an input 63 derived by way of a long time constant filter to produce a long term average value of the supply voltage, and secondly, an input 65 with a rapid response to the voltage level. The characteristic of the amplifier is such as to produce zero output for voltage variations within a predetermined extent from the average value determined by input 63 and to produce increasing and decreasing outputs for positive and negative excursions beyond these thresholds respectively.

Such an arrangement is, however, sensitive to reactive power in the supply generated by unreleated sources.

Claims

1. A reactive power compensating circuit for a variable inductive load, comprising A.C. supply terminals, an inductive reactance connected to said A.C. supply terminals by way of a phase-controlled semiconductor convertor circuit, means for controlling said convertor in dependence upon the reactive power generated at the load terminals in such manner that the variation in reactance at said supply terminals with variation of said load, is substantially reduced.

2. A reactive power compensating circuit according to Claim 1 including capacitance means of reactance magnitude comparable to that of said inductive reactance and connected to said A.C.

supply terminals, the convertor being controlled in such manner that the negative reactive power of said capacitance means tends to balance the positive reactive power of the load and of the convertor/ inductive reactance combination despite variations of load.

3. A compensating circuit according to Claim 1 or Claim 2 wherein said means for controlling the convertor comprises a thyristor firing circuit and means for controlling the firing angle in dependence upon a control signal which is derived from the difference between a reference signal and a signal representative of the reactive power generated by the load.

4. A compensating circuit according to Claim 3, wherein said reference signal is derived from a manually controllable source.

5. A compensating circuit according to Claim 3 as appendant to Claim 2, wherein said reference signal is derived from current drawn by said capacitance means.

6. A compensating circuit according to Claim 3, wherein said reference signal includes a component responsive to variations of voltage at said A.C.

supply terminals.

7. A compensating circuit according to Claim 6 including means for determining a relatively long term average magnitude of the A.C. supply voltage and for detecting relatively short term deviations, beyond a predetermined extent, of the supply voltage from this average magnitude.

8. A compensating circuit according to Claim 3, wherein said reference signal is responsive to the power factor at the A.C. supply terminals in such manner as to tend to decrease the effective reactance of said inductive reactance in response to a fall in power factor.

9. A compensating circuit according to Claim 8 and Claim 5, and comprising means providing a reference signal from said capacitance means by way of amplifying means, and means for controlling the gain of said amplifying means in dependence upon a current average value of said power factor.

10. A reactive power compensating circuit substantially as hereinbefore described with reference to the accompanying drawings.