GB2057796A - Ac transmission line reactive power compensation system - Google Patents

Abstract

A damping arrangement for thyristor switched capacitors in reactive power compensation systems. Large inrush currents are possible where capacitors are re-connected after a time of disconnection. The invention consists of incorporating in series with the thyristor switching bank a large damping resistor bypassed by thyristors which may in fact be a series part of the main switching bank. The bypass is opened prior to closing the main thyristors and closed a short time after thus encouraging decay of transients. The re-closing of the bypass may be done in stages (sections of the damping resistor) or progressively in successive cycles.

Description

SPECIFICATION Current damping arrangements This invention relates to damping arrangements for high power a.c. transmission systems in which a system component, such as a capacitor bank, is connected to the system by means of a solid state switching arrangement, such as a series-parallel thyristor bank, the system component being thereby subject to large transient currents on operation of the switching arrangement. It is thus of particular application to capacitor banks connected to the a.c. network through anti-parallel connected thyristor valves; each valve generally has one or more thyristors connected in series and parallel.By using a thyristor switch instead of conventional switches, circuit breakers or contactors, much higher speeds of controlling the ON or OFF conditions can be achieved so that the magnitude of capacitance connected to the network can be rapidly varied, for example within a cycle of power frequency, by controlling the number of capacitor banks connected. Such rapid control is of importance in networks where rapid changes and fluctuations in load current flows, and resulting reactive power and voltage variations occur, for example in industrial systems or in transmission systems. The thyristor switched capacitors then act as a form of static compensator, of which other known examples are saturated reactor compensators and thyristor-controlled reactor compensators.

Such capacitors can be used on their own or in conjunction with other reactive compensation devices.

One of the major problems associated with capacitors is the sudden inrush of currents which occurs if a capacitor is connected to the a.c. network at an instant when the connection of the capacitor produces an immediate rate of change of voltage on the capacitor which is not equal to zero. This would happen when an un-energised capacitor bank is first switched on, or if the capacitor is being connected after a lapse of time after last energisation so that the charge left on it at the last disconnection has had time to decay fully or in part, or if the crest value of the alternating network voltage has changed. The inrush would occur even if the thyristor were switched to connect the capacitor bank when the voltage across the thyristor switch was passing through zero and instantaneously equal to it.

It is normal practice to include a relatively small current limiting reactor in series with each thyristor switch/capacitor combination, as shown in Fig. 1 of the accompanying drawings. Although the reactor limits the maximum magnitude of the transient inrush current, the transient takes on an oscillatory nature having a frequency greater than the supply system operating frequency (50 or 60 Hz.) Typical values of reactor result in oscillation frequencies ranging from about 2 to 10 (or more) times the supply frequency. The oscillatory current is superimposed on the desired power frequency capacitor current and it is not desirable from the point of view of a.c. network conditions and of the thyristor switched capacitor bank itself to allow the oscillation to persist for more than a few tens of milliseconds.Some means of damping the transient is necessary and some proposed or used methods employ resistors permanently in series or parallel with the reactor as shown in Fig. 2. The different arrangements permit different levels of power frequency continuous losses in the resistor. These arrangements still require substantial size of resistors to achieve the necessary rapid damping of transients and substantial size of auxiliary capacitor components, and the supply frequency tuned case of Fig. 2(d) works adequately over only a narrow band of supply frequency variations.

Accordingly the present invention consists in a current damping arrangement for a high power a.c. transmission system in which a system component is connected to the system by means of a solid state switching arrangement and in which the system component may be subjected to large transient currents on operation of the switching arrangement, the current damping arrangement comprising resistive damping means connected in parallel with bypass switching means, the parallel combination being connected in series with the system component in such manner that, when the bypass switching means is open the whole of any transient currents are carried and damped by the damping means, and further comprising control means arranged to control the bypass switching means in such manner as to remove the damping means from the current path in steady operation, after a predetermined time from the closing of the switching arrangement.

The bypass switching means preferably consists of solid state components and may comprise a series connected part of the switching arrangement.

The bypass switching means may be controlled so as to grade the increase in current bypassed following the closing of the switching arrangement.

The damping means may be in series connected sections and the bypass switching means may then be effective to bypass the sections successively.

Alternatively, the bypass switching means may be phase-controlled by the control means so as to close for a progressively increasing proportion of successive half-cycles of the supply voltage following the closure of the switching arrangement.

The said predetermined time may be determined by the reduction of the transient cur rent to a predetermined level.

Preferably the switching arrangement and the bypass switching means both comprise thyristors.

In a system having a neutral conductor, where the resistive damping means and the system component are connected between the neutral conductor and a line conductor, the resistive damping means is preferably on the neutral side of the system component which may comprise a capacitor or capacitor bank, or a reactor or reactor bank.

Several embodiments of a current damping arrangement in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a diagram of a known thyristorswitched capactor system employing current limiting reactors; Figure 2 shows diagrams of different known resistive current-limiting arrangements; Figure 3 shows a diagram of a current limiting arrangement according to the invention, and Figure 4 is a diagram of a modification of Fig. 3.

Figs. 1 and 2 have already been described briefly. Fig. 1 shows a bank of capacitors C, each unit of the bank being in series with an anti-parallel thyristor valve arrangement T. A current limiting reactor L completes the series connection of each capacitor unit between the lines of the system (one of which may be the neutral). A control system 3 controls the gating of the thyristor valves to switch in the capacitor bank to the required extent.

Figure 2(a) shows a capacitor unit C in series with a current limiting reactor L and a damping resistor R.

Figure 2(b) shows a similar arrangement but with the damping resistor shunting the reactor L.

Figures 2(c) and 2(d) show arrangements with auxiliary capacitors CA which provide selective damping of frequencies outside the supply frequency by tuning arrangement. All of these arrangements have permanently connected damping resistors.

Referring now to Fig. 3, a capacitor bank represented by a capacitor C is connected between system lines 1 and 2 by means of a bank of thyristor switches T comprising antiparallel thyristors, each series arm of which may comprise a plurality of thyristors as shown, and each anti-parallel pair of series arms of which may be replicated for current carrying purposes. Each unit of the capacitor bank C may be series connected with a respective anti-parallel unit of the thyristor bank T.

A current limiting reactor L is connected in series, as in the known systems, and then a damping resistor R in parallel with a shunting thyristor pair Ts completes the series connection.

A control circuit S controls the gating of the thyristors. It is required that the damping resistor R should be in circuit for a predetermined period following the firing of the thyristor bank T and that it (or at least a major part of it) should then be bypassed for normal operation after the decay of the major part of the transient current.

The bypassing of the damping resistor R is effected by the thyristor unit Ts under the control of the control circuit S. A period ranging from one half cycle to several cycles of the supply frequency (approximately 10-40 milliseconds at 50Hz) is a sufficient period to leave the damping resistor in circuit.

The control circuit S is controlled to delay the firing of the thyristors Ts by a predetermined number of half cycles after the capacitor bank T has been fired. It will be appreciated that only the thyristors with a positive anode to cathode voltage at the instant of triggering will fire. The anti-parallel arrangement ensures that conduction can occur in every half-cycle.

In an alternative control arrangement, the control circuit S is made responsive to the magnitude of the transient current and the delay period before firing the bypass thyristors Ts is controlled to vary with the.transient current magnitude.

The damping resistor may be switched out of circuit, or bypassed, in stages, by physically dividing it into sections r as shown in Fig. 4. In this case each section r has its own thyristor bypass unit Ts.

The bypass thyristor unit or units Ts may be one or more series connected sections of the main thyristor bank T.

In an alternative method of grading the bypass current, where there is a single damping resistor as in Fig. 3, the control circuit S is arranged to fire the bypass thyristors T5 at a progressively earlier point in each half cycle throughout the several half-cycles of the damping period.

The two methods of grading, sectioned damping resistor, and progressive thyristor firing, can also be combined. The different sections may each be progressively fired, but the progression staggered from one section to the next.

The damping resistors of Figs. 3 and 4 remain in circuit for a short time only and therefore substantial resistance values can be used to achieve rapid damping without incurring excessive continuous losses. For example if the critical value of damping resistance for obtaining a non-oscillatory transient in the R L-C circuit is Rc = 2 < L/C), suitable values of damping resistance R range from 0.1 Rc to 0.5 Rc although values outside this range may be used in some cases. Such values of series damping resistance would be prohibitively 'lossy' if permanently connected, e.g. as in Fig. 2. The thyristor-switched damping resis tors achieve the desired transient damping for the oscillatory current components, while costly continuously losses are avoided and the physical size of the resistors is also kept small.

The bypass switching means does require additional thyristors, but their number is likely to be small in relation to the number of thyristors in the main part of the valve.

If the capacitor bank is connected from phase to neutral of the a.c. system it may be advantageous from insulation considerations to provide the damping resistor and the bypass switch towards the neutral end, particularly if the neutral is earthed in some manner.

The resistor itself may be of any of the suitable conventional constructions, e.g. metallic, metal grid, metal wire, ceramic etc. The invention applies equally to single phase and multiphase a.c. networks.

It is normal practice, as a means of avoiding, or at least reducing, inrush currents, to maintain the capacitor bank at full voltage when it is disconnected and to reconnect it when called for, at the peak voltage of the same polarity. It is any discrepancy between the maintained (or re-charged) voltage of the capacitor bank and the peak system voltage to which it is connected that causes the inrush current.

The need to keep the capacitors charged up during their 'off' period produces a requirement for expensive d.c. capacitors designed to withstand d.c. stress. In addition complex control systems are required to recharge the capacitors in their 'off' or 'reserve' period to the opposite polarity periodically to reduce this same d.c. stress.

A thyristor-switched capacitor bank according to the invention avoids these requirements and thus provides a major advantage. The combination of the current limiting reactor and the thyristor-switched resistor damps inrush current transients to such an extent that the main thyristor valve can be switched on whenever the valve voltage is zero or nearly zero within each half-cycle of the system frequency. Consequently, there is no need to maintain the capacitor banks charged up in their 'off' periods and, therefore, no need to use expensive d.c. capacitors. Conventional a.c. capacitors can therefore be used which incorporate internal discharge resistors which reduce the residual voltage on disconnection of the capacitors to a low value within a few minutes. There is thus no need for the complex control systems for reversing the capacitor standing charge polarity.In addition the periodic recycling current that would be drawn from the a.c. network is absent.

It is permissible however to incorporate such controls in conjunction with the damping arrangement of the invention. The normal operation is to close the main thyristor switch at the instant of zero switch voltage, when a command instruction is received to connect a capacitor bank. When, due to pre-charge on the capacitor, or due to network voltage changes, such a zero voltage instant is not achieved, the main thyristor switch is closed at the minimum voltage instant. After allowing the damping resistor to remain in circuit for one half cycle (for example), its bypass switch would be closed. A longer or shorter duration of damping resistor insertion may be chosen (as described above) as suited to the circumstances.

Although the invention is particularly suited for application to thyristor switched capacitors, the bypass-switched resistor method is also applicable to suppression of inrush current problems in other thyristor valve applications, such as a.c. valves for thyristor-controlled reactor schemes and for d.c. valves for hvdc (or even lower voltage d.c.) converters i.e. rectifiers and inverters. In such cases the inrush current is caused by the relatively small capacitances of the supply transformers, linear reactors and connecting busbars and bushings. The inrush frequencies are however higher than in the switched capacitor schemes because the circuit inductance is also usually not very large. The invention can be used for these applications but since the required amount of damping is smaller, more conventional means such as inrush limiting reactors having relatively large high frequency resistance may be adequate in many cases, or provision of uni-directional damping in d.c.

valves for transient oscillatory currents in a direction opposite to that of the main d.c.

current. The present invention may however be useful in difficult cases and add to the overall economy of the equipment.

In order to ensure that the bypass part of the switch is operating to prevent excessive losses and damage in the series damping resistor, the latter may be fitted with a suitable fuse, or other conditions may be designed to give an alarm, or trip action, or automatic electronic firing of the bypass valve thyristors.

Claims (12)

1. A current damping arrangement for a high power a.c. transmission system in which a system component is connected to the system by means of a solid state switching arrangement and in which the system component may be subjected to large transient currents on operation of said switching arrangement, the current damping arrangement comprising resistive damping means connected in parallel with bypass switching means, the parallel combination being connected in series with said system component in such manner that, when said bypass switching means is open the whole of any said transient currents are carried and damped by said damping means, and further comprising control means arranged to control said bypass switching means in such manner as to remove said damping means from the current path in steady operation, after a predetermined time from the closing of said switching arrangement.

2. A damping arrangement according to Claim 1 wherein said bypass switching means consists of solid state components.

3. A damping arrangement according to Claim 1 or Claim 2, wherein said bypass switching means comprises a series connected part of said switching arrangement.

4. A damping arrangement according to any preceding claim, wherein said bypass switching means is controlled so as to grade the increase in current bypassed following the closing of said switching arrangement.

5. A damping arrangement according to Claim 4, wherein said damping means is in series connected sections and said bypass switching means is effective to bypass said sections successively.

6. A damping arrangement according to Claim 2, wherein said bypass switching means is phase-controlled by said control means so as to close for a progressively increasing proportion of successive half-cycles of the supply voltage following the closure of said switching arrangement.

7. A damping arrangement according to any of Claims 1, 2 and 3, wherein said predetermined time is determined by the reduction of the transient current to a predetermined level.

8. A damping arrangement according to any preceding claim wherein said switching arrangement and said bypass switching means both comprise thyristors.

9. A damping arrangement according to any preceding claim, in a system having a neutral conductor, wherein the resistive damping means and the said system component are connected between the neutral conductor and a line conductor in such manner that the resistive damping means is on the neutral side of the system component.

10. A damping arrangement according to any preceding claim wherein said system component comprises a capacitor or capacitor bank.

11. A damping arrangement according to any of Claims 1-9, wherein said system component comprises a reactor or bank of reactors.

12. A current damping arrangement substantially as hereinbefore described with reference to Fig. 3 or Fig. 4 of the accompanying drawings.