JPS58215932A - Var generator - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field of the Invention This invention relates to controlling the residual charge of switching capacitors used in reactive power (VAR) generators of the type that supply reactive power to electrical systems, particularly static MAR generators. V for
This relates to an AR generator.

Prior Art The use of switching capacitors in static WAR generators is disclosed in U.S. Patent No. Q, 30Z3. ) No. 1 and No. 17.23
No. 17.1173 ζ is disclosed. In these types of static VAR generators, a number of capacitors are used in series with a bidirectional thyristor switch (which can be used in conjunction with a surge current limiting inductor), as found in the US patents described. In a conventional method, when the capacitor voltage and the AC network voltage are equal, that is, the voltage across the thyrisk switch is zero,
The switch is typically fired in response to a VAR demand signal. However, the moment the voltage across the capacitor bank equals the peak of the AC network voltage, the capacitor bank is disconnected. Therefore, the capacitor bank remains charged by its voltage ζ after disconnection.

Since the capacitor bank is charged to the peak of the applied AC voltage, the voltage across the thyristor switch is the sum of the applied AC voltage and the capacitor voltage, which means that once every cycle the peak AC voltage is double maximum value is reached and the thyristor switch must be able to withstand or block this voltage.

This typically does not present a problem in maintaining the integrity of the thyristor switch.

However, under certain conditions of an AC supply network, the AC voltage can transiently rise above its nominal value to very high voltage levels. If you were to disconnect the capacitor bank when this high voltage level is present, the thyristor would be exposed to extremely high voltages. One solution well known in the field is to utilize a non-linear clamping element connected in parallel with a thyristor switch. Since the breakover voltage level of current nonlinear clamping elements is approximately twice the normal peak operating voltage that the nonlinear clamping element will experience, the maximum residual voltage across the capacitor bank is high enough to prevent normal operation. is one times the voltage level encountered. Therefore, under severe overvoltage conditions when utilizing current technology such as can be found in the above-mentioned US patents, both the capacitor bank and the thyristor switch are subjected to a voltage stress that is twice the normal operating voltage stress. Furthermore, if the thyristor is fired consciously or involuntarily, a severely overcharged capacitor can be reconnected to the AC network, but this will cause a very large surge current to flow through the thyristor ψ switch and This will cause significant transient disturbances in the AC network.

It would be desirable to provide such a device that ensures free operation for the thyristor switch in overvoltage conditions by limiting the residual voltage in the capacitor bank below the levels encountered in normal operation. It would also be desirable to reduce the required surge rating of the thyristor switch, thereby reducing cost and size. It would also be desirable to provide an apparatus that limits the residual charge on a capacitor to a low value without requiring impractically low clamping voltage levels.

DISCLOSURE OF THE INVENTION According to the present invention, there is provided a VAR generator of the type for supplying reactive power to an electrical system, comprising a capacitive reactance element interconnectable to said electrical system for supplying said reactive power to said electrical system for a predetermined period of time. at least one controllable switch means interconnected in series with the capacitive reactance element for connecting the capacitive reactance element into reactive circuit relationship with the electrical system during the period; and at least two non-linear clamps. a clamping element, each non-linear clamping element being connected in parallel with each of said switch means such that the voltage across said switch means is less than a maximum predetermined tolerance value while said switch means is in an off state. A VAR generator is provided which energizes the discharge current of the capacitive reactance element only when the voltage is high and limits the voltage across the capacitive reactance element and the switch means to a predetermined safe level for a period subsequent to the period of time. be done.

Conveniently, the capacitive reactance element is interconnected with the electrical system and supplies reactive power to the electrical system for a predetermined period of time. At least/a controllable bidirectional switch means, which may consist of a pair of unidirectional elements, is interconnected in series with the capacitive reactance element and for the same period of time connects the capacitive reactance element to the electrical system and the reactance circuit. Connect to relationships. At least one pair of non-linear clamping elements, each of which is connected in parallel with each switch means, conducts a current that is a discharge current of the capacitive reactance element while the switch means is in the off state, The voltage across the capacitive reactance element and the switching means is limited to a predetermined safe level only when the voltage across the capacitive reactance element and the switching means exceeds a maximum predetermined tolerance and for a period subsequent to the previous period.

The invention will now be described in detail with reference to the accompanying drawings.

Prior Art Figure 1 is a circuit diagram of a VAR generator as disclosed in U.S. Patent No. 3. It will be appreciated that the number of capacitor banks may vary depending on the transient characteristics and/or VAR issues on the above and/or below sides of the circuit.

Therefore, a large number of capacitors/Nl sink G1 capacitors O/, Ou...On are connected to surge current limiting inductors L/, L, 2...I, n and bidirectional thyristor switches SW/, respectively. , sw, z・1・SW
It is connected in series with n. As usual, the Nyristor switches SW/-SWn are fired or gated in response to the VAR demand signal at the time when the capacitor voltage VC (FIG. 2) and the AC network voltage V are equal. The voltage across the thyristor switch is therefore zero. The moment the current in the thyrisk switch crosses the zero point, the capacitor is disconnected. At this moment, capacitor voltage VQ
is the peak 1 of the AC network voltage V/ (equal to this, h
The capacitor voltage after capacitor disconnection is equal to the peak voltage of V/.

Figures sA--1D are diagrammatic representations of voltage and current waveforms characterizing the connection (switch-in) and disconnection (switch-out) of a capacitor in different permissible switching states. FIG. 2A illustrates the conditions for switching at zero crossings of the AC network voltage V/.

In this case, when V/ is zero, a capacitor such as O/ in FIG. Generates VQ. A typical example of this state is when starting up or when the capacitor C
Exists when /~On is completely discharged. Figures 2B and 20 show the alternating current network voltage, i.e. the applied voltage V/
The peaks of , respectively, indicate the switching state of a positively and negatively charged capacitor. Note that in Figure 28, switching occurs at the positive peak of V/, and in Figure @20, switching occurs at the negative peak of V/. Note that the switch-out also occurs at the appropriate positive and negative peaks, respectively. Figure 2D illustrates the situation when a discharging capacitor is switched on. Note that θ) at which switch-in occurs is when the capacitor voltage VC is lower than the peak value of V7 ( sea bream.

FIG. 3 and FIG. 3A are a circuit diagram and a waveform diagram, respectively, of a combination of a capacitor and a thyrisk switch when the surge current limiting inductor is removed. As mentioned above, capacitor C3 is connected to the alternating current network voltage, i.e. the applied voltage v
3 peak, so any thyristor switch that can be used, such as SWJ, must be rated to block 1x this voltage).

This is the voltage VSWj across the thyrisk switch
is the sum of v3 and capacitor voltage VO3, and 7 times during each cycle, the maximum value of v3 peak value v3 maX multiplied by 2 V, ? This is to reach max.

This is shown in Figure 3A. Thyristor switch voltage VSW,? Note that the first maximum value of is half a cycle after capacitor 03 is disconnected, following the first polarity change and at the peak of <v3.

Under certain conditions in the AC network, for example when a short circuit or a load is removed, ■3 can transiently exceed its normal peak value significantly, thereby causing the connected capacitor C3 and associated capacitors to rise to a high voltage level. Charge. It is well known that during this overvoltage condition capacitor MAR compensation of the AC network is undesirable and therefore the capacitor should be disconnected. However, if this were attempted, the silicon risk switch SWJ would be exposed to a high overvoltage during the next half cycle of AC voltage reversal.

In order to protect the SIRISK switch SWj from the high voltage stress caused by the overcharged capacitor 03#, conventional protection devices have attempted to prohibit disconnection of the capacitor in high AC network voltage conditions (SIRISK switch SWj , ? by keeping it conducting). However, this has the following drawbacks. In other words,
The connected capacitor C3 has already reached a high network voltage due to the lead current that can be drawn through the inductive AC network and creates a dangerous oscillation condition in the network that can further aggravate the overvoltage problem. may also occur. Therefore,
Requirements for AC circuit network and Cyrisk switch SW,? There is a conflict between the safety operation of That is, the former requires fast disconnection of capacitor C3, while the latter requires thyristor switch BW3 to conduct until such time as 3 falls to its nominal level.

A typical way to solve this problem is to use Cyrisk Switch SW,? The idea was to reduce the residual overvoltage of capacitor C3 by using a non-linear clamping element across it. Although this configuration can disconnect capacitor OJ in an overvoltage condition, the maximum residual voltage VOJ of the capacitor remains high. This capacitor voltage vC3 is one times the voltage level encountered in normal operation.

This is because the breakover voltage level of current nonlinear clamping elements is approximately a times higher than the normal peak operating voltage stress to which the nonlinear clamping element is subjected. Therefore, under severe overvoltage conditions, capacitor C3
and thyristor switch 19WJ are subjected to a voltage stress that is twice the normal operating voltage stress. This condition for capacitor C3 typically lasts for a long period of time (in seconds) until an internal discharge resistor (usually integrated into the unit) reduces the residual voltage. During this idle period, if an overcharged capacitor is accidentally or intentionally reconnected to the AC network, a large surge current will flow through the thyristor switch SW3ζ and cause a significant transient disturbance in v3. It can be transmitted.

Examples of the invention FIG. 9 shows a preferred embodiment of the invention.

The circuit configuration shown is such that the signal received from the firing request control circuit is connected to the thyristor switches 5WII-/ and 5WV-, 2.
control the sequence. A suitable firing request control circuit is disclosed in US Pat. No. eta Core 441Jk. The capacitor is part of the VAR generator disclosed in US Pat. No. 1423Q, g'lJ.

Therefore, the signal from the ignition request control circuit is transmitted to the ignition input terminal 2.
It is accepted as 0. This signal is input to the co-input OR gate 30
and is input to one of the three input terminals. This signal is connected to a D-type flip-flop 2.2 and a pulse expander 2.
It is also input to the input terminal of DA. The output terminal of the pulse expander, 24/, is connected to one input terminal of a co-input AND gate. The Q output terminals and output-capable terminals of flip-flops J and 2 are connected to the other input terminals of AND gates 2A and 2g, respectively. AND game), 26°, 2g
The output terminals of each are connected to the other input terminal of the OR gate 1-30.3J. The output terminals of the OR gates 3θ and 3 are the first firing pulse generator 3da, the first firing pulse generator, and ?, respectively. Connected to the input terminal of B. 7th
The output terminal of the ignition pulse generator 3Q- is connected to the gate of each thyristor in the thyrisk switch 5WtI-/. Similarly, the output terminal of the second ignition pulse generator 3A is connected to the gate of each Xyrisk in the Syrisk switch SW<Z-λ. Thyristor switch 81
11-/ and swp-co consist of semiconductor thyristors. A large number of anti-parallel connected thyristor pairs are connected in series to form two "half" switches, each of which is a thyristor switch BWy-7 or 5w1I-,2. Nonlinear clamping elements R/, R, 2 are connected in parallel with each of the Gairristor switches. In this embodiment, the non-linear clamping elements R/ and R,2 exhibit a very high resistance below a voltage level known as the clamping voltage or breakover voltage.
It is a conventional zinc oxide element or voltage surge arrester with voltage/current characteristics such that it exhibits a very low resistance (ideally close to zero) above the voltage levels mentioned above. The serially connected cyrisk switches EIW+-/ and 8W4/-1 connect line terminals 3g and 3? Further, the capacitor ap and the inductor L& are connected in series between the capacitor ap and the inductor L&. It will be appreciated that a combination of capacitors, thyristor switches and inductors can be connected in parallel between line terminals 3 and 39 in the MAR generator.

Ensuring free operation of thyristor switches 5WV-/ and swp-,z in overvoltage conditions, capacitor C
II or other capacitors that may be present in the VAR generator, and limit the residual voltage of the capacitor II or other capacitors that may be present in the VAR generator, and
z-/and swt! In order to reduce the required surge rating of -,z, or other thyristor switches included in a VAR generator, thyristor pairs such as those described above are divided into "half" switches by center taps "I'/-Tj.

First firing pulse generator 3q, second firing pulse generator 36
are Cyrisk switches BWII-/, 5Il, respectively.
- Separately control the upper and lower halves of j. The clamping voltage level of the non-linear clamping elements R/, R, 2 is the same as that of the thyristor φ switch s during normal operating conditions.
w<z −/ , 5wl1. − is chosen to be higher than the peak voltage appearing across it. Therefore, two "half"
The clamping voltage level of all the thyristor switches consisting of the switches 5VI-/ and 5WII- is determined by the sum of the clamping voltage levels of the non-linear clamping elements R/ and Ru connected in series. Ru. Therefore, during the period when capacitor a+ is discharging, one nonlinear clamping element R/
or by effectively shorting Rλ, the effective clamping voltage level is reduced to the clamping voltage level of R/or R,2. This will limit the residual charge on capacitor Cp to a low value without requiring impractically low clamping voltage levels for non-linear clamping elements with normal operating voltages of middle age. Since it is the clamping voltage level of a single non-linear clamping element that determines the residual charge on capacitor C+, one non-linear clamping element in series maintains a normal operating voltage.

In the embodiment of FIG. Pulse expander, thyristor by two
A control technique is used in which the switches EIWV-/ and 5WI-, are extended for two half cycles. Therefore,
The thyristor switches ell-/ and 5W4-1 alternately discharge the capacitor CI/, thereby ensuring that both non-linear clamping elements R/ and R,2 experience the same number of average current surges. do.

FIG. S diagrammatically represents the operation of the thyristor switched capacitor in cooperation with the non-linear clamping elements R/ and R,2, which are assumed to be identical.

If the AC network voltage V4z is fairly constant before the time to and the AC network requires capacitor compensation before the time tQ, then the capacitor a<Z is switched in and the AC network voltage current 141
is in a normal steady state as shown. At time tO, the alternating current network voltage V41 is suddenly increased, for example due to a transient disturbance or a load changeover. This increased v4i is generally necessary to reduce the supplied capacitive MAR by blocking the signal from the ignition input terminal U0 of FIG. 1 to the thyristor switch and isolating capacitor C1l. be. However, in this example, one and a half"
Only the firing signal to the switch is blocked. Thus, for example, the firing signal G5811-/ to the thyristor switch S'tl-/ will be blocked before the current zero crossing at time t, as shown in FIG.

The ignition signal GS2 is applied to the thyristor switch SW<Z-1 for another half cycle as shown in FIG.

This ignition signal G55wll-co is blocked just before time t3. The thyristor switch 'In-/ turns off at time 1/ at the current zero crossing since the ignition signal is blocked. This instantaneous capacitor voltage VO<t reaches the peak overvoltage value and the voltage across the entire thyristor switch SWI -/plus 5Wf-u is zero. As the AC network voltage begins to change polarity, the voltage across all thyristor switches begins to increase. Furthermore, the AC circuit v4 voltage v4 reaches the clamping voltage level of the non-linear clamping element R/ across the zero risk switch (in this example WP-/). In this embodiment, the clamping voltage level of the non-linear clamping element R/ can be chosen to be one times the normal peak value of v4. After reaching the clamping voltage level, the non-linear clamping element R/ breaks down into conduction and exhibits a low resistance. As V+ increases further, a discharge current 1R flows from the capacitor cq through the nonlinear clamping element R/, the thyrisk switch 5w1I-r and the inductor L&. The discharge of capacitor C Hiro is
Completed at the time tJ when vp reaches its peak value 0 Before this, the firing signal to the thyristor switch 5w4t-2 will have been blocked and the capacitor residual voltage Va will be reduced as shown in FIG. <Z and the sirisk switch voltage VSW settles to normal values.

FIG. 6 shows an operation similar to FIG. 5 when a severe overvoltage condition occurs in the AC network at time to. Thyristor
The switch voltage VSW is limited to the peak value of v4, and the residual capacitor voltage VOy is reduced to zero. For a limited number of times, the AC network voltage is at normal value 7
If the residual capacitor voltage VCi,t, which varies between 1 and 1 times v4, is accordingly limited to the normal maximum value of V+. The peak value of the thyristor switch voltage, which can vary between one and two times the normal value of V+, is reduced to less than one time the average value of ■4/.

Variations of the Invention It will be appreciated that many variations of this invention are possible without departing from its spirit and scope. For example, a single non-linear clamping element can be utilized to discharge the associated capacitor. Therefore, a single and therefore identical 1-half switch is kept conductive during the second half cycle of termination of the firing request, so that the maximum clamping voltage stress is only applied to the same 6-half" switch, so that the thyristor switch Only one half needs to be rated for the clamping voltage level used.In addition, three or more non-linear clamping elements can be used across the sections of the Cyrisk switch, and the selected conduction The delay allows variable level control of the residual capacitor voltage. Additionally, a small resistor can be connected between the center tap T/ and TΩ to limit the peak current flow. Moreover, different logic circuits can also be used to control the firing of the thyristor.

Effects of the Invention In short, according to this invention, overvoltage caused by the risk can be prevented.

[Brief explanation of the drawing]

No. 11-21 is a circuit diagram showing part of the VAR generation in Juto, the first! A-, 2D diagram shows the capacitor connection and uJ l') 1ilI j,
Figure 3 is a circuit diagram of a representative circuit consisting of a capacitor and a thyrisk switch; FIG. 9 is a partial block diagram of a preferred embodiment of the invention; FIG. S is a voltage and current waveform diagram associated with the circuit of FIG. 9 during operation; FIG. , Figure 2 is a waveform diagram taken from the circuit of the temperature Q-diagram during a transient state and/or overvoltage state of the AC network. C4 is a capacitor as a capacitive reactance element, 5
WV-/ and S'tl-,2 are thyristor switches as switching means, and R/ and R,2 are non-linear clamping elements. Patent applicant's agent Teru Soga Michi:'FIG,
1

Claims (1)

Claims: 1. A MAR generator of the type for supplying reactive power to an electrical system, comprising: a capacitive reactance element interconnectable with the electrical system for supplying the reactive power to it for a predetermined period of time; at least j controllable switching means interconnected in series with the capacitive reactance element for connecting said capacitive reactance element to said electrical system and actance circuit relationship during said period; and at least j non-linear clamping means. each non-linear clamping element is connected in parallel with each of said switch means, and the voltage across said switch means is higher than a maximum predetermined tolerance while said switch means is in an off state. A VAR generator which energizes the discharge current of said capacitive reactance element only when said time period and limits the voltage across said capacitive reactance element and said switch means to a predetermined safe level for a period subsequent to said period. The VAR generator according to claim 1, wherein the nonlinear clamping element is a zinc oxide element. 3. The VAR generator according to claim 1, wherein the nonlinear clamping element is a voltage surge arrester. 8. The VAR generator according to claim 7, wherein the switch means is a thyristor switch. S. A VAR according to any one of claims 1 to 4, in which each switch means consists of at least one pair of thyristors connected in antiparallel, each thyristor being energized with a current in the opposite direction to the other thyristor. generator.