SE460818B - Semiconductor coupling for HV use - Google Patents

Abstract

The converter (SR) includes a three phase bridge with sixa valve branches (V1-V6). The bridge has AC outlets (R,S,T) for connection to a three-phase AC power mains via inductors (LR,LS,LT) and DC outlets (P,N) which are connected to a capacitor (C1). Each of the bridge valve branches comprises several series-coupled thyristors which can be switched off. These thyristors are anti-parallel coupled with diodes (D11,D12...Dn). Each thyristor has a control (T1F) for switching it on, and one (T1E) for switching it off. A control (SD) emitting pulses for the switch-on and off processes has sixa (FP) outlets transmitting switch-on impulses for the valve branches. It also has six outlets (EP) for transmitting switch-off impulses to the thyristors in the bridge valve branches. A damping inductor (L1) is incorporated between the thyristor bridge and one DC output (P) of the converter. A clamping capacitor (C2) is connected between the two DC voltage rails: of the converter. (Provisional basic advised week 89/48)

Description

___., .2 êdåtkaníkgagas with a single thyristor per valve branch. Try to connect in series extinguishable thyristors in the manner well known in conventional thyristors have shown that very high voltage stresses occur during extinguishing of such a valve branch. Extinguishable thyristors are unavoidable variations in the extinguishing time, i.e. the time that elapses from that a quench pulse has begun to be applied to the thyristor until the thyristor takes up voltage. A typical extinguished thyristor has a storage time ("storage time") of about 25 ps with a spread of about 10 ps. This means that under the extinguishing of a valve branch, which contains a plurality of extinguishable thyristors, large irregularities in the voltage distribution between the thyristors will occur. which in turn means that one of the thyristors (or some thyristors) will be exposed to very high voltage stresses during the extinguishing process. It has turned out practically black or impossible to sufficiently reduce the irregularities in the voltage distribution by means of voltage dividers connected in parallel with the thyristors. capacitances, RC circuits or other voltage divider elements. Tension the sub-components would in fact have unreasonably large dimensions and / or unreasonably high power losses.

It would in principle be conceivable to design the thyristor's controls so that extinguishing pulses are delivered to the various thyristors in a valve branch at some point different times, selected in such a way that the spread in extinguishing time compensated§ However, a thyristor's turn-off time is not constant without varies during operation depending on, among other things, the operating temperature of the thyristor. For To avoid this problem, it would be conceivable to arrange measurement and signal processing means for measuring each individual thyristor extinguishing time. either at each start of the semiconductor connection or also continuously during clutch operation. Try to measure and compensate for the variations in the thyristors' off-peak times extremely high demands on measurement and signal processing. Such a equipment would further be exposed due to the rapid processes very serious disturbance problems. It would therefore - if at all is practically feasible - become very complicated and expensive.

For these reasons, the use of extinguishable resistors has hitherto been used \ i has been limited to such low operating voltages that can be managed by a single one thyristors in each valve branch in the semiconductor connection. 0 acc O oo Rßnöaoaatsa FOR THE INVENTION _ The invention intends to provide a semiconductor connection initially specified stroke. which can be used at arbitrarily high operating voltages.

This is achieved by the invention enabling a series connection of one any number of extinguishable thyristors in a valve branch. This is made possible in in turn by according to the invention an even voltage distribution between an arbitrary number of series-connected extinguishable thyristors are obtained on one extremely simple and reliable way.

What characterizes a semiconductor connection according to the invention is clear from attached claims.

DESCRIPTION OF FIGURES The invention will be described in more detail in the following in connection with attached figures 1-5.

Figure 1 shows the invention applied to a voltage rigid converter for phase compensation in an AC network.

Figure 2 shows the design of a valve branch at the inverter according to Figure 1.

Figure 3 shows the extinguishing pulses of some of the thyristors in the one in Figure 2 showed the valve branch.

Figure Q shows an alternative way of determining the temperature of it voltage-dependent resistor connected in parallel with a thyristor.

Figure 5 shows three alternative ways of influencing a thyristor's extinguishing time through control of the time derivative of the extinguishing pulse.

DESCRIPTION OF EMBODIMENTS Figure 1 shows a self-commutated voltage-rigid type converter for control phase compensation of a three-phase power grid. The converter SR consists of one three-phase bridge with six valve branches V1-V6. The bridge has AC sockets R, S, T for connection to a three-phase AC power supply via inductor LR, LS, LT and the DC sockets P, N, which are connected to one capacitor Cl. Each of the bridge's six valve branches consists of one v n. u nu 460 818. “- several series-connected extinguished thyristors. Thus, the valve branch V1 consists of the series-connected thyristors T1, T2 ... Tn. The thyristors are anti- connected in parallel with the diodes D11¿ D12 ... Dn. The thyristor T1 is equipped with a TIF control for turning on the thyristor and a control T1E for switching off of the thyristor and the same is the case with the others in the drive input thyristors. The drive has an SD controller which pàloch is known for methods emit pulses for ignition of the thyristors and for switching them off.

The controller has six FP-designated outputs, one for each of the six valve branches of the inverter. Via these outputs are emitted from the control device ignition pulses for ignition of the valve branches. A certain output. eg FP1, is connected to and leaves ignition pulses to all thyristors in a particular valve branch (V1). The control unit SD also has six slack outputs marked with EP. Via these outputs, quench pulses are provided for quenching the thyristors in the bridge's six valve branches. Each of the exits. eg EPI, is connected to and leaves quench signals to the thyristors in a certain valve branch (V1).

Between the thyristor bridge itself and the inverter one DC voltage output P is a damping inductor L1 connected. To limit the valve voltage at quenching is a so-called clamping capacitor C2 connected between the inverter both DC rails. The capacitor is connected to the socket of the inductor the bridge facing side via a number of series-connected diodes D1, D2 ... Dn and to the side of the inductor L1 towards the direct voltage output P facing side via a resistor R1, through which the capacitor C2 can be discharged.

The inverter can mainly exchange reactive power with the mains. The reactive the magnitude and sign of the current are determined by the amplitude difference between the network and the terminal voltage of the inverter (R, S. T) and the size of the inductor between mains and inverters (Fig. 1). The control of this reactive power flow can be on per se known manner is undertaken by controlling the voltage across the DC voltage rails (P, N) and / or by pulse width control (PWM).

Figure 2 shows in more detail the design of the valve branch V1 at the position shown in fig 1 showed the inverter SR. The figure shows three of the valve branches' thyristors, namely T1. T2 and Th. Parallel to the thyristor T1 is a diode D11 connected to enable a current flow through the valve branch in a direction opposite to the direction of the thyristor. In parallel with the thyristor T1 and diode D11 is a damping circuit arranged in a manner known per se. Steam the circuit consists of a capacitor C11 connected in series with a parallel connection consisting of a diode D21, with the same direction as the thyristor T1, and a u o ø Q now 5- 460 818 resistance Rll. Furthermore, a nonlinear is connected in parallel with the thyristor resistor R21. This is preferably a voltage-dependent_to-A stand, which exhibits high resistance at voltages below one predetermined value and low differential resistance at voltages exceeded rising said value. According to a preferred embodiment of the invention this resistor is a zinc oxide varistor. Such a varistor has a lot high resistance under a predetermined knee tension, a very low differential tiell resistance over the knee tension and exhibits a sharp knee. ie en abrupt transition between these two states. The varistor is preferably so dimensioned that its knee tension by a suitable margin exceeds it maximum thyristor voltage that occurs during normal operation and during setting a completely uniform distribution between the valve branch thyristors of the voltage occurring across the valve branch.

The varistor R21 is equipped with a temperature sensor TT1 for sensing varistor temperature. It can consist, for example, of a thermistor or of one thermocouple. It is preferably arranged so that it is as close as possible to senses the temperature of the active material of the varistor. The sensor TT1 emits an à output signal 61 which is a measure of the temperature of the varistor.

The extinguishing signal EP1 from the converter's controller SD is applied to the thyristor T1 via a delay circuit TF1 and a drive SPDI. Circuit TF1 delay time ii 11 is controllable and is varied depending on the delay circuit supplied If the measurement signal 01 from the sensor TT1. The delay 11 may, for example, be ' proportional to the varistor temperature 01. ie fl = k-01.

The thus delayed extinguishing signal EPI is applied to the driver SPD1 and initiates there an extinguishing current pulse EP11 for extinguishing the thyristor T1. On this way, an increase in the varistor temperature will cause one delaying the extinguishing current pulses to the thyristor T1.

Correspondingly, each of the valve branch's other thyristors equipped with an aptiparallell diode, a damping circuit. a varistor with tempera- sensor, a delay circuit and a drive.

The other valve branches of the inverter are designed in the same way as the one above described valve branch V1. 4460818 ~ 6 "f At a valve branch with a plurality of series-extinguishable extinguishable thyristors and with simultaneous quenching pulse to all the thyristors comes the thyristor that has the shortest extinguishing time to pick up voltage first. The current through the valve the branch will then flow through this damping circuit of the thyristor. and steam circuits sens capacitor is charged. which in turn causes the voltage to exceed the thyristor tends to exceed the value that would apply to an equal shaped voltage distribution between the thyristors. The excitement over this one first extinguished thyristors will then exceed the knee tension of the one with thyristor connected in parallel varistor. whereby a strong current flows through Q varistorn. Due to the high power losses in the varistor during repeated extinguishing, the temperature of the active material of the varistor will rise.

The measuring signal from the temperature sensor of the varistor will then increase in corresponding degree and to gradually increase the delay of the extinguishing pulse applied to the thyristor. This delays the thyristor 'Q' extinguishing, which results in a lowering of the behavior occurring during the extinguishing process the thyristor voltage. The system now described is a feedback v "_s- circuit which, as soon as the thyristor voltage tends to exceed the knee span-V Q none of the varistors connected in series with the thyristor. reduces voltage-A the stress on the thyristor via a postponement of the thyristor lacquering.

The gain of this feedback control circuit can easily be made so high that the varistors are effectively protected against overtemperature.

Figure 3 shows an example of the slack current pulses of the three shown in Figure 2 the thyristors. The slack signal EPI is assumed to occur at time t0. Tyris- tower T1 receives its extinguishing current pulse EP11 at time t1. thyristor T2 sin extinguishing current pulse EP12 at time t2 and the thyristor Tn its extinguishing current pulse Å EP1n at time tn. The delay of each of the extinguishing current pulses relative to the extinguishing signal fEP1 is proportional to the temperature of the tive thyristor varistor. The figure shows the assumed case of the thyristor T2 has the shortest extinguishing time. thyristor Tl the longest extinguishing time and thyristor Tn an extinguishing time which is between the aforementioned extinguishing times. The above described closed control systems for the various thyristors will therefore automatically to give the longest delay to the extinguishing current pulse EP12 to thyristor T2, a slight delay of the slack current pulse EP1n to the thyristor tower Tn and the shortest delay for the extinguishing current pulse EP11 to »F thyristor T1. ~ v .gsï fi. .fli- In the now described converter according to the invention, it already comes with each thyristor connected the varistor in parallel to effectively protect the thyristor 46089 . O against overvoltage in all conditions. Without other measures than however, the parallel connection of the thyristors with varistors would the varistors belonging to the thyristor or thyristors having the shortest the extinguishing time be exposed to a very large power load. Without a lot strong and, for practical and economic reasons, unreasonably large ring of the varistors would therefore one or a few varistors quickly destroyed. By according to the invention the temperature step of each varistor ring is sensed and counteracted by the delay of However, the quenching pulses will be affected by the scattering of the thyristors extinguishing times to be effectively counteracted. and any oversizing of the varistors are not required. Through this return of the temperature, which is the sizing parameter of a varistor, the varistors will to be used optimally. The varistors come from the system described above to be automatically controlled against equal working temperatures. Deviation the operating temperatures between the varistors can be reduced to the desired degree I by adjusting the gain of the above-mentioned closed control circuits. , ie by adjusting the constant k in the expressions ri = k-Bi.

The feedback systems place legal demands on linearity and accuracy of the temperature sensors used which can therefore be performed easily and cheap. In the simplest case, a thermostat can be used as a temperature sensor.

The thermostat is then arranged to switch on when the associated varistor temperature exceeds a predetermined value and thereby increase the delay of the extinguishing current pulse to the thyristor by a predetermined amount.

The described system works completely independently and does not require that some measurements of the thyristors' switching times are made before or at commissioning of the equipment. For the same reason, the equipment compensates automatically for such variations in thyristor turn-off times that can occur during operation of the equipment, t ek due to variations in operating temperature of the equipment. The equipment can therefore be started immediately and work at full power directly at the turn-on.

It has been described above how in a preferred embodiment of the invention they with the thyristors connected in parallel-non-linear or voltage-dependent the resistors are zinc oxide varistors. However, it is conceivable that natively use other types of voltage limiting resistors. for example silicon carbide varistors or suitable semiconductor devices. So can others ~ 460818, _ ß. types of temperature sensors other than those mentioned above are used. Those shown in Fig. 2 the drives and delay circuits (eg SPD1 and TF1) can of course be included as an integral part of the equipment controller (SD in Fig. 1). delay the circuits (eg TF1) can be performed on a large number known per se way, for example using a counter that counts a predetermined one number of pulses from a pulse oscillator whose pulse frequency is controlled by the associated varistor working temperature. A combination of temperature sensors and delay circuit can be obtained by using a temperature dependent inductance (ferrite). This is then arranged in thermal contact with the varistor sa that its temperature will vary with the varistor temperature. Inductance then connected to such a circuit for delaying the extinguishing signal that the delay becomes dependent on the magnitude of the inductance and thus on the tor temperature. In cases where a temperature sensor for the varistor is installed _ on the outside of the varistor housing or on its cooler, the current can, if desired through the varistor is measured and used to compensate for the temperature drop between the active material of the varistor and the temperature sensor.

It has been described above how the temperature of the varistor is determined by use _of a temperature sensing sensor. A determination of the working temperature of the varistor temperature can be done in other ways. Figure Ä shows an example of this. The figure shows how the current through a variator can be measured with the help of a low-resistance measuring resistor R3 connected in series with the varistor.

The voltage ui across the measuring resistor is proportional to the varistor current and ~ integrated in an RC circuit RU-C3. Since. when current flows through a variable its voltage is approximately constant, the varistor current becomes 'proportional to the effect developed in the varistor. The integral over time of the power developed constitutes a mat on the energy supplied to the varistor gin and thus on the temperature rise of the varistor. The voltage across the condenser tower C3 will therefore constitute an approximate measure of the temperature of the varistor. peratur G1. The device shown in Figure 4 is only schematic and can will of course be replaced by a more or less refined thermal model of tower, by means of which the varistor temperature can be determined with the desired degree of accuracy based on a continuous measurement of the varistor current.

Optionally, a combination of a thermal model according to Fig. U and a temperature sensors are used.

It has been described above how the thyristors' switch-off times are affected by control of the times at which extinguishing current pulses are applied to thyristor 9,460,818 An alternative option is to allow the temperature of the tor parallel connected varistor affect the growth rate / time derivative of the extinguishing current supplied to the extinguishing control of the thyristor. This derivative namely affects the turn-off time of the thyristor. A higher derivative (steeper flank of the extinguishing current pulse) namely gives a shorter extinguishing time than a lower vata. In this alternative embodiment, all thyristor in a valve branch extinguish current pulses simultaneously. but the derivative of each thyristor extinguishing current pulse is controlled individually depending on the operating temperature. the turn of the varistor connected in parallel with the thyristor. Figure 5 shows schematically some alternative ways of achieving this effect.

Figure 5a shows how a thyristor T1 is applied to an extinguishing current pulse by means of a coupling means SW, a voltage source U1 and an inductor L2. Extinguishing the signal EPI from the equipment controller initiates the extinguishing current pulse through closure of the coupling member SW. The growth rate of the extinguishing current (its derivative) is determined by the magnitude of the voltage of the voltage source U1 and by the inductance of the inductor L2. A control of the growth current of the extinguishing current speed can be achieved by the operating temperature of the varistor at the appropriate is brought to control the magnitude of the voltage of the voltage source.

This can be easily accomplished. for example, by receiving the voltage source U1 consists of an amplifier, the input of which is supplied to a temperature corresponding signal 01. The device is then designed so that a increase in temperature gives a decrease in voltage, ie a reduction in the growth rate of the extinguishing current and a postponement of the extinguishing time.

Figure 5b shows an alternative way of controlling the time derivative of the extinguishing current depending on the operating temperature of the varistor. The voltage source - U2 - has constant voltage and varistor temperature may affect the inductance at the inductor L3. This can, for example, consist of one in thermal contact with varistor arranged temperature-dependent inductance (ferrite).

Figure 5c shows a further alternative embodiment for controlling the growth rate of the extinguishing current. Here, as in Fig. 5b, a clamping source voltage U2 with fixed voltage and an inductor - LS - with variable input ductans. Here too, a suitably arranged or controlled temperature dependent inductance (f) is used.

Claims (5)

iso sas g m PATENTKRAV 1. Semiconductor connection. comprising a plurality of series-extinguishable extinguishable thyristors (T1, T2, Th), each of which is connected to a control device (SD) for extinguishing the thyristor, characterized in that each thyristor (e.g. T1) is provided with a non-linear resistor (B21) for limiting the voltage across the thyristor and with means (TT1) for determining the temperature of the resistor (1), said temperature determining means being connected to the control device (SD) of the thyristor and arranged to actuate the control device so that the switch-off time of the thyristor is delayed at a increase in the temperature of the resistor. 2. A semiconductor connection as claimed in Claim 1, characterized in that it comprises delay means (TF1) arranged to delay the extinguishing signal (EP11) to the thyristor, the magnitude of the delay being dependent on the temperature of the resistor (01). A semiconductor connection according to claim 1, which comprises means (e.g. U1, L2, SW) arranged to emit a current pulse for switching off the thyristor, characterized in that it comprises means (U1, L3, L5) arranged to influence the time derivative of the extinguishing current pulse. depending on the temperature of the resistor (01). Semiconductor connection according to one of the preceding claims, characterized in that the resistor (R21) is provided with a temperature sensor (TT21) for supplying the temperature of the resistor (beer). Semiconductor connection according to one of the preceding claims, characterized in that it is provided with current measuring means for sensing the current through the voltage-dependent resistor (R21) and with means (R4, C3) for forming a temperature of the resistor. corresponding quantity (01) in dependence on the sensed current.