SU1350744A2 - Apparatus for overvoltage protection of thyristors in converters - Google Patents

Abstract

The invention relates to electrical engineering, in particular, to devices for protecting semiconductor converters against overvoltages, and can be used for creating power supplies on semiconductor valves. The aim of the invention is to increase the reliability of the device under the conditions of long-lasting overvoltages. During overvoltage, the current pulses are transmitted through the current transformer 5 to the functional converter 7, which simulates the thermal time of the protected thyristors. When the threshold of the threshold element 8 is exceeded, a signal is sent to the actuating element 9, which removes the control pulses. By introducing superior cells 6, the operation conditions of the device are independent of the duration of the overvoltage. 2 Il. with SS (L with with Sd O 4 4 N)

Description

The invention relates to electrical engineering, in particular, to devices for protecting semiconductor converters against overvoltages, can be used to create power sources on semiconductor fans, and is an improvement of the invention according to Aut. No. 1288812.

The purpose of the invention is to increase the reliability of the device under conditions of prolonged overvoltages.

Figure 1 shows the electrical circuit of the device; FIG. 2 shows timing diagrams explaining the principle of its work.

The transistor thyristor device of converter 1 contains voltage limiters 2-4 connected in parallel to converter valves, current sensor 5, rectifier 6, functional converter 7, threshold element 8 and protection actuating element 9. Current sensor 5 is made in the form of a current transformer with three primary windings (according to the number of valves and voltage suppressors), each of which is connected to the circuit of the corresponding voltage limiter, and the secondary winding through rectifier 6 is a functional transformation. Tel 7 and threshold element 8 are associated with an actuating security element. A switch 10 is turned on in the power circuit, which can also be connected to the protection actuator 9.

The device works as follows.

Under normal operating conditions and nominal voltages at the converter valves, the current through the voltage limiters is extremely small and the signal on the secondary winding of the current transformer 5 is close to zero (Fig. 2). The magnitude of the voltage of the limiters 2-4 voltage is selected by the maximum allowable voltage of the valves of the converter 1, and in the event of emergency conditions, accompanied by an excess voltage on the valves of the maximum allowable value, the limiters 2-4 voltage come into operation. At the same time, they limit the voltage at the valves at the maximum allowable level Ufl and through the primary windings of the current transformer 5 a current of a significant amount (tens to hundreds of amperes) flows in the form of unipolar

five

five

0

five

0

five

0

five

pulses (figure 2). An alternating current flows in the secondary winding of the transformer, reproducing in its form the sum of current pulses flowing through the primary windings. Thus, during overvoltages, even on only one valve, a signal is generated at the output of current sensor 5. This signal, with the help of a rectifier transformed into unipolar pulses, is fed to a functional transducer 7, made, for example, in the form of an integrating RC circuit. When the output signal reaches the functional converter and the threshold of the element 8 is triggered, the latter triggers the pulse signal Ug, which includes the protection actuator 9, which can either remove the control pulses from the converter 1 valves, either send a continuous signal to unlock them, or disconnect the converter from the power supply ( Fig. 1, a connection shown by a dashed line.) The prohibition of the supply of control pulses leads to a stop in the operation of the converter and, consequently, the elimination of emergency overvoltages, If they are caused by internal processes transducer (e.g., buildup of stresses in the autonomous inverter). The triggering of the unlocking signal leads to the forced opening of the valves and the removal of voltage from them; in this case, it is necessary to simultaneously limit the valve current or disconnect the converter from the power source. In the third variant, termination of the emergency mode is ensured by disconnecting the converter from the power source.

The functional converter plays the role of an analog thermal model of a voltage limiter, the input signal of which is the energy of the heat loss, which is distinguished in it, and the output is the temperature of the limiter.

For reliable operation of the device, it is essential to correctly select the structure of the model and its parameters. The heat loss energy W in the voltage limiter is amplified through the voltage U and the current i flowing through the limiter using formula i

W 5 Uo-i ,, pdt, (1)

About where t - time.

Since that is nnom.

can be considered as pic

t

o J icrp o

(2)

In the absence of heat transfer to the environment, the temperature of the limiter T is equal to

T T,

W

ha

(3)

where T is the ambient temperature

environments;

sieve specific heat and mass of the voltage limiter. Taking into account the heat transfer, neglecting the radiation losses and using the emissions (2) and (3), we write

 +

uJiorpdt-Y It-Tjdt

.

 ogre

, Il

m

-,(four)

Where

V is the thermal conductivity of the voltage limiter - the environment.

Differentiation expression (4), after some transformations we get

.l.cs (Tt,). (5)

T

Tm

Functional transducer 7 is described by a similar equation.

dUc

dt -

iR,

(6)

Uj. - voltage across the capacitor;

i is the input current proportional to the current of the voltage limiter;

R is the resistance of the resistor;

С - capacitor capacitance.

Obviously, equality must be respected.

RC S2

Yt

(7)

R-K

Tm

t.

(eight)

where K is the transformation ratio;

m is the scale factor. Thus, when choosing the parameters of the functional converter 7 in accordance with the formulas

with . (7) and (8) the output signal Uj, U., of the functional converter is proportional to the overheating temperature of the voltage limiters. The trigger voltage of the threshold element is selected in accordance with the maximum permissible superheat temperature of the voltage limiter, which allows

0 avoid irreversible thermal breakdown of the latter.

Figure 2 (timing diagram) shows that with the passage of current pulses 123, + through the limiters

15 2-4 voltages (or through one of them) the signal U-, Uc changes at the output of the functional converter 7, reflecting on the scale m an increase in the temperature of the structure of the voltage limiter. When reaching the signal

20

)

e

25

thirty

35

and the threshold of the threshold element 8 (U then), the latter gives a signal to activate the actuating element of protection. The purpose of the protection actuator 9 is to reduce the voltage on the converter valves for a safe level at which the voltage limiters do not pass the current and do not heat up. Figure 2 shows the timing diagrams of the signal of the protection actuator 9, made in the form of a gate control circuit. Starting from the moment t (Fig. 2), voltage limiters 2-4 come into operation, and at time t, the output signal is function

five

The national converter reaches the threshold, and as a result of the actuation of the actuating element 9, the supply of control pulses to the valves (Fig. 2, diagram Ug) stops and

the converter stops working.

Due to the discharge of the switching capacitor, the voltage across the valves decreases to a safe value. .

Rectifier 6 is essential for the normal operation of the device. By introducing rectifier 6 into the circuit, a pulse signal, whose amplitude is proportional to the current of the voltage limiters, arrives at the input of the functional converter 7 not only during the transient process of increasing the magnetization of the transformer, as in the known device , but also for arbitrarily long lasting action of overvoltages. To measure the total amplitude of the current can be used

A voltage doubled rectifier circuit, in the case of using conventional power supply circuits, the output signal has an error equal to the average value of the primary windings signal, reduced by the same transformation factor times. One-way conductor of the rectifier prevents the discharge of the capacitor of the functional converter 7 through the secondary winding of the transformer 5 "In the absence of the rectifier, the constant component voltage on the capacitor of the functional converter leads to an increase in the magnetizing current on the secondary winding of the transformer, which forces unreasonably increase the size of the transformer in order to avoid its saturation, and also leads to the discharge of the capacitor of the functional converter through the secondary whence. In addition, the use of two half-timed circuits of rectifier 6 simplifies the setup of the device, since

Editor S.Pekar

Reg2

Compiled by V. Shirokov

Tehred.Morgental Proofreader L. Patay

Order 5291/52 Circulation 618 Subscription

VNIIPI USSR State Committee

for inventions and discoveries 113035, Moscow, Zh-35, Raushsk nab., d, 4/5

Production and printing company, Uzhgorod, st. Project, 4

in this case, the signal is not required to be phased, the secondary winding of the transformer 5, i.e. its inclusion may be arbitrary.

The use of the invention improves the reliability of the device and reduces the installed power of the current transformer. Increased reliability is ensured by device operability at any duration of overvoltage,

Claims (1)

Invention Formula Device to protect the thyristors of the converter from overvoltages according to the author. No. 1288812, characterized in that, in order to increase reliability under the conditions of prolonged overvoltages, a rectifier is inserted connected between the secondary winding of the current transformer and the input of the functional converter.