DD257533B5 - Method for operating a sub-synchronous converter cascade - Google Patents

Description

For this 1 page drawings

Field of application of the invention

The invention relates to the operation of a sub-synchronous power converter cascade in case of power failure or other failure cases that can lead to a tilting of the inverter.

Characteristic of the known state of the art

For many applications in which the actuator is required only in a limited speed range, the sub-synchronous power converter cascade is still an uncomplicated and cheap alternative. As is known, sub-synchronous power converter cascades consist of a three-phase slip ring rotor motor, the slip power in a rectifier in DC is converted. The slip power is fed back to the grid via a network-deleted inverter. By changing the ignition angle of the inverter, the speed of the asynchronous machine can be set. In case of faults in the network, z. As a short-term power failure or ground fault, there is a risk of tilting the inverter, which can cause a breakdown that would lead to melting of the fuses or even the destruction of thyristors.

Measures are already known for rapid interruption or limitation of the overcurrent that occurs. Thus, according to DE-OS 2018025 (H02 P7 / 62) proposed to insert in the current path of the DC link bridged with a resistor contact of a high-speed switch. In case of emergency, the contact opens and limits the current through the ohmic resistance. After returning to a stable operating state, the switch is closed again. The solution requires a mechanical quick-switch that is subject to wear, requires some maintenance, and also incurs high costs.

With DE-AS 2518235 (H 02 P7 / 62) has been proposed that the ignition angle of the inverter is reduced to a value corresponding to the inverter tread limit and the DC reactor is short-circuited by a parallel thyristor. The solution requires very specific voltage relationships between DC and inverter. However, since the counter-voltage reserve is dependent on the speed of the asynchronous machine, a safe operation of the drive is not guaranteed at all times.

Furthermore, it has already been proposed in DE-OS 2633046 already to execute the rectifier as a fully controlled thyristor rectifier. If there is a risk of tilting the inverter, the DC voltage can be reduced. However, the solution requires a very high effort not only by the arrangement of a second fully controlled bridge rectifier circuit, but also by the need for a special control electronics for the rotor rectifier, since it operates at a variable frequency.

Object of the invention

The aim of the invention is a measure that allows the safe operation of a sub-synchronous power converter cascade in abnormal operations in the network at a reduced cost compared to the model solutions.

-2- 257 533 Features of the invention

The object of the invention is to provide for the operation of a sub-synchronous power converter cascade a simple, with contactless devices to be implemented method for interrupting an occurring in case of failure overcurrent that provides a blocking signal for a power interruption in the inverter to indicate.

According to the invention the object is achieved by the overcurrent occurring in the event of an accident is interrupted by acting on the inverter per se known extinguishing device, at the same time the firing pulses are blocked for all thyristors of the inverter, and released the locked firing pulses for the inverter thyristors after a brief accident immediately become. The quenching device may consist of a quenching capacitor, which is connected via decoupling diodes to the inverter bridge circuit. The quenching capacitor is charged by a charging device. The tripping occurs by detecting the overcurrent in the DC link or on the AC side of the inverter or by detecting the voltage drop of the mains voltage. The voltage on the quenching capacitor may be limited by a nonlinear resistor (e.g., metal oxide varistor). After elimination of a short-term fault, the inverter can be controlled again immediately. If the fault takes longer, so that the engine speed has dropped above a certain limit, the motor can be started up via a resistance start before the inverter is activated again.

The solution has the advantage that a fast and safe interruption of the overcurrent with contactless components is possible without a complicated control electronics is needed. Another and important advantage is that with the monitoring of main network, auxiliary network and the control state (existing technology) is an automatic reclosing of the normal operation of the sub-synchronous power converter cascade to realize a volatile fault cause.

embodiment

The invention will be explained below with reference to an exemplary embodiment. In the drawings shows

Fig. 1: a schematic diagram of the invention

2 shows the current profile in the intermediate circuit and the course of the voltage at the quenching capacitor.

The subsynchronous converter cascade is realized by an asynchronous motor M, the rotor rectifier LGR, the DC reactor L _d and the inverter WR, which returns the slip power via the transformer T to the network. The invention has the following function:

When tilting of the inverter WR by an external cause, such as a lowering of the mains voltage U _n , there is a failure of one of the thyristors of the inverter WR, eg the thyristor V 3. The current l _d is not taken over by the thyristor V3 and flows thereon via the previously fired thyristor V1 and the circuit - DC choke L _d - rotor rectifier LGR - thyristor V6 or V2 transformer T- on, where it rises. By detecting the over-current l _u in the intermediate circuit or on the AC side of the inverter WR of the emergency condition is detected by the information part IT2 and a firing signal ZS for the thyristor LT triggered. Thereupon, the quenching capacitor LKo charged by the charging device LE discharges via the quenching thyristor LT, the thyristor V1 and the coupling diode VH, whereby the thyristor V1 is deenergized. Since all ignition pulses of the inverter thyristors are simultaneously blocked by the information part IT 1, further firing of the thyristors V1, V3, V 5 are prevented. The current l _d continues to flow through the phase R - coupling diode V11 - quenching capacitor LKo - quenching thyristor LT - DC reactor L _d rotor rectifier LGR and one of the inverter thyristors V 6 or V 2. At the same time, the quenching capacitor LKo reload. At the zero crossing of the voltage of the quenching capacitor LKo the keep-free time T _{F of} the thyristor V1 is completed. If the voltage of the quenching capacitor LKo reaches the response voltage of the varistor R 2 after the Z ₁ , the current is transferred to it. After time t ₂ , the current I _{d has} decayed and thus the commutation error has been eliminated.

A disturbance which is due to an impermissible voltage change of the mains voltage U _n can also be detected by detecting the mains voltage reduction AU _n by the information part IT2 before the inverter WR is tilted and eliminated by extinguishing the thyristors V1, V3, V5 via the quenching thyristor LT. If the information part IT2 simultaneously evaluates the control state of the inverter WR from evaluation of the control voltage Us _{1 of} the information part IT2, the deletion is only initiated if the inverter operating point (α) no longer permits safe inverter operation during the voltage reduction AUn. Unnecessary tripping of the quenching thyristor LT are thus avoided. In the information part IT1 takes place by processing the control variables actual current l _i5 "target speed n _so n and actual speed n, _s , the generation of the drive pulses for the inverter WR. Depending on the duration of the fault and the technological conditions, various operations can be initiated:

a) shutdown of the drive by opening the contacts Q 1, Q4 and closing the contact Q3,

b) reclosing at a sufficiently high engine speed by restarting the inverter WR with an ignition angle of α = 150 °,

c) Restart by means of the starting resistor R1 by opening the contact Q4, closing the contact Q3 and setting the starting resistor R1 to a range below the vertical limit of the sub-synchronous power converter cascade, starting with the starting resistor R1, closing contact Q4 and taking over the regulated operation by Control of the inverter WR (sequence as in normal approach).

Claims (4)

1. A method for operating a sub-synchronous power converter cascade, at the slip-ring rotor of a rectifier and an inverter inverter is connected, wherein an occurring in case of overcurrent or an unacceptable mains voltage reduction provides a blocking signal for a power interruption in the inverter, characterized in that the power interruption is carried out by a per se known extinguishing device (LE) in the inverter (WS), simultaneously the firing pulses for all the thyristors (V 1 to V6) of the inverter (WS) are blocked, and the locked firing pulses for the inverter thyristors (V 1 to V6) be released immediately after a short-term accident. 2. The method according to claim 1, characterized in that the Sperrisgnal is obtained by detecting the overcurrent in the inverter intermediate circuit. 3. The method according to claim 1, characterized in that the blocking signal is obtained by detecting the overcurrent on the AC side of the inverter (WS). 4. The method according to claim 1, characterized in that the blocking signal is obtained by detecting the mains voltage reduction.