Characteristics and application of thyristor electronic switch module in low voltage reactive power compensation

Reactive power compensation is a basic requirement for the operation of power systems. In order to balance reactive power during the operation of power systems, the reactive power required by various power loads must be compensated. There are several methods of reactive power compensation, such as phase shifter compensation and capacitor group compensation. The most effective and easy-to-implement method is to perform local reactive power compensation near the load point. Since reactive power compensation is connected to the power grid mainly through automatic switching on and off of power capacitors to achieve compensation effects, the performance of the switch element that controls the switching of capacitors plays a very critical role in the quality and stability of the entire device. At present, domestic reactive power compensation product controllers generally use AC contactors or thyristors as switch elements to control the on and off of capacitors.

1 Switching mode and characteristics of capacitor compensation device

The switching switches of the capacitive compensation device mainly include ordinary AC contactors, special contactors with preset resistances , and thyristor electronic switches.

1.1 Ordinary AC contactor

AC contactors are low-priced and highly versatile, but they will generate large surges and pulse overvoltages when used to switch capacitors, which may sometimes cause insulation breakdown or burn out of the contactor contacts, easily causing damage to the contactor and thus affecting the use of the compensation device.

1.2 Special contactor for capacitor switching

The capacitor switching contactor is a contactor with a current limiting impedance device installed on the main contact of the ordinary AC contactor. This improvement can play a certain role when the capacitor switching is not frequent, but its effect of suppressing the capacitor inrush current is not ideal. When the current is large, its current limiting resistor and main contact are often burned, especially when the reactive load fluctuates greatly and the capacitor is frequently switched, the actual service life is often only about one year. Therefore, this special contactor is only suitable for ideal working conditions that meet the basic stability and basic balance of three-phase voltage.

1.3 Thyristor electronic switch module

The thyristor electronic switch makes full use of the thyristor characteristics such as voltage zero-crossing triggering, current zero-crossing removal, switch contactless, and fast response speed, which can make the voltage on the capacitor rise quickly from zero to the rated working voltage. When disconnected, the current on the thyristor is zero-crossing removal. It can achieve fast dynamic compensation functions such as no inrush current when the capacitor is put into operation, no overvoltage when removed, and no arc when switched on. Therefore, it can better solve the transient impact problem generated when the capacitor is switched on. However, the thyristor has a large tube voltage drop (about 1 V) in the on state, so when working, the power consumption and the large amount of heat generated and dissipated should be considered, which will increase the cost of operation and maintenance.

1.4 Performance comparison between contactor and thyristor controlled compensation equipment

The performance comparison of the compensation equipment controlled by contactor and the compensation equipment controlled by thyristor is shown in Table 1.

2 Internal functions of thyristor electronic switch module

The thyristor electronic switch module mainly includes an anti-parallel thyristor, a zero-crossing detection trigger module, a resistor-capacitor absorption device for suppressing overvoltage, and a heat dissipation device.

2.1 Zero-crossing trigger module

Since there is usually residual voltage on the removed capacitor, and the voltage at both ends of the capacitor cannot change suddenly, when the difference between the system voltage and the residual voltage of the capacitor is large, the triggering thyristor will generate a large impact current, which may directly damage the thyristor. In order to achieve fast response of the dynamic reactive power compensation device and ensure that there is no impact current during switching, it is necessary to detect the capacitor voltage and the grid voltage. Only when the two are equal in size and polarity can the capacitor be instantaneously put into use. Therefore, it is necessary to install a zero-crossing trigger module. At present, the typical trigger circuit for obtaining zero-crossing signals from both ends of the thyristor is MOC3083. There is a zero-crossing trigger judgment circuit inside the MOC3083 chip. It is a special chip designed for 220V grid voltage. The bidirectional thyristor withstand voltage of the chip is 800 V. When the voltage at both ends of 4 and 6 is lower than 12 V, if the trigger current is input, the bidirectional thyristor inside it is turned on. Figure 1 shows the internal circuit diagram of the zero-crossing trigger module of two MOC3083 in series in the TSC circuit of the 380 V grid .

In fact, when non-zero-crossing triggered, its current impact can often reach more than 3 times the stable value, while when zero-crossing triggered, its current impact is generally only 1.5 times the stable value, and its capacitor voltage impact is only 1.15 times the stable value.

2.2 Overvoltage suppression resistor and capacitor absorption device

Module overvoltage protection can also generally adopt the RC absorption method. For overvoltages with a short duration and low energy, a RC absorption circuit can generally be connected in parallel at both ends of the module, and the electromagnetic energy of the overvoltage can be converted into electrostatic energy for storage by the absorption capacitor. The use of the absorption resistor can not only prevent circuit oscillation, but also limit the turn-on loss and di/dt value caused by the discharge of the capacitor when the thyristor is turned on. The RC value connected in parallel to the thyristor switch module can be referred to Table 2.

It should be noted that the capacitor in the RC absorption circuit should be an AC capacitor. When the input voltage is 380 V, a 630 V capacitor should be used. When the input voltage is 220 V, a 500 V capacitor can be used.

2.3 Heat dissipation device

During the operation of the thyristor switch module, the junction temperature of the thyristor chip will rise. In order to keep the junction temperature below the maximum rated value of 125℃, a heat sink must be used. Moreover, the quality of heat dissipation conditions also directly affects the safe, stable and reliable operation of the module. At present, the heat dissipation methods include water cooling, air cooling (forced and natural air cooling) and heat pipe cooling. Here is a brief introduction to the selection method of air-cooled heat sinks.

The main circuit of the thyristor switch module consists of two anti-parallel thyristors. Based on the average on-state current IT(AV), on-state peak voltage VRM, module contact heat RTHCH, and average power (PT(AV)=0.85VRMIT(AV) of a single thyristor in the module, the thermal resistance RTHCA of the heat sink can be calculated:

Where: TC is the shell temperature (i.e. the temperature of the heat sink), TA is the ambient temperature.

The maximum temperature TCMAX of the heat sink can be obtained by the following formula:

Where: TVJ is the maximum junction temperature of the thyristor (125°C), and RTHJC is the thermal resistance from the junction to the heat sink.

After finding the heat sink thermal resistance RTHCA, select the corresponding heat sink model specification (DXC-450 DXC-616 DXC-573). In fact, the heat sink thermal resistance should be smaller than the calculated thermal resistance, that is:

If several modules are mounted on a heat sink, ∑PT(AV)=nPT(AV) can be substituted into equation (3) to find ∑PT(AV) and then select the heat sink. The appropriate heat sink length can be selected based on the thermal resistance value of the optional heat sink (or the thermal resistance curve of the heat sink).

In the actual application of reactive power compensation, the thyristor electronic switch module can be used in TSC low-voltage reactive power compensation to replace the contactor. In this way, the compensation capacitor can be put into use when the switch voltage passes through zero, thereby avoiding the generation of impact current and instantaneous grid voltage fluctuations. Figure 2 shows its working waveform.

3 Conclusion

Through the analysis and comparison of the two switching switches of low-voltage reactive compensation devices, it can be seen that the thyristor electronic contactless switch not only has the advantages of small zero-crossing switching inrush current and no overvoltage, but also can solve the heat dissipation problem during operation. In actual work, its operating life is almost unlimited, it can be frequently switched, and the switching moment can be precisely controlled. Therefore, it can achieve smooth switching and removal without transition process, and its dynamic response time is only 0.01~0.02 s, so it is an ideal switch for capacitor switching.