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The thyristor is a kind of power semiconductor device. It can also be called silicon controlled rectifier. The thyristor has one more control gate than the rectifier diode, and it is unidirectional controllable. Because of its high efficiency, good control characteristics, long life, and small size, it has been widely used. The application technology of thyristor is mainly for power conversion and control, which can be roughly divided into the following aspects:

1 Controllable rectification

The rectifier composed of thyristors can not only rectify AC to DC like a rectifier diode rectifier but also can conveniently control the DC output voltage under the condition of unchanged AC voltage, which can be controlled rectification.

2 AC voltage regulation and power regulation

The switching characteristics of the thyristor replace the old-fashioned contact voltage regulator, induction voltage regulator, and saturation point reactor voltage regulation. The thyristor is used to realize the conversion from AC to variable AC, which is mostly used for light control, temperature control, and voltage regulation and speed regulation of AC motors.

3 chopped flow regulation

Chopper voltage regulation is the conversion from DC to variable DC and is widely used in vehicle speed regulation transmission of tributary power sources, such as urban trams and electric locomotives.

4 Non-contact power static switch

Thyristors are used as power switching components instead of contactors and relays for frequent operation and high-frequency occasions.

The wide application of thyristors has brought us a lot of convenience in production and life. The following focuses on the overcurrent protection of thyristors. When the components of the thyristor device are misconducted or broken down, the circulating current inverter fails in the reversible transmission system, and the transmission device production machinery is overloaded and the mechanical failure causes the motor to lock up, etc., all will cause the current flowing through the rectifier component to greatly exceed its normal operating current. The current overload of the thyristor is much worse than that of ordinary electrical equipment, and overcurrent is inevitable, so more attention should be paid to the overcurrent protection of the thyristor. The task of over-current protection is to quickly eliminate the over-current phenomenon before the component is burnt out once the circuit has over-current. There are four main types of overcurrent protection for thyristors:

⑴ Sensitive overcurrent relay protection

The relay can be installed in an AC or DC brake. When an over-current fault occurs, it will act and trip the traffic power switch. Since the over-current relay power switch takes about 0.2S to act, it must cooperate with measures to limit the excessive short-circuit current value, otherwise, it will not be too late to protect the thyristor.

⑵ Current limit and pulse phase shift protection

The AC current transformer forms an AC current detection circuit through the rectifier bridge to obtain a voltage signal that can reflect the magnitude of the AC current to control the trigger circuit of the thyristor. When the rectifier output terminal is overloaded and the DC current increases, the AC current also increases. The output of the detection circuit exceeds a certain voltage, which causes the Zener tube to break down, and the trigger pulse of the control thyristor is increased to reduce the output voltage. The overload DC current is reduced to achieve the current limiting purpose, and the load current limiting value can be adjusted by adjusting the potentiometer. When there is a severe overcurrent or short circuit, the fault current rises rapidly. At this time, the current limit control may not be able to take effect, and the current has exceeded the allowable value. In order to eliminate the fault current as soon as possible when fully-controlled rectifying a large inductive load, the trigger pulse of the thyristor can be controlled to rapidly increase beyond the phase shift range of the rectification state, and a negative voltage appears at the output terminal instantaneously, and the circuit enters the inverter state to reduce the fault current It quickly decays to zero.

⑶ DC fast switch protection

In occasions with large capacity, high requirements, and frequent short circuits, the DC fast switch installed on the DC side can be used for overload and short circuit protection on the DC side. This kind of fast switch is specially designed, its switching time is only 0.2ms, and the total arc-off time is only 25ms~30ms.

⑷ Fast fuse protection

The fuse is the simplest and most effective protection component. Aiming at the poor over-current capability of thyristors and silicon rectifier components, a fast-acting fuse is specially manufactured called fast-acting. It has fast-acting characteristics and can achieve 5 times the rated current when flowing. When the fusing time is less than 0.02s, in the normal short-circuit current, it can ensure that the short-circuit current is quickly fused before the transistor is damaged, which is suitable for short-circuit protection occasions.

In short, overcurrent protection is based on the allowable overcurrent capability of the thyristor, trying to limit the peak value of the short-circuit current with sensitive protection measures, so that the duration of the short-circuit current is as short as possible, so as to protect the thyristor.

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