1、 What is flash electricity?

In the process of power system operation, the phenomenon of voltage instantaneous drop and return to normal caused by lightning strike, short circuit to ground, fault reclosing, standby automatic switching, abnormal power grid, large equipment start-up, etc., which makes the voltage fluctuate rapidly or power failure and return to normal, is called "power swing".

2、 What's the cause of the electricity shaking?

The power grid is composed of substations at all levels, especially in 10kV circuit, users are widely distributed, which is prone to short circuit or over-current fault. After a fault occurs in a circuit, the relay protection device of the circuit acts and removes the fault circuit. The time from the fault occurrence to the fault removal is about 500 ~ 1200 Ms. during this period, the voltage of other normal circuits will cause temporary power swing. In addition, summer is the season of frequent lightning, because lightning will also make the grid voltage cause temporary electricity shaking phenomenon.

3、 What's the impact of shaking electricity?

AC contactor tripping caused by power swing

AC contactor is widely used in low-voltage motor control system, which accounts for a large proportion. Due to the working principle and characteristics of electromagnetic AC contactor, when the power grid is shaken, the working coil of electromagnetic AC contactor will be short-time power-off or the voltage is too low, which leads to the dynamic and static core suction which is maintained by current is less than that of the spring This is an important reason for the contactor to trip. According to the international IEC standard, the action characteristics of electromagnetic AC contactor should be reliably closed between 85% and 110% of the rated control power supply voltage. The range of release action voltage is between 20% and 75% of rated control power supply voltage. However, in the field use, electromagnetic AC contactor is usually released at 50% of its rated control power supply voltage, which causes the electromagnetic AC contactor to trip out of control. Therefore, the working principle of electromagnetic AC contactor determines that it can not avoid the phenomenon of "electricity shaking" on the power grid.

The influence of power swing on the control circuit of power supply circuit

With the development of power grid and the continuous expansion of capacity and scale, the frequency of power swing phenomenon is more and more. As the scale of production equipment in modern chemical and mining enterprises is larger and larger, the power swing time is relatively short, but the impact on production is very huge. The instantaneous voltage fluctuation will cause hundreds of motors to trip and equipment to stop. After the power grid voltage is restored, the motors can not resume operation by themselves, resulting in the disorder of continuous production process, and may cause production and equipment accidents. For large-scale plants, if they are restored manually, it will take a long time. For unattended field plants, the restoration time will be longer. For continuous production plants such as petroleum, chemical and pesticide plants, a series of losses such as safety, environmental protection, waste products, waste of raw materials, reduction of output and low efficiency will be very huge.

4、 The solution of shaking electricity

First of all, we need to make it clear that all the current anti shake technologies are remedial measures in case of power shake, that is, to use the best way to avoid the loss or harm caused by power shake. If we want to fundamentally control the power swing and avoid the occurrence of power swing, it is impossible at present.

1. Because the short-circuit capacity of the power supply system is too large, if the compensation is made from the primary system, it can be realized in theory: each 10kV circuit should be equipped with current limiting compensation equipment, however, the investment of a 10KV circuit is millions, so the investment is quite large. For this, the investment of a substation is tens of millions, and at the same time, it also takes up several times of the area of the substation Therefore, the use value of one-time system compensation is not high for general enterprises, and it is not easy to realize.

2. Take measures on the secondary circuit of the electrical system

At present, measures are generally taken in the secondary circuit of the electrical system to prevent electric shaking. That is to say, when the power swing occurs, the voltage of the secondary circuit is basically unchanged, or the control appliance of the primary circuit is disconnected with a delay, so as to ensure its normal closing after the power swing, so as to avoid the influence of the instantaneous low voltage accident of the power supply system. At present, there are mainly the following ways in the market:

① Anti power swing restart scheme: when power swing occurs in the circuit system, the voltage decreases to release the contactor; if the voltage recovers within the anti power swing time set by the restart device, the relay contact of the restart device is closed to make the contactor pull in again, so as to ensure that the power supply circuit continues to work.

Disadvantages: if it is a heavy-duty motor, the electromagnetic energy stored in the motor is quickly consumed by the load when the power is shaking. When the voltage is restored, the motor speed has dropped to 0. When the starting contactor of the restart device is closed, it is full voltage starting. The starting current can reach 5-8 times of the rated current of the motor, and the starting current is too large. If it is a light load motor, the motor speed can not be zero when the voltage is restored when the power is shaking. When the starting contactor of the restart device is closed, the existence of motor residual voltage is easy to lead to reverse phase closing, causing greater closing impact.

② AC contactor anti shake power maintenance scheme: when the shaking occurs, the contactor will not release, so as to ensure that the power supply circuit continues to work. For the motor, this process is only a short under voltage operation process, there is no large starting current or closing impact.

Therefore, the power swing holding type is preferred for the two schemes,

The anti sway electric holding scheme is divided into external delay breaking module and permanent magnet anti sway electric contactor.

The use of the delay disconnect module is mainly used as the power supply to protect the contactor, that is, the power supply of the control circuit is connected to the delay module, and the delay module supplies power to the contactor. Therefore, the requirements for modules are high. First of all, the capacity of the module should be large enough to meet the requirements of the power supply contactor capacity, otherwise, it will make the module generate heat phenomenon and affect the service life; secondly, the module should have the exit function in case of failure, that is, when the module itself fails, the module itself should automatically exit the operation, but it can not affect the normal opening and closing of the contactor.