Application of a motor restart method in petrochemical enterprises

The existing restart cabinet with PLC as the core has high cost and complicated wiring. The single restart relay only depends on the bus voltage where the motor is located and the position of the auxiliary contact of the contactor to judge the power loss, and it needs an auxiliary power supply, so the restart failure probability is high. In this paper, Shi Yong, an engineer, introduces a restart method of motor (group) in motor protection device when power is lost. It can reliably distinguish the situation of blackout and normal shutdown, has rigorous logic, flexible setting of fixed value, and supports motor restart with various control modes, without increasing wiring and cost. This method has been applied in petrochemical enterprises, and it can act correctly in the process of many times of electricity shaking.

Restart refers to the process in which the motor switch trips and loads are dumped due to the instantaneous voltage drop in the process of power interruption. After the voltage returns to normal, the motor starts automatically in batches according to the preset sequence time.

Petrochemical industry is a continuous production enterprise, and many motors in its low-voltage auxiliary power system are not allowed to trip and stop in the process flow. Once these key motors trip and stop, they will cause unplanned shutdown and bring great economic losses to production. However, in actual operation, there are many uncertain factors (such as lightning strikes, equipment failures, etc.), which are easy to influence the power grid, so that the power supply voltage of the internal distribution network of the enterprise is reduced or interrupted for a short time, and then the power supply is resumed (usually called power interruption), which causes the low-voltage motor to trip and stop, and the motor in the internal power grid of the enterprise is also easy to be affected by this, which leads to the production stoppage, thus causing great losses.

At present, the common restart method is to use special self-starting cabinet or single restart relay. The hardware cost of self-starting cabinet is high and the wiring is complicated. The cost of the single restart relay is slightly reduced, but the wiring needs to be increased, and the probability of restart failure is also high, and false start will occur after normal parking.

PCS－9692 and PCS－9626 microcomputer programmable motor protection devices provide a reliable and economical motor (group) restart method in combination with the motor operation characteristics of petrochemical enterprises.

1 power-off restart logic

The restart logic described in this paper draws lessons from the principle of automatic reclosing in high-voltage line protection, and the restart logic is rigorous. The setting value of all-digital restart process is flexible, and it supports motor restart with various control modes (such as direct start, positive and negative start, high and low speed motor start, etc.). The on-and-off of the device can be configured, and the logic is realized in software, which makes the field wiring very simple. 24-point full-wave Fourier algorithm is used for analog sampling, which has high accuracy. The device adopts high-performance 32-bit CPU, and each sampling interval is 0.833 ms for real-time calculation. The reaction speed is much faster than the traditional restart device and PLC.

1.1 restart logic in case of power loss

When the motor (group) is in a stationary state, it will impact the power grid, lower the bus voltage, cause the under-voltage protection to trip or burn out the motor due to too low voltage. After the normal motor loses power, it will continue to rotate for some time due to inertia. The starting current of the rotating motor is small, and the voltage will not be lowered too much, so the motor can be started immediately after the power supply is restored. The stopped motors need to be restarted in batches according to the load of auxiliary power. If auxiliary power allows 10 motors to be started at the same time, 10 motors will be restarted in batches each time.