"Shocking power" refers to the short-term loss of power grid voltage, short-term large fluctuations in grid voltage, short-term power failure for a few seconds, etc. Power quality events. Chemical companies have high requirements for the reliability of system power supply. Once the power supply system shakes, it will cause undervoltage protection of the protection equipment and unexpected shutdown of the production equipment. The safety of personnel poses a threat.
1 Commonly used anti-shake measures and applications
(1) UPS anti-shake system
The control system, such as DCS, PLC, etc., is connected by UPS power supply to achieve the purpose of anti-shake. The working principle block diagram of the online UPS is shown in Figure 1. When the grid voltage is working normally, it supplies power to the load and charges the energy storage battery at the same time. When the mains power is undervoltage or suddenly loses power, the UPS power supply starts to work, and the energy storage battery supplies power to the load.
When an electric shock occurs in the system, the coil of the contactor relies on the UPS to supply power to work normally, keeping the main contact closed, and avoiding the motor shutdown caused by the electric shock. When the busbar loses power for more than a certain period of time, the output will be disconnected according to the time set by the secondary control part to avoid accidents after the voltage is restored.
(2) DC-BANK anti-shake system
There are the following methods to deal with the inverter's anti-shake:
Method 1: Cancel the inverter's low-voltage protection setting and set a quick restart. The disadvantage is that the stop and restart of the key motor will affect the continuity of production and cause the increase of defective products. In addition, low-voltage often manifests as the inverter's overcurrent protection. Canceling the overcurrent protection will increase the hidden danger of damage to the inverter itself. This method is rarely used in petrochemical companies with high continuous production requirements.
Method 2: DC-BANK system, DC-BANK system is mainly used in variable frequency motors and PLC/DCS power supply systems. When the power grid is normal, the inverter is powered by the AC bus, and the DC-BANK system is in hot standby state. When the power grid is switched off or the standby automatic switch is switched, the grid voltage drops, and the DC-BANK is converted to the DC bus of the inverter. The inverter maintains normal operation. Its working mode is shown in Figure 3, the control logic diagram of a single unit.
(3) Anti-shake measures of the motor
AC contactors are widely used in low-voltage motor control systems. Commonly used motor control circuits are shown in Figure 5. After the electric shock occurs, the contactor is disconnected, which will cause the motor to stop.
The anti-shake of the motor is mainly the anti-shake of the contactor, and the anti-shake method of the AC contactor:
Method 1: Use anti-shake electric contactors. Contactors with delayed release/avoid the bounce zone are called anti-shake electric contactors. When the electric shock appears, the contactor does not release immediately, nor does it work in the critical bounce zone. The control circuit installation wiring is shown in Figure 6.
Method 2: Add a delay module to the original AC contactor. The specific control circuit is shown in Figure 7.
Method 3: Install the restart controller. The start control circuit of the self-start controller with the restart module is shown in Figure 8.
Method 4: Use a motor protector with anti-shake function. The motor protector has protection functions such as overload, open phase, unbalance, locked rotor, blockage, starting overtime, overvoltage, undervoltage, grounding, leakage, etc.; with current Measurement, voltage measurement, frequency measurement, power measurement and other measurement functions; with control functions such as start control, anti-shake function, and restart after loss of voltage; with DC 4-20mA transmission output, MODBUS, PROFIBUS communication functions, which can improve production automation To reduce equipment investment, the anti-shake control circuit of Ankerui ARD series motor protector is shown in Figure 9.
The working principle is as follows: "95, 96" contacts are used as protection contacts. The protector is normally closed after power-on, and becomes normally open when there is a fault or the auxiliary power supply is cut off. The "7, 8" contact is connected in parallel with the start button SB2. After the power is shaken, the "7, 8" pulls in to maintain the control circuit in the starting state, and the restart function can be executed after the voltage is instant. Fig. 9 is a wiring example of the anti-shake function of the protector in the protection mode, and Fig. 10 is a schematic diagram of the anti-shake control with a direct start method.
The working reason is as follows: "95, 96" contact is the same as the function description in Figure 9. The "7, 8" contact is the start control relay. The protector receives the "SB1" and sends out a jog start signal, "7, 8" is sucked and self-maintained, and the contactor KM coil is energized to connect to the main circuit of the motor. The protector receives the "SB2" and sends out the jog stop signal, the "7, 8" output is disconnected, the contactor KM coil loses power, and the main circuit of the motor is disconnected. After the electric shock occurs, the main circuit of the motor when the coil KM loses power is cut off, and the protector automatically selects to perform "immediate restart", "batch delayed self-start" or "start prohibited" according to the length of the electric shock time.
The motor protector industry standard (JB/T 10736) describes the requirements for the function of anti-shake (restart after loss of voltage): "A protector with undervoltage (loss of voltage) restart protection function, due to main circuit undervoltage failure or loss of voltage Stop, if the voltage returns to normal (above the allowable restart setting value) within the "immediate restart time", the protector can immediately restore the motor to the operating state before the motor stopped (without starting delay, voltage drop, etc.) Process); if it exceeds the “immediate restart loss-of-voltage time” and within the “delayed restart delay time” setting time, the voltage returns to the under-voltage (loss-of-voltage) restart setting value, then the motor presses the “delay Restart delay time" delay start (the same as the normal start process), the allowable error of the delay time is ±10%; if the voltage recovers after the "delay restart delay time", the motor will no longer automatically restart Start. The error of the restored voltage value is not more than ±10%."
2 Motor protector anti-shake working principle and related parameter settings
Take Ankerui ARD series motor protection as an example, combined with Figure 10 to introduce the working principle of the anti-shake function of the motor protector. In order to realize the anti-shake function, the motor protector needs to have an anti-shake module. Connect the AC input to the input end of the anti-shake module, and connect the output of the anti-shake module to the auxiliary power input of the motor protector. Motor protection The voltage measurement signal of the sensor is taken from the upper level of the contactor to prevent the contactor from disengaging when the power is shaken, and the recovery voltage cannot be measured. When the line is normally powered, the internal energy storage device of the anti-shake module is in an energy storage state, and during the period of the electric shock, the energy storage device of the anti-shake module supplies power to the motor protector to maintain the normal operation of the motor protector. When the system voltage is restored to the "restart voltage", the motor protector judges the duration of the electric shock, and the time is less than the "immediate restart power failure time" and immediately turns on the "output relay 7, 8" to start the motor; the electric shock time is longer than "Immediately restart power-loss time", but less than "allowable power-loss time", implement delayed restart; power-shake time longer than "allow power-loss time", do not perform start.
The use of a motor protector can realize immediate restart after power-shake, delayed restart after loss of voltage, and locked start after a long loss of voltage, and has comprehensive protection functions such as overload, phase loss, locked rotor, blockage, overvoltage, and underpower, etc. It can ensure the smooth operation of the motor and reduce the system investment. It is of practical significance to use in the chemical industry. The settings of related parameters are shown in Table 1.
