Discussion on the Control Scheme of VFD (Variable-frequency Drive) System in Refining and Chemical Plants

1 Overview??

In industrial enterprises, motors are one of the most widely used and most numerous electrical equipment. At present, a large number of AC motors operate at a fixed speed, which is increasingly unable to meet the automation requirements of the production process. At the same time, it operates under the condition of low power factor and low efficiency, which is a great waste of electric energy.

Due to the increasingly fierce market competition in the petrochemical industry, new requirements have been put forward for the model, quality and quantity of our petrochemical products. In order to meet these technological requirements, a VFD (Variable-frequency Drive) system is added to the original drive motor. In this way, the conveying volume of materials can be smoothly changed, which meets the requirements of the production process and achieves the effect of energy saving.

The VFD (Variable-frequency Drive) system used in our factory's Chang 1 and Chang 2 is not only a power source but also an actuator for changing process parameters. It replaces the original actuator-regulating valve, making the medium transmission process Control has changed.

2 Energy-saving principles of VFD (Variable-frequency Drive) speed regulation operation

In the actual production process, the load selection of various pumps is greater than the actual flow required for production. In actual operation, the required flow is often much smaller than the designed flow. If the motor used cannot adjust the speed, usually only By adjusting the valve to control the flow, the result will cause a lot of energy loss on the valve. If instead of valve adjustment, the motor is allowed to run with speed regulation, then, when the required flow decreases, the motor speed will decrease, and the energy consumed will be significantly reduced.

H(n1), H(n2) represent the Q=f(H) curve during speed regulation, and R1 and R2 represent the pipeline resistance curve during valve regulation. When the valve is controlled, due to the need to reduce the flow rate and close the valve, the frictional resistance of the valve becomes larger, and the power consumption P1 during Q2→Q1, A→B, HA→HB valve control is represented by 0HBBQ1. In speed control, Q2→Q1, A→C, HA→HC, power consumption P2 is represented by 0HCCQ1. If P1>P2, it means that the power consumption in speed control is less than the power consumption in valve throttling.

P=rQH pump shaft power

Q Flow rate H Head r Liquid weight

When running at points B and C, PB-PC=Q1(HB-HC)r is the saved electric energy.

For the pump load, there is the following expression:

Q1/Q2 = n1/n2

H1/H2 = (n1/n2)2

P1/p2 = (n1/n2)3

It can be seen from the above formula that when the speed drops by 1/2, the flow rate drops by 1/2, the pressure drops by 1/4, and the power drops by 1/8, that is, the relationship between power and speed drops to the third power. If the method of closing the valve is not used, but the speed of the motor is reduced, then as the output pressure of the pump decreases, the power consumed on the valve can be completely avoided under the same flow rate. When VFD (Variable-frequency Drive) is not installed, the outlet flow of the pump is controlled and regulated by the outlet valve. When the flow rate is small, it is adjusted by closing a small valve, which increases the pressure difference of the pump pipe, and causes part of the energy to be wasted on the outlet valve. After using VFD (Variable-frequency Drive), the speed of the pump can be reduced, the head of the pump will also be reduced accordingly, and the output power of the motor will also be reduced, thus eliminating the pipe pressure difference originally consumed on the pump outlet valve.

3 VFD (Variable-frequency Drive) system control scheme

The motor power of the first-line pump B109 and B114 of our factory is 75kW and 55kW respectively, the speed is 2982 rpm, the rated voltage is 380V, the rated current is 132A and 103A, and the rated outlet flow is 28.520M3/h and 20M3/h.

Under normal working load conditions, the motor works at a rated speed of 2982rpm, and the speed cannot be adjusted. In order to keep the flow rate stable, the method of controlling the outlet valve is used for control, that is, the flow signal detected by the differential pressure transmitter is sent to the PID regulator, and then the PID regulator outputs a 4-20mA control signal to control the opening of the outlet regulating valve, thereby Control the outlet flow and keep the flow stable. In the actual operation of the original system, there are the following problems:

(1) The throttling amount is relatively large, and the throttling amount of the pump outlet valve is close to half of the rated flow rate of the pump, which wastes a lot of electric energy.

(2) The control precision is low, and the outlet flow fluctuates greatly (about 3%).

(3) The motor works at the rated speed, and the power consumption remains unchanged.

(4) The noise of the motor is high, and the pressure of the pump and pipeline valves is high, which may easily cause leakage.

According to the above process requirements of the system, we followed the following principles when designing the VFD (Variable-frequency Drive) system:

a. Keep the outlet flow stable; b. The control accuracy of the outlet flow is 0.5%; c. The speed range of the motor should be 0-2982 rpm; d. According to the working characteristics of the pump, the system design should be carried out according to the principle of constant torque; e. Energy saving and consumption reduction; f. The system design adopts power frequency and frequency conversion double switching to ensure the continuity and reliability of production, which can be used as backup for each other; g. Two DCS output contacts are used to control the original regulating valve one way.