Application of high-voltage VFD (Variable-frequency Drive) in energy-saving transformation of water pumps in power plants

Abstract: This article introduces the principles and technical characteristics of the new generation of high-voltage VFD (Variable-frequency Drive), as well as the technical scheme, application and energy saving of the mortar pump frequency conversion transformation in Guodian Shuangyashan Power Plant.

1 Introduction

The high-voltage VFD (Variable-frequency Drive) using new high-voltage and high-power power electronic devices and direct "high-high" mode has the characteristics of small size, high efficiency, simple structure, and reliable operation. The VFD (Variable-frequency Drive) device uses uncontrollable 24-pulse phase-shifting rectification and fully controlled devices for switching modulation. It has a high input-side power factor, excellent speed regulation performance and torque control performance. High-voltage VFD (Variable-frequency Drive) can achieve good power-saving effects by changing the operating frequency of the motor and performing high-efficiency speed regulation within a wide speed range. It has been widely verified in the energy-saving transformation of fans and water pumps.

Units 3 and 4 of Guodian Shuangyashan Power Plant are 210MW thermal power units, and units 3 and 4 are equipped with six 6kV/570kW mortar pump motors. The motor model is JS512-8, with a rated current of 69A and a rated speed of 730r/min. Among them, the 6# mortar pump is a secondary pump and is used in conjunction with the 5# mortar pump. Before installing the VFD (Variable-frequency Drive), the 6# mortar pump started and stopped based on the height of the liquid level in the front pool. There are two problems in this way: on the one hand, in order to adapt to the requirements of the production process, it is necessary to continuously switch, start and stop the motor according to the needs of the front tank liquid level and the ash flushing pipe every day. The liquid level in the front tank cannot be well controlled, and frequent work is required. Frequency starting of the motor has a great impact on the motor; on the other hand, there is throttling loss, resulting in a waste of electrical energy. In order to further optimize the operating conditions of the mortar pump and save electric energy, the 6# mortar pump motor was modified with high-voltage frequency conversion.

After the high-voltage VFD (Variable-frequency Drive) modification of the 6# mortar pump motor, the height of the front pool liquid level is adjusted by adjusting the operating frequency (motor speed) of the 6# mortar pump VFD (Variable-frequency Drive), so that the 5# mortar pump The pump can always run at the best efficiency at power frequency, thereby reducing the number of opening and closing operations of the 6# mortar pump switch, reducing the impact caused by the power frequency startup of the motor, further optimizing the production process, and saving electricity.

2. Mortar pump operation process and frequency conversion transformation technical plan

2.1 Operation status of 6# mortar pump and frequency conversion transformation technical plan

(1) At the mortar pump operation site, single-phase discharge to ground or single-phase direct grounding often occurs in the high-voltage cable between the VFD (Variable-frequency Drive) and the motor. In this case, it is necessary to ensure that the VFD (Variable-frequency Drive) cannot be damaged, and the VFD (Variable-frequency Drive) must be able to send out an alarm shutdown signal so that on-site personnel can handle it in time. Therefore, the VFD (Variable-frequency Drive) output is required to be able to withstand single-phase grounding. The neutral point of the output filter capacitor of the corresponding VFD (Variable-frequency Drive) cannot be directly grounded, but needs to be grounded through a capacitor.

(2) Since the 6# mortar pump is a secondary pump, before starting the 6# mortar pump VFD (Variable-frequency Drive) operation, the 5# primary mortar pump is usually already running and will drive the 6# mortar pump motor to run. , VFD (Variable-frequency Drive) is equivalent to flying start. Therefore, when starting the VFD (Variable-frequency Drive), it is necessary to detect the motor operating frequency in real time and drive the motor to start based on the operating frequency.

(3) The frequency conversion operation of the 6# mortar pump requires closed-loop control of the liquid level in the forepool and automatic adjustment of the motor speed.

(4) Since the mortar pump is running and the liquid level in the forepool is very low, it may cause overload or even stalling. Therefore, the VFD (Variable-frequency Drive) is required to have overload capability and overcurrent protection measures.

Based on the above factors, the three-level neutral point clamping topology based on IGCT was selected from the two main high-voltage VFD (Variable-frequency Drive) topologies in China and abroad. The three-level topology has the following advantages: small number of switching power devices, large IGCT switching current, strong overcurrent capability, simple structure, high reliability, and suitable for applications with large load impact.

In terms of control, the liquid level in the front pool of the mortar pump is equipped with a pressure water level sensor, which converts the measured water level height signal into a 4~20mA standard signal, which is sent to the VFD (Variable-frequency Drive) through the current loop interface; VFD (Variable-frequency Drive) Drive) calculates the deviation between the current water level and the control water level, changes the output frequency of the VFD (Variable-frequency Drive) through the built-in digital PID regulator of the VFD (Variable-frequency Drive), adjusts the speed of the motor, and then controls the front of the mortar pump. The height of the pool liquid level.

2.2 Principle structure diagram of three-level neutral point clamp circuit

The high-voltage VFD (Variable-frequency Drive) based on IGCT's three-level neutral point clamp has a simple structure, and its main body consists of a rectifier, an inverter and a filter. As shown in Figure 1, the rectifier uses 24-pulse uncontrolled rectification and consists of a 24-pulse phase-shifted rectifier transformer with a 15° phase shift and a quadruple three-phase rectifier bridge, which can meet the current harmonic requirements at the input end. The DC link consists of common mode reactance, IGCT protection and charging current limiting resistor, and DC capacitors (C1, C2).