Application of Altivar38 VFD (Variable-frequency Drive) in water treatment

Introduction

Principle of frequency conversion energy-saving technology: Generally, the design of electrical driving equipment considers short-term overload operation, and the power configuration of the motor is often greater than about 10% of the maximum power of the load, or even greater.

1. Raising the question

The water treatment center in the steel rolling area is part of the second phase of technical transformation of Xinyu Iron and Steel Co., Ltd. It is mainly responsible for processing and supplying the required turbid ring water and clean ring water to the medium and heavy plate mill and wire rod. The turbid water system of the medium and thick plate plant operates 24 hours of uninterrupted water supply. Since the operation, due to frequent roller changes and different production process requirements, the water consumption has been unstable, resulting in the water level of the cold water pool being high and low, causing unstable and unstable operation. It is safe and often overflows, causing waste of resources and environmental pollution. To this end, the water level in the pool must be adjusted by frequently operating the backflow valve and starting and stopping the pump. According to statistics, from June to August 2006, the return flow was adjusted at least 18 times a day on average, and the pump was started and stopped at least 3 times. Frequent power-frequency starting of the pump shortens the service life of the motor. The current impact during power-frequency starting, the pressure impact on the pipe network, and the water hammer effect during instantaneous stops result in high equipment failure rates, frequent maintenance, and increased maintenance costs. A large part of the electric energy consumed by the motor during power frequency operation is used to recirculate the system, doing useless work and increasing power consumption. The original system is an open-loop system, which only functions to start and stop the pump and cannot automatically adjust. It is difficult for operators to monitor and they need to monitor at all times. If they are not careful, the water level adjustment will be affected. In addition, the water level fluctuates greatly, the liquid level gauge is improperly selected, and the computer display cannot faithfully reflect the on-site conditions. The operator must often go to the site to check, which cannot provide adjustment data in a timely and accurate manner, causing the adjustment to lag and increase the workload.

2 Design and determination of system transformation plan

2.1 Selection

The current operation method is that pumps No. 1 to 4 in the oil removal pump room supply water to the medium-thick plate turbid ring cold water pool. According to production needs, it was decided to add a frequency conversion device to implement frequency conversion transformation, adopting the "one-to-two" form, that is, one set of frequency conversion devices It can interchangeably drive two water pump motors for the No. 3 and No. 4 pump motors in the oil removal pump room. In this way, one unit is running with variable frequency and the other is in standby. Unit maintenance will not affect the work of the frequency conversion system. The selected motor models are all Y2-3l5M-4 (380 V 132 kW 240 A), and the frequency conversion device is Schneider Telemecanique's Altivar38 energy-saving VFD (Variable-frequency Drive) with a power of 132 kW. It complies with standards such as IEC, UL and CSA, has a wide speed range, good acceleration and deceleration braking performance, adaptive adjustment and energy saving, as well as protection functions such as power supply overvoltage, motor phase loss, motor overheating, VFD (Variable-frequency Drive) overheating, etc. .

There are many ways to set the variable frequency speed regulator (VVVF for short). Here, the VFD (Variable-frequency Drive) panel PI setting and the current feedback detected by the liquid level meter are used to form a closed-loop automatic PID adjustment by setting the internal parameters. Control System.

2.2 Control loop

The original control loops of the two water pump motors are retained as the power frequency bypass of the new control system, which is determined by the power frequency/variable frequency selection switch on the frequency conversion cabinet. There are electrical interlocks between the frequency conversions of the two water pump motors and between the frequency conversions of each other to avoid misoperation.

2.3 Main circuit

Since the original main circuit of the power frequency uses Changsha Otto’s model QB4200 AC motor soft starter, in order to ensure that the smooth start of the power frequency is not affected after the transformation, the output terminal of the VFD (Variable-frequency Drive) motor main circuit must be connected from Soft starter output.

2.4 Operation method

The operation of this system can realize three-place control of the frequency conversion panel/main control room/on-site, and all three can control the start and stop of the water pump motor. The frequency conversion panel control can also realize speed adjustment, parameter setting and LED display of system operation status, which is mainly convenient for debugging. The main control room control is PLC remote control. The operation mode under normal circumstances is convenient for the operator to control. The on-site control is equipped with an operation box next to the motor, which is mainly convenient for staff to observe the restart of the water pump and motor after maintenance, as well as the shutdown in case of accidents.

2.5 Monitoring system

The original power frequency has been controlled by PLC, using Siemens' S7-300 PLC and the STEP7 special software of ladder diagram visual programming language to process the signals from the electric control cabinet and on-site operation box.