Energy-saving transformation of water supply system of M701F combined cycle unit

introduction

A power plant introduced the M701F gas-steam combined cycle unit jointly produced by Mitsubishi and Dongqi, which uses a three-pressure, reheat, horizontal, non-supplementary, natural circulation waste heat boiler. The original boiler feed water system uses a fixed-speed feed water pump with a high- and medium-pressure combined pump. This method has a small feed water valve opening during the start-up and shutdown of the unit and low-load stages, resulting in large throttling losses. As a two-shift peak-shaving power plant, it is necessary to carry out energy-saving transformation of the feed water system. Commonly used energy-saving technologies include motor frequency conversion technology, hydraulic coupling technology, and permanent magnet speed regulation technology. Through comparative analysis, the power plant selected the motor frequency conversion solution, and the high- and medium-pressure feed water system was changed from a single high- and medium-pressure feed water pump to a separate pump operation. Two medium-pressure feed water pumps were added, of which the high-pressure pump motor was operated with variable frequency and the medium-pressure pump motor was operated at industrial frequency.

1. Introduction to the high and medium pressure water supply system before and after transformation

1.1 Abnormal changes in high and medium pressure water supply systems

The main function of the high and medium pressure water supply system is to replenish water for the high and medium pressure drums, and also to provide cooling water for the high pressure superheater, reheater and high pressure bypass desuperheater. Before the energy-saving transformation, the high pressure feed water and high super desuperheating water were taken from the outlet of the high and medium pressure feed water pumps, and the medium pressure feed water, reheater desuperheating water and high bypass desuperheating water were taken from the middle tap of the high and medium pressure feed water pumps. After the high pressure feed water pump frequency conversion transformation, considering that the high and medium pressure feed water demand cannot be met at the same time when running at low frequency, a new medium pressure water supply system is added, and the medium pressure feed water and reheater desuperheating water are supplied by the medium pressure feed water pump, and the high bypass desuperheating water is still taken from the middle tap of the high and medium pressure feed water pump. The system flow is shown in Figure 1.

1.2 Wiring characteristics after frequency conversion

The original design has one high and medium pressure water supply pump for operation and one for standby, and the two high and medium pressure water supply pump motors are powered by two 6kV switches respectively. The frequency conversion transformation adopts the "one to two plus bypass" method, that is, one frequency converter can drive two water supply pumps respectively, and a 6kV switch is set to supply power to the high-voltage water supply frequency converter separately. The output end of the frequency converter is divided into two paths and led to the outlet of the high-voltage water supply pump power switch respectively. The frequency conversion wiring is shown in Figure 2.

The manufacturer of the inverter is Guangzhou Zhiguang, and the model is Zinvert-A6H2500/06B. GSA and GSB are manual knife switches, and 0FA and 0FB are vacuum contactors. When the water supply pump is driven by the inverter, the corresponding 6kV power switch of the water supply pump is closed through the logic interlock. The standby water supply pump can still be started through its own power switch. The inverter cannot drive two water supply pumps to run at the same time. When the inverter fails, both water supply pumps can be started at the power frequency. The new frequency conversion circuit wiring method is added under the condition of retaining the original wiring. The wiring is simple and flexible, but a reliable logic interlock system needs to be configured. The main electrical interlock relationships are shown in Table 1.

In order to realize the "five-protection" locking of the electrical equipment of the frequency conversion system, electromagnetic locks are added to the original two water pump power switches (0F1 and 0F2 respectively) and the newly added 0F3 switch ground switch. These electromagnetic locks are DC electromagnetic locks, and their power supplies are the control power supply of the switches. The operating principle of the electromagnetic locks is to unlock with power on.

Since this renovation involves many interlocking and locking circuits, many locking circuits use switch transfer contacts. The locking will only work when the control power of the inverter, inverter bypass cabinets, 0F1, 0F2, 0F3, etc. is supplied. Therefore, the control power of the above circuits must be supplied when performing various operations.

2 Changes in related control logic after frequency conversion

2.1 Drum water level control logic

The high and medium pressure drum water level control adopts the three-impulse regulation consisting of drum water level, steam flow and feed water flow. Single impulse regulation and three-impulse regulation are adopted according to the steam flow [3]. The control core is three proportional integrators. Single impulse regulation adopts one proportional integrator alone, and three-impulse regulation adopts two proportional integrators. In single impulse regulation, the proportional integral input is the drum water level deviation. In three-impulse regulation, the difference between steam flow and feed water flow is superimposed as the proportional integrator input. The control output value is the output of the actuator. The control logic block diagram is shown in Figure 3.

The frequency control of the inverter also uses this control algorithm, that is, the frequency output also adopts the three-impulse regulation mode. However, the high-pressure drum water level control is more complicated during frequency conversion operation, and the high-pressure feed water regulating valve and the inverter are used for joint control.

During frequency conversion operation, there are two control modes: closed-loop control mode and open-loop control mode. Under closed-loop control, the minimum frequency of the frequency conversion is a function of the load, and the water level is mainly adjusted by the valve. In this mode, there is still a certain throttling loss, so it is generally not used: in open-loop control mode, the frequency converter is set to run at the minimum frequency during the low-load stage, and the water flow is adjusted by the water supply valve. At this time, the frequency converter frequency is in a locked state. As the load increases, the water demand increases, and the valve is continuously opened to 90%. At this time, the valve will be locked and the water flow is adjusted by the frequency converter frequency. If there are abnormal conditions such as the water level deviating from the normal water level, the standby pump is started, and the frequency conversion exits automatically during the adjustment process, the high-pressure water supply valve will automatically unlock and participate in the water level adjustment. Joint adjustment minimizes the throttling loss of the valve while ensuring safety, thereby achieving the purpose of energy saving.

The minimum open-loop operation frequency is set as a variable value, which is mainly determined by factors such as feedwater pump motor cooling, motor vibration, desuperheating water pressure, and the difference between feedwater pressure and drum pressure. It is generally between 30 and 38 Hz. When the difference between the feedwater main pipe pressure and the drum pressure is less than 0.28 MPa, the minimum pressure increases at a certain rate until the pressure difference meets the requirements.

The medium-pressure system has not undergone frequency conversion transformation, and the original water level control logic remains basically unchanged. The feedwater regulating valve selects three-impulse or single-impulse medium-pressure drum water level regulation according to the operating conditions.

2.2 Water supply pump interlock function

After the frequency conversion transformation, the original interlocking logic of the water supply pump remains unchanged, that is, when the water supply pump is running at the rated frequency, the interlocking function between the original high-pressure water supply pumps remains unchanged, and the original high-pressure water supply main pipe low pressure interlocking function to start the standby pump only works when it is running at the rated frequency.

During frequency conversion operation, the original logic of interlocking the start of the standby pump due to low outlet pressure of the feedwater pump is no longer applicable, and this part of the logic is blocked. Two new interlocking conditions for starting the standby pump are added: (1) The condition for the closed-loop mode to interlock the start of the power frequency standby pump is that the feedwater main pipe pressure exceeds the drum pressure by less than 0.2MPa. This pump interlocking method is prone to cause the standby pump to be interlocked when the working conditions change, and is generally not used now: (2) The condition for the open-loop mode to interlock the start of the power frequency standby pump is that the drum water level is lower than -450mm. Under normal water level fluctuations in the high-pressure drum, the standby pump will not be interlocked.

3. Abnormal conditions after energy-saving transformation

3.1 Inverter failure causes water pump tripping

Inverter faults are divided into minor faults and major faults. Minor faults will trigger an alarm, while major faults will directly trip the inverter. Although the inverter can continue to operate when a minor fault occurs, it must be handled in time to prevent it from developing into a major fault. Over the years of frequency conversion transformation, the plant has experienced many water pump tripping caused by inverter failures, but fortunately, the accident did not expand. There are many reasons for the inverter tripping, but most of them are power module failures. After replacing the faulty module, it returns to normal.