Application of high-voltage VFD (Variable-frequency Drive) on condensate pumps in power plants

This paper takes the application of China's domestic multi-level high-voltage VFD (Variable-frequency Drive) in the condensate pump of the Luanhe Power Plant in China as an example. The principle and precautions are explained, and through the power consumption comparison test, the energy-saving effect of the frequency conversion adjustment of the condensate pump and the traditional baffle adjustment is compared. Recently, it has been shown that the power plant adopts the domestic high-pressure VFD (Variable-frequency Drive) application method for speed regulation and energy-saving transformation of condensate pumps and other equipment, and has the characteristics of low investment and quick results.

China State Power Luanhe Power Plant is located in Chengde City, Hebei Province, and has two 100MW Chinese-made condensing steam turbine generator sets. They were put into operation in 1993 and 1997 respectively. In March 2005, China State Power Luanhe Power Plant carried out frequency conversion transformation on a large number of equipment. Sunwoele HFV-A06/130 high-pressure VFD (Variable-frequency Drive) is used for the condensate pump renovation project of two 100MW units. At present, the VFD (Variable-frequency Drive) of the condensate pump operates stably, and the energy saving effect is obvious.

1 Operating conditions of the condensate pump

After the steam that has done work in the steam turbine is cooled and condensed in the condenser, it is concentrated in the hot water well. At this time, the function of the condensate pump is to send the condensate to the deaerator in time. Maintaining continuous and stable operation of condensate pumps is an important aspect of maintaining safe and economical production of power plants.

Monitoring and adjusting the water level in the condenser is a major task in the operation of the condensate pump. Under normal operating conditions, the water level in the condenser should not be too high or too low. When the unit load increases, the amount of condensed water increases, and the water level in the condenser rises accordingly. When the unit load decreases, the water level in the condenser decreases accordingly.

The motor of the condensation pump is a 6KV/1000KW motor, which is designed with a certain margin. Each unit is equipped with two condensation pumps, one for operation and one for standby.

Before VFD (Variable-frequency Drive) was used, the water level adjustment in the condenser was carried out by changing the opening of the outlet valve of the condensate pump. The linearity of the adjustment was poor, and a large amount of energy was lost on the valve. At the same time, due to the frequent operation of the valve, the reliability of the valve is reduced, which affects the stable operation of the unit.

After using the high-pressure VFD (Variable-frequency Drive), the outlet valve of the condensate pump is fully opened, and the motor speed is changed by adjusting the output frequency of the VFD (Variable-frequency Drive), so as to adjust the outlet flow to meet the requirements of the operating conditions.

2 Principle and characteristics of HFV-A06/130 high voltage VFD (Variable-frequency Drive)

HFV-A series high-voltage VFD (Variable-frequency Drive) adopts the unit series multi-level PWM topology (referred to as CSML). Direct high-voltage output is achieved by connecting several low-voltage PWM frequency conversion power units in series. The high-voltage main circuit and the controller are connected by optical fiber, which is safe and reliable; accurate fault alarm protection; with power electronic protection and industrial electrical protection functions, to ensure VFD ( Variable-frequency Drive) and the safety and reliability of the motor in normal operation and failure.

The power units are connected in series instead of the power devices. The highest voltage that the devices can withstand is the voltage of the DC bus in the unit. The devices do not need to be connected in series, and there is no voltage equalization problem caused by the series connection of devices. Directly use the low-voltage IG BT power module, the device works in a low-voltage state, and is not prone to failure; 6kvVFD (Variable-frequency Drive) uses a total of 42 pairs of 1200V low-voltage IG BT, the low-voltage IGBT gate drive power is low, the drive circuit is very simple, the switch The frequency is very low, so there is no need to adopt a voltage equalizing circuit and a surge absorbing circuit, and the system efficiency is high. At the same time, the power unit adopts a capacitor filter structure, and the overall technology is mature and reliable. VFD (Variable-frequency Drive) can withstand 30% power supply voltage drop and continue to run. When the 6K V main power supply of VFD (Variable-frequency Drive) is completely lost, VFD (Variable-frequency Drive) can not stop within 3 seconds , can fully meet the needs of VFD (Variable-frequency Drive) power bus switching without stopping. In addition, when the 6KV main power supply is undervoltage, it can automatically derate without stopping, and return to the original speed after the voltage is normal. Adopt diode uncontrollable rectification circuit structure, VFD (Variable-frequency Drive) has a strong ability to withstand surge voltage, and the surge voltage caused by lightning strike or switching operation can generate surge through the transformer (the impedance of the transformer is generally about 8%) The current, through the rectifier diode of the power unit, charges the filter capacitor, which is sufficient to absorb the surge energy entering the unit. In addition, a varistor surge absorber is installed on the primary side of the transformer for further protection.

The power units are multi-pole modules connected in series. When a module fails, it will automatically bypass operation, which is convenient for taking corresponding measures on site; that is, a bypass circuit is connected in parallel between the output terminals of each power unit. When the power unit fails, the corresponding power unit is blocked. The trigger signal of the IGBT, and then the bypass SCR is turned on to ensure that the motor current can pass through and still form a path, which greatly improves the reliability of the system operation.

The motor can realize soft start, soft brake, and automatic speed control; the starting current is less than the rated current of the motor; the starting time of the motor can be continuously adjusted, which reduces the impact on the power grid. VFD (Variable-frequency Drive) is pre-installed with all-Chinese operation and monitoring software with independent copyright. Local and remote start-stop operation, function setting, parameter setting, fault query, operation record query, etc. all use Chinese WINDOWS operation Interface; equipped with a 12.1" color LCD touch screen, which can realize the complete general-purpose VFD (Variable-frequency Drive) parameter setting function, and can print out the operation report; adjust the touch panel to display voltage and current waveforms, frequency and motor at any time Rotational speed, which can intuitively display the real-time state of the motor at any time; has strong diagnostic and indication capabilities: it can detect the operating status of each part of the VFD (Variable-frequency Drive), complete fault monitoring circuit, accurate fault location, All power modules are intelligently designed. When a fault occurs, the fault information will be returned to the main control unit, and the main control unit will turn off the main power component I G BT in time to protect the main circuit. At the same time, the Chinese man-machine interface The precise positioning on the screen shows the fault location and category, so that the fault point is clear at a glance, and it is suitable for the skill level of general operators and maintenance personnel.

When an external analog signal is used to control the output frequency of VFD (Variable-frequency Drive) (VFD (Variable-frequency Drive) is used as the actuator of DCS), if the analog signal is disconnected or short-circuited, VFD (Variable-frequency Drive) can provide an alarm signal while keeping the original output frequency unchanged. VFD (Variable-frequency Drive) control power supply can receive AC 220V and DC 220V input, and is equipped with UPS, which can continue to run when the control power supply fails, and provide alarm at the same time.

3 Issues that should be paid attention to

The pressure of the condensed jellyfish pipe should not be too low to prevent the air from going through the drain valve through the condensed water.