Application of Variable Frequency Driven Synchronous Motor in Domestic Compressor Unit

In order to solve the problems of irrational structure and environmental protection of the national energy industry, China began construction of the West-East Gas Pipeline project in 2000. After that, long-distance, large-caliber gas pipelines entered the peak period of construction. In order to improve the conveying capacity of the long-distance pipeline and compensate for the pressure loss on the gas pipeline, a compressor station needs to be established in the middle of the long-distance pipeline, and the main equipment of the compressor station is the compressor unit. The traditional compressor station compressor unit is driven by a gas turbine. It has been widely used in China's long-distance pipeline compressor stations due to its high reliability. In recent years, due to its low investment cost, low maintenance cost, and remote control and unmanned operation, the electric drive system has begun to be applied in the domestic long-distance pipeline compressor station, and to some extent has the tendency to replace the gas turbine drive.

At present, the compressor units in almost all the compressor stations, including the first and second lines of the West-East Gas Pipeline, are imported units abroad, which greatly limits the development of the Chinese pipeline industry. In order to break this technological monopoly, many domestic units have participated in the research and development of domestic electric drive compressor units in recent years.

In December 2011, the first domestically produced electric drive compressor unit with a capacity of 20MW was delivered. On November 16th, 2012, the 72h on-site operation test of the compressor unit was successfully completed at the Gaoling sub-pressure station of the Second West-East Gas Pipeline and officially put into operation. This marks that China's natural gas long-distance pipeline industry has broken foreign monopoly, and China's natural gas pipeline industry has entered a new era.

The domestic electric drive compressor unit is composed of a unit serial multi-level PWM voltage source type inverter, a synchronous motor, a centrifugal compressor and the like having a capacity of 20 MW. In this paper, the technical characteristics of the electric drive part consisting of frequency converter and synchronous motor will be briefly introduced in the background of the unit.

1Compressor group electric drive system composition and characteristics First: Hao Xing, born in 1985, male, Xi'an, Shaanxi, graduated from Northwestern Polytechnical University in 2010, majoring in mechanical manufacturing and automation, assistant engineer.

1.1 System structure The electric drive system is mainly composed of a phase shifting transformer with damping start, a unit series multi-level PWM voltage source type inverter, a synchronous motor and a compressed air from the transformer I group secondary coil. There are 4 different switch combinations in each unit: VA and VD are turned on at the same time, the positive bus voltage is +Ud; VB and VC are turned on at the same time, the negative bus voltage is -Ud; VA and VC are simultaneously turned on, Or VB and VD are turned on at the same time, the output voltage is 0; therefore, the output voltage of each power unit is three levels of +Ud, 0, -Ud. Each unit is superimposed with 8 units, which can produce 17 different levels, respectively +-7Ud, +-. Since the initial moment of switching on the transformer, the series excitation multi-level inverter circuit of the transformer uses multi-electric translation phase. The configuration of the supporting system of the station is shown in Fig. 1.2. The main features of the electric drive unit are unit series multi-level PWM voltage source type inverter, 2-pole explosion-proof brushless excitation high-speed synchronous motor, the inverter and motor power are both 20MW. The rated voltage is 10kV, the motor has a positive pressure ventilation and explosion-proof system, the speed range is 3 4800r/min, the maximum speed can reach 5040r/mm, and it is directly driven by the centrifugal compressor.

2 compressor unit electric drive system main equipment 2.1 phase-shifted transformer unit with damping start series multi-level PWM voltage source type inverter input side consists of two identical primary side 10kV input phase shifting transformers, which respectively give 12 power Unit power supply. Each transformer has 12 isolated three-phase secondary windings, each of which has a rated voltage of 660V, and each of the two sets of secondary windings has a phase angle of 10 electrical angles offset from each other. The angle is shown in Table 1.

Table 1 Phase shifting transformer phase shift angular flow will be abrupt to 4 times the rated current of the transformer, causing the 10kV bus voltage to drop instantaneously too much, affecting other electrical equipment.

Take the 10kV to explain the pre-charge of the transformer secondary through a damping resistor. After the power supply is about 1s~10s, the resistor is bypassed, so that the 10kV bus voltage drops within 5% during pre-charging, thus overcoming the isolation transformer is turned on. The surge effect generated at the initial moment. The resistor and circuit breaker are installed in the same switch cabinet and installed indoors.

2.2 Inverter 2.2.1 Inverter structure and principle Each phase of the inverter adopts 8 power units with rated voltage of 660V in series, and a total of 24 power units are formed. Each power unit can be interchanged and insulated. The three-phase Y-connection method has an output phase voltage of up to 660Vx8=5280V and a line voltage of up to 9144V. The power unit is a three-phase input, single-phase output consisting of uncontrollable diodes, capacitors, resistors and low-voltage IGBT power devices. The AC-DC-AC PWM voltage source inverter structure is shown in the power unit system diagram. Each power unit is realized by three uncontrollable diodes for three-phase full-wave rectification, and the phase shifting transformer is composed of input side 36-pulse rectification, which can eliminate harmonics of 35 times or less.

The rated input voltage of the inverter power unit is 660V, and the DC bus voltage Ud after rectification is 900V left stone. In order to make the system more stable, 35 filter capacitors with a capacity of 1100uF and two energy consumption resistors are connected in parallel on the bus. . The inverter side consists of four IGBTs with a rated voltage of 1700V and an H-bridge single-phase inverter circuit. The main control system consists of the DSP main control board, PLC and related electrical components. The DSP implements SPWM control for each unit through optical fiber, and its output is a single-phase 0V660V AC voltage. The output frequency is matched with the synchronous motor up to 84Hz. PWM technology, the so-called 'phase shifting' refers to the same phase power. The carrier signal of the unit is staggered by a certain electrical angle, so that the number of steps of the output voltage increases after the superposition, the equivalent switching frequency is increased, the voltage waveform is closer to the sine wave, and the current harmonic is very small.

2.2.2 Advantages of Inverter Design a) The design has made great progress in reducing harmonic components. There is no need to separately set the filter, there are no torque ripple caused by harmonics, and motor heating and noise; b) The power units are connected in series, there is no problem of device voltage equalization, each power unit is subjected to full current, only accepts 1/8 of output phase voltage and 1/24 of output power; c) each power unit adopts modular design with the same structure And the parameters can be exchanged. The connection between each power unit and the system is only 3 AC input line rows, 2 AC output line rows and 2 fiber plugs for convenient unit maintenance. The system has a unit bypass function. When an individual unit fails, the unit can be short-circuited under the control of the bypass control board. The system can still operate normally or derated, and up to 2 power units can be bypassed per phase.

2.3 High-speed synchronous motor adopts explosion-proof brushless excitation high-speed synchronous motor, rated power is 20MW, rated voltage is 10kV, Y-type wiring, series 2, rated speed 4800r/min, maximum speed 5040r/min, power factor up to 10. excitation The machine stator Y-connection method, the rotor double winding Y connection method, is directly connected with the main motor rotor, and is equipped with a rotating rectifying plate composed of 12 uncontrollable diodes in the middle to provide adjustable DC power to the host rotor. The motor has auxiliary systems such as positive pressure ventilation, air-to-water cooling and high pressure jacking.

Rotary rectifier synchronous motor schematic diagram a) Positive pressure ventilation explosion-proof system: Before starting the machine, clean and replace the gas in the motor cavity with clean compressed air to prevent dangerous gas in the motor casing. Keep the machine cavity 50Pa when the unit is running. The positive pressure of Zuo Shi makes the combustible gas or dust not enter the inside of the motor; b) Air-water cooling system: The top of the synchronous motor is equipped with an air water cooler with a power of 500 kW, and the inside is equipped with a water leakage detecting device. It is equipped with 2 sets of 4 fans with a power of 22kW, which makes the hot air exchange heat with the cooling water in the air cooler and blows the cooling air into the stator and rotor of the motor in the correct way. Cooling effect; c) High-pressure jacking device: 1 pair of 380V voltage and 15kW top lift oil pump are installed on both sides of the synchronous motor. It provides a nominal pressure of 20MPa and a nominal flow rate of 2.5L/min for the sliding bearing when the synchronous motor runs at low speed. The lubricating oil causes the rotor to be lifted by the lubricating oil, thus ensuring that the rotor operates well with the bearings at low speeds.

3 The matters needing attention in the construction and operation of the unit combined with the experience of the construction and operation of the first-line compressor unit of the West-East Gas Pipeline, the following aspects should be noted in the construction and operation of the unit: a) The inverter is cooled by deionized water to prevent water. Jelly, causing equipment damage. In order to ensure that the deionized water will not freeze when the temperature is low in winter, the temperature of the inverter should not be lower than 5°, and it can be equipped with air conditioning and other indoor heating, and equipped with emergency security facilities such as sudden power outage; b) Both the unit and the motor are cooled by circulating water. It is recommended to use non-rusted pipes to expose the ground section to set insulation and electric heating, system set low point sewage, high point exhaust, external piping, water discharge in the motor and air blowing. Sweep the facility. When the unit is shut down in winter, the cooling water should be drained to prevent the pipeline from being cracked. c) From the historical statistics of the operation failure of the West-East Gas Pipeline, the proportion of the unit failure caused by the auxiliary system is more than 70%, so it is necessary to Pay attention to the operating status of the auxiliary system. For example: to ensure the internal temperature and pressure of the unit, the temperature, flow and pressure of the external lubricating oil and circulating water system of the unit to meet the operation requirements of the unit; to ensure that there is no water, oil and solid particles in the compressed air, so that the quality of the compressed air is qualified.

On the 16th of November, 2012, the Gaoling sub-gas transmission station of the West-East Gas Pipeline Project officially completed the 72h operation test of the unit, marking the official commissioning of the domestically produced compressor on the long-distance natural gas pipeline. The first self-developed 20MW electric drive compressor unit is a domestically produced electric drive compressor unit with the highest power and highest speed on the long-distance natural gas pipeline. The system is complex and huge, and it has condensed the ingenuity of the Chinese people. It makes China's natural gas pipeline. The transportation industry has taken a new step and opened up a new direction for the development of China's national industry.