Application of high voltage inverter in increased safety brushless excitation synchronous motor

Shandong Haihua Coal Industry Chemical Co., Ltd. covers an area of ​​more than 900,000 square meters and has more than 2,100 employees. It has built 6 large modern coke ovens and supporting chemical systems, a coal washing plant with a capacity of 900,000 tons of raw coal, and a cogeneration power plant with an annual power generation capacity of 13MW. The company has an annual production capacity of 1.2 million tons of metallurgical coke. Its main products include more than 10 kinds of metallurgical coke, foundry coke, crude benzene, tar, coal gas, ammonium carbonate, etc. The products are exported to countries and regions such as Japan, India and Southeast Asia.

The company is located in the hinterland of Zaoteng Coalfield with a coal reserve of 1.45 billion tons, and is rich in coal resources. It also has a 60,000-ton coal storage yard, a 4-track railway line with a total length of 2,500 meters connected to Zaozhuang West Station of Beijing-Shanghai Railway, and three 1,000-meter mechanized loading and unloading platforms that can accommodate 89 cargo spaces. The company has 15 chemical product storage tanks with a total capacity of up to 1,400 tons, and 20 tar and crude benzene self-owned vehicles. It has an independent water supply facility with a daily water supply of 20,000m3, a 35kv double-circuit 5,600kva power distribution facility, and complete mechanical processing, sewage treatment and transportation facilities.

This time, Shandong Haihua Coal Industry Chemical Industry Co., Ltd. renovated two 2400kw/10kv increased safety brushless excitation synchronous motors, and the load was a gas compressor. This high-power brushless excitation synchronous motor frequency conversion renovation is the first in China, and it is also rare in the world. After receiving the order, our company was not afraid. With the determination of "if it can be done abroad, we can do it too", we overcame difficulties such as lack of information and made full preparations. The hard work paid off, and finally the power-on and commissioning were successful. Let's discuss with you several issues in the high-voltage frequency conversion renovation of increased safety brushless excitation synchronous motors.

2 Structure and power frequency operation process of brushless synchronous motor

2.1 Structure of brushless synchronous motor

The structure of the brushless synchronous motor is shown in Figure 1.

In FIG. 1 , 1 is a sliding bearing, 2 is a brushless synchronous motor winding, 3 is a cooler, 4 is a rotating rectifier, and 5 is an excitation generator.

2.2 Structure of the excitation system of a brushless synchronous motor

The structure of the brushless excitation synchronous motor excitation system is shown in Figure 2, in which the excitation generator rotates coaxially with the synchronous motor.

Among them, the rotating rectifier is responsible for the demagnetization and excitation logic of the motor starting process, and its internal structure is shown in Figure 3. When the motor starts, the rotating rectifier controls the demagnetization thyristor t4 to connect the demagnetization resistor rf to the rotor excitation winding of the brushless synchronous motor to provide a larger starting torque and reduce the voltage at the excitation winding terminal. At this time, the rectifier thyristors t1~t3 are cut off; when the motor reaches the subsynchronous speed and meets the quasi-angle condition, the controller triggers the rectifier thyristors t1~t3, rectifies the armature voltage of the excitation generator and adds it to the excitation winding of the synchronous motor, providing a continuous excitation current for the synchronous motor, and at the same time turns off the demagnetization thyristor t4. At this time, the rotating rectifier is equivalent to a three-phase diode uncontrolled rectifier.

2.3 Power frequency steady-state operation of brushless synchronous motor

When the brushless synchronous motor is running in steady state at industrial frequency, the exciter passes appropriate excitation current to the stator excitation winding of the excitation generator, inducing a three-phase AC voltage at the end of the rotor armature winding of the excitation generator, which is rectified into a DC voltage by a rotating rectifier (equivalent to a diode rectifier) ​​and applied to the rotor excitation winding of the brushless synchronous motor to provide it with continuous excitation current.

According to the physical characteristics of the excitation generator, its output armature voltage is approximately proportional to the product of the motor speed and the excitation current of the excitation generator. Therefore, the exciter can adjust the stator excitation current of the excitation generator by adjusting the trigger angle of the thyristor to achieve the purpose of adjusting the rotor excitation current of the brushless synchronous motor.

2.4 Power frequency starting and excitation process of brushless synchronous motor

The industrial frequency starting and excitation process of the brushless synchronous motor is shown in Figure 4.

During power frequency starting, the high-voltage circuit breaker is closed first, and the demagnetization circuit of the rotating rectifier connects the demagnetization resistor to the excitation winding of the synchronous motor according to the induced voltage on the excitation winding of the synchronous motor, and the synchronous motor gradually accelerates.

After the high-voltage circuit breaker is closed, the exciter triggers the thyristor and passes a certain excitation current to the stator excitation winding of the excitation generator. As the motor speed increases, the voltage of the rotor armature winding of the excitation generator gradually increases. When it is higher than the minimum working voltage of the rotating rectifier, the rotating rectifier controller powered by it is powered on. The rotating rectifier monitors the induced voltage on the excitation winding of the synchronous motor. When its period is greater than the preset value (indicating that the synchronous motor has reached subsynchronous speed) and reaches the reverse zero crossing point, the rectifier thyristor is triggered, the demagnetization thyristor is turned off, and the rotor armature voltage of the excitation generator is rectified and added to the excitation winding of the synchronous motor to complete the excitation. After a short stepping process, the synchronous motor enters a stable synchronous operation state, and the motor starting process is completed.

2.5 Power frequency shutdown process of brushless synchronous motor

When the power frequency is shut down, the high-voltage circuit breaker is disconnected, and the exciter adjusts the trigger angle of the thyristor to the active inverter area, and the stator excitation current of the excitation generator is quickly reduced to zero. The rotor armature winding voltage of the excitation generator drops rapidly. When it is less than the minimum working voltage of the rotating rectifier, the rotating rectifier control power is cut off, and its rectifier thyristor is cut off. The freewheeling diode connects the demagnetization resistor to the excitation winding of the synchronous motor, and the excitation current of the synchronous motor drops rapidly to zero. The synchronous motor gradually stops under the action of load and resistance torque.

3 Variable frequency operation of brushless synchronous motor

3.1 Characteristics of the excitation generator during speed regulation operation

Unlike the direct excitation of the brushed synchronous motor by the slip ring, the excitation current of the brushless synchronous motor is generated by the rotating excitation generator. Since the voltage generated by the excitation generator is proportional to the product of the motor speed and the stator excitation current of the excitation generator, when the motor speed is much lower than its rated speed, the voltage generated by the excitation generator is low. At this time, even if the exciter outputs the maximum excitation current to the excitation generator, the excitation current of the brushless synchronous motor will be less than its rated value. When the speed is very low at the initial start, the brushless synchronous motor will not be able to obtain the excitation current.

When the synchronous motor is started with variable frequency without excitation current, its stator armature winding will absorb a large inductive reactive current from the inverter (typical value is about 2 to 3 times the rated current of the motor). This current only flows between the inverter and the synchronous motor and is not injected into the power grid, but it will cause short-term heating of the inverter and the stator armature winding of the synchronous motor. Therefore, when the motor speed is low, the maximum possible current should be applied to the stator excitation winding of the excitation generator to minimize the starting current of the motor.

3.2 Excitation and stepping process during variable frequency starting

The variable frequency starting process of the brushless synchronous motor is shown in Figure 5.

After the high-voltage circuit breaker is closed, the inverter is powered on with high voltage. After receiving the "start" command, the inverter starts to output voltage to the stator armature winding of the synchronous motor from 0.5 Hz, and gradually increases the frequency and amplitude of the output voltage according to the preset acceleration time and V/F curve, and the synchronous motor starts without load.

While the inverter outputs voltage to the stator armature winding of the synchronous motor, it notifies the exciter to start outputting strong excitation current to the stator excitation winding of the excitation generator. This current is greater than the rated excitation current of the excitation generator and less than the maximum short-time excitation current of the excitation generator. At this time, the rotor armature winding current of the excitation generator is approximately zero.

After the inverter is started, the synchronous motor relies on its salient pole torque and rotor residual magnetism, and enters the synchronous operation state after a short asynchronous acceleration and full step process (about 1s to 2s). Since the synchronous motor has no excitation current at this time and only relies on salient pole torque and rotor residual magnetism to operate, its stator side current is relatively large, about 2 to 3 times the rated current of the motor.

As the motor accelerates, the voltage induced by the rotor armature winding of the excitation generator gradually increases. When it is higher than the minimum working voltage of the rotating rectifier, the rotating rectifier controls the power supply to be powered on. Since the synchronous motor is working in a synchronous operation state at this time, the small swing of its rotor angle induces a low-frequency voltage on the excitation winding, and the period of this voltage meets the slip frequency criterion. After detecting this voltage, the rotating rectifier immediately triggers the rectifier thyristor and inputs the excitation current to the rotor excitation winding of the synchronous motor. Since the motor speed is still low at this time, the armature voltage of the excitation generator is low, and thus the excitation current output to the synchronous motor is also low. After the excitation is input, the stator armature current of the synchronous motor will be reduced. As the motor accelerates, the voltage output by the excitation generator gradually increases, the excitation current obtained by the synchronous motor also gradually increases, and the stator armature current of the synchronous motor gradually decreases to below the rated current.

When the motor speed increases to the minimum operating speed (lower limit of the speed regulation range), the frequency converter notifies the exciter to adjust the stator excitation current of the excitation generator to the maximum continuous working excitation current (or rated excitation current) of the excitation generator. The motor starting process is completed and the motor can be loaded and operated at this speed, or accelerated to the desired speed according to process requirements.

Since the motor starting process is relatively short (its typical value is about 30s), and the winding temperatures of the excitation generator and the synchronous motor are usually not high before starting, this process will not cause overheating of the excitation generator and the synchronous motor.

3.3 Speed ​​range

For brushless synchronous motors, since the voltage emitted by the excitation generator is low at low speed, the excitation current obtained by the synchronous motor is small, and its maximum output torque (out-of-step torque) is small. Therefore, it is necessary to determine its minimum operating frequency based on the excitation-torque characteristics of the motor.

Generally speaking, in order to increase the speed regulation range of the motor, when the speed is low, the exciter outputs the maximum continuous working excitation current of the excitation generator to the stator excitation winding of the excitation generator. At this time, the rotor armature winding of the excitation generator will output the maximum induced voltage at this speed (still less than the rated armature voltage at its rated speed), and the synchronous motor can also obtain the maximum excitation current at this speed.

According to the output voltage of the excitation generator at its maximum continuous working excitation current, the resistance of the synchronous motor rotor excitation winding, and the excitation-torque characteristic curve of the synchronous motor, the maximum output torque (out-of-step torque) of the synchronous motor at each speed can be calculated. Generally, the minimum operating speed of the synchronous motor can be determined according to the principle that the maximum output torque of the synchronous motor is not less than 1.3 times the peak value of the load torque under the speed condition (typical value is 60% to 70% of the rated speed of the motor).

3.4 Excitation Regulation

(1) When the synchronous motor runs at the lowest speed, it will absorb a certain amount of inductive reactive current from the inverter, and the power factor will lag. As the speed increases, its power factor will gradually increase until it reaches unity power factor (pf=1). In order to reduce losses and improve system efficiency, the exciter should output its maximum continuous working excitation current to the stator excitation winding of the excitation generator before the power factor reaches unity power factor.

(2) When the speed of the synchronous motor increases further, it will send inductive reactive current to the inverter and the power factor will lead. At this time, in order to reduce losses and improve system efficiency, the inverter will communicate with the exciter based on its output power factor, reduce the excitation current output by the exciter to the excitation generator, and make the synchronous motor run at unity power factor.

3.5 Frequency conversion shutdown process

After receiving the "immediate shutdown" command, the inverter will stop outputting voltage to the stator armature winding of the synchronous motor, and at the same time notify the exciter to demagnetize. The excitation current of the excitation generator will decay rapidly, and the excitation current of the synchronous motor will decay rapidly through the demagnetization resistor. The motor will gradually stop under the action of load and resistance torque.

3.6 Out-of-step protection

When the inverter detects that the synchronous motor has lost step, it immediately stops outputting voltage to the stator armature winding of the synchronous motor, and at the same time notifies the exciter to demagnetize and reports the fault.

4 Power saving

The attached table is the data recorded on site, which shows that the power saving effect after the high-voltage frequency conversion transformation is very good.

5 Conclusion

Based on the structure and principle of brushless synchronous motor, this paper analyzes the problems that may be encountered when brushless synchronous motor is running with variable frequency in detail, and proposes the variable frequency operation mode of brushless synchronous motor. Under this operation mode, the inverter can drive the brushless synchronous motor to run with reliable and economical speed regulation. In addition, when selecting the inverter, the capacity must be appropriately considered and enlarged. This time, our company selected the model hivert-t10/192 with a rated current of 192a.