Design and application of high voltage soft start on compressor load

Abstract: High-voltage and high-power motors have a relatively large impact on the load and the power grid when starting. Thyristor series high-voltage soft starters are widely used for their superior starting performance and other advantages. This paper analyzes the working principle of high-voltage soft starters on the load of high-power compressors on site, introduces the starting function, and makes a relatively comprehensive introduction to the high-voltage soft start of thyristor series.
Keywords: thyristor, motor, trigger, soft start

1. Introduction

AC asynchronous motors have been widely used in China's coal mining, petroleum, chemical industry, steel, power generation, water supply, cement building materials and other fields due to their simple structure, extremely high operating reliability, strong environmental adaptability and excellent drag performance. However, the fatal disadvantage of AC asynchronous motors is that the starting shock is large, which will have an adverse effect on the system. The main impact is reflected in two aspects:

(1) Electrical problems:

The rated current can reach 5-7 times during starting, causing the motor winding to overheat due to the current, thereby accelerating its insulation aging, causing voltage fluctuations in the power supply network, and affecting the normal operation of other equipment in the same power grid.

(2) Mechanical problems:

Excessive starting torque produces mechanical shock, which causes great impact force on the driven equipment, shortens the service life and affects the accuracy. For example, the coupling is damaged and the belt is torn. It causes abnormal wear and impact of mechanical transmission parts, accelerates aging, shortens the service life and increases the maintenance workload.

Such problems were encountered on the air compressor of the nitrogen production line of Shougang Cold Rolling Plant. Due to the relatively large motor power (10KV/3730KW and 10KV/2250KW), firstly, the mechanical impact on the motor and compressor during the starting process is relatively large, which increases the workload of system maintenance; secondly, the power grid drops seriously when the motor is directly started. When the power grid voltage drop reaches 85%, other equipment cannot work normally. In view of the above reasons, it was decided to use the PowerEasy thyristor soft starter.

2. The composition and principle of thyristor series high-voltage soft starter

2.1 Main circuit composition

The thyristor series soft starter is a fully digital intelligent starting device designed by combining power electronics technology, optoelectronic control technology and microprocessing technology. The wiring diagram of the main circuit of the thyristor series soft starter is shown in Figure 1. It is mainly composed of a high-voltage thyristor series valve group and a bypass contactor. The high-voltage thyristor series valve group is a power conversion actuator, which is composed of multiple thyristors in series and parallel, and is supplemented by an absorption and voltage-sharing clamping circuit to ensure its reliability in a high-voltage environment. When the incoming line is powered, the conduction angle of the thyristor is controlled to chop the AC three-phase power supply and control the amplitude of the output voltage. After the starting process is completed, the bypass contactor is closed, the soft starter is switched to the bypass state, and the thyristor is turned off at the same time.

Figure 1 Thyristor series soft start main circuit wiring diagram

2.2 Control Circuit

As shown in Figure 2, the control circuit of the thyristor series high-voltage soft starter is generally composed of a main control circuit, a trigger device, a signal acquisition circuit and a human-machine interface.

Figure 2 Soft start control block diagram

2.2.1 Main control circuit

The main control circuit is mainly composed of digital circuits composed of DSP microprocessors and CPLD large-scale gate arrays. Its main function is to generate pulse instructions to trigger thyristors. The trigger commands of thyristors are issued by digital signal processors (DSP). The main control circuit can adjust the conduction angle of the thyristor according to the voltage and current signals collected by the signal acquisition circuit, so as to adjust the motor current and realize closed-loop control of the motor, that is, current limiting starting. While sending out the trigger pulse, it can monitor the operating status of the motor and the soft starter itself through feedback signals such as voltage, current, and temperature, protect the entire system, and ensure the safe operation of the system.

2.2.2 Trigger system

The trigger system is a key part of the stability and reliability of the system. It must have the ability to resist noise interference and be able to turn on the thyristor in a timely and accurate manner. There are two main technical problems here:

1. The trigger pulse can trigger the thyristor in a timely and accurate manner.

Thyristor is a current-controlled bipolar semiconductor component. It requires the gate drive unit to be similar to a current source, which can provide a particularly steep peak current pulse to the thyristor to ensure that the thyristor can be accurately and reliably triggered at any time. Therefore, the gate trigger pulse characteristics of the thyristor have a very strong influence on the normal operation of the thyristor.

When the thyristors are used in series, we require the thyristors in series to be turned on together as much as possible, because the slow opening may be subjected to overvoltage and damage the components. At this time, the opening time difference of the thyristors in the same group of series is required to be as small as possible.

The opening speed of the thyristor is mainly related to the amplitude of the gate trigger pulse and the steepness of the pulse rising edge. That is to say, the larger the amplitude of the gate trigger pulse, the steeper the pulse rising edge, and the shorter the thyristor opening time. Therefore, in the field of motor soft starting, most manufacturers use strong triggering to trigger thyristors. That is, the current amplitude of the trigger pulse IG is greater than or equal to 10IGT, and the pulse rising edge time tr≤1μs. In short. In order to ensure the reliability of the thyristor valve string, IG should be much larger than IGT.

2. Triggering method of triggering thyristors

There are three triggering systems for high-voltage isolation, pulse transformer, direct light triggering and fiber optic triggering. The pulse transformer has a simple structure and low cost, but its electromagnetic compatibility is poor and it is not suitable for use in high-voltage environments; the direct light triggering system is reliable and accurate, but it is expensive and rarely used; the fiber optic triggering system has good reliability. Although it adds some complexity to the circuit compared to the direct light triggering system, it is much cheaper and the triggering is accurate and reliable, so it is the first choice for high-voltage applications. At present, the triggering system of high-voltage soft start devices on the market basically adopts the fiber optic triggering system.

In high-voltage soft starting, the working power supply of the triggering system is generally obtained by high-level energy extraction, that is, the working power supply on the triggering circuit board is supplied by the capacitor on the dynamic voltage balancing (i.e., RC absorption circuit). This not only avoids the high cost brought by the use of high-voltage transformers, but also realizes electrical isolation and avoids interference with the triggering signal. It should be pointed out that the high-level energy acquisition method is only applicable during the soft start process. In the soft stop process designed for some pump loads, the current in the main circuit becomes smaller and smaller in the second half of the stop process, which will affect the acquisition of electric energy on the trigger board, thereby affecting the triggering of the thyristor. Therefore, some manufacturers' thyristor soft starters will provide additional power to the trigger system when producing products with soft stop functions, which increases the cost. The soft starters of some foreign companies such as AB are more sophisticated in this regard. They use a technology called low-level energy acquisition, that is, a low-voltage and high-power transformer is used on the low-voltage side, and a set of current sensing devices is used on the trigger board. The transformer line passes through the current sensing device on the trigger board, and the induced current is obtained on the trigger circuit board to act as a power supply. In this way, the trigger board can obtain enough power to continue to trigger the thyristor in the later stage of soft stop, which can make the later parking process of soft stop more stable and reduce the impact of the water hammer effect of pump loads on the pipeline and impeller.

2.2.3 Signal acquisition circuit and human-machine interface

The main function of the signal acquisition circuit is to collect various signals and pre-process them, and then feed the processed signals back to the main control circuit. At present, there are two main types of collected signals, voltage signals and current signals. Some soft start manufacturers also collect temperature and speed signals. Voltage and current signals are generally collected through mutual inductors, and fed back to the main control circuit after processing. The operating status of the motor and soft start device is judged based on the feedback data, and a trigger pulse is issued or when the overall system is abnormal, the fault is judged through the feedback signal and a protection command is issued. The

human-machine interface circuit is generally composed of an LCD screen and a keyboard, which are used to complete the user's parameter settings, select the starting mode, and display the operating status of the system. In some working conditions that require remote control, some soft start devices are also equipped with a remote communication module.

3. Starting mode of thyristor soft starter

3.1 Full voltage starting

In this state (Figure 3), the soft starter is equivalent to a solid-state contactor. The motor is subjected to all current shocks and torque shocks, just like the direct starter. In general, the thyristor full-opening time is controlled within 0.25s:

Figure 3 Full pressure starting

3.2 Voltage ramp start

This mode is a relatively long-used start mode. It reduces the impact of the starting torque to achieve smooth, continuous and stepless acceleration of the motor, thereby reducing the friction of gears, couplings and belts to a minimum. The user can adjust the initial torque of the motor. During the acceleration ramp time, the input voltage of the motor rises linearly from the voltage corresponding to the set initial torque, changing the traditional step-down start to stepless, so that the motor can start smoothly and reduce mechanical impact.

Figure 4 Voltage ramp starting

3.3 Current-limited

starting As the name implies, current-limited starting is to limit the starting current to a certain set current value during the motor starting process. It is mainly used for relatively light load starting and certain requirements for grid impact. The input voltage increases rapidly from zero until the current reaches the preset current limit. Then, while ensuring that the output current is not greater than the current limit, the conduction angle of the thyristor is changed to gradually increase the voltage until the rated voltage. At the same time, the speed of the motor is also gradually increasing and reaches the rated speed. The advantage of this type of starting is that the starting current is small, which can minimize the impact of the motor starting on the grid, and the current limit value can be set as needed. However, when setting the current limit, it must be set according to the initial torque of the motor, otherwise the setting will fail to start or burn the motor if it is too small. This starting method takes a relatively long time to start.

Figure 5 Current-limited starting

3.4 Sudden start

This starting method is mainly used in working environments with relatively heavy loads. On the basis of torque control, a sudden torque is used at the moment of starting to overcome the static torque of the load, and then the torque gradually increases until the motor reaches a normal working state. The advantage of this starting method is that it can shorten the starting time and start heavier loads, but it will have a certain impact on the power grid during starting, affecting the operation of other loads in the same power grid.

Figure 6 Kick start

3.5 Soft Stop

Soft stop is actually equivalent to the opposite soft start process. Its main function is to eliminate the anti-inertia impact of the system. For pump loads, it overcomes the "water hammer" effect. Its main process is that when the motor is soft stopped, the bypass contactor of the soft start device is disconnected, and the thyristor starts to work, so that the motor voltage gradually decreases and the speed decreases, achieving the effect of soft stop. (As shown in Figure 7)

Figure 7 The whole process of soft start and soft stop

3.6 Pump-controlled starting and stopping

Due to the relatively special mechanical characteristics of water pump loads, some soft start manufacturers have specially designed pump-controlled starting and stopping methods based on the characteristic curves of pump loads. This starting method can reduce water hammer impact during the start and stop of centrifugal pumps through smooth acceleration and deceleration of the motor. The main control circuit analyzes the various operating parameters of the motor by collecting signals, and controls the motor state by changing the thyristor conduction angle, thereby reducing the possibility of surge impact on the system. (Figure 8)

Figure 8 Pump control start and stop process

4. Application effect of soft start in compressor load

In the high-voltage and high-power compressor load system (10KV/3730KW and 10KW/2250KW) of Shougang Cold Rolling Plant, direct motor starting will not only have an impact on the power grid, but also put forward high requirements for the mechanical maintenance of the entire system. After using PowerEasy soft start, the effect is good and the expected purpose is achieved. The main application effects are reflected in the following aspects:

1) Compared with the 10KV/3730KW compressor on site, its rated current is 253A, and the current will reach about 7 times the rated current when it is directly started. After using PowerEasy soft start, according to the on-site working conditions, the starting mode is set to current limiting start, the current limiting multiple is 3.2 times, and the current is about 810A when starting, and the current control is accurate and stable. This not only reduces the impact of motor starting on the power grid, but also improves the safe operation level of the power grid; from another aspect, it relatively reduces the capacity of the power grid, reduces the investment cost, and shortens the construction period.

2) While reducing the impact of the compressor direct start on the motor itself, it also reduces the mechanical impact of the start on the bearings, pipelines and the entire system. Because the motor direct start acceleration is too fast, it is a very severe test for the life and reliability of the transmission system. The soft start process is smooth and stable, which can effectively eliminate the adverse effects on the transmission system and pipelines, reduce maintenance workload, and extend the service life.

3) The main circuit of the PowerEasy soft starter uses imported thyristors in series, the power components have good consistency, and sufficient margin is reserved in the design. When starting 3-5 times continuously, the temperature rise is relatively small, within 50°C, and high reliability can be guaranteed in harsh working environments.

4) The PowerEasy soft starter mainly uses a current closed loop to control and monitor the motor and the entire system. The starting process has good consistency under reasonable voltage fluctuations. In addition, the soft starter itself has a relatively complete protection system, which can protect and monitor the entire system at any time according to the feedback signal, fully ensuring the safe operation of the system.

5) During the debugging process, various protection parameters and starting parameters can be set according to the on-site working environment, which can better adapt to the on-site working conditions.

6) The volume is relatively small, the mechanism design is reasonable, and the noise is low.

5. Conclusion

In the application of square torque loads such as compressors, the thyristor series high-voltage soft starter can effectively change the starting characteristics of the motor and reduce the starting current. With its superior performance, it provides comprehensive services for the motor and ensures the reliability of the entire transmission system. While reducing the overall maintenance workload of the system, the thyristor soft starter itself can be maintenance-free, which reduces the overall production cost, has a good return on investment and social benefits, meets the requirements of the times, and is an inevitable trend of technological development.