Abstract: This paper introduces the application of INVT CHH100 high-voltage frequency converter as a soft starter for large motors, which enables the separate starting of multiple motors.
Keywords: High-voltage frequency converter, soft starter, synchronous switching
I. Introduction
In recent years, high-voltage variable frequency speed control technology has been increasingly applied in various industries to achieve the goals of saving energy, improving motor system lifespan, and enhancing product quality. The application of high-voltage frequency converters worldwide started later than that of low-voltage frequency converters. However, high-voltage frequency converters are used less frequently as starting devices for large motors.
Starting AC motors has always been a topic of concern, especially with the rapid increase in the use of high-voltage, high-capacity AC motors, the problem of soft starting has become even more prominent.
As is well known, when a common squirrel-cage motor is started directly under no-load full voltage, the starting current can reach 5 to 7 times the rated current. When the motor capacity is relatively large, this starting current will cause a sharp drop in the grid voltage and changes in the voltage frequency. This can disrupt the normal operation of other equipment on the grid and may even cause the grid to lose stability, resulting in a larger accident.
The large current during full-voltage starting of an electric motor generates a significant impact force on the stator coils and rotor squirrel cage bars, which can damage the winding insulation and cause the squirrel cage bars to break, leading to motor failure. The large current also generates a large amount of Joule heat, which damages the winding insulation and reduces the motor's lifespan.
When an electric motor is started directly at full voltage, the starting torque is about 7-8 times the rated torque. For gear transmission equipment, the large impact force will accelerate gear wear or even break the gears; for belt transmission equipment, it will increase belt wear or even break the belt.
For pump-type loads, when the motor starts at full pressure, the water flow reaches full speed in a very short time. When encountering a bend in the pipeline, the high-speed water flow impacts the pipe wall, generating a large impact force and creating a water hammer effect, which can damage the pipeline. If the pipeline in front of the pump is relatively long, when the pump motor suddenly stops, the high-speed water flow will impact the pump impeller, generating a large impact force that can deform or damage the impeller.
All of the above factors will increase equipment downtime, affecting normal production and increasing maintenance costs. 3-10KV motors generally have large capacities, typically above 200KW. In recent years, the production capacity of many industries has increased significantly, leading to larger drive motors for their production equipment. For example, in the steel and chemical industries, the use of motors exceeding 10,000KW is becoming increasingly common. As a result, the aforementioned problems have become more severe, and people are paying increasing attention to them.
3.1 The system mainly consists of three parts:
The first part consists of motor 1 and motor 2;
The second part is the bumpless handover section: KM4, L1;
The third part is the frequency converter and its bypass section: KM1, KM2, KM3, KM5, KM6, KM7.
Designed strictly in accordance with interlocking requirements, it can fully guarantee the safe operation of the variable frequency speed control system: KM5 and KM6, KM7 and KM3, KM2, KM3 and KM5 are mutually electrically interlocked;
3.2 M1 Disturbance-Free Handover Process
Variable frequency operation: QF1, KM1, KM2, and KM6 are engaged, KM3, KM6, KM5, and KM7 are disengaged, and motor M1 operates in variable frequency mode.
Seamless switching process: When the M1 inverter reaches 50Hz, KM4 is closed, KM2 is closed after a delay, and KM5 is closed simultaneously. KM4 and KM6 are closed after a delay, and the motor runs at the mains frequency. The switching process for M2 is similar.
3.3 Precautions and advantages of synchronous, non-disruptive handover:
Precautions:
Before seamless switching, the frequency converter controller detects the amplitude, frequency, and phase of the grid voltage, and then controls the high-voltage frequency converter to output a voltage with the same frequency, phase, and controllable amplitude before soft switching can proceed. During the switch from high-voltage frequency converter to power frequency, a phase-locked loop (PLL) locks the phase and frequency of the power frequency power supply, controlling the high-voltage frequency converter to allow the motor to operate at a slightly higher voltage and frequency than rated, providing more time for switching. This enables soft switching between the high-voltage frequency converter and the power frequency power supply.
Advantages of seamless switching:
1) Effectively reduces the impact of direct switching on the power grid, which could cause the upstream distribution switches to trip;
2) It reduces the impact of excessive shock from direct switching on motor insulation and heat dissipation;
3) It avoids the impact on the pipeline network and production caused by a sudden load increase due to direct switching;
4) Sequential soft start of multiple motors was achieved, protecting the motors.
IV. Conclusion
The successful implementation and operation of the INVT frequency converter soft starter retrofit demonstrates the broad application prospects of frequency converter technology, reflects Shenzhen INVT Electric Co., Ltd.'s technical solution capabilities in high-voltage frequency converter comprehensive retrofitting, and verifies the mature reliability of domestically produced high-voltage frequency converters. It also shows potential in starting loads such as large water pumps and blast furnace blowers.
References:
[1] CHH100 Series High Voltage Frequency Converter Product Manual, Shenzhen Invt Electric Co., Ltd.
[2] Yi Peng, Principles and Applications of High Voltage High Power Frequency Converter Technology, Posts & Telecom Press, February 2008.
About Invt:
Founded in 2002, INVT is committed to becoming a leading and respected global supplier of products and services in the fields of electrical drives, industrial control, and new energy. It was listed on the Shenzhen Stock Exchange A-share market in 2010 (stock code: 002334). INVT is a key high-tech enterprise under the National Torch Program, currently owning 12 holding subsidiaries and nine R&D centers across China. It holds over 390 patents and its main products cover high, medium, and low voltage frequency converters, intelligent elevator systems, servo systems, PLCs, HMIs, motors and electric spindles, SVG, UPS, and photovoltaic inverters. INVT currently has over 1,600 employees, four large-scale production bases, and a marketing network covering more than 60 countries and regions both domestically and internationally.
