Abstract : High-voltage, high-capacity variable frequency speed control systems often experience momentary power drops or outages in certain applications. Power loss transients are an essential function of high-voltage, high-capacity variable frequency speed control systems. This paper proposes a new power loss transient control strategy based on the design of a practical control and protection system for a high-voltage, high-capacity frequency converter. The principle, control, and protection logic of this strategy are analyzed in detail, and the results of prototype testing are presented.
Keywords: high-voltage frequency converter; power failure bypass; variable frequency speed control system
1 Overview
Due to its excellent control performance, significant energy-saving effect, and the increasing maturity of high-voltage, high-capacity variable frequency speed control technology and the decline in product prices, the application of high-voltage, high-capacity frequency converters is becoming increasingly widespread. Large fans and pumps with high requirements for regulation quality are primarily driven by medium- and high-voltage asynchronous motors.
In certain applications, high-voltage, high-capacity variable frequency drive (VFD) systems often experience momentary voltage drops or disappearances. This disturbance is very common in power supply systems, accounting for up to 92% of all fault disturbances. If the VFD stops immediately after a momentary power loss fault, it can lead to significant economic losses in industrial production. Power loss bypass function, primarily referring to power loss bypass (PLRT), is one of the essential functions of high-voltage, high-capacity VFDs.
Previous research on pulse-light relay (PLRT) for frequency converters has been extensive, including adding auxiliary power failure bypass devices, structural improvements, adding energy storage elements, and using highly precise closed-loop control strategies. However, research on PLRT for high-voltage frequency converters is scarce because their switching frequencies are low, closed-loop control is not precise enough, and they have large capacities and high pulse power, making many methods effective for small and medium capacities unsuitable for large capacities. One approach is to stop the pulse after detecting a power failure, thus maintaining the original bus voltage essentially unchanged. After the main power is restored, the motor speed drop and rotor flux phase need to be estimated before directly starting the motor at the corresponding frequency. This method is relatively complex and requires precise closed-loop control.
This paper proposes a new power failure bypass control strategy for high-voltage frequency converters, based on the design of a practical voltage-type high-voltage three-level frequency converter control and protection system. The paper analyzes the PLRT principle in high-voltage, high-capacity variable frequency speed control systems, introduces the design and implementation of related functions, and describes the protection logic in the PLRT process. Finally, the prototype test results are presented. The frequency converter used in the test was a 1250kW three-level high-voltage frequency converter with 6000V input and output. Its inverter structure is a three-level neutral-point clamped type, as shown in Figure 1.
For details, please click: A Novel Power Loss Crossover Control Strategy for High-Voltage, High-Capacity Frequency Converters
