1 Introduction
With the increasing use of frequency converters, engineers have found that when a frequency converter malfunctions and requires maintenance, the motor must be stopped, which cannot meet the requirements of continuous production on site. Therefore, engineers have added a switching device between the frequency converter, the motor, and the busbar to meet the requirement of continuous motor operation. Depending on the type of switching switch, the bypass scheme of the frequency converter is divided into manual bypass and automatic bypass. These are described below:
2. Introduction to two bypass methods
2.1 One-to-one manual bypass method
2.1.1 Detailed Introduction
(1) The disconnecting switches are selected as GN19 series single-throw and double-throw indoor high-voltage disconnecting switches, with a phase spacing of 210mm; the three insulated terminals of the incoming line of the single-throw disconnecting switch are the three sensors of the high-voltage live display device;
(2) The lighting is a cabinet door type lighting;
(3) The surge arrester adopts a three-phase combined type;
(4) Added input and output terminals; power frequency and frequency conversion indicators.
(5) Standard cabinet dimensions (length × width × height): 1200mm × 1200mm × 2320mm
2.1.2 Advantages and disadvantages
(1) Advantages
When overhauling the frequency converter, there is a clear power outage time, which can ensure personal safety. At the same time, the load can be manually connected to the power frequency grid. Manual bypass allows for manual fault diagnosis before switching, which is relatively safe. It is also inexpensive.
(2) Disadvantages
The load must be manually intervened when running at the power frequency, which does not meet the requirement that some field conditions cannot be stopped.
2.2 One-to-one automatic bypass method
2.2.1 Advantages and disadvantages
(1) Advantages
When a serious fault occurs in the frequency converter, the system can automatically switch to the mains frequency grid and disconnect the frequency conversion speed control system without stopping the load, thus meeting the requirement that the system cannot be shut down on site.
(2) Disadvantages
The price is relatively high and the operation is complex. When switching a motor from inverter to mains frequency operation, the automatic bypass switch generally doesn't cause problems. However, there is an exception: if the inverter shutdown is caused by a fault in the motor or its load, re-bypassing may exacerbate the problem. For example, when the inverter experiences overcurrent, it cannot determine whether the problem is with itself or with the motor (e.g., stalling, rotor rubbing, inter-turn short circuit). Directly connecting the motor to the mains frequency in this situation will cause even greater damage. Furthermore, when the motor is running at a speed below the mains frequency, an automatic switch to mains frequency can cause a sudden increase in motor speed, leading to abrupt changes in parameters such as furnace negative pressure and airflow. Users need to consider the potential impact on production in the short period before taking countermeasures. With manual bypass, countermeasures can be taken beforehand before restarting the motor, avoiding these problems.
2.3 Introduction to Manual One-to-Two Bypass Method
The system's main circuit uses a frequency converter with a matching manual bypass switch cabinet, providing dual power supply and a one-to-two control configuration. This allows one frequency converter to control the soft start of two motors and the variable frequency operation of either motor, or to achieve power frequency operation of the motors via the bypass cabinet.
2.3.1 Signal interlocking between the frequency converter and the high-voltage switch
To ensure the safe operation of the high-voltage frequency converter, the frequency converter and the high-voltage switch have the following signal interlocks:
(1) The frequency converter provides each high-voltage switch with a pair of "closing permission" contacts (passive contacts, effective closure). The high-voltage switch is allowed to close only when the contacts are closed.
(2) The frequency converter provides each high-voltage switch with a pair of "emergency disconnect" contacts (passive contacts, effective when closed). When the contacts are closed, the high-voltage switch must disconnect immediately.
(3) Each high-voltage switch provides a pair of "switch closed" contacts (passive contacts, effective closure) to the frequency converter for operation of the electromagnetic lock circuit of the high-voltage disconnect switch.
Comparison of the advantages and disadvantages of the two bypass schemes
According to statistics from Leadway's nearly 2,000 operational systems, most customers choose the manual bypass solution. The differences between the manual and automatic bypass solutions are shown in the attached table.
4. Application in the power industry
Both manual and automatic bypass solutions have numerous successful field applications, but the vast majority of power industry customers choose manual bypass solutions. The main reasons are as follows:
(1) Generally, the fans of a boiler are configured on both sides. When the frequency converter on one side fails, the DCS will automatically speed up the frequency converter on the other side and execute the RB function to ensure the continuity of boiler combustion and prevent sudden fire extinguishing accidents. The boiler allows only one side of the fan to be allowed for a short period of time.
(2) The frequency converter is designed with a speed-up current limiting function. When rapid acceleration is required in a short period of time, the frequency converter will not experience overcurrent due to the drastic increase in load.
(3) When an important auxiliary machine on one side stops working, DCS will automatically execute the RB function, which is in line with the user's operating habits.
(4) Manual bypass cabinets for fan loads meet the requirements of most customers. For sites that choose automatic bypass cabinets, attention should be paid to the coordination and connection between the bypass switching system and the thermal system to avoid sudden switching causing severe disturbances to the system.
5. Conclusion
The choice between manual and automatic switching schemes should be based on the actual site conditions. Generally, we recommend customers choose the manual bypass scheme. After a frequency converter failure, the motor can be switched to mains frequency operation manually via the high-voltage disconnect switch, offering high economy and reliability. Furthermore, compared to the automatic bypass scheme, the primary and secondary circuits of the manual switching scheme are much simpler, making maintenance easier. Automatic bypass schemes should be used selectively. For example, in small-scale thermal power plants, where the boiler fan is configured with single induced draft and single forced draft, if the frequency converter needs to be stopped during operation after the motor frequency conversion, but boiler combustion does not allow the motor to stop, automatic bypass switching is very suitable. For large generator sets, important boiler auxiliary equipment such as induced draft and forced draft fans, and primary air fans are configured on both sides. If one auxiliary equipment trips, the boiler can still maintain combustion, and there is no risk of boiler shutdown in the short term. In this case, the automatic bypass scheme is unnecessary, and manual bypass switching can fully meet the actual production process requirements.
