Comparison between manual bypass and automatic bypass of frequency converter

With the increasing use of frequency converters, engineers and technicians have found that when the frequency converter fails and needs to be repaired, the motor has to stop, which cannot meet the requirements of continuous production on site. For this reason, engineers and technicians have added a set of switching devices between the frequency converter and the motor and busbar to meet the requirements of continuous operation of the motor. Depending on the switching switch, the frequency converter's bypass scheme is divided into manual bypass mode and automatic bypass mode. The following are introduced as follows:

2 Introduction of two bypass methods

2.1 One-to-one manual bypass mode

2.1.1 Basic principles

The one-to-one manual bypass mode is composed of three high-voltage disconnectors qs1, qs2 and qs3, as shown in Figure 1. It is required that qs2 and qs3 cannot be closed at the same time and are mechanically interlocked. During variable frequency operation, qs1 and qs2 are closed and qs3 is disconnected; during power frequency operation, qs3 is closed and qs1 and qs2 are disconnected.

2.1.2 Detailed Introduction

(1) The isolating switches are GN19 series single-throw and double-throw indoor high-voltage isolating switches, with a phase spacing of 210 mm; the three insulated terminals at the incoming line end of the single-throw isolating switch are the three sensors of the high-voltage live display device;

(2) The lighting is a cabinet door type lighting;

(3) The arrester adopts a three-phase combination type;

(4) Additional input and output terminals; power frequency and variable frequency indication.

(5) Standard cabinet dimensions (length × width × height): 1200 mm × 1200 mm × 2320 mm

2.1.3 Advantages and Disadvantages

(1) Advantages

When repairing the inverter, there is an obvious power-off time to ensure personal safety. At the same time, the load can also be manually put into the industrial frequency power grid for operation; the manual bypass can be switched after manual judgment of the fault, which is safer and has low cost.

(2) Disadvantages

The load must be manually intervened when it is switched to industrial frequency operation, which does not meet the requirement that some on-site working conditions cannot shut down the machine.

2.2 One-to-one automatic bypass mode

2.2.1 Basic principles

The one-to-one automatic bypass mode is composed of three high-voltage vacuum switches (vacuum contactors are used when the current is less than 300a, and vacuum circuit breakers are used when the current is greater than 300a) km1, km2 and km3, as shown in Figure 2. It is required that km1 and km2 cannot be closed at the same time as km3, and they are electrically interlocked. When the frequency conversion is running, km1 and km2 are closed, and km3 is disconnected; when the power frequency is running, km3 is closed, and km1 and km2 are disconnected.

2.2.2 Advantages and Disadvantages

(1) Advantages

When a serious fault occurs in the frequency converter, the system can automatically switch to the industrial frequency power grid and disconnect the frequency conversion speed regulation system without shutting down the load, thus meeting the requirement that no shutdown is allowed on site.

(2) Disadvantages

The price is relatively high and the use is complicated. When the motor switches from variable frequency operation to industrial frequency operation, there will generally be no problem with the automatic bypass switch. However, there is one exception, that is, if the inverter stops due to a fault in the motor and its load, bypassing it again may amplify the fault. For example, when the inverter is overcurrent, the inverter itself cannot determine whether it is a problem with itself or an abnormality in the motor (such as stalling, sweeping, inter-turn short circuit, etc.). At this time, directly putting the motor into the power frequency operation of the power grid will cause greater damage. In addition, when the motor is running at a speed lower than the power frequency, if it automatically switches to the power frequency, the motor speed will suddenly increase, and parameters such as the furnace negative pressure and air volume will suddenly change. At this time, the user needs to consider whether it will affect production in a short period of time before taking countermeasures. When manually bypassing, countermeasures can be taken in advance and then the motor can be restarted, and the above problems do not exist.

2.3 Introduction to manual one-to-two bypass mode

The main circuit of the system uses a frequency converter plus a matching manual bypass switch cabinet, dual power supply, and one-to-two control mode. This allows one frequency converter to control the soft start of two motors and control the variable frequency operation of any of the two motors, or to achieve the purpose of operating the motor at the industrial frequency through the bypass cabinet, as shown in Figure 3.

In the figure, qf2 and qf3 are high-voltage switches, qs1, qs2, qs3, qs4, qs5, and qs6 are manual isolating switches of the bypass switch cabinet, and m1 and m2 are motors.

2.3.1 Main circuit working principle

Taking the operation of motor M1 as an example, manually disconnect qs3, close qs1 and qs2, select the "variable frequency operation" position of the inverter, close qf2, and the variable frequency operation of motor M1 can be realized.

Manually disconnect qs1 and qs2, close qs3, close qf2, and the power frequency operation of m1 can be realized. Disconnect qf2 to realize the power frequency shutdown of motor m1.

Disconnect qs1, qs2, qs4 and qs5 to achieve safe maintenance of the inverter.

There is a mechanical interlocking relationship between qs2 and qs3, and between qs5 and qs6; there is an electrical interlocking relationship between qs1 and qs4, and between qs2 and qs5 to prevent short circuit.

That is, when motor m1 is operated with variable frequency using stage i power supply, qs1 and qs2 are closed, and qs3, qs4, and qs5 are interlocked and cannot be closed. When motor m1 is operated with stage i power supply, qs3 is closed, and qs1 and qs2 are interlocked and cannot be closed.

The operating requirements of motor M2 are similar to those of motor M1, and the interlocking and protection are also similar to those of motor M1.

2.3.2 Signal interlock between inverter and high voltage switch

To ensure the safe operation of the high-voltage inverter, the inverter and the high-voltage switch have the following signal interlocks:

(1) The inverter provides each high-voltage switch (qf2, qf3) with a pair of "closing permission" contacts (passive contacts, closing is effective). The high-voltage switch is allowed to close only when the contacts are closed.

(2) The inverter provides each high-voltage switch (qf2, qf3) with a pair of "emergency disconnect" contacts (passive contacts, closed and effective). When the contacts are closed, the high-voltage switch must be disconnected immediately.

(3) The high-voltage switches (qf2, qf3) each provide a pair of "switch closed" contacts (passive contacts, closed and effective) to the inverter for operation of the electromagnetic lock circuit of the high-voltage disconnector.

3 Comparison of the advantages and disadvantages of the two bypass solutions

According to the statistics of nearly 2,000 sets of operating performance of Lead Harvest, most customers choose the manual bypass solution. Compared with the automatic bypass solution, the difference between the two is shown in the attached table.

4 Application in the power industry

Both the manual bypass solution and the automatic bypass solution have a large number of successful application cases on site, but the vast majority of power industry customers choose the manual bypass solution. The main reasons are as follows:

(1) Generally, the fans of boilers are configured on both sides. When the inverter on one side fails, the DCS will automatically speed up the inverter on the other side and execute the RB function, thereby ensuring the continuity of boiler combustion and preventing sudden fire extinguishing accidents. The boiler allows the fan on one side to be turned off for a short period of time.

(2) The inverter is designed with a speed-up current limiting function. When rapid acceleration is required in a short period of time, the inverter will not cause overcurrent due to a sharp increase in load.

(3) When an important auxiliary machine on one side stops operating, 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 severe disturbances in the system caused by sudden switching.

5 Conclusion

For manual and automatic switching schemes, the selection should be made according to the actual situation on site. Generally speaking, the author recommends that customers choose the manual bypass scheme. After the inverter fails, the motor can be switched to industrial frequency operation by manually operating the high-voltage disconnector. Its economy and reliability are very high. In addition, compared with the automatic bypass scheme, the primary and secondary circuits of the manual switching scheme are very simple and easy to repair and maintain. For the automatic bypass scheme, it should be used in a targeted manner. For example, in a small thermal power production unit, the boiler fan is configured as single-induction and single-sending. After the motor frequency conversion transformation, once the inverter needs to be shut down during operation, and the boiler combustion does not allow the motor to stop, it is very suitable to choose automatic bypass switching. For large generator sets, important boiler auxiliary machines such as induced and supplied air fans and primary fans are all configured on both sides. After the auxiliary machine on one side trips, the boiler can still maintain combustion, and there is no risk of stopping the boiler in a short time. At this time, it is unnecessary to choose the automatic bypass scheme. Manual bypass switching can fully meet the actual production process requirements.