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Automatic Transfer Switches
Basics of Automatic Transfer Switches
An automatic transfer switch (ATS) forms the interface between the gen set, utility power and the consuming electrical equipment. It performs three functions:
It monitors power sources for failure.
It transfers load from one power source to another.
It can and should be used to exercise the gen set.
Proper transfer switch operation is critical to the whole power system. It provides safety to the gen set operator and protects the electrical service and the utility.
This switch must handle overloads for a short period of time while the proper protective relay begins operation.
Many considerations affect ATS selection. The unit must be sized correctly for the loads it serves as well as have the correct protective devices.
Occasionally, gen set applications require paralleling of generator power with the utility power. Utilities require that customer connected equipment (including gen sets) must not cause disturbances or degrade the quality of service to other customers on the line. The customer also cannot degrade the safety interlocks and protection devices on the power network.
Two types of automatic transfer switches
ATSE usually consists of two parts: switch body + controller. Switch bodies can be divided into PC class (integrated) and CB class (circuit breaker).
PC Grade: ATSE can connect, carry but not be used to disconnect short circuit current.
Its main body is the load (isolation) switch, which is a kind of electromechanical switching appliance, the switching mechanism is driven by a motor or excitation.
Class CB: ATSE is equipped with an overcurrent trip whose primary contacts can be connected and used to disconnect short circuit currents. As shown below, the circuit breaker (miniature circuit breaker or molded case circuit breaker) is additionally equipped with a mechanical interlocking device.
The controller is mainly used to detect the working state of the monitored power supply (two circuits). When the monitored power supply failure (such as lack of phase, undervoltage, voltage loss or frequency deviation of any phase), the controller issued action command, the switch body automatically switch from one power supply to another loaded power supply. Figure 1 is a typical ATSE application circuit. The controller is connected with the incoming line end of the switch body.
Four kinds automatic transfer switch
- Contactor;
- Circuit breaker;
- The use of power load switches (in line with GB14048.3 standard) to complete two-way power conversion products;
- Electric special transfer switch, PC grade (integrated) ATSE.
The ATSE product composed of contactor has the advantage of low price; The disadvantage is that the coil is easy to burn for a long time, the connection and breaking ability of the product is low, the contact is easy to shake and fuse, the reliability of the product is very low, especially when the load is switched, it is easy to happen explosion accident. This product has been phased out and banned from use abroad.
To overcome the shortcomings of the above products, circuit breaker manufacturers have introduced CB-class ATSE. The CB class ATSE consists of circuit breakers and mechanical interlocks. The CB grade ATSE is mechanically maintained, and the ability to connect and disconnect contacts is improved, but it is not an ideal ATSE product.
Therefore, a third product appeared on the market - power load switch. This product is composed of two load switches and electric operating devices, greatly improving the reliability of the product operation. However, the contact type arc extinguishing system is designed to break the primary arc, and there are unreasonable factors when it is used for switching the dual power supply circuit.
Therefore, the market needs ideal ATSE products.
The PC-grade (integrated) ATSE product integrates the advantages of the above products and overcomes their disadvantages. It is a more ideal dual power conversion product in the domestic and foreign market.
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