Transfer Switch Panels

The subject we will talk about today is Transfer Switch Panels. When an emergency generator is installed in order to provide power in case of a failure, a transfer switch is needed. It is designed to detect when there is a power failure, and then sends a signal to the generator for it to start, switch the load from utility to the generator and finally return the loade to utility when it has been re-established. In the image we can see a diagram of a Transfer Switch.

Fig. 9.9: Function of a Transfer Switch Panel

9.6.1 Intelligent Transfer Panel: This panel is designed to work with the Automatic Starting Control System in order to form an automatic system in case of failure from the general network. Its installation is very simple: only an interconnection of two control cables between the generator’s control panel and the transfer switch is needed.

The control system has two PLAQUES OF PRINTED CIRCUITS (P.C.I.), control switches, a state panel and automatic switches. The P.C.I. controls all circuits of failure detection in the network and surveillance. The Fuse Panel ATS of the P.C.I. contains all fuses for all functions of the P.C.I and its automatic switches. Both P.C.I.’s are mounted in the back part of the hinged folding door located in the front part of the transfer panel.

State Panel: This is installed in the front part of the door with the main control switch. This state panel has four lights as indicated on Figure 9.10. These are “Available Utility” (amber), “Utility with load” (green), “Generator with load” (red) and “Available Generator” (amber). To test the functioning of these lights, push the Lamp Testing Button located underneath and, if working properly, they will light on.

Controls: The main control switch has three positions:

AUTOMATIC SELECTOR – Normal setting for automatic functioning.

TEST WITHOUT LOAD – To test the generator without connecting charge.

TEST WITH LOAD – To test the generator with connected load. Besides the main control switch on the panel’s face, there is a three position maintenance switch to be used by service staff. In a "normal" position the Panel works normally. The other two positions allow the service staff to connect manually the load to the generator or to the general network. This switch is used when the P.C.I. is damaged.

Pos.     Description

1.            “Available Utility" light

2.            “Utility with Load" light

3.            “Generator with Load" light

4.            “Generator available" light

Fig. 9.10: Setting of the Transfer Switch Panel lights

WARNING:

·        Due to high voltage, the Inside Maintenance Switch should only be used by qualified technical staff.

How it works: The Transfer Switch works as following: When utility provides power to the load, the “Available Utility” and “Utilitywith Load” lights are on. These lights indicate that utility voltage level is above the P.C.I.’s MVS (network voltage adjustment level) and that the load is connected to utility. The MVS is calibrated by the manufacturer to the voltage corresponding to the disconnecting point of the three phases.

When the voltage of any of the utilities three phases falls below the MVS disconnecting level, the “Available Utility” light as well as the “Utility with Load” light are off and the Delay Start Timer is activated. This timer prevents false engine starts due to sudden fluctuations from the network supply. If the network supply has not been re-established by the elapsed delay time, the automatic network switch opens and a start signal is sent to the generator’s control panel.

Once the generator starts and is delivering power, the “Available Generator” light is on and the Transfer Delay Timer (AT) is activated. This timer allows the alternator to be stabilized before the Emergency Automatic Switch is locked. Once the AT Timer has calculated the delay time, and if the generator’s voltage is above the Generator’s Voltage Adjustment level, the emergency automatic switch is closed. This allows the generator to supply power to the load. At this moment the “Generator with load” light will turn on. When the utility supply is re-established above the MVS disconnection level, the “Available Utility” light will turn on and the IMT (Retransfer Delay Timer) will be activated. This timer allows some time to elapse in order to ensure that the utility supply is safe before transferring load again. If at any time the utility network voltage falls below the MVS during this period of time, the IMT will rearm and will not reactivate until the network voltage is above the MVS.

Comments

[sarkis]

This is the basis or the transfer switch. I have a problem and maybe you can help me. When the power goes the transfer works great. The generator strats and transfers the power the way it should. The problem is once the power comes back, the transfer does not go back to the power source.

What could be wrong?

[sarkis]

This is the basis or the transfer switch. I have a problem and maybe you can help me. When the power goes the transfer works great. The generator strats and transfers the power the way it should. The problem is once the power comes back, the transfer does not go back to the power source.

What could be wrong?

[Sunshine generator]

Thanks for this tips