Continuity of power supply is extremely important in any critical installations. In order to avoid any power outage, users often employ alternate sources such as DG set, UPS or integrated power generation units. This also demand a reliable power transfer scheme that switches from a preferred to an alternate source in the event of a power disruption & return back to the preferred supply when the faults are taken care of.
There are two types of transfer systems. They are:
• Manual Transfer system:
These are generally toggle / knob operated switches or circuit breakers which need to be manually switched on so that the load circuit gets transferred from one power source to the other. The manual transfer switches can be used where power outage happens quite rarely and loss of power does not cause any loss to the appliances or systems used with the electric power supply.
• Automatic Transfer system:
These automatically transfer the power to the load circuit from one power source to the other. Thus, these are more convenient to use as one does not have to manually operate to switch the power source. During normal power interruption, these switching devices will automatically transfer the load circuits to the emergency power source. Once normal power has been restored, the process is automatically reversed. Automatic transfer systems are useful where even a small loss of power can cause a lot of losses in the system. Automatic transfer systems have therefore found their popularity and utility in several industrial and commercial applications where a constant source of power is necessary.
Automatic transfer systems operate in two different methods i.e. open transition and close
transition.
1. Open Transition Transfer
• Break before make switching action. In this, the connection to one power source is opened before the connection to the other source is made and during this process of power transfer, the flow of electricity is interrupted. This change-over time can be adjusted by using different time setting in any voltage sensing controller.
• This is the most popular method used in many installations for automatic power transfer. This system is widely used in applications which can accept a small interruption of power from few msec to few seconds.
• It does not require alternate hot source (like a continuous running DG set or an UPS).
2. Closed Transition Transfer
• Make before break switching action for uninterrupted power transfer. This facilitate a seamless transfer of power supply from one source to other by momentarily paralleling both the sources (<100 msec) during the transfer period. The transfer switch monitors the phase angle difference between the two sources and when it approaches zero degree, the switch operates.
• This system is used primarily in critical installations like Hospitals, Data Centre etc where even momentary power interruption is not acceptable.
• However, this system necessarily requires alternate hot source (like a continuous running DG set or an UPS) all the time.
While the closed transition method is the best to ensure no interruption of power at all, open
transition method is more popularly used due to following reasons:
1. Most power transfer application accepts a momentary interruption in the order of 60 msec to 5 seconds.
2. Non-availability of hot sources in most applications.
3. Very high prices of close transition auto transfer switches.
4. Multiple choices available to the user for open transition power transfer & protection with a combinations of conventional switching, sensing & control devices
5. Ease of maintenance
A typical open transition auto transfer system involves:
1. Two 4 pole, mechanically and/or electrically interlocked power switching devices which can be remotely operated.
2. Voltage and/or frequency sensing accessories or controller.
3. Back up protection devices like circuit breakers or fuses in case the power switching devices have only switching capability.
As mentioned earlier, the key elements in any source transfer systems are:
1. Sensing & control
2. Switching & protection
3. Interlocking
Sensing & Control
For any ASTS, it is important to monitor the source voltage to decide on which source needs to be in service & a control system to ensure the correct logic is in place to get the most optimized power.
The different options used for this are:
• Use of Under voltage release in circuit breaker to monitor the source voltages & enable a control logic with auxiliary & trip alarm contacts
• Simple controller with separate voltage sensor, contactors, timers, logic & interlocking control circuit
• High end digital auto transfer controller with in-built voltage, frequency sensor & a complete logic controller for all power transfer control, interlocking features, multiple setting for voltage & time, digital display, communication etc.
Switching & Protection
ASTS necessarily needs two separate 4 pole switching devices suitable to offer complete isolation in OFF state. Depending on the application & installation requirement, they must have on-load or offload switching duty. In addition to the switching device, it must have the necessary protections available against any abnormal condition. The switching & protection functions can be combined into one device e.g. Air circuit breakers & Moulded Case circuit breakers. In case the switching devices like contactors, switch-disconnectors etc, separate upstream protection devices like circuit breakers or HRC fuses must be provided.
Interlocking
One of the key and a must safety feature for any open transition ASTS is to ensure that under no circumstances, both the sources will get switched on together even momentarily. Hence, reliable and failsafe mechanisms must be incorporated to ensure that the two switching devices are fully interlocked so that only one device can be closed at any point of time.
Interlocking of the two switching device can be done by following means:
1. Mechanical interlock - This is the most reliable method of interlocking. This can be done through suitable interlocking mechanisms like base plate, clutch wire or see-saw toggle interlocks.
2. Electrical Interlocking - This is generally used in addition to the mechanical interlocks. It electrically interlocks the two switching devices like circuit breakers, contactors etc and can be logically programmed to operating sequence, time delay etc. This can be done by using:
a. a combination of under voltage release with Auxiliary contacts for circuit breakers
b. Using an external controller & suitably wiring it
c. Using the NO & NC contacts with the coil in case of contactors
3. Self interlocked mechanism – This is generally adopted in the change-over SDs or Auto Transfer switches. The basic mechanism of SDs will not permit closure of both switches together.
Keeping all the above requirements of ASTS, there can be multiple combinations which can be selected. The selection of transfer system for specific installations can be optimized by keeping following parameters in mind:
1. Feeder Ratings
2. Application need in terms of maximum acceptable change-over time
3. Desired features in terms of sensing & interlocking
4. Specific safety considerations
5. Panel space
6. Life expectancy
7. Cost