Backup Power and Transfer Switches

        The basic standby or backup power system consists of an alternate power source and an isolation device capable of switching between the two power sources to provide the required power to the designated loads. The key in determining these requirements is based on the essential loads.

Planning for a backup power system can be an overwhelming process. The basic standby or backup power system consists of an alternate power source and an isolation device capable of switching between the two power sources to provide the required power to the designated loads. The key in determining these requirements is based on the essential loads.

Important Considerations

Consider the following questions when specifying your backup power needs and don't forget the future electrical requirements. In many instances, facilities have an ever-increasing need for power.

• What are the current electrical requirements?
• Single or three-phase source?
• Powering just "essential" loads or the entire facility?
• What is the voltage?
• Do the loads cycle ON and OFF, or stay ON all the time?
• Can the loads be disconnected for service work?

The answers to these questions will provide information to appropriately size the generator for the application. These answers will also provide the basic data for the transfer switch rating. Many of the major generator manufacturers offer a sizing and specifying software program on their web sites for electrical contractors to download free of charge.

Electrical Faults

A factor in any electrical system is the system's ability to recover from an electrical fault. Faults can be very destructive and possibly cause a fire. The main factor in a fault is the amount of current for a specific amount of time. One of the responsibilities of the systems designer is to size protective devices that coordinate effectively to provide isolation of a local fault, and specify that the devices can handle the "let-through" current over a specified clearing time without destructive failure. Current interrupters can be circuit breakers or fuses. Each of these devices has specific clearing times. The fuses open and clear faults much more quickly than circuit breakers. In addition, not all breakers are created equal. Some are much slower than others. This is important to transfer switches (contactor type) because they must stay closed during a fault condition and must withstand the fault current for the amount of time the interrupter takes to clear the fault.

The switching device must be designed to withstand the heating and physical forces that result from a fault current. If a contactor is designed to withstand a specified amount of current for the longest industry clearing time, then it will be able to withstand a higher fault current for an upstream device that opens more quickly.

The transfer switch market is segmented in "specific breaker" and the "any breaker" Withstand and Closing Ratings (WCR). Being able to identify the specific upstream breaker that falls within the WCR of the transfer switch "specific breaker" rating will result in the most efficient and cost effective system since it will use a power switch best suited for the installation.

Automatic Transfer Switches

Typically, transfer switches involve automatic operation, though a manual transfer switch can be used for a very basic installation. Automatic operation requires some type of drive mechanism to move the power switching contacts from one power source to the other. The drive mechanism may be electronic, solenoid or motor based. A controller monitors the electrical parameters of each power source and causes a transfer to occur at the correct time. There is a spectrum of electrical parameters concerning the term "acceptable source." At one end of the spectrum, sensing may mean only a single-phase type of under-voltage on the utility power source and an under-voltage/under-frequency on the emergency power source. These parameters may be fixed or adjustable from the manufacturer. There are a number of requirements that drive these parameters but, for the most part, are derived from the electrical limitations of the load.

Automatic Transfer Switch

At the other end of the spectrum, the installation and/or load devices may have other critical operating parameters that require a more elaborate sensing system. Such parameters may include over/under-voltage frequency, phase rotation, voltage unbalance, and detection of regenerative voltage on a lost phase. Once the sources are electrically defined, the logic and sequence of operation may incorporate fixed timing for its operation or may have complete adjustability to provide the most efficient process for the specific installation. A basic sequence allows the utility voltage to fluctuate for a short period of time before it is considered an unstable or failed power source and the generator set is started (engine start delay). This delay prevents the engine from starting and stopping unnecessarily. However, a mode of operation may be necessary to detect fluctuations over a period of time and transfer to a more stable generator power source.

After the engine is started, it must stabilize electrically and physically before effectively having load placed on it. That delay in transferring to the standby power source commences after the proper emergency electrical parameters have been established. Additional sequences associated with transferring to the emergency power source may be necessary. When sizing a generator set, voltage and frequency dips from blocked loads should be considered. These dips may be a normal function and tolerated by the load. The logic should be able to ignore these transients for a short period of time. A flexible system may give the user the ability to change these transient delays.

Retransfer Delay

When the utility power returns, it is usually prudent to remain on a stable generator power source for a period of time to ensure that the utility has been stable. This is the retransfer delay. If, after this delay the utility proves stable, the transfer can occur. Voltage transients may be generated by the loads if they are inductive. If the "off" transition of the power switching device is done quickly, the voltage generated may have a potential difference with the power source being connected to and may result in transient surge currents.

These transients may be high enough to trip circuit breakers and disrupt the normal load functioning. It may be possible to increase the transfer "off" time with another delay and allow these transient generators to decay to safe levels prior to connecting to an energized power source. The disadvantage is having no power or "dark" time, which may also negatively affect the process of the load.

Testing the Power System

After the transfer has occurred, all should be back to normal and the generator set can be shut down after having sufficient time to run unloaded and cool off. One might think that this is the extent of the standby power system. But to ensure proper operation, periodic testing is a must. Such testing may be manually performed using a test switch on the transfer switch. This may also be used to preemptively get on standby power to avoid weather related outages.

A test may be automatically performed using a controller-based programmable timer to initiate the sequence. This is called "exercising the generator" and should be performed on a weekly basis. Some exerciser requirements vary throughout a monthly period. Consider one loaded and three unloaded tests during this monthly exercising period.