In a prior Tech Brief, ASCO Power Technologies described the roles of essential time delays invoked during a load transfer cycle between two power sources. The following sections describe elements of those delays and how they are used to avoid nuisance transfers in backup power systems.
Avoiding Unnecessary Transfers
When deciding whether to transfer power, it is important to avoid switching activity that may actually be unnecessary. That is because transfers can result in (depending on power system configuration) momentary power disruptions that affect sensitive loads and their operations. Nuisance transfers also result in unnecessary wear-and-tear on electrical equipment and engine-generator sets. Avoiding nuisance switching is the best way to avoid these effects.
Assessing Stability of a Normal Power Source
Automatic Transfer Switches (ATSs) continually monitor their connected power source to verify acceptable voltage and frequency. If an ATS controller detects an absence of voltage on the normal source, it initiates a series of operations to transfer electrical load to the emergency power source. (For more information, review the ASCO document entitled Basic Control of Automatic Transfer Switches.)
Several phenomena can trigger unnecessary transfer. For example, a momentary drop in voltage could result from a fault current or a lightning strike. When a large fault occurs, line voltage can drop. If voltage drops below a preset threshold, an ATS controller could initiate a transfer. However, if an overcurrent protection device clears the fault before transfer occurs, the transfer becomes unnecessary.
Likewise, if a lightning strike produces an instant voltage fluctuation that includes a decrease to a level below a pre-set threshold, the transfer again becomes unnecessary if voltage recovers in a matter of milliseconds. Consequently, applying a delay allows a controller to monitor the persistence of the condition – if a voltage drop or frequency aberration is persistent, then the need to transfer load to an alternate power source is confirmed.
Two Elements of Transfer Delays
Two elements are required to invoke a delay: (1) a triggering value, and (2) a set time interval. Their relationship is described in the following scenarios.
In the first example, voltage is the triggering value. The system designers and managers desire to transfer to an emergency power when voltage on the normal source drops below 75 percent of the nominal circuit voltage. On retransfer to the normal source, a higher pickup voltage value of 90 percent is selected to avoid nuisance transfer to a potentially unstable normal source.
In addition, designers may determine that the facility and its operations can tolerate voltage fluctuations in these ranges for up to x seconds without undue impact in equipment and business operations. A time delay of x seconds is assigned to delays for normal-to-emergency and emergency-to-normal transfers.
Avoiding Unnecessary Transfers
When deciding whether to transfer power, it is important to avoid switching activity that may actually be unnecessary. That is because transfers can result in (depending on power system configuration) momentary power disruptions that affect sensitive loads and their operations. Nuisance transfers also result in unnecessary wear-and-tear on electrical equipment and engine-generator sets. Avoiding nuisance switching is the best way to avoid these effects.
Assessing Stability of a Normal Power Source
Automatic Transfer Switches (ATSs) continually monitor their connected power source to verify acceptable voltage and frequency. If an ATS controller detects an absence of voltage on the normal source, it initiates a series of operations to transfer electrical load to the emergency power source. (For more information, review the ASCO document entitled Basic Control of Automatic Transfer Switches.)
Several phenomena can trigger unnecessary transfer. For example, a momentary drop in voltage could result from a fault current or a lightning strike. When a large fault occurs, line voltage can drop. If voltage drops below a preset threshold, an ATS controller could initiate a transfer. However, if an overcurrent protection device clears the fault before transfer occurs, the transfer becomes unnecessary.
Likewise, if a lightning strike produces an instant voltage fluctuation that includes a decrease to a level below a pre-set threshold, the transfer again becomes unnecessary if voltage recovers in a matter of milliseconds. Consequently, applying a delay allows a controller to monitor the persistence of the condition – if a voltage drop or frequency aberration is persistent, then the need to transfer load to an alternate power source is confirmed.
Two Elements of Transfer Delays
Two elements are required to invoke a delay: (1) a triggering value, and (2) a set time interval. Their relationship is described in the following scenarios.
In the first example, voltage is the triggering value. The system designers and managers desire to transfer to an emergency power when voltage on the normal source drops below 75 percent of the nominal circuit voltage. On retransfer to the normal source, a higher pickup voltage value of 90 percent is selected to avoid nuisance transfer to a potentially unstable normal source.
In addition, designers may determine that the facility and its operations can tolerate voltage fluctuations in these ranges for up to x seconds without undue impact in equipment and business operations. A time delay of x seconds is assigned to delays for normal-to-emergency and emergency-to-normal transfers.
