In the automatic mode, the transfer system must be able . For this purpose, it is necessary for the mode logic to know the condition of the normal and alternate power sources. This is typically accomplished via undervoltage () and negative-sequence voltage () relays, as shown in Figure 1.
Frequency relaying is usually not required for utility sources as the frequency isquite stable, and not affected by changing load conditions within the facility.
Also, at the beginning of the startup cycle of a generator, the frequency is zero and will ramp up to the nominal frequency (typically 60Hz) as part of the startup process. For these reasons, under- (and possibly over-) frequency relays (device 81) are used for generator sources in addition to the under- and negative-sequence voltage relays.
Because overvoltage can be an issue (for example, if the voltage regulator fails or the generator is called upon to absorb an excessive amount of reactive power), are occasionally used as well (not shown in Figure 1).
The pickup and time delay levels for these relays are functions of the system itself and the amount of time that abnormal conditions can be tolerated. For example, the undervoltage relays are typically set to pick up when the voltage level falls to 80% of nominal. The negative sequence voltage relays are typically pre-set to respond to the loss of a single-phase but may be adjustable.
The indication from these relays to the automatic transfer logic is typically in the form of a single binary input, i.e., . Upon receipt of a “source abnormal” signal when operating on utility power, the automatic transfer logic must respond.
Typically, this response is delayed to insure that the abnormality is not a transient condition, in order to prevent an unnecessary transfer. When the system is supplied via utility power this will allow the utility system to attempt clear the fault through reclosing
Once the source failure time delay has expired, the automatic transfer system opens the circuit breaker for the affected source, starting the automatic transfer process.
Once a transfer operation has been initiated and the system has been disconnected from its normal source of supply, a suitable time delay must be given to allow the residual voltage from spinning motors to decay before the system is transferred to the alternate source.
For the example system of Figure 1, a signal is provided to initiate generator starting. In general, this will be a requirement any time one of the sources of power is a generator or generators unless the generator(s) is used for cogeneration aswell as standby power.
Other variations exist, however in general the management of the generator cool-down cycle should remain under the control of the generator(s) control system rather than the automatic transfer system.
After the alternate source is available and the required dead-bus delay has expired, the transfer system must close the alternate-source circuit breaker to supply the system from the alternate source. In the example system of Figure 1, .
For a main-tie-main system as shown in Figure 2
The normal source, when it returns, typically starts a timer in the automatic transfer system. This timer is present so that re-transfer will not occur until the normal source has been shown to be stable.
As its name implies, open-transition re-transfer entails de-energizing the system prior to reenergizing it with the normal source. This requires a dead-bus delay as discussed above. In the example system of Figure 1, the would open, a dead-bus delay would transpire, and would close to complete the re-transfer.
is that it requires the system to take a second outage in order to be restored to the normal source. However, it does not require additional equipment to accomplish.
Closed-transition re-transfer requires the paralleling of the normal and alternate power sources for a brief time period prior to separation from the alternate power source. Where a generator(s) is involved
, in order to keep the heightened exposure of the system which occurs to as brief a time as possible. This heightened exposure results from the elevated fault-current levels that exist with the sources in parallel, and also due to the exposure of the system to supply a fault on the normal source via the alternate source. If both the normal and alternate sources are separate utility services the utility may have restrictions on the ability to perform closed-transition re-transfer.
Unusual conditions can occur during the automatic transfer process. For example, the normal power source could fail, only to be restored during the dead-bus time delay before the alternate source is connected to the system.
In the automatic mode, the transfer system must be able . For this purpose, it is necessary for the mode logic to know the condition of the normal and alternate power sources. This is typically accomplished via undervoltage () and negative-sequence voltage () relays, as shown in Figure 1.
Frequency relaying is usually not required for utility sources as the frequency isquite stable, and not affected by changing load conditions within the facility.
Also, at the beginning of the startup cycle of a generator, the frequency is zero and will ramp up to the nominal frequency (typically 60Hz) as part of the startup process. For these reasons, under- (and possibly over-) frequency relays (device 81) are used for generator sources in addition to the under- and negative-sequence voltage relays.
Because overvoltage can be an issue (for example, if the voltage regulator fails or the generator is called upon to absorb an excessive amount of reactive power), are occasionally used as well (not shown in Figure 1).
The pickup and time delay levels for these relays are functions of the system itself and the amount of time that abnormal conditions can be tolerated. For example, the undervoltage relays are typically set to pick up when the voltage level falls to 80% of nominal. The negative sequence voltage relays are typically pre-set to respond to the loss of a single-phase but may be adjustable.
The indication from these relays to the automatic transfer logic is typically in the form of a single binary input, i.e., . Upon receipt of a “source abnormal” signal when operating on utility power, the automatic transfer logic must respond.
Typically, this response is delayed to insure that the abnormality is not a transient condition, in order to prevent an unnecessary transfer. When the system is supplied via utility power this will allow the utility system to attempt clear the fault through reclosing
Once the source failure time delay has expired, the automatic transfer system opens the circuit breaker for the affected source, starting the automatic transfer process.
Once a transfer operation has been initiated and the system has been disconnected from its normal source of supply, a suitable time delay must be given to allow the residual voltage from spinning motors to decay before the system is transferred to the alternate source.
For the example system of Figure 1, a signal is provided to initiate generator starting. In general, this will be a requirement any time one of the sources of power is a generator or generators unless the generator(s) is used for cogeneration aswell as standby power.
Other variations exist, however in general the management of the generator cool-down cycle should remain under the control of the generator(s) control system rather than the automatic transfer system.
After the alternate source is available and the required dead-bus delay has expired, the transfer system must close the alternate-source circuit breaker to supply the system from the alternate source. In the example system of Figure 1, .
For a main-tie-main system as shown in Figure 2
The normal source, when it returns, typically starts a timer in the automatic transfer system. This timer is present so that re-transfer will not occur until the normal source has been shown to be stable.
As its name implies, open-transition re-transfer entails de-energizing the system prior to reenergizing it with the normal source. This requires a dead-bus delay as discussed above. In the example system of Figure 1, the would open, a dead-bus delay would transpire, and would close to complete the re-transfer.
is that it requires the system to take a second outage in order to be restored to the normal source. However, it does not require additional equipment to accomplish.
Closed-transition re-transfer requires the paralleling of the normal and alternate power sources for a brief time period prior to separation from the alternate power source. Where a generator(s) is involved
, in order to keep the heightened exposure of the system which occurs to as brief a time as possible. This heightened exposure results from the elevated fault-current levels that exist with the sources in parallel, and also due to the exposure of the system to supply a fault on the normal source via the alternate source. If both the normal and alternate sources are separate utility services the utility may have restrictions on the ability to perform closed-transition re-transfer.
Unusual conditions can occur during the automatic transfer process. For example, the normal power source could fail, only to be restored during the dead-bus time delay before the alternate source is connected to the system.
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