Managing risks, benefits with closed transition transfer switches

Here are best practices for closed transition transfer switches when two live sources are connected together.


Learning objectives:

  1. Understand advantages and risks associated with using closed transition transfer switches in a standby power system.
  2. Recognize all the factors that contribute to breakers tripping during closed transition transfer.
  3. Identify methods to mitigate the risk of tripping breakers during closed transition transfer.  

Closed transition transfer switches are becoming more popular for transferring power for life safety and critical processing loads. The benefits are that the emergency power system can be tested without interrupting power to loads and power can be retransferred to the utility after a failure without interrupting power to loads. However, there are risks associated with closed transition transfer switches as two live sources are connected together.

Problems with closed transition transfer originate from a difference in voltage between the two sources at the instant when the two sources are connected. The difference in voltage can be caused by several factors:

  • A difference in root mean square (RMS) voltage between the sources
  • A phase angle difference between the two sources
  • A transient condition on one of the sources caused by a load switching on or off, or instability of one of the sources.

The instantaneous voltage difference between the sources results in a current surge from the source with the higher voltage to the source with the lower voltage at the instant of the sources’ interconnection. This current is limited only by the impedance of the sources and the current-carrying capacity of the cable or bus connecting them. It is this current surge that can trip breakers or, in more extreme cases, damage equipment.

Recommendations for minimizing risks of out-of-phase closure include:

  • Recognize that all sync check systems allow for sources to be a few degrees out-of-phase at closure, resulting in some level of surge current between the sources. Breakers, transfer switches, and cable must be sized accordingly.
  • Consider active synchronizing with voltage matching to minimize the phase and voltage differences between sources.
  • Minimize transient conditions at the moment of transfer by inhibiting multiple transfer switches from transferring at the same time and preventing other loads from cycling during the transition.
  • Use a transfer switch “fail to disconnect” or maximum parallel timer relay to shunt-trip an upstream breaker to prevent extended paralleling in the event that a transfer switch fails.

Phase difference at closure: How much is too much?

Very rarely will two sources be exactly in sync at the instant that a switch or breaker closes the two sources together, so it is reasonable to ask how far out-of-phase two sources can be and still have a reliable closed transition. IEEE 1547 allows generator sets or systems of paralleled generator sets between 1.5 and 10 MVA to be up to 10 deg out of phase with the utility when closing to the grid, with higher phase difference limits for smaller systems.

Alternators typically can handle connecting to a source that is 10 deg out of phase with it. For other equipment the answer depends on how much surge current the system can handle without tripping breakers or damaging equipment. For high reliability, one must consider the magnitude of current that can flow between the sources at the instant of transfer.

The surge current will be proportional to the voltage difference between the phases divided by the total impedance in the system. Surge current can be modeled as:

Isurge = V diff / Zsystem


V diff = the instantaneous voltage difference between the sources

Zsystem = the total impedance of the system.

Total system impedance is the sum of the subtransient reactance of the alternator, the impedance of the utility transformer, and the impedance of the cable or bus connecting the sources. In many applications where a single standby generator set is backing up the utility, the impedance at the instant of closure will be dominated by the subtransient reactance of the alternator. However, a thorough analysis will include all the sources of impedance in the calculation.

This example uses only the subtransient reactance of a single alternator. Note that in applications with paralleled generators, one must account for the contribution to the current from all the generator sets. This can be done by calculating an equivalent subtransient reactance for the paralleled generators according to the following equation:

Xd"equivalent = 1/(1/ Xd"gen1 + 1/ Xd"gen2 + ...)

Neglecting the reactance of the utility transformer results in a worst-case scenario for calculated surge current. With this assumption, surge current can be modeled as:

Isurge = V diff /Xd" 


Xd" = the subtransient reactance of the alternator, or the equivalent subtransient reactance of paralleled alternators

Figure 1: This is a representation of two voltage sine waves that are 10 deg out-of-phase and the difference between the two waveforms at each point in the cycle. Courtesy: Cummins Power GenerationAssuming that the root mean square (RMS) voltages of the two sources are identical and no other loads are being switched at that moment, then the instantaneous voltage difference between the sources will be a function of the phase difference between the sources at the moment in the cycle at which closure occurs. Figure 1 is a representation of two voltage sine waves that are 10 deg out-of-phase and the difference between the two waveforms at each point in the cycle. The dashed line represents the instantaneous voltage difference between the two sources. This line is also a sine wave at the same frequency as the two sources. The maximum voltage on this line is the worst-case scenario for the differential voltage at the instant the two sources are paralleled.

The equation for the worst-case differential voltage is:

V diff (per unit) = 2* sin(delta/2)


delta = the phase angle difference between the sources in degrees (10 deg in this case). The worst-case voltage in this case is 0.17 per unit (pu).

For example, in a 480 V system with the two sources 10 deg out-of-phase, the worst-case instantaneous voltage between the sources would be 82 V (480*0.17 = 82 V). If these two sources were paralleled, the voltage difference between them at the instant of closing could be as high as 82 V.

Whether that voltage is too high depends on how much current it causes to flow and whether the equipment in the circuit can handle it.

Figure 2: In this alternator data sheet, we see that the alternator has a subtransient reactance of 0.144 per unit based on an alternator kVA rating of 3660 kVA. Courtesy: Cummins Power GenerationConsider a 2.5 MW generator set. To calculate how much current would flow, use the alternator data sheet to determine the kVA rating and subtransient reactance of the alternator. In the data sheet in Figure 2, we see that the alternator has a subtransient reactance of 0.144 pu based on an alternator kVA rating of 3660 kVA.

Current resulting from the 0.17 pu difference in voltage is given by:

Isurge = V diff /Xd"


V diff = 0.17 and Xd" = 0.144 the surge current Isurge = 1.2 pu.

To convert the per unit current to amps, use the following:

Iamps = Ipu * alternator kVA rating/(√(3) * 480) = 5329 amps (RMS)

Note that in a thorough analysis, the reactance of the transformer would be added to the subtransient reactance of the generator or the equivalent subtransient reactance of paralleled generators. When adding per-unit quantities, the per-unit values for the alternators and transformer must be based on the same base kVA rating.

Whether this level of surge current can damage equipment or trip a breaker depends on the equipment through which the current is flowing. Circuit breakers typically have their instantaneous trip current set to 7 to 10 times the long time-trip setting. The surge will last only for one or two cycles, so as long as the level of surge current is not in the instantaneous trip range of the breaker, the breaker will not trip. Keep in mind that if the breaker is a current-limiting breaker designed to trip in the first half cycle of a fault, this will have to be considered.

A transfer switch that is listed to UL 1008 can withstand a repeated overload of 6 times rated current and maintain that current for 10 electrical cycles (167 msec) on each iteration, and continue to function at rated load after the test.

In the example above, if the load is transferred by a 2000-amp transfer switch protected by 2000-amp breakers, the maximum current surge is less than three times the long time-trip rating of the breaker and less than three times the full load current rating of the transfer switch. This will not cause a problem for the switch or the breakers. However, if this load were being transferred by a 400-amp transfer switch protected by 400-amp breakers located lower in the system, there is now a chance that the current surge will trip one of the breakers.

Surge current generated by an instantaneous phase difference between sources at transfer must be considered in the design of the system. Equipment must be sized to handle the surge current. Where this is not practical, open transition switches should be used. Loads that cannot tolerate a momentary interruption in service should be fed by an uninterruptible power supply (UPS).

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