Managing risks, benefits with closed transition transfer switches


Passive synchronizing systems

Closed transition transfer switches have successfully used passive synchronizing systems in many applications. Transfer switches use a sync check function for initiating closure to the oncoming source when the two sources are in phase. There are two basic algorithms used by sync check systems: a permissive window algorithm and a predictive algorithm.

A permissive window algorithm is commonly used in both active and passive synchronizing systems. The sync check system measures the voltage, frequency, and phase difference between the two sources. When the three parameters are within some predefined limits, the sources are said to be within a “permissive window.” When the sources have been in the permissive window for some preset period of time, the controller closes to the oncoming source. The required time in the permissive window is typically set to 0.1 to 0.2 seconds for passive synchronizing systems and 0.5 seconds for active synchronizing systems.

A predictive algorithm operates similar to a sync check system except that rather than waiting for the two sources to be in a permissive window for some period of time, it measures the rate of change of the phase angle difference between the two sources and calculates an “optimum phase angle” at which to initiate closure so that at the instant the switch closes, the two sources are as close to in-phase as possible.

Both types of algorithms have been used successfully. Generally speaking, the permissive window algorithm is more robust because the predictive algorithm is susceptible to transients on the voltage sources, which could skew the calculation of the optimum phase angle.

In many applications a slight frequency difference known as a “slip frequency” is imposed between the sources to make sure that they will come into sync with each other at a controlled rate. A slip frequency of 0.1 Hz has been used effectively.

Active synchronizing

Figure 3: A generator waveform is coming into phase with a utility waveform using an active synchronizer with voltage matching. Courtesy: Cummins Power GenerationActive synchronizing is the process of adjusting the generators’ engine governor to bring the waveform into phase with the utility waveform. Many synchronizing systems also include a voltage matching function, in which the generator sets will adjust the voltage regulator to drive the generator voltage level to match the utility voltage level. The voltage matching function is important in applications where the voltage on the utility transformer varies with load.

Figure 3 represents a generator waveform coming into phase with a utility waveform using an active synchronizer with voltage matching. Note that the utility waveform is constant and the synchronizer drives the generator set waveform in to sync with the utility. The voltage matching function forces the generator voltage to be at the same level as the utility voltage.

The synchronizer will hold the generator in sync with the utility until the synchronizer is turned off, unless a sudden load change causes a frequency change. Load changes on a system bus cause a sudden change in phase angle difference as frequency surges or sags in response to the load transient. This can cause the two sources to momentarily be out of sync until the synchronizer forces them back into synchronization. This is why for systems with multiple closed transition transfer switches, best practice is to allow only one switch to transfer at a time. With an active-phase lock-loop synchronizer, the time to synchronize is relatively short and reliable, so timing between switch operations need not be long.

Transfer and retransfer inhibit

It is common for changes in load on a generator set to cause sudden changes in the voltage and in the phase angle relationship between two sources that have been synchronized. For this reason the possibility of load transients at the moment of transfer should be minimized. For systems with multiple transfer switches, best practice is to allow only one switch to transfer at a time. This can best be achieved either by staggering transfer time delays or by using the transfer and retransfer inhibit functions.

The inhibit functions are used to prevent transfer to either the emergency source (transfer inhibit) or the normal source (retransfer inhibit). When transferring loads with closed transition transfer switches, only one transfer switch should be allowed to transfer at any given time.

The inhibit function can be controlled by a master control used in conjunction with a paralleling system. All switches initially are inhibited from transferring and the master releases the inhibit on one switch at a time.

Figure 4: This consists of two closed transition transfer switches, showing that in simple applications, one switch can inhibit another. Courtesy: Cummins Power GenerationIn simple applications one switch can inhibit another. For example, consider the system in Figure 4 consisting of two closed transition transfer switches. The normally closed aux contact from the normal side of the automatic transfer switch (ATS) 1 is wired into the retransfer inhibit input ATS 2. This will inhibit ATS 2 from beginning its retransfer sequence (including all time delays) until after ATS 1 has transferred back to the normal source.

This configuration does create a potential failure mode. If the first switch fails to transfer or if the aux contact fails, the second switch will not transfer without manual intervention. For this reason it is preferable in some cases to use staggered time delays to prevent the switches from transferring at the same time. As these time delays are typically set on-site, it is important to clearly specify the time delays in commissioning documentation.

It should also be noted that the inhibit function is only required when transferring between two live sources. It is not a requirement in the event of a utility failure, so there is no need to be concerned about not getting the emergency source online quickly enough.

Breaker shunt-trip

Many utilities require that closed transition transfer switches provide means to shunt-trip the breaker on the normal (utility) side of the transfer switch if there is a failure of the transfer switch that causes the two sources to remain paralleled in excess of 100 msec. Many transfer switches have a “fail to disconnect” output which can be used for this. It is up to the installer to connect these devices to the shunt trip of the breaker. Some utilities require maximum parallel timer and lockout relays that are separate from the transfer switch control to implement this function.

Regardless of whether this is required by the local utility, it is considered a best practice to use this function to make sure that two sources are not unintentionally paralleled for an extended period of time. Tripping either the normal side or emergency side breaker will provide the same level of equipment protection, although many utilities require tripping the normal side breaker.


Closed transition transfer switches allow for transferring loads without interruption during a test or when returning to the utility after an outage. This is a significant benefit in some applications; however, there are risks associated with closed transition transfer as two live sources are connected together. For loads that are not protected by a UPS, it is worth considering if the value of not having an interruption during a test or a retransfer to the utility justifies the risk of a closed transition transfer.

There are several methods for mitigating the risk of closed transition transfer:

  • Make sure that breakers, transfer switches, and cable are sized to handle the surge current that may result from sources being a few degrees out-of-phase at closure as allowed by the sync check system.
  • Consider using active synchronizing with voltage matching to minimize the phase and voltage differences between sources.
  • Minimize the possibility of 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 a feeder breaker to prevent extended paralleling in the event that a transfer switch fails.

Rich Scroggins is a technical specialist in the application engineering group at Cummins Power Generation. Scroggins has been with Cummins for 18 years in a variety of engineering and product management roles. He has led product development and application work with transfer switches, switchgear controls, and networking and remote monitoring products, and has developed and conducted seminars and sales and service training internationally on several products. Rich received his BSEE from the University of Minnesota and an MBA from the University of St. Thomas.

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