Coordinating Typical Utility Performance with Automatic Transfer Switch Time Delays and Devices

Any emergency power system designed to be automatic must use switching controls capable of monitoring utility voltage, monitoring generator voltage, controlling one or more contactor assemblies, and communicating a run signal to the separate control assembly of an engine generator set. All major US manufacturers of automatic transfer switches including Onan, ASCO, Kohler, Zenith, Russelectric, Generac, Magnatek, Westinghouse, Lakeshore, and Square "D" incorporate strikingly similar logic systems. With all the similarities, operational considerations such as voltage sensor and time delay adjustments, sequence of operation, and troubleshooting are simplified. In this page I will discuss features common to most automatic transfer switches as well as typical commercial power company practices and equipment.

Commercial Power

Commercial electric power companies, also referred to as utility companies, share a great many similarities all across the continental United States. All are connected to the national electric grid. This allows the companies to import or export electrical power instantly as the situation demands. Should a power shortage occur in Florida because of a local generating plant problem, the deficit may be instantly corrected by a power plant in New York with excess capacity to sell. This interconnected system requires a high degree of standardization among its subscribers including voltage, frequency, and circuit protection.

Standard frequency in the US is 60 HZ
Common delivery voltages are:
120/240 single phase
120/240 three phase delta
120/208 three phase wye
277/480 three phase wye
Common protection devices include:
Re-closers
Circuit breakers
Fuses
Transformers and capacitors are used to change and condition voltages as well as correct power factor

When local electric power problems arise, the utility will exercise one of many options. Since it has to purchase energy to make up a shortfall, it may try to stretch its available power. The utility may use a rolling brown-out during which voltage is reduced over rotating sections of customer base. It may use a stationary brown-out, or it may opt for complete outages. All of these affect emergency power systems.

Automatic Transfer Switch Sequence of Operation

Typically, during a power outage or brown-out, the utility side undervoltage sensor will sense a loss of power. This will drop out a normally energized relay to begin the following sequence of events:

The time delay on start will begin timing. After 3 to 15 seconds, this time delay will close a contact that sends a run signal to the control panel of an engine generator set.
The engine will start. The engine will be controlled by its own separate automatic control system.
After the engine starts, the generator will produce electricity.
When the electricity produced by the generator is sensed by the emergency side voltage sensor, the time delay for transfer from normal (utility) to emergency will begin.
The time delay for transfer to emergency may be set for up to 180 seconds.
After the time delay for transfer to emergency times out, a signal is given to the transfer relay to initiate a transfer.
The transfer switch main contactor will disconnect from the normal source and move to a neutral position. (If the automatic transfer switch is not equipped with a time delay neutral, the contactor continues straight through to the emergency source.)
The time delay neutral or programmed transition is engaged once the main contactor moves to the neutral position. This time delay may be adjusted for up to 60 seconds.
After the time delay neutral times out, the main contactor moves on to the emergency position.
The system will remain in the emergency mode until the utility restores or the engine generator set fails.

When the utility side undervoltage sensor senses that normal utility voltage is restored, it will pick up a relay which will begin the following sequence of events:

The time delay, on transfer from emergency to normal, will begin timing. This may be adjustable up to 30 minutes.
When the time delay from emergency to normal times out, the main contactor will disconnect from the emergency source and move to the neutral position. (If the automatic transfer switch is not equipped with a time delay neutral, the contactor continues straight through to the normal source.)
Again, the time delay neutral is engaged, creating a pause in the neutral position.
After the time delay neutral times out, the main contactor moves on to the normal source.
After the contactor switches the load back to utility, the stop or cool down timer begins timing. This will keep the engine generator set running for up to 15 minutes without load.
After the stop timer times out, the run signal to the engine generator set control panel is discontinued and the engine will stop.

Controls

Utility Undervoltage Sensor: The utility undervoltage sensor monitors the normal utility voltage line to line. It is connected either directly or through a utility interface board so that it measures A to Be (for a single phase) or A to B, B to C, and C to A (for three phase voltage). The utility undervoltage sensor is usually set so that significane voltage drop in any one phase will initiate the run sequence.; The undervoltage sensor will be adjustable for both dropout and pickup with a voltage differential between the two. This differential is important because utility voltage frequently varies. If the differential is too close, the system may cycle unneccesarily.; For example: Let's say the nominal utility voltage to be monitored is 240 VAC, the dropout voltage is set for 220 VAC, and the pickup voltage is set for 230 VAC. If the utility drops to just below 220 VAC because of a heavy load or brownout on the utility before the lines reach your facility, and the generator is brought on line, the utility will have to restore all the way back up to 230 VAC before the voltage sensor will pick up. This has a stabilizing effect. It allows the generator to carry the load and avoid oscillations back to the utility.
Utility Overvoltage Sensor: As an option, some automatic transfer switches use a utility overvoltage sensor. This sensor monitors the commercial power for sustained overvoltage conditions. If it detects voltage above the user's preset parameters, it will begin the normal sequence of operation. The emergency power system will remain on line until the overvoltage condition is corrected.
Generator Voltage Sensor: This sensor monitors the generator output. It is designed to allow a transfer to emergency only after the generator voltage has reached the appropriate level and become stable. It may also have dropout and pickup adjustments. It may be identical to the utility voltage sensor.
Time Delay on Start: The time delay on start is intended to hold off the beginning of the transter sequence until after the utility has had a chance to stabilize by itself. Often a momentary outage will occur and the utility will restore within a few seconds. When this happens, it may be desirable to prevent the generator from coming on line unless it is really needed.; Time delay on start controls may be adjustable up to 15 seconds.
Time Delay on Transfer to Emergency: The time delay on transfer to emergency is usually adjustable up to 180 seconds. Its purpose is to allow the engine to run for a while before assuming the load. This may be considered a warm-up period. It also allows the utility additional time to stabilize.; With many automatic transfer switches, if the utility voltage restores before th transfer to emergency occurs, the system will not transfer to emergency. Instead, it will go immediately to the stop timer, run for that time setting, and shut down without having assumed the load.

Commercial power systems use re-closers as well as breakers.

Public utilities generate and transmit electrical power at high voltages. They then use transformers to reduce the voltages to levels usable to industry and homes. Like any electrical distribution system, these high-voltage systems must have a method for circuit protection. High voltage allows a different approach.
When a fault occurs on a high-voltage system, it is often a squirrel or tree branch touching the line. High-voltage lines are kept secured from people, pets, and livestock, so when a fault occurs the utility tries to "burn" it off. This is accomplished with an automatically closing breaker, or "re-closer." This device will open when a fault happens on a protected line and remain open for a preset time period, after which it will close again. The cycle may be repeated up to ten times. The object is to use the high voltage to burn away the branch or animal causing the fault. Typically, the re-closer is set to stay open for about ten seconds, cycle three times, then stay open if the fault has not cleared.
The action of a re-closer may interfere with timing in the automatic transfer switch. For example:

A fault occurs on the commercial power.
The re-closer opens, causing failure of the utility power.
In the automatic transfer switch, the utility coltage sensor drops out and the time delay on start begins to time.
The time delay on start times out and the engine generator set starts up.
Power from the engine generator set becomes available and the generator voltage sensor picks up. This starts the time delay to emergency timing.
The re-closer closes after 10 seconds to try to clear the fault; it cannot, so after 3 seconds it again opens the utility's circuit.
The time delay on transfer continues to time.
The re-closer closes to again try to clear the fault. This time the effort is successful and commercial power is restored.
With commercial power back on, the utility voltage sensor picks up. This stops the time delay on transfer to emergency and starts the time delay on stop.
After approximately 5 minutes, the time delay on stop times out and the signal to the engine generator set is discontinued. The engine stops.

Time Delay Neutral: With many automatic transfer switches it is possible to pause the action of the main contactor, for up to 60 seconds, in the neutral position. This is desirable in applications involving large motors, compressors (such as air conditioners), rectifiers, UPS systems, and transformers.; Many automatic transfer switches will transfer from one source to the other in less than 250 milliseconds. A pause allows for a smooth transition. When a pause is not allowed, the rapid transfer from one hot source to the other hot soure may have the following effects:; Air conditioners, compressors, mills, or other motors working under a heavy load with low enertia tend to stall when the contactor disconnects. When the transfer completes, the motor has difficulty restarting unless the load is removed. Often, these devices use a time delay relay to allow compressor pressures to equalize before restarting or motor contactors which fall out with the loss of voltage and must be manually reset.; The speed of transfer, however, defeats the built-in safety devices. The transfer may be so fast that the contactor or time delay relay fails to fall out before the transfer is complete. In this event, the motor may stall or the engine generator set may fail to perform adequately due to the extreme overload.; Large motors driving heavy inertia loads develop regenerative voltage when disconnected from a power source. This happens whent he driven equipment continues to turn the motor by means of inertia. The motor actually becomes a generator until it is allowed to slow down. A pause in neutral will allow the regenerative voltage to decay before the connection to the secondary source is made.; Rectifiers, UPS systems, and transformers, as an integral part of their design, use a large coil of wire wrapped around an iron core. When such a core is energized, an electrical field develops. When the circuit is opened and the electrical field is allowed to collapse, a high-voltage spike will develop. This spike must be routed to ground ot be allowed to dissipate if damage is to be prevented in equipment connected to the circuit. A pause in neutral allows the spike to dissipate. Whent he transfer is made without pause, damage to sensitive driven equipment may occur.
Time Delay from Emergency to Normal: When the utility side voltage sensor senses the return of commercial power, the time delay on transfer from emergency to normal will begin timing. This time delay is adjustable up to 30 minutes. When commercial power restores, it often takes some time for it to stabilize. This time delay keeps the load on the generator power until the commercial power has been at the appropriate level for the duration of the time delay setting.
Time Delay on Stop: When an engine runs under load, it builds up heat within the metal of the block, manifolds, and cylinder head. This head is dissipated through the cooling system as long as the engine runs, but when the engine stops, the cooling system stops as well. If excess heat has built up due to heavy load, it may cause damage to the engine's systems or it may evacuate coolant. This time delay allows the engine to run without load for up to 15 minutes. The extended run allows the cooling system to remove the excess heat and thus minumize the prospect of engine damage.; Note: Kohler puts its time delay on stop in the engine generator control panel rather than the automatic transfer switch.
In-phase Monitor: Some automatic transfer switches have an in-phase monitor as an option. The object of this devices is to overlap the phase rotation of he commercial source with the emergency source. This is an awkward application in that it is only appropriate where regenerative voltage is possible. Since only large motors with high-inertia loads and possibly elevators are likely to generate regenerative voltage, applications for in-phase monitors are limited. However, we find them installed in many applications. I can only assume the design engineers or owners failed to understand the function and application of in-phase monitors.; When inappropriately applied, an in-phase monitor can cause lots of trouble. It monitors the phase angle of both sources, and if improperly adjusted, will prevent a transfer. I recommend that these be taken out of the control circuit if not needed for a specific purpose.
Minimum Run Timer: Generac uses a minimum run timer in its automatic transfer switch. This timer does not affect the transfer operation. It may be set for up to 30 minutes. Its sole function is to keep the engine generator set running for a preset period of time once the automatic control issued a run signal. I recommend this be set for five minutes.

Generator Services, Inc. Recommended Timer Settings:

              Time Delay on Start              5-10   Seconds
              Time Delay Normal to Emergency   30-60  Seconds 
              Time Delay Neutral               5      Seconds 
              Time Delay Emergency to Normal   15     Minutes
              Time Delay on Stop               5      Minutes
              Minimum Run Timer              5      Minutes

Note: These recommendations are for most applications. Health care, communications, FCC, FAA, EMS, or other special applications may have different requirements.

Automatic Exerciser Clock: An automatic transfer switch is usually equipped with an exerciser clock as an option. Many different clocks are used but they generally function the same way. In the "with load" mode, the clock should engage all the time delays except possibly the time delay on start. It will often accomplish this by failing the utility undervoltage sensor. In the "without load" mode it should only run the engine generator set without transferring the load.; The clock should be a seven or fourteen day repeatable timer with multiple settings for the exercise period. I prefer to set the clock for a 15-minute run "with load." This will engage the time delay on emergency to normal (15 minutes) and the time delay on stop (5 minutes) to give the unit a 35-minute total exercise run time.; If the utility fails during an exercise cycle, the automatic controls will take over to serve the load. If the exerciser clock is set in the "without load" mode, it will transfer during an outage. The "without load" mode only affects the exercise cycle.
Test Switch: Some automatic transfer switches have a test switch. This may be a toggle switch, push button, or key-type switch. The "with load" "without load" controls apply, just like with the exerciser clock mentioned above.; The automatic transfer switch may be placed in test "with load" mode in the event a storm or other potential utility interruption is expected. This will put the emergency power system on line. It may remain on line for the duration of the event. When the emergency power system is no longer needed, the test switch may be returned to normal.

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