This month in EC&M's forum for your power quality questions, Mark McGranaghan, vice president of consulting services, teams up with several other experts from EPRI-PEAC to address the topics of transformer inrush current, grounding of UPS systems, earth leakage relays, and centrifugal blowers.
Q.
One of our customers has experienced tripping of an upstream breaker when turning on a specific machine. The breaker is rated 250A, 3-pole. We suspected that a high inrush current condition on a transformer caused the tripping. We measured a nominal inrush current of 80A. Is there an inrush current limiter available for such a high current?
McGranaghan and Arshad Mansoor's answer: Transformer inrush current can cause breakers to trip. The problem can be particularly severe with smaller transformers, such as control transformers, that can have peak inrush currents 40 times the normal load current. In many cases, this current level is high enough to blow fuses or trip breakers. The Figure shows a peak inrush current of about 230A, which led to fuse blowing on a 1.5kVA, 230V, 1:1 isolation transformer with a nominal current of 6.8A.
There are multiple solutions to combat this problem:
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You can use time delay fuses to avoid nuisance tripping. Because the inrush current is short in duration, the time delay prevents such events.
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You can also use high inrush current breakers, which are designed specifically to prevent inrush current tripping. They're best suited for protecting circuits likely to encounter high-inrush loads as much as 30 times rated current for one-half cycle at 60 Hz. The high-inrush design eliminates nuisance tripping without the need for breaker derating, thereby maintaining tight tolerances for overall circuit protection.
Q.
Should I install a new ground rod to connect the neutral and ground conductors in the secondary side of the UPS that has a primary (input) voltage of 220V (single-phase, 2-wire) and a secondary (output) voltage of 110/220V (single-phase, 3-wire plus ground)? If yes, does this mean the UPS is considered to be a separately derived system all the time?
McGranaghan, Rick Langley, and Doug Dorr's answer: The IEEE Green Book (IEEE 142-1991) has a section titled, “System Grounding for Uninterruptible Power Systems” that addresses UPS grounding design issues. The book features several illustrative examples to assist the user in better understanding the various guidelines associated with proper grounding techniques. The question of whether to bond the neutral and ground on the output of the UPS depends on several variables, especially the following:
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The configuration of the UPS bypass circuit.
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The source supplying power to the UPS.
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The input/output of the UPS.
According to the Green Book, which uses the NEC as guidance, if the “UPS is considered a separately derived source, the neutral of the UPS output module should be bonded to the equipment grounding conductor, and a local grounding electrode module should be installed, per NEC [1], 250-26.”
Your question doesn't provide us with enough information to supply a specific answer because we don't know if the UPS is really a separately derived source. The best way to make this distinction is to take a multimeter and perform a continuity test from input neutral to output neutral. If the neutral is continuous — there is no isolation transformer in the unit — it isn't a separately derived source. So you can't bond the output neutral to ground.
On the other hand, if the continuity test reveals a break in the neutral circuit, it most likely means the UPS is equipped with an isolation transformer. In this case, the UPS is a separately derived source, and you should bond the neutral and ground conductors. You can also bond this connection to building steel and to the building's grounding electrode system. However, there's generally no need for you to use a new ground. If you do use a ground rod, it should be part of the overall building grounding electrode system, which means it must be bonded to building steel at this location.
Q.
In our building, the earth leakage relay operates at different floors at different times during lightning storms and strikes. Also, the same strike does not affect all floors. What could be the cause of this, and how can we rectify it?
McGranaghan's answer: Earth leakage relays detect currents flowing in the ground path. Load currents aren't supposed to flow in the ground path. Any load current flowing in the ground path is an indication of an incorrect neutral-to-ground connection or a connection between a phase conductor and ground, such as faulty wiring. The earth leakage relay is often set at very sensitive levels for safety reasons.
Lightning transients cause momentary ground currents to flow, which can cause nuisance operation of earth leakage relays. This can occur in two different ways:
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Lightning currents can flow in the ground system. The flow of the lightning strike current depends on where the strike occurs. A stroke that actually hits a building will cause currents to flow throughout the building grounding system. Ground conductors are connected to the grounding electrode system throughout the building. There will be numerous opportunities for operation of earth leakage relays.
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Lightning-induced transient voltages could cause current to flow. Lightning strokes rarely hit buildings. However, even remote lightning strokes can cause high transient voltages, since the transients are coupled from the supply system. These transient voltages can cause operation of surge protective devices like surge suppressors located throughout the facility and at equipment locations. These surge suppressors are often connected line-to-ground and will conduct current into the ground system when they operate. If the current is sufficient, it can cause operation of earth leakage relays.
In either case, a slight delay in the operation of earth leakage relays would prevent operation on transient currents such as those associated with lightning strokes. If the cause is operation of surge suppressors, you can opt to apply surge suppression at the service entrance only. This is usually sufficient for protection of loads within the facility.
Q.
One of our customers has a wastewater plant equipped with two 20-hp pumps and three 50-hp centrifugal blowers, all rated at 480V, 3-phase. This system, with no changes or additions, has been working without any problems since it was installed in 1986. But in recent months the customer has experienced intermittent incidents where the third blower motor tries to start when all of the other motors are running. The 120V coils on all of the motor starters also start chattering. This, of course, causes significant problems with all the motors. The three blower motors alternate every time they are called for. It doesn't matter which specific blower motor is the third one to come on when the incident happens. Therefore, it's not a specific motor causing the problem. The process cycled twice while I visited the site, but the problem did not occur. I think the problem is caused by voltage sag, which occurs when the third blower motor goes through its locked rotor at start-up. It then dips the 120V power to the coils low enough to make the coils chatter. It seems to be a global problem with the whole system. I have advised the customer to contact the local electric utility and ask them to connect an analyzer on the 480V power for a week to record any sags. If chattering happens during this week, we may be able to correlate it with a sag disturbance.
McGranaghan's answer: I agree with the recommendation that you already provided for the customer. Measurement of the actual voltage sag associated with the motor starting events should help explain why the starter coils are chattering.
It's strange that the problem only occurs when the last of the three blower motors is starting, but perhaps this is because the voltage prior to switching is somewhat lower for this case since the other motors are already running. Make sure that the measurement equipment can give you some good voltage trends over the entire monitoring period, and not just during the disturbances.
It's also interesting that this has only recently become a problem. Here are some possible reasons for the change:
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Changes in the supply system could lower the voltage from the supply. This could be a problem with a voltage regulator or capacitor controls on the circuit.
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The addition of loads to the system could lower voltages within the facility.
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System changes in the supply could result in a higher source impedance. The step-down transformer dominates the source impedance, so I wouldn't expect changes in the supply to be a major factor.
Finally, take some measurements to explain what's happening on the system.
