When UPS and gensets have issues

Uninterruptible power supplies (UPS) are an example of the profound impact electronics have had on the electrical distribution world. Not only do these devices provide instantaneous backup power in the event of a power outage, but they also provide clean power on a continuous basis.

What’s more, to a generator, utility, and other sources of backup power, UPS provides a seamless transition preventing critical loads from experiencing a power interruption of any duration. While performing these tasks, a challenge presents itself in the form of high levels of reflected current harmonics. When these harmonics interact with the typical high reactance of a generator, a potential recipe for disaster is created in the form of lost critical loads—what I call the “harmonics issue.”

Specifically, the issue addressed here is a cyclical phenomenon where the UPS is unable to maintain a transition from battery power to generator power. In such a scenario, the UPS briefly makes this transition only to rapidly and automatically be returned to battery power. It repeatedly attempts to make and maintain this transition only to be thwarted in its efforts. I will discuss how this scenario can be avoided, but the primary focus is on understanding what’s going on. To facilitate this, one should first discuss the following three topics:

• Generator fundamentals

• UPS fundamentals

• Key electrical concepts.

Having established this foundation, we are ready for a step-by-step analysis of the cyclical phenomenon.

Generator fundamentals

A generator is a motor operating in reverse. But whereas a motor receives ac electrical power and converts it into mechanical energy, a generator converts mechanical energy (e.g., from a diesel engine) into electrical power. The generator accomplishes this by turning a set of coils through a magnetic field, thereby inducing current in those coil.

A coil has an impedance consisting primarily of inductance. As this is characteristic of a coil, so too is it characteristic of a motor or generator. A typical generator impedance is 11%. Contrast the generator’s impedance with that of a pad-mounted or substation transformer, which typically have impedances of between 5% and 6%.

UPS fundamentals

This article’s discussion of UPS is limited to the on-line double-conversion type. UPS work by converting ac power to dc power and then inverting it back to ac power to feed the load (see Figure 1). The primary reason for this “double conversion” is to create a dc link via which batteries are able to provide replacement power in the event of a primary power loss. Because the batteries are always connected into the dc bus, there is no power interruption at the output of the UPS when they are called upon to feed the load.

Figure 1 – Uninterruptible power supply systems work by converting ac power to dc power and then inverting it back to ac power to feed the load in order to provide replacement power in case of a power loss.

Let’s take a closer look at the UPS front end—the converter. Three-phase or single-phase ac power is provided to the converter. Integrated circuit switches, either silicon-controlled rectifiers (SCR) or insulated-gate bipolar transistors (IGBT) switch on and off at speeds of 450 Hz to 4 kHz and in a manner that converts the ac voltage to dc voltage.

A byproduct of this switching is current harmonics. The percent of harmonics generated is inversely proportional to switching speed. The phenomenon that is of concern here is most pronounced with the slower SCR-type converters. These originate at the converter and affect everything upstream of it. Consequently, these harmonics are commonly referred to as “reflected harmonics”—reflecting back on and affecting the source of power.

The current harmonic orders that are most significant (in percent of harmonic distortion) depend on the particular design of the converter. For converters with the slowest switching speeds, these would be the fifth and seventh harmonics.

Finally, it should be mentioned that most UPS include a static bypass switch. This is a device that automatically bypasses the UPS in the event of a UPS failure and assures that power from the source will be able to get around the disabled unit. Static switches operate at approximately 4 milliseconds or one-quarter of an electrical cycle. This is sufficiently fast for an electronic load to literally not notice the interruption. When a UPS needs to use its static switch, the UPS itself is considered to have failed. The load it is protecting is one outage away from being lost.

Key electrical concepts

At the top of the list of rudimentary electrical theory is Ohm’s law: voltage is equal to resistance times current (V=RI). Related to this is the relationship during a generator’s transient response to a UPS load and pertaining to harmonics, namely that:

V _n = n x Z ₁ x I _n

where

n = order of the harmonic (i.e., n = 2, 3, 4, etc. for the 2nd, 3rd, 4th, etc. harmonic order)

Z ₁ = impedance of the generator to fundamental frequency current

V _n = nth harmonic voltage

I _n = nth harmonic current

The relationship between the individual orders of voltage harmonics and the total voltage harmonic distortion (V _THD ) is as follows:

V _THD = SQRT[V ₂ 2+ V ₃ 2+ V ₄ 2+… V _N 2+…] x 100/V ₁

Note that V ₁ is the fundamental (60 Hz) voltage. Because generators have negligible resistance and relatively high reactance, one can assume that for all intents and purposes, Z = X. Because we are concerned with what happens the instant a generator responds to the added load of a UPS, we are interested in the reactance during the first cycle of this response which is, by definition, its subtransient reactance or X"d.

Given the above, the engineer may modify the relationship between individual order, voltage, and current harmonics to the following:

V _n = n x X" _d x I _n

Inasmuch as a higher X" _d results in a higher V _n , the aggregate effect, taking all orders of harmonics into consideration, is that as X" _d increases, so too does V _THD (Statement 1).

Understand that, as is the case with all harmonic-generating electronics, UPS generate current harmonics (I _THD )—not V _THD harmonics. Voltage harmonics result from the interaction of current harmonics with the circuits’ impedance, which in this case is predominantly reactance. As can be understood by considering Statement 1, reactance acts as a sort of volume control for V _THD . For any given I _THD , the higher the reactance, the higher the V _THD .

A UPS continually monitors the quality of the power source feeding into it. If one or more of the parameters being monitored does not fall within acceptable limits, the UPS rejects the source and goes to or remains in battery mode. Among these parameters is V _THD .

The automatic transfer switch (ATS) between the generator and UPS acts more or less as a traffic cop between the two primary sources. In the scenario being explored, it sets things in motion, but once having done so, plays no further role.

The harmonics issue

How do these principles and equipment attributes combine to create the cyclical phenomenon under investigation? Many of the events occur instantaneously or nearly so, yet we can break them down by steps as follows:

1. Utility power is lost. Instantaneously, the UPS goes to battery power.

2. The generator receives a start signal from the ATS. Once up to full frequency and voltage, the ATS switches to generator power.

3. The UPS senses acceptable voltage and frequency from the generator and switches internally from battery power back to line (now generator) power. This is the state of things in Figure 2, which shows the schematic of a typical UPS/generator system.

4. Upon making the transition, high harmonic current from the UPS interacts with the high subtransient reactance of the generator. This results in high voltage harmonic distortion which is immediately seen by the UPS monitoring circuitry.

5. The UPS deems VTHD to be at unacceptable levels prompting it to automatically switch back to battery power.

6. Once back on battery power, the voltage from the generator looks good again, and returns to Step 3.

And so, an unending cycle ensues. Of course, it’s not completely unending, as the batteries eventually run out and the UPS goes to static bypass (assuming it has one). Either way, the critical load is left without protection and is likely to be “dumped.”

Figure 2 – The schematic of a typical UPS/generator system shows how the UPS senses acceptable frequency and voltage and switches to generator power.

The occurrences of loads being dumped as a result of this phenomenon have decreased over the years due to improvement in UPS front-end designs, lower generator reactances, and a general awareness that non-linear loading on generators needs to be limited. None of this should result in your lowering your guard against the possibility of this harmonic issue occurring in a system.

Here are some things to consider in any design involving a UPS downstream of a generator:

1. One manufacturer, in its engine and generator sizing publication, recommends that SCR loads be kept to no more than 66% of their generators prime power ratings. This is doubtless a safe threshold designed to keep the designer out of trouble regardless of the UPS used.

2. If circumstances require that you exceed the above threshold, consider a UPS with an IGBT-designed converter in lieu of an SCR type. The faster switching speeds of IGBTs afford a design with significantly lower reflected current harmonics. One manufacturer whose UPS front ends are predominantly of the IGBT variety boasts an ability to match the kVA rating of its UPS to the kW rating of a generator feeding them for many of its models. 

Be aware of the potential problem and work closely with your UPS and generator vendors to assure compatibility under a worst case loading scenario.

Author Information

Shields has nearly 20 years of electrical engineering experience on residential, commercial and, municipal projects.

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