By Denzil Merrill, Vice President, Standard Three Phase Products, MGE UPS SYSTEMS, INC.

Many claim s have been made about how one UPS technology may be superior over another when it come s to generator compatibility. To under stand the integrity of the se claim s, it i s nece s sary to define what characteri stic s are important for generator compatibility and how the variou s technologie s operate. Thi s will ultimately reveal the mo st favorable or “generator friendly” technology – not ju st ba sed on compatibility, but al so from a complete sy stem reliability standpoint.

Causes of Generator Incompatibility

THD (Total Harmonic Di stortion): The fir st characteri stic of a load that influence s generator performance i s the THD reflected back by the load (a UPS in thi s case) onto the generator. The part of the UPS that the generator see s a s the load i s the rectifier.

Two different rectifier technologies are currently used in the market today dealing with true double conversation design of a UPS system. It is important to understand each design in order to apply the correct technology to the conditions of each site. The two technologies are: designs using an SCR rectifier and designs using IGBTs as the switching device of the rectifier.

SCR Based Designs: Most large UPSs (100 kVA and above) today u se SCR-ba sed rectifier s, which can reflect around 40% THD on six pul se rectifier s and 14% on 12 pul se rectifier s. Thi s THD value i s often insignificant a s mo st SCR rectifier s are equipped with input filter s that lower the THD level to around 5%. It i s only the final filtered THD which ha s reflected onto the generator input that i s a significant THD value. What is significant is the ability of the input filter to handle loads over a wide range. Since input filters are designed “tuned” to a fixed output value of the UPS, changes in the load percentage from the tuned number (normally sized at 100% load) will affect the values of the input filter, which will result in a higher THD reflected back onto the generator.

The only real effect that THD ha s on generator s i s that a load with very high di stortion will require more kW to power it than the same load with no di stortion. High THD level s (50% or greater) are a real threat to generator compatibility. High THD generally means that the generator will have to be oversized to combat the higher THD. If the THD can be reduced to 5% or even 10% by filtering, the THD reflected from the input of a UPS will have little or no effect on the requirement for oversizing a generator. The key to this statement is the actual design of the input filter. Not all input filter designs are friendly to generators.

The UPS input THD i s not the only relevant factor in the generator sizing equation. Some consideration should be given if and when the UPS tran sfer s to bypa s s and the generator power s the load s directly. A s mo st critical load s are non-linear, like switch mode power supplie s u sed for example in computers and servers, these device s will reflect upward s of 40 to 90% THD in a typical data center, and will also have a different power factor than does the input of the UPS. It is therefore important that the THD level of the load powered by the UPS itself be con sidered for generator sizing.

THD a s a di sturbance to critical load s: The industry-wide acceptable input THD of large UPS s i s between seven to 10% at nominal load, with mo st manufacturer s offering filter solution s to thi s value. It i s important to under stand that the con sequence of five, seven and even 10% of the input THD of a UPS ha s no effect on the UPS output load s. The output THD (load side of the UPS) cau sed by the UPS it self and the reflected THD of the load will not be di scu s sed in thi s paper; however we will addre s s the reflected THD cau sed by the input of the UPS effecting the operation of the generator.

Reactive Current : The second and most important factor influencing generator compatibility is the amount of reactive current (kVAR) generated by the UPSs and critical loads. Reactive current is created from loads with high capacitance, such as the capacitor banks of UPS input filters.

When a generator see s highly reactive load s (tho se with leading power factor s), the capacitor s feed current backward to the generator. Thi s will cau se the generator’ s voltage regulation sy stem to see a much higher voltage and eventually shut the generator down due to an overvoltage error.

Thi s problem i s mo st apparent when the UPS module i s lightly loaded and the UPS power factor i s the mo st leading. A s the rectifier ramp s up with UPS load, the capacitor s become saturated and the net power factor of the UPS becomes le s s leading. The le s s leading the power factor, the le s s of an effect the UPS will have on the generator voltage regulation sy stem.

The problem may also occur during generator start-up conditions when the UPS rectifier output is intentionally lowered to provide a lighter and more gradual load for the generator to walk into. The limiting of the rectifier current again causes the UPS load to become highly leading, recreating the conditions that lead to loss of generator voltage regulation.

Once UPS load s exceed approximately 40%, mo st UPS sy stem s will have a power factor that will be manageable for mo st generator set s. Even the wor st leading power factor load s can be overcome with enough generator oversizing. However, when it come s to large UPS s and multi-module UPS sy stem s, greatly oversizing generator s i s economically unpractical, and other solution s mu st be explored.

A s with THD, it i s not ju st the power factor characteri stic s of the UPS that should be con sidered for generator compatibility. The combined power factor of the critical load s and the UPS sy stem need to be evaluated to account for the po s sibility of the generator directly powering the load s when operating in bypass.

Peripheral load s that are not powered by the UPS and operate directly off the utility and the generator in time of an outage mu st be con sidered in the generator compatibility equation. Often the se peripheral load s are inductive, like the compre s sor motor s on a chiller sy stem. If inductive, the load s will have a lagging power factor, re sulting in a compen sating affect on any reactive current s the UPS may generate at lower load s. By calculating the net load profile of the UPS and peripheral load s, it i s po s sible to determine the net critical bu s power factor, which will typically be le s s leading or lagging than the power factor s of many UPS sy stem s analyzed individually. The con sequence of a le s s leading net sy stem power factor i s that the generator sy stem will not need to be over sized beyond the standard margin of safety to account for the UPS. Thi s mixture of peripheral load s and reflective THD of SCR-ba sed UPS de sign s i s normally seen with sy stem s of 100 kVA and above. Smaller sy stem s (under 100 kVA) generally do not have a mixture of peripheral load and UPS, but normally have only the UPS a s a load.

Industry Respon se s for UPS/Generator Compatibility:

Now that we have examined what cau se s generator incompatibility (leading power factor s and, to a much le s ser extent, THD), we can evaluate the effectivene s s of variou s solution s for optimizing generator/UPS compatibility.

Load variable filter s: The load variable filter operate s by di sengaging the input filter via a contactor prior to generator start-up and when the UPS i s lightly loaded. Di sengaging the filter remove s the capacitor s from the line, preventing the UPS from becoming a leading power factor load. Once the generator ha s walked in and i s powering the UPS, the filter contactor may clo se only when the UPS load goe s above 30 to 40%. At the se higher load s, the capacitor s on the input filter are more saturated and will not create a s much of a leading power factor.

The danger with thi s sy stem i s that if the contactor ever fail s to di sengage the input filter, the UPS s reactive load characteri stic s will cau se the generator to lo se voltage regulation and go off line. Additionally, when the input filter i s disengaged, there i s no mitigation of harmonic s, which may be a s high a s 45%, reflected from the rectifier. Another unfavorable re sult of u sing thi s type of filter control i s that every time you switch the filter you are hard switching a large capacitor bank in side the UPS input filter. Thi s action causes notching on the utility bu s, di sturbing all the load s sharing the bu s – a bad effect that often can propagate to your di stant neighbor’ s utility bu s.

IGBT Designs

 Over the past several years, the industry has made a turn toward using IGBTs as the switching device of the rectifier in small to mid size UPS design (10 to 100 kVA). IGBTs offer the advantage of being able to rectify AC power without creating much THD, avoiding the need for input filter s and their a s sociated capacitor bank s. IGBT rectifier s can al so create a stable power factor as high as a 0.99 pf at 100% load and 0.98 pf at 50% load. IGBT rectifiers can also be designed as ACTIVE devices, while SCR designs are PASSIVE devices.

The two major penalties of IGBT rectifiers in larger UPS sizes (over 150 kVA) are lower efficiency and the cost of the components (IGBTs) themselves. Since IGBTs switch at much higher frequencie s than SCR rectifier s, the switching lo s se s are higher. Looking at the efficiency profile of a UPS with an IGBT rectifier, we can see that efficiency suffer s a s the load level decrea se s. Thi s can re sult in severe operating co st implication s, particularly with larger UPSs. Therefore, most UPS designs using pure IGBT rectifiers are reserved for small to mid size three-phase UPS systems (10 kVA to 150 kVA). As technology advances, the use of IGBTs in the larger rectifier design will be more common.

Con sidering that state-of-the-art SCR-ba sed rectifier s and input filter a s semblie s can offer practically identical performance characteri stic s (five percent THD, no leading power factor / 0.95 pf) with the added bonu s of high efficiency, IGBT or power factor corrected rectifier s offer little advantage over modern rectifier s in large kVA sized UPS applications.

Hybrid IGBT Rectifier Designs

A sub class of rectifier design has emerged using a very old technology of utilizing Diodes as the switching element of the rectifier, with a separate IGBT buck / boost battery charger. This design requires the use of an input filter, with its limitations, but does achieve higher efficiencies with large size UPS modules. The design appears to be a bridge of technology until the industry develops larger, more cost effective IGBTs with a more effective switching technology to equal the efficiency of the SCR designs in the large size UPSs.

Line Interactive UPS: Line Interactive UPSs (or faults on-line) may claim to be generator compatible; however, thi s technology ha s some seriou s generator compatibility shortcoming s. The ba sic theory of operation of a line interactive UPS i s that under stable utility condition s (V i s +/- 10 to 15%), utility power simply pa s se s through the UPS to the critical load. At the same time, voltage and harmonic s are regulated by a parallel current that i s injected into the output. Since the input and output are on the same bu s, the parallel current being u sed to control the harmonic s and voltage al so create a stable power factor and relatively low harmonic s on the input of the UPS. Thi s create s a compatible load profile for the generator.

Unfortunately, the Achille s Heal of line interactive UPS generator compatibility i s that it doe s not have frequency regulation unle s s switched to battery mode (generating 100 percent of the output from the inverter of the UPS, which i s powered by the battery bank).

The problem occurs when the UPS is on generator and the generator hits a block load (i.e., a chiller motor starts or inrush from a transformer). When this occurs, the generator frequency will typically drop. Since the UPS has no way to regulate the output frequency, it must let the output frequency drop to the generator’s frequency or switch to battery mode and let the inverter generate the output power.

If the UPS i s heavily loaded in relation to the generator size, it will take the load off the generator when the UPS switche s to battery mode. The generator will then stabilize the frequency, cau sing the UPS to switch back to generator power. Thi s will block load the generator, forcing the UPS to revert to battery mode in an uncontrollable cycle until the batterie s are dead, and the UPS will eventually drop the load.

Some manufacturer s have attempted to remedy thi s cycling by programming the UPS not to switch to battery unle s s the frequency drop s below four to six Hz. The down side of thi s i s that the UPS will feed the critical load with power that may be four to six Hz out of tolerance. While thi s method may work (unless the generator i s sub stantially oversized), it i s po s sible that a block may still force the UPS to battery mode, a s deviation s greater than four to six Hz are very po s sible. Thi s mean s that line interactive UPS s may not be free from significant oversizing requirement s in mi s sion critical application s.

Low kVAR Input Filter s: Another more effective input filter de sign i s the low kVAR filter. The principal behind the low kVAR filter i s to reduce the filter capacitance and thu s limit the reactive current to manageable level s for the generator. The con sequence of reducing the capacitance i s only a slight increa se of a few percent in the THD (typically six to seven percent). The extra few percent of current di stortion experienced with thi s filter ha s no effect on generator output sizing requirements, nor i s it large enough to be a di sturbance to neighboring load s. Becau se thi s i s a pa s sive, solid state solution with no switching involved, it i s a preferred solution.

Shunt Inductor: For situation s demanding a low input THD (five percent) and ab solutely no leading power / reactive current factor under all load condition s, a shunt inductor filter may be ideal. The technology u se s compen sating inductor s in line with the input filter capacitor s. The inductor a s semblie s are carefully tuned to provide enough inductance to cancel the capacitance (reactive current) of the input filter capacitor s. Thi s inductive compen sation en sure s that the UPS will never become a leading power factor load (generate significant reactive current), even when the UPS ha s no load or during generator walk-in. The shunt inductor filter i s a 100% solid state solution u sing only pa s sive inductive components, making it extremely reliable.

Becau se the shunt inductor limit s THD to around five percent and eliminate s reactive current s, it can theoretically permit UPS to generator ratio s a s low a s 1:1 (providing allowance s are made for the UPS battery charging current and load characteri stic s when the UPS i s on bypa s s).

Efficiency and Input Filter s : Well designed input filter s are tuned to filter specific current harmonic s and only have a marginal effect on sy stem efficiency con suming around 0.5% of the total load power. Compared to the alternative technology of power factor corrected rectifier s, input filter s offer a huge efficiency advantage in large UPS systems. Because larger systems generally have mixed loads of UPS and peripherals, current available technology using SCR based rectifiers with low kVAR designs are the best choice for large systems above 150 kVA; while for mid size systems below 150 kVA a true IGBT design is the best choice.

In conclusion, while there are various UPS technologies available, factors such as load variables and UPS sizing should be considered to determine the best solution for optimizing generator/UPS compatibility.

Denzil Merrill is Vice President, Standard Three Phase Products at MGE UPS SYSTEMS, INC., 1660 Scenic Drive, Costa Mesa, CA 92626. For more information, call 800-523-0142 or e-mail denzil.merrill@mgeups.com.