TECHNOLOGY

 
OVERCOMING UPS/GENERATOR INTERFACE PROBLEMS

 

 

50kVA to 500kVA range

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By JOHN G. TRACY
June 13 - 19, 2005

 

 

It is a fairly common misconception that a generator alone is adequate for power protection needs. But a generator which is protecting a critical load may cause other problems, especially with sensitive electronic equipment. A UPS is normally designed to condition power to help ensure damage to a critical load does not occur. However, the ability of a UPS to work well with the unique operating characteristics of a generator requires special consideration which is the subject of this article.

Since over 90 percent of power outages last up to only five seconds, a UPS alone with a 10 or 15 minute battery can provide enough quality power to allow time for an orderly shut down. A UPS is not a power generating device, so the ability to provide longer duration power, beyond the 10 to 15 minutes of battery power, requires the use of a standby generator set. The seemingly straightforward task of sizing an engine-generator for use with a UPS, however, is complicated by the complex nature of the resulting system. Some UPS manufacturers solve the issues, others leave the task to the generator supplier.

UPSs range in size from less than 300 watts to more than 3 million watts. The scope of this article is applied to the range of 50 thousand watts to a half million watts. In this range, most UPS manufacturers use a thyristor rectifier for battery charging with a passive filter to improve power factor and reduce harmonic feedback. While many engineers ideally would prefer a charger section capable of synthesizing a linear load, the increased complexity of such a converter with its decrease in efficiency and reliability make this solution undesirable.

When engine generators and UPSs are put together in a system, problems arise that do not normally exist with a UPS or generator alone. This article deals specifically with problems that arise only when the UPS and generator are required to operate together. These are issues that neither the UPS nor generator exhibits when operating without the other. Neither is at fault, and a solution may be provided by either or both. UPS manufacturers, however, can offer robust, low cost solutions. Engineers at Powerware have identified the following common problems and solutions when applying a standby generator to a UPS:

LINE NOTCHES AND HARMONIC CURRENT: Most uninterruptible power supplies use a method of charger control (rectifier) that causes notches on the power feed (i.e. generator, or utility). These notches can play havoc with some types of generator controls. Additionally, chargers typically do not draw sine wave current from the line. The extent that the current differs from a sine wave is often referred to as Total Harmonic Distortion or THD. These harmonic currents may cause excessive heating in the generators. The THD of a charger with a 12 pulse rectifier is typically 12 percent with 11th and 13th harmonics dominant. The THD of a 6 pulse rectifier is typically 30 percent with 5th and 7th harmonics dominant. The THD of the 12 pulse rectifier is generally low enough to avoid the generator heating problems. However, 12 pulse rectifiers are becoming increasingly rare below 500kVA since it requires an input transformer which increases the size, cost, and weight of the unit.

 

 

The problem of both line notches, and harmonic currents can be solved by the UPS manufacturer using a properly designed passive filter. Most generator manufacturers have derating information to address harmonic heating problems. However, an input filter on the UPS that reduces the harmonics for less than 10 percent at full load eliminates the need for derating the generator. This filter should have an input series inductor of about 5 percent to de-tune it for other disturbances on the line.

STEP LOADING: When a generator turns on and the switch connecting it to the UPS closes, the instantaneous application of the load to the generator can cause sudden swings in both frequency and voltage. This situation can generally be avoided by assuring that the UPS has a walk-in feature. This requires that the UPS rectifier have some means of controlling power flow (usually thyristors) so that the power draw of the UPS can gradually be applied to the generator over 10 to 20 second period.

VOLTAGE RISE: This is an application problem that occurs when a generator is closely sized to the UPS and there is little or no other load on the generator. When the UPS is first connected to the generator by a transfer switch, its charger has turned off so that it may begin its power "walk-in" routine. If the input filter is the only load on the generator, it may provide excess excitation energy for the generator. Most generators' exciter controls have no way to deal with this excess. The result is that the voltage wanders up without control to approximately 120 percent by some fundamental generator design constraint, usually magnetic saturation of the generator iron. Most generator suppliers have preloads that can be attached to the generator to counter this effect. However, a UPS that disconnects its filter when its charger is off avoids this problem altogether.

FREQUENCY FLUCTUATIONS: Generators have inherent limitations on how closely they can control frequency and on their response due to changing loads. The function is complex and not only involves generator features, such as rotational inertia and speed of governor response, but also involves the load's reaction to frequency changes. The UPS charger, on the other hand, also has inherent limitations on how closely it can regulate its power needs from a source with fluctuations in voltage and frequency. Since both the generator controls and the UPS charger controls are affected by and respond to the frequency, an otherwise small frequency fluctuation may be exasperated. The most noticeable affect of this fluctuation is a chronic alarm on the UPS announcing that it cannot synchronize to bypass.

Good control design from both the engine-generator supplier and the UPS supplier are needed to minimize or eliminate frequency fluctuation problems. The engine should have a responsive governor, properly sized and adjusted for the system. The generator's voltage regulator should not be more responsive than the governor; otherwise an unstable condition will occur with the UPS battery charger. The UPS supplier should have a control responsive to fast frequency fluctuations. The UPS charger should be able to function properly with a slew rate on input frequency fluctuations greater than 5 Hz per second.

SYNCHRONIZING TO BYPASS: Some applications require the UPS to synchronize to bypass so that the critical load may be transferred to the generator. This generally places tighter demands on the generator for frequency and voltage stability. When this is the case, the system integration problem may be exacerbated. Good control design as described above can generally nullify this problem. Additionally, where it is acceptable to the load, the UPS supplier should be able to increase the acceptance window for bypass frequency deviation and slew rate.

AUTOMATIC TRANSFER SWITCH: Most generator-UPS installations include automatic transfer switches that switch the UPS back to utility power once it becomes available again. The speed of operation can be a problem and may result in a failed transfer. If the transfer switch also has motor loads, such as HVAC systems, the UPS input filter will supply excitation energy during the transfer. This excitation source turns these motors into generators using their inertia as an energy source. If the transfer occurs too fast, causing an abrupt phase change in the voltage, the results can be devastating for both the motors and the UPS. Only a few UPS suppliers, including Powerware, solve the problem by providing a fast means of detecting the transfer and disconnecting the filter.

Finally, it's important to note that depending on the specific application, any or all of the preceding seven features may be necessary to assure good generator interface, and avoid unnecessary risks.

EATON POWERWARE: A globally recognized industry leader for more than 40 years. Powerware designs and manufactures a full line of Uninterruptible Power Systems (UPS), DC power systems, power management software/connectivity products. Powerware products are typically used in networking, PC, financial, medical, industrial, voice and data communications, military and aerospace applications — wherever continuous power is essential to daily operations. Powerware is headquartered in Raleigh, NC, and is part of Eaton Corporation.

The company offers a full range of UPSs ranging in size from 300 volt-amperes (VA) for personal computers to greater than 1,000 kilovolt-amperes (kVA) for mission-critical applications including data centers, Internet service providers, and server farms. Network-connectivity devices, extended battery cabinets, power distribution modules and other power-related devices further enhance Powerware UPS hardware offerings. The Powerware product portfolio also includes a full line of DC Power Systems ranging in size from mini systems for roadside cabinets or customer premises equipment to large systems for major switching installations. Power management software and IT infrastructure management software complete the company's product line. Basic power management software provides industry-standard monitoring, alarm and shutdown capabilities. More advanced software packages offer expansive data archiving, trending and regression analysis tools with predictive capabilities. With innovative software that can predict the type and timing of possible future events. Powerware is leading an industry shift from reactive power protection to proactive power management.

EATON CORPORATION: Eaton Corporation is a diversified industrial manufacturer with 2003 sales of $8.1 billion. Eaton is a global leader in fluid power systems and services for industrial, mobile and aircraft equipment; electrical systems and components for power quality, distribution and control; automotive engine air management systems and powertrain controls for fuel economy and intelligent drivetrain systems for fuel economy and safety in trucks. Today, the company classifies its business into four distinct segments, which are, by size: Fluid Power, Electrical, Automotive and Truck. Eaton has 51,000 employees, 207 manufacturing sites and sells products to customers in more than 100 countries.

GEMNET: GemNet is a power solutions company, providing quality power products and services. Our philosophy has always been to combine its local expertise and commitment with quality products, technical competence and the support of its principal. Since inception, it has been our goal to build deep and long lasting relationships with our customers and alliance partners by providing them top-quality power products and services. This has been the single most important success factor for GemNet, earning it the reputation as one of the leading and most professional power products and solutions companies. Today, GemNet enjoys an established track record of excellence with its customers in Pakistan and the United Arab Emirates. GemNet is the authorized distributor for Eaton Powerware's range of AC and DC Uninterruptible Power Systems, and offers a large range of specialized power related products and services. The company has local offices in Karachi, Lahore and Islamabad (GemNet Pvt. Limited) and Dubai, United Arab Emirates (GemNet Gulf).

AREAS OF EXPERTISE:

•Service plans to fit the clients needs, including plans that cover the client's existing Power Systems and UPSs.

•Services also include: Needs Analysis, Comparative Product Analysis, Cost Justification, End-User Training, Maintenance, After Sales Support, Data/telecom center electrical contracting and site preparation

•Power Consulting & Critical Environment Site Audit (CESA): Audit of existing UPS and recommendation of a power solution. Analysis of the quality of power feeding the systems. Facility/Site inspection, identification and correction of the problem.