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How to match the generator set and UPS power supply?

How to match the generator set and UPS power supply?

Most people is more and more curious on UPS. They hope to combine generator with UPS to make a system. However, how to choose generator or UPS? How to match the generator set with UPS power supply? It is a inevitable question.

In recent years, the construction of large-scale data centers has grown rapidly, and more and more high-power UPS will be used in those center. Due to the need to control the number of batteries used in high-power UPS which has the backup time of basically 15-30 minutes, those centers requires  the matching power generation which provides continuous power supply for the equipment. Based on the above reasons, it is necessary to face the matching and compatibility problems of high-power UPS and generator sets. The following is an introduction.

UPS (Uninterruptible Power System/Uninterruptible Power Supply), that is, uninterruptible power supply, is a system that connects batteries (mostly lead-acid maintenance-free batteries) to the host, and converts DC power into mains power through the host inverter and other module circuits equipment. Uninterruptible power supply is mainly used to provide stable and uninterrupted power supply for single computer, computer network system or power electronic equipment such as solenoid valve and pressure transmitter. When the mains input is normal, the UPS supplies power to the load after it is stable. Voltage. At this time, UPS is an AC voltage regulator, which also charges the battery in the machine. When the city is out of power (accidental power outage), UPS will convert the inverter power supply to continue to provide 220vac, and immediately provide battery DC power to the load. The current maintains the normal operation of the load and protects the load’s software and hardware from damage. Uninterruptible power supply equipment usually provides high or low voltage protection.

A generator set (English name: Generators Set) is a complete set of mechanical equipment that converts other forms of energy into electrical energy. It consists of a power system, a control system, a silencer system, a shock absorption system, and an exhaust system. It is composed of water turbines, steam turbines, diesel engines or others. Power mechanical drive is used to convert the energy generated by water flow, air flow, fuel combustion or nuclear fission into mechanical energy and transmit it to the generator, which is then converted into electrical energy and output to electrical equipment for use. Generators are widely used in industrial and agricultural production, national defense, science and technology and daily life. In recent years, with technological progress, as a high-quality choice for home emergency power supplies and power supplies for outdoor travel, lightweight portable small generator sets have also begun to enter the daily lives of residents. The generator set is calibrated with two parameters when it leaves the factory, one is the common power and the other is the available power. The concept is defined in this way. For every 12 hours a generator set operates at the usual power, it is allowed to operate at the available power for 1 hour. If you are selling a new unit, you mainly consider the common power. Because the actual band impedance of the unit is 80% of the common power, for example, when the common power is calibrated at 300KW, the actual band impedance is 240KW.

In the past, there would be coordination problem between generator set and UPS. Manufacturers and users of uninterruptible power supplies have long noticed the problem of coordination between generator sets and UPS, especially the current harmonics generated by the rectifier to the power supply system, such as the voltage regulator of the generator set, and the synchronization circuit of the UPS. The adverse effects are very obvious. Therefore, UPS system engineers designed the input filter and applied it to the UPS, successfully controlling the current harmonics in the UPS application. These filters play a key role in the compatibility of UPS and generator sets.

In fact, all input filters use capacitors and inductors to absorb the most destructive current harmonics at the UPS input. The design of the input filter takes into account the percentage of the maximum possible total harmonic distortion inherent in the UPS circuit and under full load. Another benefit of most filters is to improve the input power factor of the loaded UPS. However, another consequence of the application of the input filter is to reduce the overall efficiency of the UPS. Most filters consume about 1% of UPS power. The design of the input filter always seeks a balance between favorable and unfavorable factors. 

In order to improve the efficiency of the UPS system as much as possible, UPS engineers have recently made improvements in the power consumption of the input filter. The improvement of filter efficiency largely depends on the application of IGBT (insulated gate-level transistor) technology to UPS design. The high efficiency of the IGBT inverter led to the redesign of the UPS. The input filter can absorb some current harmonics while absorbing a small part of the active power. In short, the ratio of the inductive factor to the capacitive factor in the filter is reduced, the volume of the UPS is reduced, and the efficiency is improved. However, the new problem is that the compatibility of UPS and generator has appeared again, replacing the old problem.

After solving the cordinate problem, we also need to pay attention on the problem of power factor. Usually, people pay attention to the working state of UPS power supply at or near full load. Most engineers can express the UPS operating characteristics under full load, especially the characteristics of the input filter, but few people are interested in the condition of the filter at or near no load. After all, the current harmonics of UPS and its electrical system under light load have little impact. However, the operating parameters of the UPS under no load, especially the input power factor, are very important for the compatibility of the UPS and the generator.       The newly designed input filter has a good effect in reducing current harmonics and improving power factor under full load. However, under no-load or very small load conditions, a very low power factor with a capacitive lead is derived, especially those filters that meet the 5% maximum current distortion. In general, when the load is less than 25%, the input filter of most UPS systems will cause a significant power factor reduction. Nevertheless, the input power factor is rarely lower than 30%, and some new systems have even reached the no-load power factor lower than 2%, which is close to the ideal capacitive load.

This situation does not affect the UPS power output and critical loads, and the mains transformer and power transmission and distribution system are not affected. But the generator is different. The generator engineer knows that the generator will work abnormally with a large capacitive load. When it is connected to a low power factor load, typically less than 15% to 20% capacitive, the system is out of adjustment. , May cause the generator to stop. This kind of shutdown occurs after the mains power failure? The emergency generator system drives the UPS system load will cause catastrophic accidents. Shutdown brings danger to critical loads due to the following two reasons: First, the generator needs to be restarted manually, and must be before the end of UPS battery discharge; second, the generator may cause system “overvoltage” before shutdown, it It may damage telephone equipment, fire alarm systems, monitoring networks and even UPS modules. To make matters worse, after an accident, it is difficult to distinguish responsibilities, identify the problem and correct it. The UPS manufacturer stated that the UPS system was tested well and pointed out that similar problems did not occur with the same equipment elsewhere. The generator manufacturer said it was a load problem and could not adjust the generator to solve the problem. At the same time, the user engineer explained his specifications and hoped that the two manufacturers would be compatible with each other. To understand why accidents occur and how to avoid them (or how to find solutions in critical applications), you first need to understand the working relationship between the generator and the load.

Resonance problem also needs to be considered for long term use. The problem of capacitor self-excitation may be aggravated or covered by other electrical conditions, such as series resonance. When the ohmic value of the generator’s inductive reactance and the ohmic value of the input filter’s capacitive reactance are close to each other, and the resistance value of the system is small, oscillation will occur, and the voltage may exceed the rated value of the power system. The newly designed UPS system is essentially 100% capacitive input impedance. A 500kVA UPS may have a capacitance of 150kvar and a power factor close to zero. Shunt inductors, series chokes and input isolation transformers are conventional components of UPS, and these components are all inductive. In fact, they and the capacitance of the filter together make the UPS appear capacitive as a whole, and there may already be some oscillations inside the UPS. Coupled with the capacitive characteristics of the power lines connected to the UPS, the complexity of the entire system is greatly improved, beyond the scope of the analysis of ordinary engineers.

Two additional factors have recently made these problems more common in critical applications. First of all, according to users’ high-reliability data processing requirements, computer equipment manufacturers provide more redundant power inputs in their equipment. Typical computer cabinets now have two or more power cords. Secondly, equipment managers require the system to support online maintenance, and they hope that critical loads will be protected when the UPS is shut down for maintenance. These two factors increase the number of UPS power installations in a typical data center, and the load capacity of each UPS decreases. But the increase in generators has not been consistent with UPS. In the eyes of equipment managers, generators are usually standby and easy to arrange for maintenance. In addition, financial pressure limits the number of expensive high-power generator sets in some large projects. The result is that each generator carries more UPS, which is a trend that makes UPS manufacturers happy and troubled by generator manufacturers.

The best defense against self-excitation and oscillation is the basic knowledge of physics. The engineer should carefully determine the power factor characteristics of the UPS power system under all load conditions. After the UPS equipment is installed, the owner should insist on a comprehensive test and carefully measure the working parameters of the entire system during commissioning and acceptance. When problems are found, the best solution is to set up a project team composed of manufacturers, engineers, contractors, and owners to fully test the system and find a solution.

After aboving all words, let us make the brief summary. Before chosing the generator set, we need to take few tips into consideration. Firstly, if the generator you choose can cordiante with the used UPS and work well together. Secondly, we need to check the power loss and possible power factor. Then after deducting the possible lost power, we need to check if the generator’s power can match the actual load machines’ needed power. If we can help to solve the cordinate problem , match the power factor and can meet the load machines’s power demand. There is the last issue being paid more attention. It is resonance problem.  These all problems cannot be viewed sperately and should be checked with together because they will influence each other. Please note, after selecting the suitable generator set and building the power supplying system, we also need to use time and pay attention to maintain the generator sets. Otherwise, there is no meaning for check all aboving factors. Hope that everyone who reads this article can find the suitable generator and build the available power system for your used UPS. Both generator set and UPS can be matched and compatial for your whole system.