Views: 0 Author: Site Editor Publish Time: 2011-05-06 Origin: Site
In recent years, large data centers have grown rapidly and will be applied to more and moreHigh power UPS, due to the need to control the amount of batteries used in UPS, the delay time of high-power UPS is basically 15-30 minutes, so it is necessary to match the generator set to provide continuous power supply for the equipment.Based on the above reasons, we have to face high-power UPS andGenerator set的match and compatibleQuestion, here are some personal suggestions for reference for people in the power supply industry:
High power UPS and generator set power supply truck
Manufacturers and users of uninterruptible power supply systems have long noticed the coordination problem between the generator set and the UPS, especially the current harmonics generated by the rectifier on the power supply system such as the voltage regulator of the generator set and the synchronous 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 current harmonics in UPS applications.These filters play a key role in ensuring the compatibility of the UPS and the generator set.
Virtually all input filters use capacitors and inductors to absorb damaging current harmonics at the UPS input.The input filter design takes into account the percentage of total harmonic distortion inherent in the UPS circuit and possible under full load conditions.Another benefit of most filters is to improve the input power factor of the loaded UPS.However, another consequence of the application of input filters is that the overall efficiency of the UPS is reduced.Most filters consume around 1% of UPS power.Input filter design always strikes a balance between advantages and disadvantages.(Copyright © WWW.HXDLKJ.COM)
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 depends largely on the application of IGBT (insulated gate transistor) technology to UPS design.The high efficiency of IGBT inverters has led to the redesign of UPS.The input filter absorbs some of the current harmonics while absorbing a very small portion of the active power.In short, the ratio of inductive factors to capacitive factors in the filter is reduced, the size of the UPS becomes smaller, and the efficiency is improved.Things in the field of UPS seem to have been solved, but a new problem is the compatibility of UPS and generators has appeared again, replacing the old problem.
Usually, people focus on the working status of the UPS when it is fully loaded or close to full load.Most engineers can describe the UPS operating characteristics under full load, especially the characteristics of the input filter, but few are interested in the conditions of the filter at no load or near no load.After all, the UPS and its electrical system have little impact on current harmonics under light load conditions.However, the operating parameters of the UPS at 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 better effect in reducing current harmonics and improving the power factor under full load conditions.However, under no-load or very small load conditions, an extremely low power factor with capacitive lead is derived, especially for those filters that are designed to meet the maximum current distortion of 5%.In general, the input filter of most UPS systems causes significant power factor reduction when the load is below 25%.Despite this, the input power factor rarely falls below 30%, and some new systems have even achieved no-load power factors below 2%, close to ideal capacitive loads.
This situation does not affect the UPS output and critical loads, nor are the mains transformers and power transmission and distribution systems affected.But the generator is different. Experienced generator engineers know that the generator will not work properly when it has a large capacitive load. When a lower power factor load is connected, typically less than 15% to 20% capacitance, Generator shutdown may occur due to system imbalance.If this kind of shutdown occurs after a power outage, the emergency generator system will drive the load of the UPS system, which will cause a catastrophic accident.Shutdown brings danger to critical loads due to the following two reasons: First, the generator needs to be restarted manually, and before the UPS battery is discharged; second, the generator may cause 'overvoltage' of the system before shutting down, which May damage telephone equipment, fire alarm systems, surveillance networks and even UPS modules.Even worse, after an accident occurs, it can be difficult to separate blame, identify what went wrong, and correct it.The UPS manufacturer said that the UPS system tested well and pointed out that similar problems did not occur with the same equipment elsewhere.The generator manufacturer said it was a load issue and there was no way to adjust the generator to fix the problem.At the same time, the user engineer explained his specification requirements 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 how the generator works in relation to the load.(Copyright © WWW.HXDLKJ.COM)
2.1 Generator and load
Generators rely on voltage regulators to control output voltage.The voltage regulator detects the three-phase output voltage and compares its average value with the required voltage value.The regulator draws energy from an auxiliary power source within the generator, usually a small generator coaxial with the main generator, and delivers DC power to the field excitation coils of the generator rotor.The coil current rises or falls, controlling the rotating magnetic field of the generator stator coil, or the size of the electromotive force EMF.The magnetic flux in the stator coil determines the output voltage of the generator.
The internal resistance of the generator stator coil is represented by Z, including the inductive and resistive parts; the generator electromotive force controlled by the rotor excitation coil is represented by the AC voltage source and is represented by E.Assuming that the load is purely inductive, the current I lags the voltage U by exactly 90° electrical phase angle in the vector diagram.If the load is purely resistive, the vectors of U and I will coincide or be in phase.In fact most loads are somewhere between purely resistive and purely inductive.The voltage drop caused by the current passing through the stator coil is represented by the voltage vector I×Z.It is actually the sum of two smaller voltage vectors, the resistor drop in phase with I and the inductor drop 90° ahead.In this case, it happens to be in phase with U.Because the electromotive force must be equal to the sum of the voltage drop of the generator's internal resistance and the output voltage, that is, the vector sum of the vectors E=U and I×Z.The voltage regulator changes E to control voltage U.
Now consider what happens to the internal conditions of the generator when a purely inductive load is replaced by a purely capacitive load.The current at this time is exactly the opposite to that of the inductive load.The current I now leads the voltage vector U, and the internal resistance voltage drop vector I×Z is also in the opposite phase.Then the vector sum of U and I×Z is less than U.
Since the same electromotive force E produces a higher generator output voltage U with a capacitive load as with an inductive load, the voltage regulator must significantly reduce the rotating magnetic field.In reality, the voltage regulator may not have enough range to fully regulate the output voltage.The rotor of all generators contains a magnetic field that is continuously excited in one direction. Even if the voltage regulator is fully turned off, the rotor still has enough magnetic field to charge the capacitive load and generate voltage. This phenomenon is called 'self-excitation'.The result of self-excitation is overvoltage or voltage regulator shutdown, and the generator's monitoring system considers it to be a voltage regulator failure (i.e. 'loss of excitation').Either situation will cause the generator to shut down.The load connected to the output end of the generator may be independent or connected in parallel, depending on the timing and settings of the automatic switching cabinet.In some applications, the UPS system is the first load connected to the generator during a power outage.In other cases, the UPS and mechanical load are connected simultaneously.Mechanical loads usually have starting contactors, which require corresponding time to re-close after a power outage. There is a delay for inductive motor loads that compensate for the UPS input filter capacitor.The UPS itself has a period of time called a 'soft start' cycle, which shifts the load from the battery to the generator, causing its input power factor to increase.However, the input filter of the UPS does not participate in the soft start process. They are connected to the input end of the UPS and are part of the UPS. Therefore, in some cases, the main load first connected to the output end of the generator during a power outage is the input filter of the UPS. devices, they are highly capacitive (sometimes purely capacitive).
The obvious solution to this problem is power factor correction.There are a number of ways to do this, roughly as follows:
Install an automatic switching cabinet so that the motor load is connected before the UPS.Some switching cabinets may not be able to implement this method.In addition, during maintenance, plant engineers may need to separately debug the UPS and generator.
To add a reactive reactance to compensate for the capacitive load, a parallel wound reactor is usually used, connected to the EG or generator output parallel board.This is easy to achieve and less expensive.But whether under high load or low load, the reactor is always absorbing current and affecting the load power factor.And regardless of the number of UPS, the number of reactors is always fixed.
Install an inductive reactor in each UPS to compensate for the capacitive reactance of the UPS.Under low load conditions, the contactor (option) controls the input of the reactor.This method of reactor is more accurate, but the quantity is large and the cost of installation and control is high.
Install a contactor in front of the filter capacitor to open it at low load.Since the timing of the contactor must be precise and the control is complicated, it can only be installed at the factory.
Which method should be determined based on the site conditions and equipment performance.
2.2 Resonance problem
Capacitive self-excitation problems may be exacerbated or masked 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.Newly designed UPS systems have essentially 100% capacitive input impedance.A 500kVA UPS may have 150kvar capacitance and a power factor close to 0.Shunt inductors, series chokes and input isolation transformers are common components of UPS, and these components are all inductive.In fact, together with the capacitance of the filter, they make the UPS appear capacitive overall, and there may already be some oscillations inside the UPS.Add to this the capacitance characteristics of the power lines connected to the UPS, and the complexity of the entire system is greatly increased, beyond the scope of the average engineer to analyze.
Two additional factors have recently made these problems more common in critical applications.First of all, according to users' requirements for high-reliability data processing, computer equipment manufacturers provide more redundant power inputs in their equipment.Today's typical computer cabinets come with 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 have led to an increase in the number of UPS installations in a typical data center and a decrease in the load capacity of each UPS.But the increase in generators has not kept pace with UPS.Generators are often seen as backup in the eyes of facility managers and are easily scheduled for maintenance.In addition, in some large projects, financial pressure limits expensive high-powerGenerator setquantity.The result is more UPS per generator, a trend that pleases UPS manufacturers and troubles generator manufacturers.
Defense against self-excitation and oscillation is basic knowledge of physics.Engineers should carefully determine the power factor characteristics of the UPS system under all load conditions.After the UPS equipment is installed, the owner should insist on comprehensive testing and carefully measure the working parameters of the entire system during commissioning and acceptance.When a problem is discovered, the plan is to form a project team consisting of manufacturers, engineers, contractors and owners to test the system and find a solution.(Copyright © WWW.HXDLKJ.COM)
The following is an example of a UPS and generator compatibility issue that occurred during commissioning of a new data center built by an online service provider.It shows how vendors, engineers, and users identify and resolve problems.
There are 3 sets of MGE UPS 3000kVA systems installed on site, each consisting of 4 sets of 75 kVA IGBT width and frequency modulation modules, which can be expanded to 6 sets.The design load rate of the module is 65%, and the UPS module is equipped with an input isolation transformer and a 5% input current harmonic filter.All modules are connected to two sets of generator parallel buses. Each set of buses has three 1600 kW generators and can be expanded to six generators.Each generator is equipped with an electronic voltage regulator.The power conversion plan for each parallel bus is to wait for the two generators to be connected in parallel before the first loads are connected.The first loads included a UPS in each system and some air conditioning loads.As subsequent generators are incorporated, the same loads as the first are subsequently added.During the fault mode test, the operator found that when one of the two generators with the first load failed, the other generator would have an overvoltage alarm and shut down after 2 seconds.But the first load is much less than the capacity of one generator because the load on the UPS is very light at this time.Further testing was scheduled to determine the impact of the UPS on individual generators.Because it is suspected that the input link of the UPS interferes with the voltage regulator, the UPS tested was not loaded, or the inverter of the UPS was turned off.The test setup consists of DC voltage and ammeters that directly monitor the field excitation coils, since these parameters are controlled by the voltage regulator and can immediately reflect the action of the voltage regulator.At the same time, use the generator's own instrument to monitor the load's power (W), current and voltage (VA), and var (var).
Test with a purely resistive load to establish a baseline.It shows that as the load increases the field current and voltage rise as we would expect.The larger load current produces a larger voltage drop I×Z on the internal resistance Z of the generator, which must be overcome to keep the output voltage U stable.Then test the impact of the UPS on the generator, adding one unit at a time.The UPS is not loaded, and the UPS rectifier soft start process is observed.The test results clearly indicate that the voltage regulator behaves in the opposite manner to a purely resistive load.After two UPSs were connected, the voltage regulator was close to the edge of the allowable range. Adding another UPS caused the generator to enter an overload state after 2 seconds.
Please pay attention to the load value corresponding to a single 750kVA UPS.It causes the generator to shut down, but there is essentially no real load. The capacitive reactance of each UPS is close to 230kvar, making the power factor 0.
The project team, consisting of engineers, owners, contractors, suppliers and manufacturers, after considering all possibilities, chose the option of installing a reactive reactor on each capacitive load.Based on the previous test data, the manufacturer designed a 200kvar shunt reactor for each UPS, which was controlled by a contactor. The contractor installed it in parallel with the input filter of the UPS on site. The engineer designed an external control circuit, which measures the generator. load, the reactor is only allowed to be connected if the UPS is powered by a generator and the total kW load of the generator is lower than an (adjustable) set value.The project team retested by connecting the modified UPS to a generator.
At this time, the influence of capacitance still exists, and the reactor can only balance part but not all capacitance.Therefore, as the UPS increases, the excitation current slowly decreases, but this does not cause a problem.Because 6 UPSs have exceeded the capacity of one generator, the voltage regulator is still normal and controlling the output voltage.
