Matters needing attention when cooperating with high-power UPS uninterruptible power supply and generator

With the continuous increase of various large-scale power equipment, many equipment will use higher power UPS uninterruptible power supply . At present, the maximum power of high-power UPS uninterruptible power supply can reach 800KVA, and multiple parallel machines can also be realized. The backup time of the conventional high-power UPS power supply is 15-30 minutes. The user can also equip it with a longer time according to his needs, but this needs to be matched with the generator set in order to provide a more continuous and continuous power supply. The high-power UPS power supply What should be paid attention to when cooperating with the generator set? The following are the issues that need attention when cooperating with high-power UPS power supply and generator?

 1. The coordination problem between the generator set and the UPS

Manufacturers and users of uninterruptible power supply systems have long noticed the coordination problem between generator sets and UPS, especially the positive sine wave of the current generated by the rectifier on power supply systems 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, and successfully controlled the current harmonics in the UPS application. These filters play a key role in the compatibility of the UPS with the generator set.

In order to maximize the efficiency of the UPS system, UPS engineers have recently made improvements in the power consumption of the input filter. The improvement in filter efficiency depends largely on the application of IGBT (Insulated Gate-Level Transistor) technology to UPS designs. The high efficiency of the IGBT inverter led to a redesign of the UPS. The input filter can absorb some current harmonics while absorbing a small portion of active power. In short, the ratio of inductive factor to capacitive factor in the filter is reduced, the size of the UPS is reduced, and the efficiency is improved. In the field of UPS, things seem to be solved, but the new problem is that the compatibility between UPS and generator has appeared again, replacing the old problem.

2. Problems with power factor

Generally, people pay attention to the working condition of the UPS under or near full load. The vast majority of engineers can describe the operation of the UPS under full load, especially the characteristics of the input filter. However, few people are interested in the condition of the filter under no load or near no load. After all, UPS uninterruptible power supply and its electrical system under no-load state of the current harmonic impact is small. However, UPS uninterruptible power supply operating parameters at no load, especially input power and other factors are very important for the compatibility of UPS and generator.

The newly designed input filter has made great improvements in reducing current harmonics and increasing power factor at full load. However, under no load or light load conditions, a very low power factor with a capacitive advance is derived, especially those filters to meet the 5% maximum current distortion. Under most conditions, the input filter of most UPS systems will cause a noticeable reduction in power factor when the load is below 25%. Nevertheless, the input power factor is rarely below 30%, and some new systems have even reached no-load power factor below 2%, which is close to the ideal capacitive load.

This situation does not affect the output and critical load of the UPS, and the mains transformers and transmission and distribution systems will not be affected. But the generator is different. The experienced generator engineer knows that the generator will work abnormally when the generator has a large capacitive load. When a lower power factor load is connected, it is typically less than 15% to 20% capacitive. At this time, the generator may stop due to system failure. Such a shutdown occurs after the mains power fails, and the emergency generator system driving the UPS system load will cause a catastrophic accident. Due to the above two reasons, the shutdown brings danger to the critical load: first, the generator needs to be restarted manually, and the UPS battery must be discharged before the discharge is finished; Damage to telephone equipment, fire alarm systems, monitoring networks and even UPS modules. What's worse, after the accident, it is difficult to distinguish the responsibilities, find out the fault and correct it. The UPS manufacturer said 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 fault in the load, and there was no way to 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 an accident occurs and how to prevent it from happening again (or how to find a solution in a critical application), you first need to understand the working relationship between the generator and the load.　

Generator and load

The generator controls the output voltage by means of a voltage regulator. The voltage regulator checks the three-phase output voltage and compares its average value with the specified voltage value. The regulator obtains energy from an auxiliary power source inside the generator, usually a small generator coaxial with the main generator, and transmits DC power to the magnetic field excitation coil of the generator rotor. The increase or decrease of the coil current will control the rotating magnetic field of the generator stator coil or the magnitude of the electromotive force. The magnetic flux of the stator coil determines the output voltage of the generator.

Consider what happens to the internal conditions of the generator when a pure capacitive load is used instead of a pure inductive load. At this time, the current and the inductive load are exactly the opposite. The current I now leads the voltage vector U and the internal resistance voltage drop vector I × Z, which is also exactly inverted. Then the vector sum of U and I × Z is less than U.

Because the same electromotive force E as in the inductive load produces a high generator output voltage U in the capacitive load, the voltage regulator must significantly reduce the rotating magnetic field. In practice, the voltage regulator may not have enough range to fully regulate the output voltage. The continuous excitation of all generator rotors in one direction contains a permanent magnetic field. Even if the voltage regulator is fully closed, the rotor still has enough magnetic field to charge the capacitive load and generate a voltage. This phenomenon is called "self-excitation". The result of self-excitation is over-voltage or the voltage regulator is shut down. The monitoring system of the generator considers the voltage regulator to be faulty (ie, "de-energized"). In either case, the generator will stop. The load connected to the generator output may be independent or parallel, depending on the timing and setting of the automatic switch cabinet work. 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 at the same time. The mechanical load usually has a start contactor. It takes a certain time to reclose after a power failure. There is a delay in compensating the inductive motor load of 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 to increase its input power factor.

The solution to this problem is obviously to use power factor correction. There are several ways to achieve this, roughly as follows:

Install an automatic switching device so that the generator load is connected before the UPS. Some switching devices may not be able to implement this method. In addition, during maintenance, engineers may need to debug the UPS and generator separately.

Add a permanent reactive reactor to compensate for the capacitive load, usually using a parallel winding reactor, connected to the EG or generator output parallel board. This is easy to implement and less expensive. But no matter under the condition of 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.

Adding an inductive reactor in each UPS just compensates the capacitive reactance of the UPS. Under low load conditions, the input of the reactor is controlled by the contactor (option). This method is more accurate, but the number is large and the cost of installation and control is relatively high.

Install a contactor in front of the filter capacitor and disconnect it at low load. Because the time of the contactor must be precise and the control is more complicated, it can only be installed at the factory.

Which method is the best depends on the site conditions and equipment performance.