Diesel Generator Set is Matched With UPS

This article analyzes and explains the impact of UPS input power factor and input filter on the power generator in order to clarify the cause of the problem, and then find a solution.

1. Coordination between diesel generator set and UPS.

Manufacturers and users of uninterruptible power supply systems have long noticed the coordination problems between generator sets and UPS, especially the current harmonics generated by rectifiers are generated on power supply systems such as voltage regulators of generator sets and synchronization circuits of UPS. The adverse effects of this 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.

Virtually 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 Transistor) technology to UPS design. The high efficiency of the IGBT inverter has led to a 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 inductive factors to capacitive factors in the filter is reduced, the volume of the UPS is reduced, and the efficiency is improved. Things in the UPS field seem to have been solved, but the compatibility of the new problem with the generator has appeared again, replacing the old problem.

2. Resonance problem.

The problem of capacitor self-excitation may be aggravated 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. 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 behave as 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 increased, beyond the scope of the analysis of ordinary engineers.

3. Diesel generator set and load.

Diesel generator sets rely on a voltage regulator to control the output voltage. The voltage regulator detects the three-phase output voltage and compares its average value with the required voltage value. The regulator obtains energy from the 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 coil current rises or falls to control the rotating magnetic field of the generator stator coil, or the size of the electromotive force EMF. The magnetic flux of the stator coil determines the output voltage of the generator.

The internal resistance of the stator coil of a diesel generator set is represented by Z, including inductive and resistive parts; the electromotive force of the generator controlled by the rotor excitation coil is represented by E by an AC voltage source. 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 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 resistance voltage drop in phase with I and the inductor voltage 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 vector E=U and I×Z. The voltage regulator can effectively control the voltage U by changing E.

Now consider what happens to the internal conditions of the generator when a purely capacitive load is used instead of a purely inductive load. The current at this time is just the opposite 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 as in the inductive load produces a higher generator output voltage U in the capacitive load, the voltage regulator must significantly reduce the rotating magnetic field. In fact, the voltage regulator may not have enough range to fully regulate the output voltage. The rotors of all generators are continuously excited in one direction and contain 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 voltage. This phenomenon is called "self-excitation". The result of self-excitation is overvoltage or shutdown of the voltage regulator, and the generator's monitoring system considers it to be a failure of the voltage regulator (ie, "loss of excitation"). Either of these conditions will cause the generator to stop. The load connected to the output of the generator may be independent or parallel, depending on the timing and setting of the automatic switching cabinet. In some applications, the UPS system is the first load connected to the generator during a power failure. In other cases, UPS and mechanical load are connected at the same time. The mechanical load usually has a starting contactor, and 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 "soft start", which shifts the load from the battery to the generator to increase its input power factor. However, UPS input filters do not participate in the soft-start process. They are connected to the input end of the UPS as part of the UPS. Therefore, in some cases, the main load first connected to the output of the generator during a power failure is the input filter of the UPS. They are highly capacitive (sometimes purely capacitive).

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

1. Install an automatic switching cabinet to make the motor load connected before the UPS. Some switch cabinets may not be able to implement this method. In addition, during maintenance, plant engineers may need to separately debug UPS and generators.

2. Add a permanent reactive reactance to compensate the capacitive load, usually using a parallel winding reactor, connected to the E-G or generator output parallel board. This is very easy to achieve, and the cost is low. But no matter in 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.

3. Install an inductive reactor in each UPS to just compensate for the capacitive reactance of the UPS. In the case of low load, the contactor (optional) controls the input of the reactor. This method of reactor is more accurate, but the number is large and the cost of installation and control is high.

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

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

If you want to know more about diesel generators, welcome to consult Dingbo Power by email  dingbo@dieselgeneratortech.com,and we will be at your service at any time.