2.1 Generator and load
The generator relies 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 draws energy from an auxiliary power source inside the generator, typically a small generator coaxial with the main generator, and delivers a DC power source to the magnetic field excitation coil of the generator rotor. The coil current rises or falls, controlling the rotating magnetic field of the stator coil of the generator or the magnitude of the electromotive force EMF. The magnetic flux of the stator coil determines the output voltage of the generator.
The internal resistance of the generator stator coil is denoted by Z, including the inductive and resistive portions; the generator electromotive force controlled by the rotor excitation coil is denoted by E with 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 pure resistive and purely inductive. The voltage drop caused by the current passing through the stator coil is represented by a voltage vector I x Z. It is actually the sum of two smaller voltage vectors, the voltage drop in phase with I and the inductor voltage drop of 90° ahead. In this case, it happens to be in phase with U. Since the electromotive force must be equal to the sum of the voltage drop of the internal resistance of the generator and the output voltage, that is, the vector sum of the vectors E=U and I×Z. The voltage regulator changes E to effectively control the voltage U.
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 exactly 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 exactly inverted. Then the vector sum of U and I×Z is smaller than U.
Since the same electromotive force E at the time of the inductive load produces a higher generator output voltage U at 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 continuous excitation of the rotor of all generators 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 overvoltage or voltage regulator shutdown, and the generator's monitoring system is considered to be a voltage regulator fault (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 settings of the automatic switch cabinet operation. In some applications, the UPS system is the first load to be connected to the generator during a power outage. In other cases, the UPS and the mechanical load are simultaneously connected. The mechanical load usually has a starting contactor. It takes a certain time to re-close after power failure, and 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 the "soft start" cycle, which shifts the load from the battery to the generator, increasing its input power factor. However, the input filters of the UPS do not participate in the soft start process. They are connected to the input of the UPS as part of the UPS. Therefore, in some cases, the main load that is first connected to the output of the generator when the power is cut off 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 several ways to do this, as follows:
● Install the automatic switch cabinet so that the motor load is connected before the UPS. Some switchers may not be able to implement this method. In addition, plant engineers may need to separately commission UPS and generators during maintenance.
• Add a permanent reactive reactance to compensate for the capacitive load, usually using a parallel wound reactor connected to the E-G or generator output parallel board. This is easy to implement and costs less. But in the case of high load or low load, the reactor is always absorbing current and affecting the load power factor. And regardless of the number of UPSs, the number of reactors is always fixed.
● Add an inductive reactor to each UPS to compensate for the capacitive reactance of the UPS. The reactor input (option) controls the input of the reactor under low load conditions. This method is more accurate, but the number is large and the cost of installation and control is high.
● Install the contactor before the filter capacitor and disconnect it at low load. Since the time of the contactor must be precise, the control is complicated and can only be installed at the factory.
Which method is optimal depends on the situation on site and the performance of the equipment.
2.2 Resonance problem
Capacitor self-excitation problems may be aggravated or masked by other electrical states, such as series resonance. When the ohmic value of the inductive reactance of the generator 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. Parallel inductors, series chokes, and input isolation transformers are common components of UPS, and these components are inductive. In fact, together with the capacitance of the filter, the UPS is generally capacitive, and there may be some oscillation inside the UPS. Coupled with the capacitive characteristics of the transmission line connected to the UPS, the complexity of the entire system is greatly improved, beyond the scope of analysis that can be analyzed by general engineers.
Two additional factors in key applications have recently made these problems more common. First, computer equipment manufacturers provide more redundant power input in their equipment, depending on the requirements of the user's highly reliable data processing. Typical computer cabinets now come with two or more power cords. Second, the equipment manager asked the system to support online maintenance, and they wanted to protect the critical load during UPS shutdown maintenance. These two factors increase the number of installations of typical data center UPSs and reduce the load capacity of each UPS. However, the increase in generators is not consistent with the UPS. In the eyes of the equipment manager, the generator is usually spare and easy to arrange for maintenance. Also in some large projects the financial pressure limits the number of expensive high-power generator sets. The result is that each generator has more UPS, which is a trend that makes UPS manufacturers happy and generator manufacturers trouble.
The best defense against self-excitation and oscillation is the 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 adhere to the comprehensive test and carefully measure the working parameters of the entire system when adjusting the test. When problems are discovered, the best solution is to set up a project team of vendors, engineers, contractors, and owners to fully test the system and find solutions.
