Laboratory Exercise Report LAB INFO Lab. name The Synchronous Generator Date of Lab. class August 14 2009 Lab. Assistant Janaina Gonçalves de Oliveira AUTHORS Name Student 1 Student 2 Student 3 E-mail Student1@student.uu.se Student2@student.uu.se Student3@student.uu.se REPORT DELIVERY Date 2009- Date 2009- Comments APPROVAL Signature 1(7)
1 Purpose The purpose of this laboratory exercise was to study three different modes of operation for the synchronous generator. These modes were: 1. No-load operation 2. Island operation 3. Grid-connected operation At rated no-load operation, the generator operates at nominal speed and voltage but does not deliver any power. During island operation, the generator is connected to an isolated load. In this operational mode, the generator is the only (or at least one of very few) generating unit(s) in the system. This usually implies that it is possible for the generator to influence the operating frequency and the voltage level of the AC system quite easily via adjustments of the turbine governor and the level of excitation respectively. Finally, during grid-connected operation, the generator typically has little or no possibility to influence the grid voltage and frequency, provided that the grid is a strong one (i.e. it contains many generating units). The primary aim of the laboratory exercise was to illustrate a number of typical features of each of these operational modes from the perspective of the generator. 2 Theory The synchronous generator consists of a stationary part, called the stator, and a rotating part, referred to as the rotor. The stator is usually equipped with a three-phase AC winding, called the armature winding, while the rotor is equipped with a DC winding, the field winding. The latter is arranged in magnetic poles of altering polarity. The number of rotor poles is always an even number. In an AC system with a fixed frequency, the number of rotor poles determines the operational speed of the synchronous generator according to f n = 60 [rpm], (1) 2P where n is the mechanical speed of rotation, f is the electrical frequency and P is the number of rotor poles. It is seen in (1) that a higher the number of poles implies lower generator speed. No-load operation of a synchronous generator means that the generator is operated at its rated speed and that the level of DC excitation in the rotor winding is such that rated voltage is induced in the open-circuited armature winding. This is the usual mode of operation of the generator before it is connected to a load or is synchronized with an external power system. The power provided to the generator from the prime mover of the energy conversion system (i.e. for hydro applications: the hydraulic turbine) needs only to overcome the no-load losses of the generator. Load operation of the generator, that is, when the generator delivers power, can be realized in two different ways: 1) The generator is connected to a single, isolated load. 2) The generator is part of a greater electric power system with many loads and many generating units. 2(7)
While the single generating unit has a decisive influence upon the voltage level and frequency in the first case, no generator is in general large enough too cause any substantial changes in voltage level and frequency during grid-connected operation. If changes in power generation from a single unit does affect the frequency we speak of weak grid, otherwise, we speak of a strong grid. In power systems terminology, an infinitely strong grid is also referred to as an infinite bus. 3 Method 3.1 Experimental Setup The experimental setup used in the laboratory exercise is schematically illustrated in Figure 1. A DC motor powers a 220V, 2 kva synchronous generator via a common shaft. The DC voltage fed to the armature winding of the motor controls the amount of power fed into the synchronous generator. This voltage level (and thus power transfer) is regulated with a variable resistor. The magnetizing current to the field winding of the synchronous generator can also be manually controlled. A torsiometer provides a reading of the torque on the shaft. A resistive load can be connected to the three phase terminals of the generator in order to simulate island operation. Further, the setup holds a synchronizing unit which must be employed prior to grid-connection of the generator. 3.2 Experiments 3.2.1 No-load Operation The no-load curve (field current vs. terminal voltage) was recorded at two different frequencies, 47 and 52 Hz. Six pairs of data points for each test frequency were collected. 3.2.2 Island Operation A purely resistive load was connected to the terminals of the synchronous generator and it was studied how the frequency and terminal voltage changed when the load was decreased / increased. 3(7)
3.2.3 Grid Connected Operation Figure 1. Schematic illustration of the experimental setup. An electronic synchronizing unit was employed to equal the voltage level, the frequency, the phase sequence and phase position of the generator vis-à-vis the grid. Once synchronized, the active power and the field excitation was modified one at a time and the corresponding changes in stator current and power factor were observed. 4 Results 4.1 No-load Operation The no-load curves recorded at 47 and 52 Hz are shown in Figure 2. When the field current was decreased to 0 A, the reading on terminal voltage voltmeter was slightly above 0 V. The non-zero terminal voltage is attributable to a small residual (or remanent ) magnetic field in the rotor poles which remains even when the external excitation is removed. In Figure 2, it can be seen that a given excitation current at 52 Hz produces a higher voltage level than at 47 Hz. This agrees with the Generator Formula, which states that the induced voltage is proportional to the (angular) frequency. As the field current is increased, the iron parts of the generator become more saturated, which in turn means that it will become harder to increase the magnetic flux in the machine. In the no-load curve, the saturation phenomenon manifests itself by a decreased slope at higher excitation levels (i.e. the curve flattens out). When the field current was increased, we further noticed that the frequency decreased slightly. Hence, we had to increase the active power input from the DC motor to stabilize the frequency. The reason for this is that a greater field current produces a higher magnetic field inside the generator. This, in turn, implies higher iron losses that need to be covered for if the frequency is to be kept constant. 4(7)
180 160 140 47 Hz 52 Hz Teminal Voltage [V] 120 100 80 60 40 20 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Field Current [A] Figure 2. No-load curves for the synchronous generator used in the experiment for the frequencies 47 and 52 Hz. 4.2 Island Operation We found that the load during island operation can be increased by decreasing the load resistance. If the power delivery to a resistance R at the voltage level U is P, then the power delivered to a resistance R/2 at the same voltage level U is 2P. Note however that the voltage level must be kept constant for this to hold true. In our island operation circuit with one generator and one load, the terminal voltage level will start to decrease when we increase the load demand. This is a consequence of the increased voltage drop over the internal impedance of the generator that takes place when the load calls for a larger current. In order to keep the voltage constant at higher loads, it is necessary to increase the excitation level (i.e. increase the field current). When the load was increased, we noticed that the frequency dropped. This can be expected since the active power demand was increased while the active power fed into the generator remained unchanged. Moreover, the voltage dropped slightly when the load was increased. This means that power delivered to the load will become smaller since 2 P U. (2) Because the power input from the DC motor did not change and the frequency and voltage level eventually stabilized at lower values, the same amount of power was delivered to the load both before and after the load was modified. In order to come back to the same frequency and voltage as before the change, the active power input from the DC motor needs to be increased. 4.3 Grid-connected Operation The grid frequency was measured and was found to be 50 Hz, as expected. When the active power input was increased and the field current was kept constant, the magnitude of the stator current increased. In the same way, an increase of the field current while the active power was kept constant also led to an increase in the stator current. Moreover, we could also observe that the power factor changed in both when the field current was changed and when the active power input was changed. 5(7)
Figure 3. Phasor diagram illustration of an increased active power input to a synchronous generator when the field current is kept constant. Figure 4. Phasor diagram illustration of an increase of the field current of a synchronous generator when the active power input is kept constant. The observations confirm the simple phasor diagram reasoning shown in Figs. Figure 3 and Figure 4 respectively. The phasor diagram in Figure Figure 3 illustrates a change in active power while the field current is kept constant. Similarly, the phasor diagram in Figure Figure 4 illustrates a field current change while the active power is kept constant. In both figures, the black set of phasors (subindices 1) corresponds to the situation before the change and the red set of phasors (subindices 2) corresponds to the situation after the change. E is the internal generator emf and is a quantity directly proportional to the field current, U is the terminal voltage, I is the stator current and X s is the synchronous reactance. Since active generator power is given by EU P = sinδ, (2) X s 6(7)
it can be concluded from Figure 3 that the active power output from the generator is increased when the power from the DC motor is increased. This is also manifested by an increased stator current (I 2 is larger than I 1 ), as we observed in the experiment. Further, the phasor diagram confirms that the reactive power output from the generator also changes after a change in active power input. This is because not only the stator current, but also the power factor angle changes. If the active power input is increased, as in the experiment, the power factor increases and hence the reactive power output decreases. Finally, Figure 4 readily shows that the stator current increases when the excitation (E) is increased. However, the entire current increase is attributable to a reactive current component, since the active power is kept constant. The load angle δ has hence decreased to compensate for the increase in E, and hence the active power output remains unchanged (see (2)). Moreover, the power factor angle increases. 5 Conclusions In this laboratory exercise, we have studied no-load operation, island operation and gridconnected operation of a synchronous generator. The saturation phenomenon could be clearly seen in the recorded no-load curves, where it manifested itself as a decreased slope at higher excitation levels. Further, a residual flux was found to induce a voltage even at zero magnetization. A decrease of the load resistance implies that the load (or power demand) is increased. If the load is increased during island operation, an increase in active power input from the prime mover is required to restore the nominal frequency of the system. A change in active power input affects the power factor and hence the reactive power output from the generator. A changed excitation affects only the reactive power input, provided that the prime mover active power input is held constant. 7(7)