SG LEC 1-6 PDF - Synchronous Machine Lectures

University of Mosul College of Engineering Electrical Engineering Department 3rd class Sub: Electrical Machines Synchronous Machine Lecture: 1 Basics of Synchronous Generator...

University of Mosul College of Engineering Electrical Engineering Department 3rd class Sub: Electrical Machines Synchronous Machine Lecture: 1 Basics of Synchronous Generator Dr. Yasir M.Y. Ameen 1 2 Synchronous Machine Synchronous machines are ac machine that have a field circuit supplied by an external dc source. DC field winding on the rotor, AC armature winding on the stator Origin of name: syn = equal, chronos = time Synchronous machines are called ‘synchronous’ because their mechanical shaft speed is directly related to the power system’s line frequency. 3 4 5 6 7 8 9 10 11  Various Types ❑ Salient-pole synchronous machine ❑ Cylindrical or round-rotor synchronous machine 12  Salient-Pole Synchronous Generator 1. Most hydraulic turbines have to turn at low speeds (between 50 and 300 r/min) 2. A large number of poles are required on the rotor d-axis N Non-uniform air-gap D  10 m q-axis S S Turbine N Hydro (water) Hydrogenerator 13 Salient-Pole Synchronous Generator Stator 14  Cylindrical-Rotor Synchronous Generator Turbine D1m L  10 m Steam d-axis Stator winding  High speed N Uniform air-gap  3600 r/min 2-pole Stato  1800 r/min 4-pole r q-axis Rotor winding  Direct-conductor cooling (using hydrogen or water as coolant) Rotor  Rating up to 2000 MVA S Turbogenerator 15 Cylindrical-Rotor Synchronous Generator Stator Cylindrical rotor 16 LECTURE #2 THE TOPICS THE SPEED OF ROTATION OF A SYNCHRONOUS GENERATOR THE INTERNAL GENERATED VOLTAGE OF A SYNCHRONOUS GENERATOR THE EQUIVALENT CIRCUIT OF A SYNCHRONOUS GENERATOR 17  Operation Principle The rotor of the generator is driven by a prime-mover A dc current is flowing in the rotor winding which produces a rotating magnetic field within the machine The rotating magnetic field induces a three-phase voltage in the stator winding of the generator 18  Electrical Frequency Electrical frequency produced is locked or synchronized to the mechanical speed of rotation of a synchronous generator: P nm fe = 120 where fe = electrical frequency in Hz P = number of poles nm= mechanical speed of the rotor, in r/min 19 20 21 22 23 24 25 26 27 28 29 LECTURE #3 30 31 32 33 34 35 36 37 38 ∞ 39 40 41 42 43 44 45 PARALLEL OPERATION OF AC GENERATORS ∞ Why are synchronous generators operated in parallel? There are several major advantages to such operation: 1. Several generators can supply a bigger load than one machine by itself. 2. Having many generators increases the reliability of the power system, since the failure of anyone of them does not cause a total power loss to the load. 3. Having many generators operating in parallel allows one or more of them to be removed for shutdown and preventive maintenance. 4. If only one generator is used and it is not operating at near fun load, then it ( will be relatively inefficient. With several smaller machines in parallel, it is possible to operate only a fraction of them. The ones that do operate are operating near full load and thus more efficiently. 46 The Conditions Required for Paralleling 1. The rms line voltages of the two generators must be equal. 2. The two generators must have the same phase sequence. 3, The phase angles of the two a phases must be equal. 4. The frequency of the new generator, called the oncoming generator, must be slightly higher than the frequency of the running system 47 Frequency-Power and Voltage-Reactive Power Characteristics of a Synchronous Generator Regardless of the original power source, all prime movers tend to behave in a similar fashion-as the power drawn from them increases, the speed at which they turn decreases. The decrease in speed is in general nonlinear, but some form of governor mechanism is usually included to make the decrease in speed linear with an increase in power demand. Whatever governor mechanism is present on a prime mover, it will always be adjusted to provide a slight drooping characteristic with increasing load. The speed droop (SD) of a prime mover is defined by the equation where nnl s the no-load prime-mover speed and nfl is the full -load prime-move speed. Most generator prime movers have a speed droop of 2 to 4 percent 48 49 Since the shaft speed is related to the resulting electrical frequency The relationship between frequency and power can be described quantitatively by the equation 50  A similar relationship can be derived for the reactive power Q and terminal voltage VT. As previously seen, when a lagging load is added to a synchronous It is important to realize that when a single generator is operating alone , the real power P and reactive power Q supplied by the generator will be the amount demanded by the load attached to the generator- the P and Q supplied cannot be controlled by the generator's controls. Therefore, for any given real power, the governor set points control the generator's operating frequency fe and for any given reactive power, the field current controls the generator's terminal voltage VT. 51 An infinite bus is a power system so large that its voltage and trequency do not vary regardless of how 'much real and reactive power is drawn from or supplied to it. The power frequency characteristic of such a system is shown in Figure Curves for an infinite bus: (a) frequency versus power and (b) terminal voltage versus reactive power. 52 Operation of Generators in Parallel with Large Power Systems To understand the behavior of a generator connected to such a large system, examine a system consisting of a generator and an infinite bus in parallel supplying a load. Assume that: * the generator's prime mover has a governor mechanism, * but that the field is controlled manually by a resistor. It is easier to explain generator operation without considering an automatic field current regulator, so this discussion will ignore the slight differences caused by the field regulator when one is present. Such a system is shown When a generator is connected in parallel with another generator or a large system, the j frequency and terminal voltage of all the machines must be the same, since their output conductors are tied together. Therefore, their real powerfrequency and reactive power- voltage characteristics can be plotted back to back, with a common vertical axis. Such a sketch, sometimes informally called a house diagram, is shown 53 (a) A synchronous generator operating in parallel with an infinite bus. (b) The frequency versus power diagram (or house diagram) for a synchronous general or in parallel with an infinite bus. 54 55 56 57 58