Introduction to Synchronous Generators | What Is 3-Phase Power?, Part 8 - MATLAB & Simulink
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    Introduction to Synchronous Generators | What Is 3-Phase Power?, Part 8

    From the series: What Is 3-Phase Power?

    The fundamental operational characteristic of a synchronous generator is that the electrical frequency is directly proportional, or synchronized, to mechanical rotational speed. This characteristic is a consequence of establishing a magnetic field on the rotor that stays aligned with the rotor orientation as the rotor rotates. You will learn:

    1. The relationship between rotor magnetic field and induced stator voltage
    2. The relationship between stator voltage and frequency
    3. How changing the number of pole-pairs changes the ratio of mechanical speed to electrical frequency
    4. How a speed governor controls electrical frequency
    5. How an automatic voltage regulator (AVR) controls stator voltage

    Published: 8 Jul 2022

    Hello, everyone. My name is Graham Dudgeon and welcome to part 8 in a series of tutorials on three-phase power. The aim of the video series is to build up our engineering knowledge on the design analysis and operation of three-phase electrical power systems.

    Today I'll give an introduction to synchronous generators. Here's a system that we're working towards in this tutorial. Our focus will be on the fundamental behavior of wound rotor synchronous generator. And we'll also introduce how the generator is controlled in terms of frequency and voltage.

    The speed governor and the automatic voltage regulator are control systems. And the prime mover, field circuit, wound rotor generator and load are physical plant components. We'll explain these components more as we work through the tutorial.

    The fundamental operational characteristic of a synchronous generator is that the electrical frequency is directly proportional, or synchronized, to mechanical rotational speed. This characteristic is a consequence of establishing a magnetic field on the rotor that stays aligned with the rotor orientation as the rotor rotates.

    There are two ways to establish an aligned magnetic field. You can use permanent magnets shown on the upper left, or you can use a wound coil that's energized or excited with a DC source shown on the lower right. Wound rotor synchronous generation offers a controllable magnetic field by changing the DC voltage through the use of an excitation system.

    Note on a three-phase system, there are three coil windings on the state are separated by 120 degrees. As the magnetic field passes through the coils, AC voltages are generated as a consequence of electromagnetic induction. Here we see our representation of the magnetic field formed on a wound rotor.

    I've kept the wound rotor fixed in this view, although it is in fact rotating. The DC supply to the winding is known as the field supply. And so we adjust field voltage, EFD to adjust the magnetic field strength. If we increase field voltage, then the magnetic field strength increases. This in turn causes a three-phase voltage induced on the stator to increase.

    If we decrease field voltage, then the magnetic field strength decreases, which causes the induced state of voltage to decrease. In a synchronous generator, for a fixed rotor field and no electrical loading, the relationship between induced stator voltage magnitude and rotational speed is directly proportional.

    If rotational speed is 0, then there is no stator voltage. And if rotational speed is nominal, then stator voltage is also nominal. Here you can see a plot of stator voltage and rotational speed for an example synchronous generator with no electrical loading, where I'm using per unit or normalized values, and ramp rotational speed up from 0 to 1 per unit.

    I'm showing only phase A stator voltage for clarity. You can see that the induced stator voltage magnitude tracks rotational speed as expected. What I'm showing here is the root mean square or RMS of stator voltage as a function of electrical loading for a fixed-field voltage and different act of power loading.

    I ramp frequency up from 0 to 1 per unit and use different resistive laws to draw different levels of active power. Note that the greater the power demand, the lower the stator voltage for a given field voltage. In this example, I have no control systems active, and have the generator initially spinning at a frequency of 1 per unit.

    In the view in the left, I'm applying a constant mechanical torque to slow the shaft down. And on the right, I'm applying an electrical load to slow the shaft down. Note that when I slow the shaft down mechanically, the stator voltage and frequency are overlaid on a per unit basis.

    But when I slow the shaft down electrically, the state of voltage is lowered in frequency due to the increased level of current being drawn from the generator. While electrical frequency and mechanical speeds are directly proportional, they are not necessarily equal.

    By introducing additional windings to the rotor and stator, we can increase the electrical frequency relative to the mechanical speed. We do this by introducing a certain number of pole pairs into the generator architecture. Here we can see the difference between one pole pair on the left, two pole pairs in the middle, and four pole pairs on the right.

    You can see that with one pole pair, the electrical frequency is equal to mechanical frequency. For two pole pairs, electrical frequency is two times mechanical frequency. And for pole pairs, you guessed it, electrical frequency is four times mechanical frequency.

    More generally, electrical frequency is n times mechanical frequency, where n is the number of pole pairs. The reason that we increase the number of pole pairs is to reduce the mechanical speed necessary to achieve a certain electrical frequency. Reducing mechanical speed also reduces mechanical stress on the rotor.

    A synchronous generator requires a mechanical source of energy to drive the rotor. This mechanical source is commonly referred to as a prime mover. Examples of prime movers include diesel engines, gas turbines, and steam turbines. The speed of a prime mover, and hence the electrical frequency, is controlled by a speed governor.

    A speed governor adjusts the talk of the prime mover, such that a given electrical load can be supplied while maintaining required system frequency. If electrical active power demand increases, then the speed governor will increase mechanical tork as can be seen in these results.

    Reactive power demand causes frequency to drop, as energy initially comes from the rotating inertia. The speed governor then increases fuel flow to the prime mover, which increases the mechanical torque, hence raising the rotor speed. The speed governor then regulates fuel flow to maintain rotor speed, and hence system frequency at 1 per unit.

    Note in these results that the terminal voltage reduces due to the increased current supplied by the generator to meet the active power demand. In this case, I have not yet included active voltage regulation. With an automatic voltage regulator, or AVR, you can see that we now regulate terminal voltage to 1 per unit.

    Look at the AVR detects an initial drop in terminal voltage, and responds by increasing the field voltage in order to bring the terminal voltage buck to normal value of 1 per unit. I showed a similar response earlier. I'd like to reemphasize this point.

    If we do not actively control torque or field voltage, then an increase in electrical active power will cause the mechanical speed to reduce, as the required energy comes only from the stored rotational energy of the shaft. Voltage will also reduce due to rotor speed and voltage magnitude being proportional when we have a fixed field voltage.

    What we are seeing here is called an open loop response. And why we never run generators open loop in actual operation, observing an open loop response does help reinforce that we are seeing the expected operational characteristics of a synchronous generator.

    Here I'm showing all the elements of a closed loop synchronous generation system that I showed at the start of this tutorial. To recap, voltage is controlled by an automatic voltage regulator which adjust field voltage, and frequency is controlled by a speed governor, which are just mechanical torque.

    There is a relationship between voltage and reactive power, and between frequency and active power. Depending on the operational mode, synchronous generators of flexibility in how they control voltage and frequency, and active and reactive power. I'll discuss generator control in more detail in the next tutorial.

    For more information on active and reactive power, please refer to my video on what is complex power, which you can find by searching for what is complex power MATLAB in Google. In summary, the fundamental operational characteristic of a synchronous generator is that the electrical frequency is directly proportional, or synchronized, to mechanical rotational speed.

    A wound rotor synchronous generator provides control of rotor magnetic field strength through a DC supply. And a synchronous generator for a fixed rotor field and no electrical loading, stator voltage magnitude and rotational speed are directly proportional.

    If we have n pole pairs, then the electrical frequency is n times mechanical frequency. The mechanical energy source that drives a generator is known as a prime mover. The rotational speed of a prime mover, and hence the electrical frequency, is controlled by a speed governor. A speed governor can also control active power.

    The terminal voltage of a wound rotor synchronous generator is controlled by an automatic voltage regulator, or AVR. An AVR can also control reactive power. I hope you found this information useful. Thank you for listening.