ELECTRICAL GRID

Power quality


Power quality
Diurnal wind variations         The buyer of a wind turbine does not need to concern himself with local technical regulations for wind turbines and other equipment connected to the electrical grid. This responsibility is generally left to the turbine manufacturer and the local power company.
        The main issue when connecting to grid is power quality. The term power quality refers to the voltage stability, frequency stability and the absence of various forms of electrical noise (e.g. flicker or harmonic distortion) on the electrical grid. More broadly speaking, power companies and their customers prefer an alternating current with a nice sinusoidal shape, such as the one in the image.




Starting and stopping a turbine
        Most electronic wind turbine controllers are programmed to let the turbine run idle without grid connection at low wind speeds (if it were grid connected at low wind speeds, it would in fact run as a motor, as you can be sawn in the the generator page). Once the wind becomes powerful enough to turn the rotor and generator at their rated speed, it is important that the turbine generator becomes connected to the electrical grid at the right moment.
        Otherwise there will be only the mechanical resistance in the gearbox and generator to prevent the rotor from accelerating and eventually overspeeding (there are several safety devices, including fail-safe brakes, in case the correct start procedure fails (look on Wind Turbine Safety).



Soft starting with thyristors
        If a large wind turbine is switched on to the grid with a normal switch, the neighbours would see a brownout (because of the current required to magnetise the generator) followed by a power peak due to the generator current surging into the grid. The same effect can possibly be seen when you switch on your computer and the transformer in its power supply all of a sudden becomes magnetised. Another unpleasant side effect of using a "hard" switch would be to put a lot of extra wear on the gearbox, since the cut-in of the generator would work as if you all of a sudden slammed on the mechanical brake of the turbine.
Thyristors        To prevent this situation, modern wind turbines are soft starting, i.e. they connect and disconnect gradually to the grid using thyristors, a type of semiconductor continuous switches which may be controlled electronically (a thyristor may also found in a typical home, if there is a modern light dimmer, where the voltage on the lamps can be adjusted continuously). Thyristors waste about 1 to 2% of the energy running through them. Modern wind turbines are therefore normally equipped with a so called bypass switch, i.e. a mechanical switch which is activated after the turbine has been soft started. In this way the amount of energy wasted will be minimised.
        Large power thyristors in wind turbines get very hot when they are activated. They have to be equipped with aluminium heat sinks and fans as you see in the picture to the right.





Weak grids, Grid reinforcement
        If a turbine is connected to a weak electrical grid, i.e. it is vary far away in a remote corner of the electrical grid with a low power-carrying ability, there may be some brownout / power surge problems of the sort mentioned above. In such cases it may be necessary to reinforce the grid, in order to carry the fluctuating current from the wind turbine.
        The local power companies have experience in dealing with these potential problems, because they are the exact mirror-image of connecting a large electricity user, e.g. a factory with large electrical motors, to the grid.



Flicker
        Flicker is an engineering expression for short lived voltage variations in the electrical grid which may cause light bulbs to flicker. This phenomenon may be relevant if a wind turbine is connected to a weak grid, since short-lived wind variations will cause variations in power output. There are various ways of dealing with this issue in the design of the turbine, mechanically, electrically and using power electronics.


Preventing "Islanding"
        Islanding is a situation which may occur if a section of the electrical grid becomes disconnected from the main electrical grid, e.g. because of accidental or intended tripping of a large circuit breaker in the grid, e.g. due to lightning strikes or short circuits in the grid. If wind turbines keep on running in the isolated part of the grid, then it is very likely that the two separate grids will not be in phase after a short while.
        Once the connection to the main grid is re-established it may cause huge current surges in the grid and the wind turbine generator. It would also cause a large release of energy in the mechanical drive train, i.e. the shafts, the gear box and the rotor of the wind turbine, much like "hard switching" the turbine generator onto the grid would do.
        The electronic controller of the wind turbine will therefore constantly have to monitor the voltage and frequency of the alternating current in the grid. In case the voltage or frequency of the local grid drift outside certain limits within a fraction of a second, the turbine will automatically disconnect from the grid and stop itself immediately afterwards (normally by activating the aerodynamic brakes as explained on wind turbine safety page).



Reactive power control
Capacitors bank         Voltage and current are typically measured 128 times per alternating current cycle, i.e. 50 x 128 times per second for 50 Hz electrical grid frequency. On this basis, a so called DSP processor calculates the stability of the grid frequency and the active and reactive power of the turbine (the reactive power component is basically a question of whether the voltage and the current are in phase or not).
        In order to ensure the proper power quality, the controller may switch on or switch off a large number of electrical capacitors which adjust the reactive power, i.e. the phase angle between the voltage and the current. As can be seen in the image, the switchable capacitor bank is quite a large control unit in itself in a MW sized machine.






Video frames
- The grid: considerations
- Harmonics, flicker and reactive power: graphical example