Understanding Power Quality Issues in Wind Engineering: Electromagnetism & AC Generators, Slides of Environmental Law and Policy

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Wind Engineering

Module 7.2: Power Quality Issues

Electromagnetism

Induction

• The light bulb flashes the moment you connect the switch to the battery.
• The explanation is, that the magnetic field coming from the upper electromagnet flows through the lower iron core.
• The change in that magnetic field, in turn induces an electric current in the lower coil.
• The current in the lower coil ceases once the magnetic field has stabilized.
• If you switch off the current , you get another flash, because the magnetic field disappears. The change in the field induces another current in the lower core, and makes the light bulb flash again.

Source: http://www.tpub.com/content/doe/h1011v3/css/h1011v3_111.htm

Simple AC Generator

Alternating Current

• With an alternating current in the electrical grid,

the current changes direction very rapidly, as

illustrated on the graph above.

• Ordinary household current in most of the world

is 230 Volts alternating current with 50 cycles

per second = 50 Hz

• "Hertz" named after the German Physicist H.R. Hertz (1857-1894).
• The number of cycles per second is also called

the frequency of the grid.

• In USA household current is 130 volts with 60

cycles per second (60 Hz).

Phase Angle

• Since the voltage in an alternating current system varies with time one cannot connect a generator safely to the grid, unless the current from the generator oscillates with exactly the same frequency, and is exactly "in step" with the grid, - i.e. that the timing of the voltage cycles from the generator coincides exactly with those of the grid. - Being "in step" with the grid is normally called being in phase with the grid.
• If the currents are not in phase, there will be a huge power surge which will result in huge sparks, and ultimately damage to the circuit breaker (the switch), and/or the generator.
• In other words, connecting two live AC lines is a bit like jumping onto a moving seesaw. - If you do not have exactly the same speed and direction as the seesaw, both you and the people on the seesaw are likely to get hurt.

Stationary Armature 3 Phase

Generator

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.

Effects of Sudden Starts

• If you switched a large wind turbine on to the

grid with a normal switch, the neighbors would

initially see a brownout

• This is because of the current required to magnetize the generator.
• This is followed by a power peak due to the

generator current surging into the grid.

• 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.

Soft Starting with Thyristors

• To prevent this situation, modern wind turbines are soft starting.
• They connect and disconnect gradually to the grid using thyristors, a type of semiconductor continuous switches which may be controlled electronically. - You may in fact have a thyristor in your own home, if you own a modern light dimmer, where you can adjust the voltage on your lamps continuously.
• Thyristors waste about 1 to 2 per cent of the energy running through them.
• Modern wind turbines are therefore normally equipped with a so called bypass switch, i.e. a mechanical switch
• This is activated after the turbine has been soft started, and the thyristor is bypassed.

Power Quality Issues: 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 occur 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

Power Quality issues: 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

Conclusions based on Field Data

• The generating rating of the Blue Canyon Wind Power Project is about 6% of WFEC’s peak load, and during light load periods it may approach 14% of the load.
• At such levels, the data show that on average the fluctuations of wind power only increase the short-time frame variability of system apparent load by 8%.
• For a longer-time frame, the increase in system apparent load variability is even less.
• The available data show that wind generation has less impact on system regulation requirements than system load.

Field data Conclusions (Continued)

• The uncertainty of wind power availability complicates the day- ahead resources scheduling and hour-ahead adjustment processes.
• Better wind power forecasting can help improve the system performance. - Improve how system operators will incorporate the information into scheduling and operating decision processes.