Wind Turbine Generator: Part 1

Electromagnetism Fundamentals

Electric Charge

The most fundamental quantity in electricity is electric charge. Energy transfer is accomplished by the motion of charge.

Circuit

In electricity, the path through which charge flows is called a circuit.

Current

The rate of motion of charge in a circuit is called current. The unit of current is the ampere (A) – defined as the flow of one Coulomb of charge per unit of time:

I=dq /dt

Where i is the current, q is the charge and t is the time in seconds.

• Electric charge q is similar to displacement in mechanics. Current i is a vector quantity in that it possesses both a magnitude and direction.
• The direction of positive current is the direction of positive charges. Motion of charges is accompanied by energy transfer.
• The energy transfer associated with the motion of 1 unit (Coulomb) of charge from one point to another in a circuit is measured as one volt (V).

Ohms Law

• Voltage v across an element of a circuit is proportional to the current i flowing through it or

v = ir

where r is the constant of proportionality (or resistance) measured in ohms.

Energy

The energy associated with moving a charge of q Coulombs through a potential difference of v volts is given by energy w in Joules where

w = qv

Power

Power is the rate at which energy is transferred or the time derivative of energy.

P= dw/dt

where w is the energy related to moving a charge q through a potential difference v.
. Combined with the energy equation we get

P=iv

DC vs AC

Direct current (DC) is when the flow of charge is in one direction and alternating current is when the direction of flow changes continuously.

The number of direction changes per second is called the frequency for simply cycles per second.
Frequency is measured in Hertz (Hz). The electrical system in the US uses 60Hz while most other countries use 50Hz.

Faraday’s Law

When current exists in a circuit, in addition to the electric field, a magnetic field is generated in the vicinity of the circuit.

When a wire/coil moves inside a magnetic field a voltage is generated in the wire resulting in the flow of current.

Induced Voltage

When the current is changing with time, as a result of the magnetic field, the circuits in the vicinity will generate a potential difference called induced voltage v.

The induced voltage is

where is the magnetic flux in a single coil or loop. Thus a coil with N loops will induce a voltage given by

Generators and Motors

• Generators convert mechanical power to electrical power.
• Motors convert electrical power to mechanical power.
• Wind turbine generators can operate as motors or generators.
• Wind turbines use induction or synchronous AC generators as well as DC generators.

AC Generator

• The single loop of wire is rotated by the high-speed shaft in the magnetic field.
• The ends of the loop are connected to slip rings that rotate with the loop and are in contact with stationary brushes constantly.

• As the loop rotates in the magnetic field, the magnetic flux through the area enclosed by the loop changes with time.
• According to Faraday’s Law, this induces a current and a voltage (emf) in the loop.
• Each coil loop makes a phase and most utility-grade wind turbines generate power in three phases.

Components of Wind Turbine Generator

• The gearbox increases the rotational speed to that which is appropriate for the generator which uses magnetic fields to convert the rotational energy into electrical energy.
• The power output goes to a transformer which converts the electricity from the generator at around 400-700V to the appropriate voltage for the power collection system typically 11kV or 33KV.

Requirements For Wind Electric Generation

• Two main requirements for electric power generation include good constant speed characteristics with variations in torque and speed no more than approximately one percent and matching speeds with the rotor and generator rotational speeds.
• In conventional power generation both of the above requirements are met easily.
• By definition in wind electric generation both conditions are not met in general.

Wind Electric Generation

• The AC generators used in wind turbines are classified as synchronous and asynchronous (also known as induction) generators.
• Asynchronous generators are induction or permanent magnet generators.
• All power generation (steam, gas, diesel generator, or wind) is a fixed frequency alternating current (50Hz in Europe or 60Hz in the US).
• The drive train must be able to absorb the variations in torque and speed.
• There is control for the rotor and a control for power quality and regulation.
• The control characteristics of the electric generator and rotor blades (blade pitch or stall regulation) need to be considered collectively as a system for power quality.
• In weak grids, grid reactions in the form of power and voltage fluctuations or harmonics are important.

Generators and Rotation

• A synchronous generator has to rotate at a design or synchronous speed.
• An asynchronous motor runs at a slightly lower RPM than the synchronous speed.
• An asynchronous generator must run at a slightly higher RPM than the synchronous speed.

Squirrel Cage Generators

• The simplest asynchronous generator is also known as the squirrel cage induction generator is employed in fixed-speed wind turbines.
• The rotor consists of metal bars (copper or aluminum) mounted on rings on both ends of the bar giving it a cage-like structure.
• Three-phase squirrel cage generators are commonly used in wind turbines.

For constant RPM operation, squirrel cage induction generators work well.

• A simple induction generator consists of a four-pole stator, where magnetic fields of the stator are supplied by the utility grid (3 phase) and a rotor is in the center which is in the form of a cage made up of copper or aluminum bars.
• The RPM is tied to the frequency of the grid as the grid supplies the reactive power for the field coils of the generator.
• Induction generators drop offline during utility grid faults because the power for the field coils comes from the grid.
• Induction generators are the most common generators for wind turbines from 25kW to MW scales because the controls for synchronization to the grid are simple.
• In addition, they are mass-produced, inexpensive, and have reduced operation and maintenance costs.

Other Induction Generators

• A wound rotor induction generator is used in modern wind turbines where electricity is supplied to the coils on the rotor.
• Another popular variable speed induction generator used in wind turbines is the doubly-fed induction generator (DFIG).

Generator Summary

• Squirrel Cage rotor
  • Directly connected to an AC system that operates at a fixed speed.
• Wound rotor operates at fixed speed or variable speed with fully rated power electronic system
  • Rotor resistance slip control provides for a variation in speed in the design range
• Doubly Fed Induction Generators (DFIG)
  • Can deliver power to the grid through the rotor and stator while the rotor can also absorb power

Asynchronous Generator

• An asynchronous generator can act both as a motor and as a generator.
• The AC voltage across the coils in the stator creates a rotating magnetic field which in turn produces a rotation of the rotor center (120 degrees).
• The rotating magnetic field in the stator induces a current in the rotor coil which in turn exerts a torque on the rotor and rotates it as a motor.
• When it is forced to rotate past the synchronous speed (900,1200,1800RPM) it becomes a generator.
• Some wind turbines with lower starting torque use the induction generators as motors to turn the wind turbines as the cut in wind speed is reached and until synchronous speeds are reached.

Induction generators are essentially constant RPM with a small variation called the slip.
The motor mode can be used to start the turbine in order to overcome the starting torque requirement of the blades.
The rotational speed (in RPM) of the generator for zero shaft torque no for an asynchronous generator is

On a stall-regulated wind turbine, an asynchronous generator is often used whereby the rotational speed is almost constant and determined by the torque characteristics of the generator.

Slip

The relative difference between the actual rotational speed n and no is defined as slip

For a stall-regulated turbine, slip is 1 to 3%. This means that the rotational speed of the rotor is almost constant and the possibility of using the rotor as a flywheel to store energy (from a gust) is small.
Changes in the rotor torque QR (from turbulence and other causes) are immediately transferred to the generator torque Q, and thus to the power produced

Notes on Asynchronous Generator

• The rotor does not need electricity and hence there are no brushes.
• Squirrel cage induction generators are coupled to the power system through a transformer and consume reactive power so it is conventional to provide power factor correction capacitors at each turbine.
• They also have a soft starter to help build up the magnetic flux slowly and to maintain transient current during the energization of the generator.

Power Transmission

From the definition of Power and Ohms Law:

This is the power lost to heating. This implies that power needs to be transmitted at low current and high voltage.

Transformers change the voltage so wind farms have a transformer with every large turbine to increase the voltage for transmission.

Power Factor

The real power generated or consumed in an AC circuit is given by

where cos(phi) is the power factor.

The power delivered in an R-L circuit is lower than the power from a pure resistance circuit by the factor of cos(phi)

The power factor is the ratio of the true power to the apparent power.

• Adding a number of induction generators to the grid can increase the overall power factor but reduce the actual power delivered because of induction. generators also require reactive power supplied by the grid.
• This is the reason why capacitors are added to the wind turbines or substations.

Control and Power Quality

• Compared to conventional power methods, wind electric generation is not controllable and fluctuates randomly.
• Wind turbines often use converter-based generation systems.
  • As a result of differences, there are local and system-wide impacts on the network. Local impact includes circuit power flows and bus bar voltages as well as protection schemes, fault currents, and switchgear rating.
  • Power quality is also affected by voltage flickers and a harmonic voltage distribution.
  • System-wide impacts include power system dynamics and stability, reactive power and voltage stability, and frequency support. To remedy these issues two options are available: load following or storage.
• Wind turbines generate at low voltage (-400-600V) where losses are very high. Higher voltage lines are necessary to minimize losses.