Power Quality Standards.

Voltage provided by Wind turbines must comply some requirements specified in the IEC 61400-21 standard. Here we are some power quality parameters that power energy from wind turbines must be under control:

Rated Data

• Rated Power (P_n) (or Active Power): Maximum continuous electric output power for a wind turbine under operating conditions.
• Rated Reactive Power (Q­­­­­_n): Reactive power under rated power, nominal voltage and frequency.
• Rate Apparent Power (S_n): Apparent power from the wind turbine while operating at rated power and nominal voltage and frequency.
• Rated Current (I_n): Current from the wind turbine while operating at rated power and nominal voltage a frequency.

Maximum Permitted Power (P_mc)

It is the 10-minute average power from a wind turbine. Wind turbines with active control of output power (blade pitching and/or speed control) typically provide P_mc = P_n. Wind turbines with passive control of output power (fixed-speed or stall-controlled wind turbines) are fixed with P_mc some 20% higher than P_n.

Maximum Measured Power (P₆₀ and P_0.2)

Maximum measured power as a 60 and 0,2 seconds average value. They can be used as protection settings. For variable-speed wind turbines P_0,2=P₆₀=P_n. For fixed-speed wind turbines, stall or pitch controlled P_0,2 > P_n.

Reactive Power (Q)

It must be specified as a 10-minute average value as a function of 10%,…, 90%, 100% of P_n, P_mc, P₆₀ and P_0,2. Wind turbines employing modern frequency converters can control the Reactive Power (Q) to zero (depending by the size of the converter.)

Flicker Coefficient c(Ψ_k, ν_a)

Power fluctuations in wind turbines can have several consequences in the grid. Therefore, it is important to measure these fluctuations, “flickers”, with a flickermeter and to fulfill the IEC 61000-4-15 standard. Variable-speed wind turbines have usually low flicker coefficients, whereas fixed-speed wind turbines swing from average (stall-controlled) to high coefficients (pitch-controlled). The formula is;

Where;

• Ψk: Network impedance phase angle.
• νa: Annual average wind speed.
• Pst : Flicker emission from the wind turbine.
• Sn: Rated apparent power of the wind turbine.
• Sk: Short-circuit apparent power of the grid.

Maximum Number of Wind Turbine Switching Operations (N₁₀ and N₁₂₀).

The following switch operations may cause voltage variations:

• Start-up at cut-in wind speed.
• Start-up at rated wind speed.
• Switching between generators (measurement of the worst case) in the case the wind turbine has more than one generator. So, N₁₀ is the maximum number of the switching operation within 10-minutes period and N₁₂₀ within 2-hour period.

Flicker Step Factor Kf(Ψk)

It is the flicker emission due to a single switching operation of a wind turbine.

• T_p Duration of the voltage variation due to the switching operation.

Similar to the flicker coefficient, variable-speed wind turbines have low Flicker Step Factors, whereas values for fixed-speed wind turbines swing from average (pitch-controlled) to high (stall-controlled).

Voltage Change Factor K_u(Ψk)

It is the voltage change caused by a single switching operation of a wind turbine.

• U_max and U_min: Maximum and minimum voltage (Root Mena Square (RMS) phase-to-neutral) due to the switching.
• U_n: Nominal phase-to-phase voltage.

Therefore, variable-speed wind turbines have low Voltage Change Factors, whereas values for fixed-speed wind turbines swing from average (pitch-controlled) to high (stall-controlled).

Maximum Harmonic Currents (I_h)

It is the emission of harmonic currents of a wind turbine with an electronic converter. However, IEC 61400-21 does not require measurement of harmonic emissions from induction machines#mce_temp_url# with power electronic converters (Type A and B).

Tags: Asynchronous (Induction), Frequency Converter, Generators, Power, Requirements, Synchronous, Theory, Wind Turbines