Ana MORALES, Jean-Claude MAUN
ULB, Power Systems Department – Bruxelles
The purpose of this paper is to discuss the grid impedance assessment method at wind farm terminals. The non-intrusive method uses variations of the wind power production, and this permanent variability allows us to apply the source impedance identification process, assuring its assessment. New algorithms are developed taking as a reference the accurate GPS synchronized measurement of phase at a very stable power plant, to evaluate the source impedance at a wind farm point of common coupling (PCC) where complex variations of the voltage are expected. The instantaneous value of the grid impedance will, not only, allow assigning responsibilities but also will facilitate the design of equipments, network components, control systems tuning and protections configuration. The development of dispersed generation will be positively influenced by the capacity of measuring the network strength.
THE TRUE HARMONIC EMISSION LEVEL
Standards are ready  providing recommendations for preparing the measurements and assessment of power quality characteristics of wind turbines. One important question is raised: Are these fluctuations caused by the wind turbine or by other fluctuating loads? In other words, who is responsible? Generally, the influence of a wind turbine on the voltage quality of the grid depends not only on the turbine but also on the grid where it is connected. The European Wind Turbine Testing Procedure (EWTTP)  report has illustrated that the grid impedance angle and short circuit power have an important influence on power quality. We are then facing a very complex problem: how to assess the emission level of a particular distorted generation taking into account the instantaneous fundamental and harmonic source impedance variations of a power network.
Proposed IEC-61400-21 procedure
Proposed measurement procedures are valid to test the power quality characteristic parameters for the full operational range of a wind turbine, connected to a MV or HV network with fixed frequency within ±1Hz, sufficient active and reactive power regulations capabilities and sufficient load to absorb the wind power production. The measurements procedures are designed to be as non-site-specific as possible, so that power quality characteristics measured at for example a test site can be considered valid also at other sites. The standard specifies a method that uses current im(t) and voltage-time series um(t) measured at the wind turbine terminals distributed over the wind speed interval from cut-in to 15 m/s, to simulate the voltage fluctuations on a “fictitious” grid with no source of voltage fluctuations other than the wind turbine. This fictitious grid is presented in the next figure. [pic]
Figure 1: Fictitious grid as proposed by IEC – 61400-21
The fictitious grid is represented by an ideal phase to neutral rated voltage source u0(t) with an appropriate short-circuit apparent power Sk,fic, and four grid impedance angles ψk while the wind turbine is represented by a current generator im(t), which is the measured instantaneous value of the phase current. Thus, the fluctuating voltage that will be input to the voltage flicker algorithm in compliance with IEC-61000-4-15 to generate the flicker emission value Pst,fic is given as:
The fundamental voltage has to be determined from measured voltage at the wind turbine terminals. If voltages and currents are sampled with a constant sampling frequency, the fundamental can be found by filtering the measured voltage and determining the zero crossings of the filtered signal, taking into account phase compensation due to filtering lag.
Once this measurement and simulation procedure is done, flicker coefficients c(ψk,va) are then reported for continuous...