Behavior of quadrature polarized Mho characteristic on lines fed from DFIG based wind farms

Distance relays due to their selectivity and operating speed are applied for HV/EHV line protection. The dynamic nature of Mho characteristic, which happens to be most primitive technique in line protection implemented with distance relaying, is built by using the measurement of local voltage and current signals. These signals have been influenced substantially by fault resistance and the source impedance feeding the line. Environmental benefit of wind turbine technology is making it a potential source of energy. These wind turbine generating units (WTGU) use rugged Induction/Synchronous Machines along with power electronics converters as controlling equipment. This gives a new challenge to distance relays, as fault current contribution of these sources depends upon converter operational principle. This paper discusses comparative analysis of behavior of Quadrature Polarized Mho (QPM) relay implemented on line protection in systems having traditional thermal synchronous machine and another system with Doubly Fed Induction Generator (DFIG) based wind farms. The results are illustrated with appropriate graphs and discussed at the end with concluding suggestions.

I. Introduction

EARLY days power systems used to be focused with large generations located centrally and supplying to distant load centers. With recent development of technologies, environment concerns and continuously decreasing fossil fuels, wind generation is an inevitable solution and grids throughout the world are going with more and more wind penetration. The introduction of distributed generation makes power flow bidirectional in sub-transmission systems which need proper review of protection philosophy. There are several technical papers and working group reports in the literature which give good review of protection philosophy in the light of distributed generation [1]. These reports express concern for the distribution system protection. In present days, due to such non conventional methods of power generations are making a way into EHV grids, another thorough review of protection scheme is also needed. Line protection at HV/EHV/UHV level is primarily provided with distance protection or impedance protection due to requirements of better security and dependability. This protection uses Quadrature Polarized Mho (QPM) characteristic which is the most primitive technique employed since last five decades. The impedances presented in reference [2] need to be further verified against relay characteristic. In this paper a comparative study of QPM tripping characteristic is presented in presence of conventional synchronous machine with DFIG wind farms of same capacity. Table I Abbreviation list

	Positive, negative and zero sequence current distribution factors at relay.
,	Complex three phase to ground current at relay point.
,	Positive, negative and zero sequence relay current at relay.
	Voltage and current design constant.
,	Complex three phase to ground voltages at relay point.
,	Positive, negative and zero sequence relay voltage at relay.
,	Positive, negative and zero sequence voltage at fault point.
,	Positive, negative and zero sequence impedance of line.
,	Positive, negative and zero sequence reach impedance of line.
,	Positive, negative and zero sequence source impedance behind relay.
,	Positive, negative and zero sequence grid impedance behind at relay.
,	Relay locations at bus A and bus B respectively.
	voltage and current at relay point
	angle of operation of phase comparator,
,	Zero sequence compensation factor -, -
,	Zero sequence current at relay point.
,	Fault point from bus A.