Introduction to Wind Power Plants and FACTS Devices’ Influence on the Performance of Distance Relays

Abstract The continuous growth in power demand in the electric power system allowed the development of various power electronics-based schemes for com- pensation and optimization of transmission systems. The converter-based systems like wind power plants (doubly fed induction generator or DFIG scheme), flexible AC transmission systems (FACTS) devices such as static synchronous compensator (STATCOM), unified power flow controller (UPFC), and series compensator are power electronics-based equipment, so the inclusion of the converter-based systems in the power transmission system generates frequency components that affects the performance of the distance relays, the relays have constant pickup values which will be exposed to these new power system conditions causing fault detection problems. The modern power converters generate a wide spectral band of frequency components which compromise the quality of the energy delivered, which affects the operation of the electric power grid, power consumers, and relay protection systems. Relay operation should be established using only the fundamental signal components at the nominal frequency because these are proportionately affected by the fault location. The main purpose of the conventional digital filters in distance relays is to estimate the fundamental frequency phasor of the electric input signals required by the relay, but when frequency components as interharmonics or sub- harmonics exist in the voltage and current signals, the conventional digital filters as Cosine or Fourier will cause an error in the fundamental frequency phasors esti- mate, and by consequence an error in the estimate of apparent impedance, this will compromise the performance of the distance relay causing underreach or overreach (fault detection problems). This book chapter is intended to present the effect and compensation of nonfiltered frequency components, such as interharmonics and subharmonics in the performance of conventional distance relays. Prony method is implemented as a filtering technique as a solution of the apparent impedance measurement error due to nonfiltered frequency components; simulated cases and real fault events are evaluated to validate the proposed distance relay algorithm.

Introduction

Converter-based systems like wind power plants DFIG schemes (doubly fed induction generator) which are the most common wind turbine schemes use a wound rotor induction generator which is controlled by a back-to-back power converter. The stator of a doubly fed induction generator (DFIG) is connected to the grid directly, while the rotor of the generator is connected to the grid by electronic converters through slip rings. Self-commutated converter systems, such as IGBT-based switching converters, are normally used for this type of system. The DFIG normally uses a back-to-back converter, which consists of two bidi- rectional converters sharing a common dc link, one connected to the rotor and the other one to the grid [1, 2]. Another converter-based system known as flexible AC transmission systems (FACTS) devices, such as static synchronous compensator (STATCOM) which is used for parallel compensating of reactive power and, its inductive or capacitive current can be controlled independent of system AC voltage using a voltage source converter (48 pulse VSC), a coupling transformer, and controls [3]. The STATCOM injects a reactive current into the system to correct the voltage sag and swell [3]. The unified power flow controller (UPFC) is the most versatile and complex of the FACTS devices. UPFC which consists of a series and a shunt converter connected by a common DC link capacitor can simultaneously perform the function of transmission line real/reactive power flow control in addition to UPFC bus voltage/shunt reactive power control. The shunt converter of the UPFC controls the UPFC bus voltage/shunt reactive power and the DC link capacitor voltage. The series converter of the UPFC controls the transmission line real/reactive power flows by injecting a series voltage of adjustable magnitude and phase angle. The interaction between the series injected voltage and the transmission line current leads to real and reactive power exchange between the series converter and the power system [3]. The fixed series com- pensator (FSC) reduces the electric distance of a transmission line allowing an increment in the power transfer capability, it comprises capacitors banks and parallel metal oxide varistors (MOVs), triggered spark-gap, and a bypass switch (Circuit Breaker). The MOVs protect the capacitors bank from overvoltages during and after transmission system failures. The triggered spark-gap protects the MOVs  against excessive energy absorption and the bypass switch (Circuit Breaker), in turn, protects the triggered spark-gap. Three high-voltage disconnecting switches serve to integrate the FSC into and isolate it from the transmission line [4]. The devices described are power electronics-based equipment, where its operating characteristics interacting with the power system during a fault condition could lead in a power quality issue in the voltage and current signals causing a mal- operation of a distance relay, so the inclusion of the converter-based systems in the power transmission system generate frequency components as interharmonics and subharmonics [511]. The quality of the voltage and current signals are now a significant problem for the electric power system which affects the operation of the electric power grid, power consumers, and manufacturers of electric-electronic equipment. The main problem in the quality of current and voltage signals is caused by the inclusion of equipment that its operation is based on power electronics; this is the main reason why certain requirements are necessary for a good quality of the energy in the electric power system.

The input voltage and current signals required by the distance relay are expected to be purely sine with an assigned amplitude and frequency. The modern power converters generate a wide spectral band of frequency components which compromise the quality of the energy delivered, and by consequence there’s an increment in the power losses and the reliability of the electric power system is reduced [9].

In some cases, the large-scale power converter schemes generate in addition to the typical operational harmonics of an ideal power converter, noncharacteristic frequency components as interharmonics and subharmonics will greatly deterio- rate the power quality of the voltage and current signals [911].

The estimation of the frequency components is very important in the power system protection schemes, that is why the characterization of these frequency components allows to compensate measurement errors in the electric protection system, such as distance relay. The frequency components as interharmonics and subharmonics could not be rejected by the distance relay digital filters (Cosine or Fourier filter) [1215]. The main purpose of the conventional digital filters in distance relays is to estimate the fundamental frequency phasor of the electric input signals required by the relay, but when frequency components as interhar- monics or subharmonics exist in the voltage and current signals, the conventional digital filters as Cosine or Fourier will cause an error in the fundamental frequency phasors estimate [13], and by consequence an error in the estimate of apparent impedance; this will compromise the performance of the distance relay causing underreach or overreach (fault detection problems) [1619].

The Prony method is a good alternative to obtain the correct apparent imped- ance measurement by estimating the fundamental frequency voltage and current phasors [20].

The main contribution of this chapter is the implementation of Prony method as a filtering technique in distance relay algorithms; hence, an apparent impedance measurement error due to nonfiltered frequency components (generated by the interconnection of wind power plants and FACTS devices in the power grid) when conventional digital filters (Fourier or Cosine) are used is compensated. The solution is presented and is validated using simulated and real fault events.

The organization of the chapter is as follows: in Sect. 2 the model of con- ventional distance relay and the effect of nonfiltered frequency components is presented; in Sect. 3 the fundamental frequency phasor estimation error is characterized due to the impact of converter-based systems in the performance of distance relays. In Sect. 4 a proposed distance relay algorithm using Prony method as filtering technique is presented as a solution of the error in the fundamental frequency phasor estimation in conventional distance relays due to nonfiltered frequency components in the voltage and current input signals. Then in Sect. 5 an evaluation of the proposed algorithm is performed using simulated and real fault events. Finally Sect. 6 closes the chapter with the main conclu- sions of this work.

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