03-05-2011, 12:00 PM
Abstract
The current differential protection scheme isone of the most sensitive method for protection of transmissionlines. However, its sensitivity can be compromisedby distributed shunt capacitance of the line, presence of seriescapacitor, current transformer inaccuracies and communicationlatency as well as asymmetries in communication.This paper proposes a novel sensitive current differentialprotection scheme for transmission line protection. Ituses Global Positioning System (GPS) for time synchronizedmeasurements while fibre optic is preferred for communication.The fictitious current Iser, in the series branch of the¼-equivalent line model at local and remote ends is used tomake trip decision. Case studies on four machine systemsare used to substantiate the claims.Keywords - Current differential protection, GPS
1 Introduction
IT is a well recognized fact that differential protectionschemes provide sensitive protection with crisp demarcationof the protection zones. Such schemes when usedfor transmission system using pilot wires are called pilotrelaying schemes and are classified in Fig. 1 [1, 2].Two versions of pilot relaying schemes are mainly usedin practice, directional comparison and phase comparisonschemes. Being legacy systems, both approaches limit thecommunication requirements.Pilot RelayingDirectionalcomparisonCurrentbasedDirectionalwavecomparisoncurrent differentialcharge comparisonphase comparisonblockingunblockingtransfer tripcomposite phasecomparisonsegregated phasecomparisonFigure 1: Classification of pilot relayingIn the directional comparison scheme, distance relaysare used to determine the direction of the fault (fault detector)and this information is transmitted to other end of theline. With this additional information, it can be quickly ascertainedwhether the fault lies on the transmission line ornot. While such methods work with low communicationrequirements, they may maloperate on post-fault systemdynamics like electrical center created by power swing(primary and backup).In the phase angle comparison schemes, phase anglecomparison is made between the current phasor of twoends of the transmission line. For this purpose, usuallya square wave in phase with the line current is generatedand transmitted to the other end on the pilot wire. If theshunt charging current is negligible, then in the absenceof a fault, phase angles of line currents at the two ends ofthe line will be equal. Conversely, a large phase differenceis a signature of the fault. Segregated phase comparisonis considered to be the most accurate implementationof the phase comparison schemes. Its communication requirementis much larger than the directional or compositephase comparison schemes. However, with such systems,benefit is reaped in terms of higher sensitivity and lowerprobability of false line trips.Phase comparison pilot relaying systems can bethought of as the first generation synchronized measurementsystems because an implicit synchronization isachieved for phase angle comparison. The method is comparativelysimple as it only transmits zero crossing informationas a dc signal to the other end of the transmissionline.In 1983 Sun et. al. [3] published a seminal paper describinga current differential relay system using fiber opticscommunication. The basic idea is to transmit phasesequence current information from one end to another usingPulse Period Modulation (PPM) method. In the PPMapproach, change in the period of modulated carrier is proportionalto the change in the analog signal. An effectivetransmission rate of 55 samples per cycle at 60 Hz frequencywas achieved in [3]. Since, the differential comparisonof the local and remote end current must correspondto same time instant, a delay equalizer is used withthe local phase sequence current signal to reflect the delayof the modem process of remote quantity. For example, at50 Hz, an uncompensated delay of 1 ms in communicationwill translate into an error of approximately 13 degrees inthe phase computation.The inaccuracies of such a current differential protectionscheme arises primarily due to the following reasons:² Distributed shunt capacitance current of the line.² Approximate delay equalization between local andremote end current.² Series Compensation of transmission line.² Current Transformer (CT) errors due to saturationof the core in presence of decaying dc offset current.
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