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Sunday, August 8, 2010

Power System Analysis: Tripping of Transmission Line after restoration of Power

I am going to share my analysis on the event that happened recently on our substation. For an overview, there are four substations in radial system, first is the Substation A, next are Substations B, C, and D. The source of power is from substation A going to B, going to C and lastly to D. The distance relays of substation A to C are radially connected. It means that no looking back relay is installed on adjacent substation.

Background of the problem:
Last August 3, 2010, Tropical Depression Domeng entered the Philippine area of responsibility. On that day, the region where the substations are located is characterized by overcast clouds with isolated rains and thunderstorms. Around 14:44:46H, Transmission Line from Substation A to B autotripped but autoreclosed at 14:44:48H. Upon autoreclosing, the Transmission Line from substation C to D autotripped at 14:44:48H with the following relay indications: REZ1:TRLC (Trip Line Check); OPTIMHO:196 km, 59.17ms. The line autoreclosed at 14:44:50H. Notice that Transmission line from substation B to C was normal operation during the event of trippings.


Root Cause Analysis:
One type of distance relay is the mho distance relay. Mho type distance relays are best for long transmission lines because it is not greatly affected by the adverse affect of tripping on power swings or out of synchronism between stations. Another type of distance relay is the reactance distance relay. This type of relay is beneficial to short lines because arc resistance will affect the distance measurement of the relay. Arc resistance is a part of fault impedance due to current flowing through ionized air. The reactive component of the arc is negligible. The arc resistance comes from the heating effect of the arc. Causes of arc resistance include arc faults, and these arc faults happen when electric current flows through air spaces between conductors. The magnetic fields of the fault current make conductors to separate producing an arc. In other words, due to uncontrolled conduction of electrical current from phase to ground, phase to neutral, or phase to phase accompanied by ionization of the surrounding air, arc flash is produced . Arc flash is also the heat energy and intense light produced at the point of the arc . As per ABB product sheet, Rez-1 is a mho type distance relay. One of its features is it can operate fault currents on power swings of 10% of the rated current. Phase-to-phase faults can be identified by negative sequence mho circles. The characteristic is not applicable for balanced three-phase conditions (load or power swing) but on combination of mho circle and quadrilateral characteristic is used for single and three-phase faults (See Figure 1). By using the two characteristics in parallel, optimum performance in speed and sensitivity can be attained. Three phase faults with low fault resistance are dealt with Fast fault clearing.

Conclusion:
Last August 3, 2010,the region experienced thunderstorm due to the arrival of Tropical Depression Domeng. The tripping of Transmission Line from substation C to D maybe caused by arc faults which were likely brought by inrush currents from the transmission line load end. One of the reason is that Substation D is the load end and load demand is higher than Substations B and C. These arc faults were aggravated by the ionization of the surrounding air and could have resulted to flashover between phase to phase or phase to ground. Since arc faults are highly resistive in nature, the resultant arc resistance was added to the impedance of the line thus the distance seen by the relay was increased. This is manifested in the distance reading of the OPTIMHO relay which is 196 KM. Comparing it to 29.43 CKM which is the total length of Transmission Line from substation C to D , the OPTIMHO Reading is already beyond the length of the transmission line involved.

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