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PSSNSERC Industrial Research Chair in Detecting fault generated surges in DC line of VSC HVDC Schemes for travelling wave based fault location Fig. 2- Surge detection method Fault locations were calculated using the timing obtained by comparing the Rogowski coil output signals with a threshold. The system was calibrated by calculating the propagation velocity by applying a test fault at a known location (1 km from rectifier side). This Research investigated the detection of travelling waves in a VSC based HVDC cable for fault location purposes. Results show that travelling waves can be detected through the surge capacitor currents, if a series inductor is present between the cable and the converter terminal. If no series inductor is in the system, a small series inductor can be inserted for fault location purposes Using the surge capacitor and Rogowski coil combination, DC line faults in VSC HVDC schemes can be located accurately HVDC transmission systems that carry large amounts of power and important for system stability. Quick repairing of permanent faults HVDC transmission lines/cables is essential for minimizing the down time and outage costs Fast and accurate fault location is required to initiate repair work. Fault location in HVDC transmission systems is usually carried out using travelling wave based fault locators. Most of the existing HVDC line fault locators (LFLs) are designed for traditional line commutated convertor (LCCs) based HVDC schemes. The objective of this research is to examine the applicability of currently used LFL technology for VSC based HVDC schemes. A fault generates travelling waves that travel at a constant velocity denoted of v. Propagation of the travelling waves can be illustrated on a lattice diagram: Using the initial travelling wave arrival times at two terminals (t R1 and t I1 ) the distance to the fault can be estimated using (1). Most LFLs used in conventional LCC HVDC schemes use surge capacitor current to detect and measure the arrival of fault generated travelling waves . This works well because the large DC smoothing reactors in LCC HVDC schemes essentially decouple the line side and converter side voltages at high frequencies. The VSC topology does not require a DC smoothing reactor for its functionality VSC appears as an ideal voltage source due to large DC bus capacitor. Thus a clear surge will not be visible in the terminal voltage. In practice, an inductor is installed between the line/cable terminal and the converter to reduce the rate of change of current through IGBTβs during DC line faults. The presence of a series inductor should allow detection of travelling waves using the same arrangement even for the case of VSC HVDC. Ip l L R C Z E(t) I(t) Vr Vr dx Ξ± H A Ip Ns :number of turns A :cross-section area ( 2 ) l :length (m). Fig. 3 β Rogowski coil and Rogowski coil equivalent circuit H : magnetic field intensity dx : length of a small element along the loop Ξ± :the angle between the directions of H and dx E(t) :induced EMF on the Rogowski coil = total flux linked with the coil, M = mutual inductance between the primary conductor and the Rogowski coil. Using the equivalent circuit output voltage of the Rogowski coil can be calculated as, Ampereβs and Faradayβs laws give induced EMF on the Rogowski coil, Surge capacitor currents for a solid L-G fault at 200 km from the rectifier side. The fault occurs at 5.01s. Fig. 4- Test System (Surge capacitors: 100 nF , Series inductors: 10 mH) Fig.5- Surge capacitor currents (a) Rectifier side (b) Inverter side, with and without series Inductors 5.008 5.01 5.012 0 50 100 150 Voltage kV (a) Time S 5.008 5.01 5.012 0 50 100 150 Voltage kV (b) Time S 5.008 5.01 5.012 -10 0 10 Current A (c) Time S 5.008 5.01 5.012 -5000 0 5000 Current A (d) Time S 5.008 5.01 5.012 -0.2 0 0.2 Voltage V (e) Time S 5.008 5.01 5.012 -0.2 0 0.2 Voltage V (f) Time S Fig.6 - (a)- Rectifier side line current (b)- Inverter side line current (c)- Rectifier side line voltage (d)- Inverter side line voltage (e)- Rectifier side Rogowski coil voltage (f)- Inverter side Rogowski coil voltage (red lines show the thresholds) Sensitivity of the surge capacitor current to series inductor and surge capacitor K.P.A. N. PATHIRANA 1 , A. D. RAJAPAKSE 1 , O. M. K. K. NANAYAKKARA 1 , R. WACHAL 2 University of Manitoba 1 (CAN), Manitoba HVDC Research centre 2 (CAN) www.cigre-canada.org CongrΓ¨s 2012 CIGRΓ Canada www.cigre.ca CIGRΓ-052 Travelling wave based fault location Introduction Surge arrival time measurement system Modeling of Rogowski coil Simulation studies Results Conclusions Typical variations of the terminal voltages, currents and the Rogowski coil voltages for solid L-G fault 290 km away from the rectifier are shown in Fig. 6 Fig. 1- Travelling wave based fault location Converter side Cable Side Surge Capacitor Rogowski Coil Inductor β d d β d d Sync. Generator AC System Cable (300 km) Xf v v Rogowski coil T1 T2 E ( ) = β = β 0 . . . ( ) = β ( ) (2) ( ) = β ( ) β . ( ) β ( ) . (3) Table 2 Actual and Calculated fault locations Fault Location Error (m) Fault Location Error (m) Actual Calculated Actual Calculated 1 1.000 0 200 200.301 301 2 1.564 436 250 250.038 38 10 9.278 722 290 290.722 722 50 49.962 38 298 298.436 436 100 99.770 230 299 299.000 0 Table 1 Peak surge capacitor currents series inductance Surge capacitance 100 mH 10 mH 2 mH 200 nF 45.4 A 40.3 A 29.1 A 100 nF 22.7 A 20.2 A 14.5 A 20 nF 4.6 A 4 A 2.9 A Sync. Generator AC System T1 T2 tR1 tRI1 tI1 tI2 v v Cable Xf ( )= 1 2 [ β ( 1 β 1 ) . ] (1) Distance Time 5.009 5.01 5.011 5.012 5.013 5.014 -25 -20 -15 -10 -5 0 5 Ic (A) (a) Time S 5.009 5.01 5.011 5.012 5.013 5.014 -80 -60 -40 -20 0 20 Ic (A) (b) Time S with Ind. without Ind. with Ind. without Ind.
