CIGRÉ SC B1 International Colloquium on H.V. Insulated Cables Oct 2017, New Delhi, India

Transcription

1 CIGRÉ SC B1 International Colloquium on H.V. Insulated Cables Oct 2017, New Delhi, India Cable Overvoltage for MMC based VSC HVDC System: Interaction with Converters S. MUKHERJEE, M SALTZER, Y.-J. HÄFNER, S NYBERG ABB AB, Ludvika, Sweden

2 HVDC Cable Overvoltages Applied test schemes for extruded HVDC cables: TB496, IEC Lightning impulse surges (1.2/ 50 μs) Switching impulse surge (250/2500 μs) Investigate the temporary overvoltages (TOV) occurring in a DC link with MMC VSC converters Symmetric monopolar typology S1 IdpS1 IdpS2 S2 IdnS1 IdnS2

3 HVDC Cable Overvoltages Why? HVDC (High Voltage Direct Current), MMC (Modular Multilevel Converter) type VSCs (Voltage Source Converters) + extruded cables widely used New TOVs predicted in HVDC cable systems interacting with MMC VSC converters from Literature Input to JWG B4/B1/C4.73 a Cigré JWG dealing with «Surge and extended overvoltage testing in HVDC cable systems» S1 IdpS1 IdpS2 S2 IdnS1 IdnS2

4 HVDC Cable Overvoltages Transient Over Voltages (TOVs) occur due to faults in the HVDC system TOV depends on HVDC system arrangement Monopole, bi-pole, HVDC converter topology (half bridge, full bridge etc.) Focus: +/- 320 kv symmetric monopolar HVDC Investigate the impacts to the overvoltage by fault location, fault instant, length of cable, system loading S1 V IdpS1 V V V IdpS2 V S2 IdnS1 IdnS2

5 Model S1 and S2 are two converters with DC sides connected by sea cable Surge arresters considered Internal overcurrent and overvoltage protection For power electronic switches against high current/ voltage PSCAD Frequency Dependent Phase model (multiple-run feature) Half bridge MMC based VSC is modelled with full control as real system and simulation time step of 10 µs S1 V IdpS1 V V V IdpS2 V S2 IdnS1 IdnS2

6 S1:V100N S1:V100P S1:V0N S1:V0P S1:IdnS1 S1:IdnS2 S1:IdpS1 S1:IdpS2 Results Typical TOV Voltages and currents Pole to ground fault in the middle of the negative pole A. Rectifier and inverter current in the positive pole B. Rectifier and inverter current in the negative pole C. Rectifier positive pole voltage D. Rectifier negative pole voltage E. Inverter positive pole voltage F. Inverter negative pole voltage Healthy pole voltage rises to 510 kv in 3-4 msec. Faulty pole voltage goes to near zero by oscillation and polarity reversal Time [s]

7 Results Faults at midpoint Voltage at different healthy pole locations Different fault locations Voltage at the healthy pole midpoint 225 km 225 km

8 Results (a) Load dependence Midpoint fault for high loaded and low loaded cable Midpoint fault and midpoint voltage at healthy pole Rectifier positive pole current (IdpS1q, a.)) for the high loading case rises faster to a high value Increase in the dc currents <= Increase in the arm currents => triggers over current protection and converter blocking The faster the blocking of the converter, the lower the DC current increase, thus the lower the DC overvoltage (c.)) Higher voltage observed for the low loaded case (b) (c) (d)

9 Summary For extruded cables pole to ground fault the healthy pole midpoint is expected to see highest overvoltage due to longest distance from the station arresters The overvoltage magnitude is expected to increase with decreased load DC fault occurring in the middle of the line gives the highest over voltage on the healthy pole For TOV due to a pole to ground fault in a symmetrical monopolar HVDC system consideration of the mid point voltage of a fault in the middle of the line with low loading is sufficient

10 Results Length dependence - Midpoint fault for low loaded cable of different lengths Fault applied in the midpoint of the negative pole cables and voltage measured at the midpoint of the positive pole Rate of rise of voltage varies with the length of the cable Different length gives different cable capacitance and also different travelling wave time Figure 7: Healthy pole mid-point overvoltage for a pole to ground fault in the middle of the link. Shown is the low load case for a 225 km (solid line) link compared to a link above 1000 km (dot-dashed line).