COMMUTATION FAILURES IN HVDC SCHEMES

Commutation failures are very frequent dynamic events in high voltage dc (HVDC) transmission systems. Commutation failure is mostly observed at the inverter side of HVDC links, where large firing angles are used. The minimum value of the extinction angle required for the proper operation of a valve is specified by the valve manufacturer; however, at the inverter side of HVDC systems the extinction angle is regulated to a value higher than the valve specifications to allow control adjustments and also leave an adequate safety margin for unforeseen events in the power system, such as faults. Severe faults such as the voltage drops, phase shifts, or sudden increase in dc current may cause the commutation process to fail. A failure of one inverter station may affect the operation of other adjacent inverter stations, even the power transmission of HVDC links might be consequently blocked, and thus the stable operation of whole AC/DC transmission system might be affected.  Failure to complete commutation before the commutating voltage reverses is referred as commutation failures. Commutation failures occur in HVDC systems due to ac voltage dips (possibly caused by an ac systems short-circuit), increased direct current, late ignition or a combination of these. During the period of commutation failures, usually the fault duration, the associated valve groups cannot deliver any power into the AC network. The energy loss to the AC system during the fault is unavoidable. After fault clearing, the DC would normally be required to recover as quickly as possible to minimize the energy loss and prevent transient instability of the AC system

 Table of contents

1.              INTRODUCTION.. 3

2.               BACKGROUND……………………………………………………………………………………………………………….  4.

2.1            Working Principles of an HVDC converter……………………………………………………………………..…4.

3.              COMMUTATION FAILURES. 7

3.1             Causes of commutation failures 9

3.1.1         Voltage magnitude reduction on the A.C side. 9

3.1.2          Increase in D.C current. 10

3.1.3          Phase angle shift. 10

3.1.4          The effect of earth faults in A.C systems on voltage across valves. 11

3.1.5          Commutation failures mitigation. 13

3.1.6          Commutation failure recovery. 14

4.                TYPES OF FAULTS ON A.C NETWORK. 15

4.1              Single phase-to-ground fault. 15

5.              Conclusions. 20

                       Reference………………………………………………………………………………………………………………………20

                      Appendix A........................................................................................................................21

                      List of Figures

                      Figure 1 Equivalent circuit for a three phase full wave bridge converter………………………..4

                      Figure 2: Basic equivalent circuit during commutation…………………….. .............................4

                      Figure 3: Commutation process between valve 1 & valve 3 …………………………...................5

                      Figure 4: Voltage and current during commutation……….................................................. 6

                      Figure 5: Variation of the overlap angle µ as a function of delay angle α………………........ ..7

                      Figure 6: Commutation failure in a six pulse bridge……………………………………………………......8

                      Figure 7: Waveforms for V1 with the converter in the inverter operation…………………......8

                      Figure 8: Voltage magnitude reduction……………………………………………………………................9

                      Figure 9: Increase in d.c current…………………………………………………………………………………..….10

                      Figure10:Waveforms during a single commutation failure…………………………………………..….11

                      Figure11: Line to ground voltages in the a.c system at a single line to ground fault in R….12

                      Figure12:Phase to phase voltage on valve side of a YY converter transformer……………….12

                      Figure13: Phase to phase voltage on valve side of a Y∆ converter transformer………………13

                      Figure14: VDCOL characteristics……………………………………………………………………………………...16

                      Figure15: Single phase to ground phase to ground fault at inverter………………………....…...18

                      Figure16: Inverter valves value current during the fault………………………………………………....18

                      Figure17: Voltage waveform at R phase during line-line fault…………………………………….……19

                      Figure18: Voltage waveform at Y phase during lin-line fault…………………………………….………19