System aspects on insulation levels for HVDC converter stations

Transcription

1 Ulf Radbrandt, System aspects on insulation levels for HVDC converter stations February 3, 2012 Slide 1

2 Introduction, Insulation Coordination Optimization Cost Protection Equipment Probability February 3, 2012 Slide 2

3 Type of stresses Continuous AC - Normal operation - clean conditions - contamination - other Temporary overvoltage Abnormal system conditions Switching overvoltage Dry Rain Snow & Ice Birds Fires Vegetation System switching operations (lines, loads and equipment) Lightning overvoltage Lightning flashes to and around lines February 3, 2012 Slide 3

4 Strength and Stress February 3, 2012 Slide 4

5 Strength and Stress February 3, 2012 Slide 5

6 Principles of insulation coordination Primary objective is: Establish maximum steady state, temporary and transient overvoltages on equipment Select insulation strength and characteristics of equipment and protective devices To ensure a safe, economic and reliable installation. February 3, 2012 Slide 6

7 Insulation coordination Important parameters Minimum Short Circuit Capacity, pre and post fault X/R X1/X0 AC fault clearing times Auto-reclosing? Times? Maximum system voltage Arresters in the surrounding a.c. network characteristics DC and electrode line data Insulation margins February 3, 2012 Slide 7

8 Voltage profiles - Arrester scheme February 3, 2012 Slide 8

9 Voltages - Establish operating voltages February 3, 2012 Slide 9

10 Valve voltages CCOV=π/3*U di0absmax February 3, 2012 Slide 10

11 Surge Arresters Polymeric type PEXLIM P arrester February 3, 2012 Slide 11

12 Surge Arresters - Valve hall arrester February 3, 2012 Slide 12

13 Surge Arresters Voltage (p.u.) Min protection levels in kv (peak) according IEC Region 1 Region 2 Region Protection against lightning overvoltages Protection against switching overvoltages 1.0 x x 2 I res I cap Rated voltage (U r ) Continuous operating voltage (U c ) I res, resistive current Effect of increased block temperature on I res I cap, capacitive current (no influence from temperature) Log scale Current (Ampere) February 3, 2012 Slide 13

14 Surge Arresters Protective parameters U rw = Insulation level of equipment U p = Protective level of the S.A. Overvoltage without surge protection U rw U rw U p Protective margin Overvoltage with surge arresters February 3, 2012 Slide 14

15 Insulation Coordination Exemple of minimum Insulation margins (based on IEC ): Valves 20 % for steep front impulses 15 % for lightning impulses 15 % for switching impulses AC equipment, line side 25% for lightning impulses 20 % for switching impulses DC equipment 20 % for lightning impulses 15 % for switching impulses Converter transformer, valve side 20 % for lightning impulses 15 % for switching impulses February 3, 2012 Slide 15

16 Factors affecting on surge arrester dimensioning Continuous operating voltage Climate (ambient temp., rain, sunshine) Mechanical stresses Temporary overvoltages, TOV Transient overvoltages Protection function Energy- and current strength Outer insulation High outer pollution Short circuit proof February 3, 2012 Slide 16

17 Energy dimensioning Current amplitude Duration Time between current pulses The number of pulses before cooling Total energy February 3, 2012 Slide 17

18 Surge Arresters Difference between AC and DC AC Much higher current through arresters (coordinating current) for lightning than for switching overvoltages. That leads to higher BIL than SIL The same maximum operating voltage in both ends of a transmission line DC About the same current through arresters (coordinating current) for lightning as for switching overvoltages. That leads to about the same BIL as SIL Lower maximum operating voltage in the receiving end of a transmission line February 3, 2012 Slide 18

19 Converter Configuration for Xiangjiaba Shanghai DC Pole 800kV DC Filter DC Neutral Electrode Line February 3, 2012 Slide 19

20 Converter Configuration for North East - Agra DC Pole 800kV DC Filter DC Neutral Electrode Line February 3, 2012 Slide 20

21 Transformer voltage characteristics Steady State Transform er voltages Xiangjiaba Shanghai Transform er voltages North East - Agra tfo HV Y 500 u [kv] 400 tfo HV D tfo LV Y u [kv] 400 tfo Y tfo D 300 tfo LV D t [ms] t [ms] February 3, 2012 Slide 21

22 Transformer voltage characteristics Transient February 3, 2012 Slide 22 (kv) (ka) (kj) k 6.0k 5.0k 4.0k 3.0k 2.0k 1.0k 0.0 Arresters V135 C4 S1 UV1_4S1 UV3_4S1 UV5_4S1 IV1_4S1 IV3_4S1 IV5_4S1 EV1_4S1 EV3_4S1 EV5_4S Min Max Min Max k k k Min k Max k

23 Pros and cons with standard insulation levels for HVDC Pros: Manufacturers can standardize their product portfolios Utilities can have the same equipment for several converter stations (same voltage level) Increased insulation margins for e.g. transformers and bushings because thyristor valves will be optimized anyway (minimized insulation levels) Less need for thorough calculations/simulations? Cons: Standard levels will mean higher levels (due to e.g. different insulation margins by different utilities and different minimum short circuit capacity) which will give higher cost for equipment Higher insulation levels will limit the number of possible test institutes Higher insulation levels for transformers will lead to increased difficulties for transport Higher insulation levels for transformers will lead to increased amount for oil Higher insulation levels for transformer bushings will lead to increased size of valve halls Correction factors for external insulation might require selection of the next standard level Higher insulation levels for transformers might lead to higher commutation reactance which will lead to increased number of thyristors and increased need for reactive power compensation February 3, 2012 Slide 23

24 Ongoing work within IEC Extract from IEC TS , Insulation co-ordination Part 5 HVDC: Selection of standard withstand voltages for a.c. side equipment only. The present step is skipped for equipment on d.c. side because there are no standardized withstand voltage levels for such equipment For equipment on the d.c. side, specified insulation levels are rounded up to convenient practical values Ongoing work for the revision of IEC The work is going in the direction to even more emphasize the principle with tailor made insulation coordination and insulation margins instead of standard insulation levels within the converter The following line is added in the new draft under Clause 6.1 Essential differences between a.c. and d.c. systems: there exist no standard insulation levels in the case of d.c. systems The following is added in the new draft under Clause 12 Clearances in air: The clearances in d.c. applications are based on insulation levels of equipment which are determined to provide the appropriate margin over the protective level of the arresters rather than on standard equipment levels. February 3, 2012 Slide 24