School of Electrical and Information Engineering. ELEC High Voltage. University of Sydney. Dr Keith Mitchell. Engineering

Transcription

1 University of Sydney School of Electrical and Information Engineering Dr Keith Mitchell ELEC High Voltage Engineering

2 ELEC High Voltage Engineering 3. Switchgear

3 Types of Power System Switchgear Only fuses and circuit breakers are designed to interrupt shortcircuit currents Source: ref 2

4 Source CT and VT Connection To load

5 Circuit Breaker and Relay Wiring Schematic (1 of 3 phases) Source: Ref 1

6 Power System Protection Relays Power system protection relays accept signals representing the fault current and voltages (derived from measuring transducers known as current and voltage transformers) and then analyse these signals and send a trip signal to a circuit breaker if an abnormal condition is detected. The main types of relays are (for short-circuits): Over current Distance (impedance) Unit (differential) Other relay types are Thermal, over and undervolts, Buchholz, neg ve sequence, etc

7 Protection Relays and Circuit Breakers Upon detection of a fault, protection relays provide the trip signals for the circuit breakers. These are switching devices designed to be able to carry and interrupt safely the often very high currents, which flow during faults. Modern high capacity circuit breakers are capable of interrupting currents of up to amps at a nominal voltage of 750kV within 40 milliseconds. The purpose of the circuit breakers is To disconnect the faulty element or circuit, leaving the rest of the power system intact To prevent other healthy equipment from being damaged by the fault currents they must carry. To reduce damage at the point of fault, important because the less damage caused by the fault the greater the probability of successful repair.

8 Circuit Breaker Ratings Rated load current the normal load current it can carry indefinitely without thermal damage Rated voltage the rated voltage it can withstand indefinitely without insulation breakdown Breaking current the short circuit current it can safely interrupt without damage Making current the maximum short-circuity it can close onto (assuming an upstream device can interrupt the current) Eg 630A, volts, 21 ka Load current Rated voltage Breaking current

9 Steady-State or Asymmetric Current? Transmission system protection is designed to clear faults very quickly (sub-second), so asymmetric current is relevant Distribution system protection is often much slower and transients have usually decayed away - so steady state may be more applicable.

10 Fully Offset Fault Current

11 Calculation of Fully Offset Fault Current Source: ref 2

12 Short- Circuit Equiv Cct Source: ref 4 The total source and line R and L will determine the DC transient time constant τ = L tot /R tot

13 CB Symmetrical Interrupting Capability For CBs < 120 kv, a proportionally higher breaking capacity may be possible at reduced voltages ( K factor)

14 Circuit Breaker Fault Clearance

15 Circuit Breaker Fault Clearance.. 2

16 Current Choppin g

17 Circuit Breaker Types OIL Oil is vaporised and gas extinguishes arc Old technology maintenance intensive Explosion and fire risk AIR Arc shute designed to lengthen arc and extinguish Used for LV <1100V Air-Blast High pressure air extinguishes arc Used for HV AC and DC SF 6 Low maintenance. Soft switching Gas disposal issue VACUUM Maintenance free. Suitable for MV < 36kV Hard switching voltage transients Can t monitor vacuum condition

18 330 kv Switchgear - TransGrid Liverpool 330/132kV Substation 330kV busbars 330kV CTs 330kV Air-Blast CBs

19 132 kv Switchgear - Energy Aust 132kV Substation 132 kv low oil volume CBs The author in front gives an idea of size at these sorts of voltages.

20 HV, MV and LV CBs Voltage very much dictates size and type of circuit breaker Low Voltage >120 V and < 1.1 kv AC - Encased, Air CBs are the main type Medium Voltage > 1.1 kv and < 52 kv - Vacuum (<36 kv) and SF6 main types. - Many designs are indoor High Voltage > 52 kv - SF6 the main design. SF6 GIS switchgear up to 400 kv now available for indoor applications - Oil, low volume oil, and air-blast older types for high voltage.

21 Typical CB Ratings 120 kv Load A; S/C ka 300 kv Load A; S/C ka 800 kv Load A; S/C 40 ka

22 Oil CBs

23 SF 6 CB - Operation Source: ref 4 Arc quenching process in a puffer type SF6 CB

24 SF 6 Dielectric Properties Source: ref 4 SF 6 is a heavy largely inert gas and has superior dielectric and arc quenching properties compared to air and oil. Most SF 6 switchgear operates at a pressure of about 5-6 bars.

25 Source: Ref 4 Air-Blast CBs

26 Source: Ref 4 LV Air CBs

27 LV Magnetic Arc Extinction Source: Ref 4

28 Vacuum CBs Becoming very popular for voltages up to about 15 kv (eg motor contactors). Basically maintenancefree. Issues with hard arc extinction (current chopping) and

29 Indoor or Outdoor? Originally, most switchgear, right down to 11 kv, was placed outdoors, designed for use with aerial busbars, for both technical and cost reasons, especially at high voltages. The advent of SF 6 gas insulation has enabled the use of switchgear to 400 kv indoors. Inner city substations use indoor gear for aesthetic and space-saving reasons. Indoor subs may cause earthing design problems - why?

30 Source: ref 4 Outdoor CBs

31 Source: ref 4. A 440 kv packaged substation. The CBs are in the front and busbars at the rear. An incoming circuit is on the top right. HV GIS Switchgear

32 Other CB Design Issues Insulation level - set by system BIL Transient voltage withstand - 3 phase CBs do not open all 3 poles precisely at the same instant - over-voltages of ~1.5 times on other phases may occur.

33 CB Maintenance Traditional CB maintenance has been based on duty cycles - so many switching operations, so many fault clearing operations, etc. Normally minor maintenance interwoven with major maintenance. The latter involving complete stripping down of gear, and replacement of contacts. Newer concepts based on condition monitoring.

34 Condition Monitoring Refers to the actual amount of wear and tear a CB has experienced, and using this to determine the timing and nature of next CB maintenance. Uses sensors and microprocessors to log actual operational behaviour. Vibration analysis to monitor wear of CB contacts and operation mechanisms Oil analysis to determine amount of carbonisation caused by arcing, etc.

35 Switchgear Testing Manufacturers tests - R & D testing, type testing Commissioning tests - voltage pressure tests, tripping/closing tests, dielectric tests, etc Routine maintenance tripping tests.

36 Substation Layouts - Full Mesh Source: ref 4. Used on HV transmission substations where deliberate redundancy in current paths ensures maximum reliability.

37 Substation Layouts - Breaker and Half

38 Substation Layouts - Simple Single Bus NB: The X s are CBs and the switches are isolators.

39 HV Switchyard & Earth Grid Source: ref 4. Layout A design for a simple 132 kv sub with one incoming 132 kv feeder and 2 transformer CBs and isolators, and switchroom with the lower voltage side switchgear being indoor. Note the denser earth grid under the switchroom. The earth grid must keep voltage rise due to fault current inflow to acceptable levels.

40 Earth Grid Resistance

41 Earth Ground Rod Resistance Source: ref 4 The earth grid resistance will be specified in the substation design (usually < 0.5 ohm). Actual grid resistance tests are done as part of the commissioning tests of a new substation, by direct current injection.

42 Step and Touch Potentials

43 Allowable Human Body Currents Electrocution is caused by currents flowing through a body and interfering with the operation of key organs eg lungs, heart Body resistance varies with voltage and dryness Up to 900 ma acceptable for very short (<0.05 sec) periods; only 80 ma at 1 second and <50 ma continuous

44 Allowable Touch Potentials Source: ref 4. For example, for a 1-second fault duration, touch potential should be 60 volts, or less.

45 Typical Computer Model of Step & Touch Potentials Source: ref 4.

46 Source: ref 4. Switchyard Spacings

47 IEC Phase Clearances Source: ref 4.

48 Source: ref 4. IEC Safe Working Distances