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Insulation Coordination “the selection of insulation strength” 1 Kiski Valley Substation - 1938 2 Luxor substation – 1933…………….before lightning arresters 3 Insulation Self restoring insulation recovers after a flashover – external such as air switches, insulators, external surfaces of bushings (defined at standard atmospheric conditions) Non-self restoring insulation does not recover after a flashover – enclosed oil, paper, gas, air systems such as transformers and circuit breakers 4 Standards http://www.techstreet.com/info/astm. Voltages: ANSI C84.1- 1995 System Voltages Voltages: IEEE 1312 – 1987 AC Electrical Systems and Equipment Operating Voltages above 230kv BILs: ANSI 92 defines BILs for the equipment manufacturers to use IEEE std 4 – 1995 Standard Techniques for High Voltage Testing Circuit Breakers: ANSI C-37.06 1997 Transformers: ANSI C57.12 – 1993 Liquid Immersed Distribution, Power and Regulating Power Transformers Switches: IEEE C37.34 Standard Test Code for High Voltage Switches Insulators: ANSI C29.8 & C29.9 Cap and Pin Type & Post Type Insulators Arresters: IEEE C62.11 Metal Oxide surge arresters for AC Power Circuits Arresters: IEEE C62.22 Guide for application of Metal Oxide surge arresters for Alternating Current Systems 5 Impulse Voltages and Equipment Requirements Over-voltages Transient Waveshapes Equipment testing 6 Over-voltages what causes over-voltages on the system? Lightning Switching – breakers clearing faults Increases in power frequency voltage 7 Transient Waveforms Lightning - 1.2 x 50 u-second Front of Wave Chopped Waves Switching - 250 x 2500 u-second 60 hertz waves 8 Peak or RMS 60hz voltages are expressed in rmsph-ph Duty cycle and MCOV are in rms phase-ground Lightning, chopped wave and switching surges are expressed in peak volts phase to ground: rmsph-ph Peak = 2 3 Peak = 2 145kv = 118.4kv 3 9 Lightning Impulses Very fast rise can cause line insulation to fail – “backflashes” - rise in the 500-2000KV per u-second range and 10-20 u-sec tail (cfo of line - positive polarity waves) – Direct strokes to phase wires (cfo due to negative polarity waves) slightly slower rise and longer tail than backflash BIL: Basic Impulse Level is the electrical strength of insulation to the crest value of the standard lightning impulse (for dry conditions) – BIL = CFO – 1.28 x of (10% probability of flashover) Test wave doesn’t reflect actual lightning but its easily reproduced in test lab Lightning can be positive or negative compared to the earth – most strokes are negative. The insulation withstand10 can be different for each type Lightning Impulse Tolerances: Front: +/- 30% Tail: +/- 20% 1.2 x 50 u-seconds Virtual Zero Virtual front = 1.2 micro-seconds BIL = 1.0 .9 B Virtual tail = 50 micro-seconds v .5 .3 A t t1 IEEE std 4-1995 Standard Techniques for High Voltage Testing t2 11 Impulse Generators: + - Charging circuit + - Discharging circuit •Impulse generators are stacks of capacitors and resistors •Initially the capacitors are connected in parallel and charged up •Then the capacitors are re-connected in series •Then they are discharged to provide the high voltage impulse wave •Resistors are used to shape the front and tail of the wave 12 Chopped Wave Impulses Allows a faster front wave to be applied without flashover occurring on the tail of the wave Chopped wave impulses are standard lightning waves that are shunted or “chopped” to zero on the tail of the wave within 2 or 3 u-seconds Chopped wave tests are only used in ANSI (not IEC) standards and are meant to model surges created by the flashover of a post insulator or an arrester operation Stresses transformer turn to turn insulation Model the need for arresters at the “open breaker” 13 Chopped Wave Impulse 1.2 x 3 u-seconds 1.1 - 1.29 x BIL BIL = 1.0 v t IEEE std 4-1995 Standard Techniques for High Voltage Testing t 14 Switching Impulses BSL: Basic Switching Level is the electrical strength of insulation to the crest value of the standard switching impulse (wet conditions) o – BSL = CFO – 1.28 x f (10% probability of flashover) Switching impulses are longer duration both on the front and the tail (hundreds of u-seconds) Low line surge impedance (> 138kv overhead lines, also cable circuits) Re-closing into trapped charge on line 15 Switching Surges: Voltage doubles when closing in on an open line = 2 P.U. at open line terminal Z0= L/C Assume that High Speed Re-closing traps a negative 1 P.U. charge on the line. Then when the breaker re-closes the maximum voltage at the open end can approach a maximum of 3.5 - 4.0 P.U. for multiple reflections depending on damping (R): 3.5 P.U. Trapped charge = -1.0 P.U. 16 Switching Surges: Insulation strength of air gaps: 50% Critical Flashover Voltage CFO Strike Distance At operating voltages above 230kv switching transients become the constraint that drives system design Breakers & switches are only tested for switching surge at 345kv and above. Transformers 115kv & above. 17 Switching Impulse Tolerances: Front: +/- 20% Tail: +/- 60% 250 x 2500 u-seconds 1.0 v .5 t t1 = 250 u-sec t2 = 2500 u-sec IEEE std 4-1995 Standard Techniques for High Voltage Testing 18 Power Frequency (60hz) Maximum 60Hz Operating Voltage Nominal + 5% 500kv system is nominal + 10% 19 System Voltages ANSI C84.1 - 1995 20 System Voltages ANSI C84.1 - 1995 21 Power Frequency (60hz) Overvoltages Rise on unfaulted phases for a line-ground fault (X/R and co-efficient of grounding) Loss of ground (backfeed) Ferro-resonance – line or cable capacitance with transformer inductance Load rejection Ferranti effect – long unloaded transmission lines Transformer inrush 22 Coefficient of Grounding 23 24 Potential Rise on the Line Voltage .. Breaker open 25 26 Equipment Insulation Bus Insulators Air Switches Circuit Breakers Transformers Bushings Equipment External Clearances 27 Probability of Flashover 1.0 BIL/BSL Testing .5 .1 CFO BIL or BSL = 1.28 of KV 28 Equipment BILs (Class one equipment): 29 IEEE 1313.1 1996 Equipment BILs/BSLs for EHV (Class two): 30 Equipment Insulation 60 hz testing: 60 hz tests – wet tests are done on insulators, air switches and cijrcuit breakers to understand response during rain (IEEE std 4 – 1995) and dew conditions (ANSI C-29.1 – 1988 section 4.6) 31 Equipment Insulation Bus Insulators Air Switches Circuit Breakers Transformers Bushings 32 Bus Supports & Air Switches ANSI C37.32 Withstand (10%) 33 Standard Atmospheric Conditions Ambient temperature = 20 deg C Air Pressure = 760mm mercury (1 atmosphere) Absolute humidity = 11 gms water/Mtr3 air The test voltages are adjusted as needed when conditions vary from the standard 34 Equipment Insulation Bus Insulators Air Switches Circuit Breakers Transformers Bushings 35 Bus Supports & Air Switches ANSI C37.32 36 EHV Switches: Air Switches ANSI C37.32 37 38 Air Switch Coordination - Gap flashover - Gap withstand - Insulator flashover - Insulator withstand Make the gap withstand 10% higher than the insulator flashover! 39 Bus Supports & Air Switches ANSI C37.32 40 Air Switches Passing the 3+9 test: AP’s switch specification MS 1100-10 requires transmission switches to withstand 15 impulses without flashover for the gap withstand and also flashover for 15 impulses to establish the insulator flashover value. 41 Circuit Switcher Coordination In the model Mark V circuit switcher the interrupter closes when the switch blade reaches the full open position so the open blade must coordinate with the post insulator. In the series 2000 the interrupter stays Open when the blade opens! 42 Equipment Insulation Bus Insulators Air Switches Circuit Breakers Transformers Bushings 43 ABB 550 PM 44 Weakest point is typically the throat shield to ground Circuit Breaker Coordination Breaker closed 45 ABB 550 PM 46 ABB 550 PM Failure Doubs DL-55, July 8, 2000 – Tube Flashover 47 Circuit Breaker Coordination Breaker open Switching surge values are higher for the gap than for phase to ground 48 Circuit Breaker Design Tests Breaker switching surge withstand is higher across 49 the interrupter than to ground! Circuit Breaker 500kv PM Design Tests One minute dry withstand 60hz Ten second wet withstand 60hz Full wave impulse (BIL) Chop wave impulse – 2 u-sec to chop Chop wave impulse – 3 u-sec to chop Switching impulse – terminal to gnd (breaker open) Switching impulse – terminal to gnd (breaker closed) Switching impulse – terminal to terminal (breaker open) 860kv 775kv 1800kv 2320kv 2070kv 1175kv 1175kv 1300kv 50 Breaker Applications Closing resistors are used to reduce the switching surge created at the far end of a transmission line when it is energized – Once breaker is closed the resistor is removed from the circuit. – Not designed to handle continuous load current or fault current. – AP uses closing resistors on EHV breakers – Typical value: 400-450 ohms. 51 Closing resistors: bypass Rc Z0= L/C Rc VS Z0 V0 V0 = VS Z0 (Rc + Z0) 52 Equipment Insulation Bus Insulators Air Switches Circuit Breakers Transformers Bushings 53 Transformer BIL Levels 54 Transformer Test Levels 55 Transformers: 138-12kv delta-wye Gas Space - N2 high voltage winding low volage wintding HV & LV BILs determine the amount of line end insulation of the windings 56 Turn-Turn failure at line end 138kv disk winding – 30/40/50mva 57 Transformer Gas Space - N2 high voltage winding low volage wintding HV BIL determines the distance between the high-low windings and H-L insulation 58 Transformer Gas Space - N2 high voltage winding Induced test determines the high-ground insulation low volage wintding 59 Transformer Gas Space - N2 high voltage winding LV BIL and thru-fault mechanical forces stress low-ground insulation low volage wintding 60 61 62 Equipment Insulation Bus Insulators Air Switches Circuit Breakers Transformers Bushings 63 Bushings Condenser type bushings are built to grade the voltage from line to ground evenly. “creep” and contamination SF6 weathersheds 64 500 kV Bushing Type O 65 500 kV Bushing Type O 66 500 kV Bushing Type O 67 External Equipment Clearances: Need to compare external equipment withstands with bus support and air switch clearances Compare phase-to-ground and phase-to-phase equipment dimensions with the bus design Equipment manufacturers will set phase-phase and phaseground dimensions to pass equipment test standards but these are not coordinated with bus dimensions Typically the clearances between live parts on the equipment are too small! Flashovers will occur at the equipment before the bus. Need to specify equipment clearances that will coordinate with bus clearances 68 Transformer Clearances: 138-12kv xfmr 138kv 12kv Phase To phase 49 in. 6.5 in. Phase To ground 49 in. 6.5 in. 69 AP Transformer Design Clearances: 70 Proper Insulation Coordination If lightning strikes the phase conductor and it generates enough voltage to flash the insulation I want it flashing over the cheapest, self restoring insulation first and I want it to go to ground 71 Transformer Terminated Lines Reflected wave Then close the xfmr air switch Initial Switching surge .. When you close this breaker a switching transient is created that doubles at the other end of the line because the transformer is open on the low side Breaker open Close line breaker first 72