DEFINITION As per IEC 60071 “Insulation Coordination is defined as selection of dielectric strength of equipment in relation to the operating voltages and overvoltages which can appear on the system for which the equipment is intended and taking into account the service environment and the characteristics of the available preventing and protective devices”
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DEFINITION
As per IEC 60071
“Insulation Coordination is defined as selection of
dielectric strength of equipment in relation to the operating
voltages and overvoltages which can appear on the system
for which the equipment is intended and taking into
account the service environment and the characteristics of
the available preventing and protective devices”
T em porary O ver-voltage s Switchin g O ver-voltage s O ver-vo ltages d ue to lightning .
P o w e r S ys te m s O ve r vo lta g es
Temporary Over-Voltages
• Typically due to faults
• < 1.2 pu
• ms to tens of second or even minutes
• Not dangerous to insulation
• Decides arrester selection.
Switching Over-Voltages
• Due to system switching operations
• 1.5 pu – 5 pu depends on system voltage
• mostly damped asymmetric sinusoids
• front time of first peak – tens of s to a few ms.
• decides external insulation in EHV/UHV systems
• decides arrester duty by way of ‘long duration class’
Over-Voltages due to Lightning
• Due to ‘direct’ or ‘indirect’ lightning strokes.
• known to contribute to 50% of system outages in EHV & UHV systems
• few hundred kV to several tens of MV.
• Few kA to 200 kA
• very short duration : times to front : 1 to few tens of s
• times to tail : few tens to hundreds of s.
• Decides line insulation (BIL)
• Severely influences Transformer insulation.
Brief History of Development of Lightning Arresters (Surge Diverters) - Surge Arresters
6. ‘Auto Valve Arrester’ – Westinghouse Stephen etal
7. ‘Thyrite’ – McEachron – General Electric (1932)
V = K I 0.3 to 0.4
LnV = LnK + LnI
Slope =
‘Modern’ Surge Arrester
Series Gaps • + Magnetic Action • + Grading Resistors • + Non Linear Resistors • + Insulating Coating • + Pressure Relief
Nonlinear Resistors• Silicon Carbide based• Sic 80%• Clay• Feldspar• MnO2 • CuO• (Sodium Silicate ?) • 0.3 to 0.45
9. ‘Most Modern‘ Surge Arrester
Metallic Oxides – ZnO 85% to 90%
V= KI
SiC NLRs …. 0.25 to 0.4
n = 1/ 4 to 0.25
MOV NLRs … 1/40 – 1/50
balance
....OAl
OZr
OCO
OBi
OSb
32
32
32
32
32
1
1
2
1
2
1
2
1
2
V
V
I
I
I
I
V
V
n = 1/ = 40 to 50
MOVfor
2 to2
NLRs SiCfor
2 to2 I
I,2
V
V If
5040
2.54
1
2
1
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Series G aps N on L inea r Resis to rs
Surge A rrester
1. Should be insulating under healthy voltage conditions
2. Should sparkover whenever a dangerous overvoltage arises.
3. Should quench power follow current at the earliest.
1. The voltage developed during flow of impulse currents should be below withstand capability of insulation under protection.
2. Should limit the power follow current to a value that can be safely interrupted by series gaps.
Power Frequency Voltage Rating of Arresters
VA = Max arrester voltage = VL-L(max)
at power freqency
Kg = 1
VA = Kg . VL-L(max) VA = VP(max)
Practical Situation Kg = 1/3 0.6
Vn remains at 0 even when fault current flows
Isolated neutralIdeally grounded neutral
VA = 0.8 VL-L(max) Nearest higher standard rating slected
80 arrester
Solidly grounded(Effectively grounded system)
The max voltage across arrester/s on the healthy phases under fault-conditions (single line-ground fault is almost invariably the worst) should not cause operation of arresters.
Zinc-Oxide (Metal Oxide) Gapless Surge Arresters
HVDC ARRESTER CONFIGURATION
STEP BY STEP insulation coordination procedures
The main objectives of the insulation coordination process are to establish requirements (i.e. equipment BIL/ BSL, air clearance, and insulator creepage) for various components in the HVDC system and to determine the devices (i.e. arresters) necessary to protect equipment. This process typically involves 2 stages :
Preliminary and Final insulation coordination.
PRELIMINARY HVDC insulation coordination design
This consist of following steps:
• Step 1: Establish Areester location.
• Step 2: Assume a single-column arrester at each location and determine minimum feasible arrester rating at each location.
• Step 3: Estimate, using simplified techniques, maximum arrester crest currents for lighting surges and for switching surges.
• Step 4: Estimate, using simplified techniques, arrester absorbed energy for current discharges determined in step 3, above.
• Step 5: Compare arrester absorbed energy determined in step 4, above, with arrester energy capability.
• Step 6: Tabulate arrester protective levels based on the results of step 3 through 5.
• Step 7: Determine minimum insulation withstand requirements based on minimum protective margins.
• Step 8: For equipment other than valves, select standard BIL levels which are at least as high as the minimum insulation withstand requirements for lightning surges determined in step 7.
Establish Arrester Location
Determine Minimum Feasible Rating of Single-Column Arrester at Each Location
Determine Arrester Crest Currents for Lighting and Switching Surges
• Step 5: Where feasible, optimize the selection of protective devices (arresters) and insulation withstand requirements by considering the cost of insulation versus the cost of protection.
Verify Maximum Continuous Operating Voltage and Check for Temporary Over voltages
Change Required?
Validate / Re-Establish Limiting Cases and Limiting Case Data