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

Various types of Gaps

1880 A.D.: a)'Horn Gap’

Rarely used : Waterjets to leak charges on

Transmission Lines.

b)'Modified Gaps'

Discriminating Lightning Arrester Low Equivalent Lightning Arrester

3. ‘Aluminum Cell’ LA

4. Oxide – Film Arrester Pellet Arrester

Crosby Field & Christopher Lougee

43 52 PbOOPb Lead RedOPbInsulating(Litharge)

150

g)(Conductin

o

5. Impulse Gaps – Chester Allcut

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

2

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

Determine Maximum Arrester Absorbed Energy

Tabulate Arrester Protective Levels

Determine Minimum Insulation Withstand Requirements

Choose standard BIL Levels and BSL where possible Calculate Actual margins

Tabulate Results

Is Energy Greater Than Arrester Capability?

New Solution Required?

Increase Number of Columns

NoNo

Yes

Yes

Flow Chart of Preliminary insulation coordination process

FINAL HVDC insulation coordination design

The objectives of the final insulation coordination process can be summarized as follows:

• Verification of the adequacy of the design.

• Elimination of excessive conservatism.

• Optimization of the design considering the cost of arresters.

• Determination of data for final arrester and equipment insulation specification

The steps involved in the methodology as shown in the flow chart are as follows:

• Step 1: Verify maximum continuous operating voltages (MCOV)

• Step 2: Validate or re-establish limiting cases and limiting case data.

• Step 3: Determine minimum insulation requiremtns.

• Step 4: Adjust BIL levels as required.

• 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

Determine Minimum Insulation Withstand Requirement

Adjust BIL Level as Required

Optimization Feasible?

Adjust Protective Level

Stop

Adjust V10

No

Yes

YesNo

Flow Chart of Final insulation coordination process

End