Mitsubishi - substation insulation coordination studies-sparacino

Insulation Coordination Studies“The Selection of Insulation Strength”

March 25, 2014Adam Sparacino

MITSUBISHI ELECTRIC POWER PRODUCTS, INC. POWER SYSTEM ENGINEERING SERVICES

Definition of Insulation Coordination1

• Insulation Coordination (IEEE)– The selection of insulation strength consistent with expected

overvoltages to obtain an acceptable risk of failure.– The procedure for insulation coordination consists of (a)

determination of the voltage stresses and (b) selection of theinsulation strength to achieve the desired probability of failure.

– The voltage stresses can be reduced by the application of surge‐protective devices, switching device insertion resistors and controlledclosing, shield wires, improved grounding, etc.

MITSUBISHI ELECTRIC POWER PRODUCTS, INC. POWER SYSTEM ENGINEERING SERVICES 2

(1) IEEE Std 1313.1‐1996 “IEEE Standard for Insulation Coordination ‐ Definitions, Principles, and Rules.

Four Basic Considerations

• Understanding Insulation Stresses• Understanding Insulation Strength• Designing Methods for Controlling Stresses• Designing Insulation Systems





Definition of Overvoltages

• Overvoltage– Abnormal voltage between two points of a system that is greater than

the highest value appearing between the same two points undernormal service conditions.2

• Overvoltages are the primary “metric” for “measuring” and“quantifying” power system transients and thus insulationstress.


(2) IEEE Std C62.22‐1991 ‐ IEEE Guide for the Application of Metal‐Oxide Surge Arresters for Alternating‐CurrentSystems, 1991.

Vocabulary of Voltage


Peak line‐ground VoltageRMS Voltage line‐ground = (Vpeak/√2)

Peak Voltage line‐ground = VL‐L_rms√2/√3

Illustration of Overvoltages





Electrical Insulation

• Insulation can be expressed as a dielectric with a function topreserve the electrical integrity of the system.– The insulation can be “internal” (solid, liquid, or gaseous), which is

protected from the effects of atmospheric conditions (e.g.,transformer windings, cables, gas‐insulated substations, oil circuitbreakers, etc.).

– The insulation can be “external” (in air), which is exposed toatmospheric conditions (e.g., bushings, bus support insulators,disconnect switches, line insulators, air itself [tower windows, phasespacing], etc.).


Insulation Strength


Source: IEEE Std 62.22-1997, IEEE Guide for the Application of Metal-Oxide Surge Arresters for AC Systems

Typical Volt Time Curve for Insulation Withstand Strength for Liquid Filled Transformers

Insulation Strength

• Example for Transformers Windings– Normal system operating voltage

• 345 kVL‐L_RMS (1.00 p.u.)

– Maximum continuous operating voltage (MCOV)• 362 kVL‐L_RMS (1.05 p.u.)

– Basic switching impulse insulation level (BSL)• 745/870/975 kVL‐N_Peak

– Basic lightning impulse insulation level (BSL)• 900/1050/1175 kVL‐N_Peak

– Chopped wave withstand (CWW)• 1035/1205/1350 kVL‐N_Peak


Frequency of Different Events


Transients& Surges

Power System Control& Dynamics

milliseconds microsecondsseconds10-20 minutes Power

Frequency


• Understanding Insulation Stresses• Duty and Magnitude of applied voltage

• Understanding Insulation Strength• Ability to withstand applied stress

• Designing Methods for Controlling Stresses• Designing Insulation Systems


Potential Overvoltage Mitigation

1. Surge Arresters– Need to be sized and located properly to “clip” overvoltages.

2. Pre‐Insertion Resistors/Inductors– Need to be sized according to equipment being switched (only help

during breaker operation) to prevent excessive overvoltages frombeing initiated.

3. Synchronous‐Close/Open Control– Need to use independent pole operated (IPO) breakers and program

controller based on equipment being switched (only help duringbreaker operation) to prevent excessive overvoltages from beinginitiated.

4. Surge Capacitors– Need to be sized and located to “slow” the front of incoming surges



• Understanding Insulation Stresses• Duty and Magnitude of applied voltage

• Understanding Insulation Strength• Ability to withstand applied stress

• Designing Methods for Controlling Stresses• Designing Insulation Systems


Insulation Coordination Process

1. Specify the equipment insulation strength, the BIL and BSL ofall equipment.

2. Specify the phase‐ground and phase‐phase clearances thatshould be considered.

3. Specify the need for, location, rating, and number of surgearresters.


Insulation Coordination Studies

1. Very Fast Transients (VFT) Analysis (nanoseconds time frame)– GIS disconnected switching.– Quantify the overvoltages throughout the substation.– Primary intent of determining location and number of surge arresters

within the substation.2. Lightning Surge Analysis (microseconds time frame)

– Quantify the overvoltages throughout the substation.– Primary intent of determining location and number of surge arresters

within the substation.3. Switching Overvoltage Analysis (milliseconds time frame)

– Quantify the overvoltages and surge arrester energy duties associatedwith switching events and fault/clear operations.

– Primary intent is to verify that transient overvoltage mitigating devices(e.g., surge arresters, pre‐insertion resistors, synchronous close control)are adequate to protect electrical equipment.

– Capacitor, Shunt Reactor, Transformer, and Line Switching Studies.


Insulation Coordination Studies (cont.)

4. Temporary Overvoltage Analysis (seconds time frame)– Quantify the overvoltages and surge arrester energy duties as produced

by faults, resonance conditions, etc.– Primary intent is to verify conditions that cause problems within the

system and develop the necessary mitigation.– Fault/Clear, load rejection, ferroresonance studies.

5. Steady State Analysis (minutes to hours time frame)– Quantify voltage during various system configurations.– Power flow/stability studies.



EXAMPLE APPLICATIONSTUDY FOR INSULATION COORDINATION

LIGHTNING SURGE ANALYSIS


EAST 500 kV BUS

WEST 500 kV BUS

CB CB CB CB CB CB CB CB CB

DUMMY BUS (POSITION FOR FUTURE BREAKER)

GML00

G762W

G762E

GEB06

G752E

G752W

G3A00

B3A01

B3A00

G952E

G952W

GWB06

G962W

G962E

G972W

G972E

GLU00

G872W

BLU01

BLU00

G872E

G4A00

G772W

G772E

B4A01

B4A00

la = 30.70lb = 25.66lc = 21.76

la = 21.19lb = 20.74lc = 23.64

la = 70.62lb = 76.69lc = 82.77

la = 70.15lb = 76.25lc = 82.30

la = 26.42lb = 25.51lc = 24.59

la = 23.47lb = 22.56lc = 21.64

la = 23.47lb = 22.56lc = 20.64

la = 26.42lb = 25.51lc = 24.59

la,b,c = 8.323

la,b,c = 19.59la = 12.47lb = 11.55lc = 10.64

la,b,c = 19.59

la,b,c = 8.323

la = 9.518lb = 8.603lc = 7.689

la,b,c = 8.323

la,b,c = 5.634

la,b,c = 5.634

la,b,c = 8.323

BML00

BML01

500 kV LINE 500 kV LINE

Refer to Figure 2 for details of line terminations.

Refer to Figure 2 for details of line terminations.

XFMR Refer to Figure 3 for details of XFMR

terminations.

Refer to Figure 3 for details of XFMR

terminations.

XFMR

All lengths shown in meters.

Example for Line/XFMR Termination


Notes(1) Line traps only on phase A and C for 500 kV lines. In

EMTP model, phase B has a 2.53 m section of conductor modeled in place of line trap.

550 kV GIS

To GIS Bay #6

Line Trap1

CCVT

Gas-to-Air

Bushing

Surge Arrester

500 kV Line

350 MCM Ground Lead

(38’)

550 kV GIS

To GIS Bay

Gas-to-Air Bushing

Surge Arrester

To Transformer

350 MCM Ground

Lead (38’)

Approach for Evaluation the Insulation Coordination of the 550 kV Gas‐Insulated Substation


Step 1: A severe voltage surge was injected into the substation for variousoperating configurations to screen for maximum potential overvoltages.

Step 2: The resulting overvoltages were compared to the Basic Lightning ImpulseInsulation Level (BIL) of the equipment and the protective margin1 for theequipment was calculated.

Step 3: If overvoltages resulted in less than a 20% protective margin in the initialscreening analysis for cases with the full system in or N‐1 contingencies, a moredetailed analysis was performed to identify the protective margins resulting from areasonable upper bounds lightning surge based on the configuration of thesubstation and connected transmission lines.– For the detailed analysis, specific details of the transmission lines such as conductor

characteristics, shielding design, ground resistivity, keraunic level, etc. are considered todetermine a reasonable upper bounds to place on the lightning surge impinging on thesubstation.

(1) Protective Margin = [ BIL / Vmaximum_peak – 1] x 100%

Screen

ing An

alysis

Detailed An

alysis

Lightning Surge Incoming From 500 kV LinePhase‐to‐Ground Voltage of Incoming Lightning Surge


0

1000

2000

3000

4000

0 5 10 15 20

MLFULL_halfSRC>MLSRCA(Type 1)

Vol

tage

(kV

)

Time (us)

Peak = 3264 kV (1.2 x 2720 kV CFO)Time-to-peak = 0.5 microseconds.

Lightning surge impinges substation from 500 kV Line.

Lightning surge initiated at 1.0 microseconds.

Lightning Surge Incoming From 500 kV LineHighest Phase‐to‐Ground Voltage Observed in GIS


0

500

1000

1500

2000

0 5 10 15 20

MLFULLB>G752WB(Type 1)

Vol

tage

(kV

)

Time (us)

Peak overvoltage = 1109 kV.

GIS Basic Impulse Insulation Level (BIL) = 1550 kV

Protective Margin = 40% ([1550/1109 – 1] x 100%)



TRANSMISSION LINE SWITCHING ANALYSIS

Transmission Line Switching Analysis


• Excessive Transient Overvoltages and the Possibility of a Flashover During Energizing or Re‐Closing

• Overvoltages Exceeding Guidelines Used to Develop Line Clearances

Potential Equipment Concerns

Transmission line is energized (normal energizing or re-closing).

• Synchronous‐Close Control• Pre‐Insertion Resistors/Inductors• Surge Arresters• Shunt Reactors

Potential Mitigation Techniques

• Basic Switching Impulse Level (BSL)• Probability of Flashovers

Applicable Criteria

Statistical Switching Methodology


Tclose

Three poles closing centered around closing

time (Tclose)

3 = ¼ cycle ÷ 2 = 2.08 ms

Sliding ¼ cycle window for pole closing shifted over a half cycle

timeframe using a uniform distribution

Each pole can close at anytime within the ¼ cycle window centered around the closing time (Tclose) for

each energization. Random closing times based on a normal (Gaussian)

distribution

¼ cycle window

Source-Side Voltage

Case simulated with 200-400 energizations

Electro‐Geometric Line ModelExample 345 kV Transmission Line


14.5’ 14.5’

27’

B C A

27’

54’(24’ at midpoint)

78’ (63’ at midpoint)

CenterLine Line Length (total) = 85 mi

UntransposedGround resistivity = 37 Ohm‐m

Phase Conductor:ACSR Lapwing2/c Bundle 18” spacingOutside diameter = 1.504”RDC = 0.059 Ohm/miThick/Diam = 0.375

Shield Wire:Alumoweld 7#8Outside diameter = 0.385”RDC = 2.40 Ohm/mi

Statistical Switching Overvoltage Strength Characteristics and SOV densities of the line


Statistical Distr. Of Overvoltages Along 500 kV Line with NO Surge Arresters


0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

110%

1.00 1.50 2.00 2.50 3.00 3.50 4.00

Prob

ability to

Exceed Overvoltage (%

)

Peak Overvoltage (Per Unit on a 500 kV Base)

Statistical Distribution of Overvoltages Along Line

Sending End

1/4 Point

1/2 Point

3/4 Point

Remote End

Example CFO

Estimated insulation withstand for the

transmission line: CFO = 3.53 p.u., f/CFO =5%.

E2 is the value in which the overvoltages exceed 2% of the

switching operations.

Highest overvoltage at the remote end of the line = 2.75

p.u. (1123 kV).

98% of the overvoltages along the line are ≤ 2.62 p.u. (1070

kV).

Statistical distribution based on

the case‐peak method from IEEE Std 1313.2‐1999.

Statistical Distr. Of Overvoltages Along 500 kV Line withLine End Surge Arresters


0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

110%

1.00 1.50 2.00 2.50 3.00 3.50 4.00

Prob

ability to

Exceed Overvoltage (%

)

Peak Overvoltage (Per Unit on a 500 kV Base)

Statistical Distribution of Overvoltages Along Line

Sending End

1/4 Point

1/2 Point

3/4 Point

Remote End

Example CFO

Estimated insulation withstand for the

transmission line: CFO = 3.53 p.u., f/CFO =5%.

E2 is the value in which the overvoltages exceed 2% of the

switching operations.

Highest overvoltage along the line = 2.21 p.u. (902 kV).

98% of the overvoltages along the line are ≤ 2.16 p.u. (882

kV).

Statistical distribution based on

the case‐peak method from IEEE Std 1313.2‐1999.



SHUNT CAPACITOR SWITCHING ANALYSIS

Shunt Capacitor Switching Analysis


• Contact Wear from Excessive Inrush Current Duty

• Excessive Transient Overvoltages• Induced Voltages and Currents in

Control Circuits• Step and Touch Potentials During

Switching


Capacitor bank is energized and transient inrush currents flow through capacitor bank breaker and voltage surges propagate into the system.

• Current‐Limiting Reactors• Synchronous‐Close Control• Pre‐Insertion Resistors/Inductors• Surge Arresters


• ANSI/IEEE Inrush Current Limits• Basic Switching Impulse Level (BSL)• Breaker Capability Beyond Standards• IEEE Std 80 for grounding

Applicable Criteria

Capacitor Bank Re‐StrikeDuring De‐Energization


Current Through Switching Device Voltage on Each Side of Switching Device

Current is interrupted

First restrike occurs and current is re‐established

High frequency current is interrupted

Second restrike occurs and current is re‐established

Voltage on capacitor bank side of

switching device (DC trapped charge)

Voltage on system side of switching

device

Peak overvoltage from 1st restrike

Peak overvoltage from 2nd restrike

Voltage Magnification


• When a shunt capacitor bank is energized with a nearbycapacitor at a lower voltage, the potential for voltagemagnification may exist when the following condition is true:

1 1 2 2

• Furthermore, when C1>>C2, and L1<<L2 the condition can be exaggerated

Voltage Magnification (Cont.)


Example 1.95 p.u. overvoltage at HV bus when capacitor bank is switched.

Example 4.39 p.u. overvoltage at LV bus when capacitor bank is switched.



SHUNT REACTOR SWITCHING ANALYSIS

Shunt Reactor Switching Analysis


• Excessive Inrush Currents from Energizing

• Transient and Temporary Overvoltages from Resonance Conditions

• Generation of Harmonics• Resonance from Parallel Lines


Shunt reactor is energized and inrush current flows through the

system and circuit breaker.

• Synchronous‐Close Control• Surge Arresters• Appropriate Relay Settings• Operational Limitations


• Equipment Insulation Levels• Voltage Sag/Dip Criteria• Harmonic Distortion

Applicable Criteria

Resonance Overvoltages


345 kV Substation

Voltage Measured on Energized Line

Line in service (breakers closed at both ends)

Line out of service (breakers open at both ends)

345 kV Substation

345 kV Substation 345 kV Substation

Resonance Overvoltages


Peak overvoltage = 2.94 p.u.

It is anticipated that the line equipment would be capable of withstanding at

least 1.5 p.u. for 100 ms.

Line breakers open to trip the line at 200 ms.

The shunt reactors should be tripped within 550 ms of the line breakers

tripping to avoid excessive overvoltages for this case.

Anticipated temporary overvoltage (TOV) capability (1.5 p.u. for 100 ms).

Summary

• Insulation Coordination is the selection of insulation strength.• Determine maximum insulation stress.• Determine the minimum insulation strength with margin taking into

account stress reducers (surge arresters, pre‐insertion resistors,synchronous close control, etc.) that can withstand the maximumstress.

• Studies help in quantifying the maximum anticipated stressand determining the rating/location of overvoltage mitigatingdevices.

• A key component of insulation coordination is pairing thecorrect strength to the correct stress.• As a rule of thumb, the shorter the time the overvoltage is applied to

the insulation the greater the magnitude of overvoltage the insulationcan withstand before failure.



THANK YOU FOR YOU ATTENTION

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Mitsubishi - substation insulation coordination studies-sparacino