Substation Insulation Coordination Studies-Sparacino (1).pdf

Insulation Coordination Studies “The Selection of Insulation Strength”

March 25, 2014 Adam Sparacino

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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 the insulation 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 controlled closing, shield wires, improved grounding, etc.

(1) IEEE Std 1313.1‐1996 “IEEE Standard for Insulation Coordination ‐ Definitions, Principles, and Rules. MITSUBISHI ELECTRIC POWER PRODUCTS, INC.  POWER SYSTEM ENGINEERING SERVICES

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Four Basic Considerations • • • •

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

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Four Basic Considerations • • • •

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

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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 under normal service conditions.2

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

(2) IEEE Std C62.22‐1991 ‐ IEEE Guide for the Application of Metal‐Oxide Surge Arresters for Alternating‐Current Systems, 1991. MITSUBISHI ELECTRIC POWER PRODUCTS, INC.  POWER SYSTEM ENGINEERING SERVICES

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Vocabulary of Voltage

Peak line‐ground Voltage

RMS Voltage line‐ground = (Vpeak/√2)

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

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Illustration of Overvoltages

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Four Basic Considerations • • • •

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

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Electrical Insulation • Insulation can be expressed as a dielectric with a function to preserve 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 circuit breakers, etc.). – The insulation can be “external” (in air), which is exposed to atmospheric conditions (e.g., bushings, bus support insulators, disconnect switches, line insulators, air itself [tower windows, phase spacing], etc.).

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

Source: IEEE Std 62.22-1997, IEEE Guide for the Application of Metal-Oxide Surge Arresters for AC Systems MITSUBISHI ELECTRIC POWER PRODUCTS, INC.  POWER SYSTEM ENGINEERING SERVICES

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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

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Frequency of Different Events

10-20 minutes

seconds

Power System Control & Dynamics

Power Frequency

milliseconds

microseconds

Transients & Surges

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Four Basic Considerations • 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

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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 from being initiated.

3. Synchronous‐Close/Open Control –

Need to use independent pole operated (IPO) breakers and program controller based on equipment being switched (only help during breaker operation) to prevent excessive overvoltages from being initiated.

4. Surge Capacitors –

Need to be sized and located to “slow” the front of incoming surges

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Four Basic Considerations • 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

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Insulation Coordination Process 1. Specify the equipment insulation strength, the BIL and BSL of all equipment. 2. Specify the phase‐ground and phase‐phase clearances that should be considered. 3. Specify the need for, location, rating, and number of surge arresters.

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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 associated with 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. MITSUBISHI ELECTRIC POWER PRODUCTS, INC.  POWER SYSTEM ENGINEERING SERVICES

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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.

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EXAMPLE APPLICATION STUDY FOR INSULATION COORDINATION LIGHTNING SURGE ANALYSIS

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500 kV LINE

Refer to Figure 2 for details of line terminations.

BML00

Refer to Figure 2 for details of line terminations.

BLU00

BML01

All lengths shown in meters.

BLU01

la = 30.70 lb = 25.66 lc = 21.76 la = 23.47 lb = 22.56 lc = 21.64

500 kV LINE

la = 21.19 lb = 20.74 lc = 23.64 la = 23.47 lb = 22.56 lc = 20.64

WEST 500 kV BUS GWB06

la,b,c = 8.323

la,b,c = 19.59

G752W

G952W

CB

CB

G752E

la = 26.42 lb = 25.51 lc = 24.59

la = 9.518 lb = 8.603 lc = 7.689

G762W

G962W

CB

CB G952E

CB

G762E

G772W

la = 12.47 lb = 11.55 lc = 10.64

EAST 500 kV BUS

CB

CB

G772E

G872E

GML00

la,b,c = 8.323

GEB06

la = 70.62 lb = 76.69 lc = 82.77

G972W

G872W

G4A00

DUMMY BUS (POSITION FOR  FUTURE BREAKER)

XFMR

la,b,c = 5.634

CB

CB G962E

la,b,c = 19.59

G3A00

la,b,c = 8.323

la = 26.42 lb = 25.51 lc = 24.59

G972E

GLU00

la,b,c = 8.323

la,b,c = 5.634 la = 70.15 lb = 76.25 lc = 82.30

B3A01

B4A01

B3A00

B4A00

Refer to Figure 3 for details of XFMR terminations.

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XFMR

Refer to Figure 3 for details of XFMR terminations.

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Example for Line/XFMR Termination 500 kV Line

Line Trap1

To Transformer

Surge Arrester

Gas-toAir Bushing

CCVT

To GIS Bay #6

Gas-to-Air Bushing 550 kV GIS

350 MCM Ground Lead (38’)

Surge Arrester

Notes

To GIS Bay 550 kV GIS

(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.

350 MCM Ground Lead (38’)

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Detailed Analysis

Screening Analysis

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 operating configurations to screen for maximum potential overvoltages.

Step 2:

The resulting overvoltages were compared to the Basic Lightning Impulse Insulation Level (BIL) of the equipment and the protective margin1 for the equipment was calculated.

Step 3:

If overvoltages resulted in less than a 20% protective margin in the initial screening analysis for cases with the full system in or N‐1 contingencies, a more detailed analysis was performed to identify the protective margins resulting from a reasonable upper bounds lightning surge based on the configuration of the substation and connected transmission lines. –

various

For the detailed analysis, specific details of the transmission lines such as conductor characteristics, shielding design, ground resistivity, keraunic level, etc. are considered to determine a reasonable upper bounds to place on the lightning surge impinging on the substation.

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

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Lightning Surge Incoming From 500 kV Line Phase‐to‐Ground Voltage of Incoming Lightning Surge

MLFULL_halfSRC>MLSRCA(Type 1)

4000

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

Voltage (kV)

3000

2000 Lightning surge impinges substation from 500 kV Line.

1000 Lightning surge initiated at 1.0 microseconds.

0 0

5

10 Time (us) MITSUBISHI ELECTRIC POWER PRODUCTS, INC.  POWER SYSTEM ENGINEERING SERVICES

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

MLFULLB>G752WB(Type 1)

2000

GIS Basic Impulse Insulation Level (BIL) = 1550 kV

Voltage (kV)

1500 Protective Margin = 40% ([1550/1109 – 1] x 100%)

Peak overvoltage = 1109 kV.

1000

500

0 0

5

10 Time (us) MITSUBISHI ELECTRIC POWER PRODUCTS, INC.  POWER SYSTEM ENGINEERING SERVICES

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20

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EXAMPLE APPLICATION STUDY FOR INSULATION COORDINATION TRANSMISSION LINE SWITCHING ANALYSIS

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Transmission Line Switching Analysis Potential Equipment Concerns • Excessive Transient Overvoltages and  the Possibility of a Flashover During  Energizing or Re‐Closing • Overvoltages Exceeding Guidelines  Used to Develop Line Clearances Transmission line is energized (normal energizing or re-closing).

Applicable Criteria • •

Basic Switching Impulse Level (BSL) Probability of Flashovers

Potential Mitigation Techniques • • • •

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

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Statistical Switching Methodology

Source-Side Voltage ¼ cycle window

Case simulated with 200-400 energizations 3 = ¼ cycle ÷ 2 = 2.08 ms 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 Tclose Three poles closing centered around closing time (Tclose)

Sliding ¼ cycle window for pole closing shifted over a half cycle timeframe using a uniform distribution

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Electro‐Geometric Line Model Example 345 kV Transmission Line

14.5’

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

14.5’

78’  (63’ at midpoint) B

C

A

27’

27’

54’ (24’ at midpoint) Center Line

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Phase Conductor: ACSR Lapwing 2/c Bundle 18” spacing Outside diameter = 1.504” RDC = 0.059 Ohm/mi Thick/Diam = 0.375

Line Length (total) = 85 mi  Untransposed Ground resistivity = 37 Ohm‐m

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Statistical Switching Overvoltage Strength Characteristics  and SOV densities of the line

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

Statistical Distribution of Overvoltages Along Line 110%

Statistical  distribution based on  the case‐peak  method from IEEE  Std 1313.2‐1999.

Probability to Exceed Overvoltage (%)

100% Estimated insulation  withstand for the  transmission line:  CFO = 3.53  p.u., f/CFO =5%.

90% 80% 70%

Sending End

60%

1/4 Point

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

50% 40%

1/2 Point 3/4 Point

Highest overvoltage at the  remote end of the line = 2.75  p.u. (1123 kV).

30% 20%

Remote End Example CFO

E2 is the value in which the  overvoltages exceed 2% of the  switching operations.

10% 0% 1.00

1.50

2.00

2.50

3.00

3.50

4.00

Peak Overvoltage (Per Unit on a 500 kV Base) MITSUBISHI ELECTRIC POWER PRODUCTS, INC.  POWER SYSTEM ENGINEERING SERVICES

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

Statistical Distribution of Overvoltages Along Line 110%

Probability to Exceed Overvoltage (%)

100% Estimated insulation  withstand for the  transmission line:  CFO = 3.53  p.u., f/CFO =5%.

90% 80%

Statistical  distribution based on  the case‐peak  method from IEEE  Std 1313.2‐1999.

70%

Sending End

60%

1/4 Point 98% of the overvoltages along  the line are ≤ 2.16 p.u. (882  kV).

50%

1/2 Point 3/4 Point

40%

Remote End 30%

Example CFO

20%

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

E2 is the value in which the  overvoltages exceed 2% of the  switching operations.

10% 0% 1.00

1.50

2.00

2.50

3.00

3.50

4.00

Peak Overvoltage (Per Unit on a 500 kV Base) MITSUBISHI ELECTRIC POWER PRODUCTS, INC.  POWER SYSTEM ENGINEERING SERVICES

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EXAMPLE APPLICATION STUDY FOR INSULATION COORDINATION SHUNT CAPACITOR SWITCHING ANALYSIS

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Shunt Capacitor Switching Analysis

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

Potential Equipment Concerns • Contact Wear from Excessive Inrush  Current Duty • Excessive Transient Overvoltages • Induced Voltages and Currents in  Control Circuits • Step and Touch Potentials During  Switching Applicable Criteria • ANSI/IEEE Inrush Current Limits • Basic Switching Impulse Level (BSL) • Breaker Capability Beyond Standards • IEEE Std 80 for grounding Potential Mitigation Techniques • Current‐Limiting Reactors • Synchronous‐Close Control • Pre‐Insertion Resistors/Inductors • Surge Arresters MITSUBISHI ELECTRIC POWER PRODUCTS, INC.  POWER SYSTEM ENGINEERING SERVICES

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Capacitor Bank Re‐Strike During De‐Energization Current Through Switching Device High frequency  current is  interrupted

First restrike  occurs and  current is re‐ established

Second restrike occurs and  current is re‐established

Voltage on Each Side of Switching Device Peak overvoltage  from 1st restrike

Voltage on system  side of switching  device 

Current is  interrupted Voltage on capacitor  bank side of  switching device (DC  trapped charge) Peak overvoltage  from 2nd restrike

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Voltage Magnification • When a shunt capacitor bank is energized with a nearby capacitor at a lower voltage, the potential for voltage magnification may exist when the following condition is true: 1

1

2

2

• Furthermore, when C1>>C2, and L1