History and Detection of Transformer Over-Excitation at

CenterPoint Energy

Eric Weygandt

May 3rd, 2019

1

Disclaimer

• This presentation is being provided for informational purposes only and does not purport to be comprehensive. Neither CenterPoint Energy, Inc., together with its subsidiaries and affiliates, nor its employees or representatives, make any representation or warranty (express or implied) relating to this information.

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Introduction

• Energize a 5-limb distribution transformer with the same voltage on two phases

– BBC or ACC

• Occurs when a jumper disconnects and is fed from the phase below

• 9 events and 42 damaged transformers

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4

VA

VB

VC

VA

VB

VC

VB

• A-phase disconnects, is fed by B-phase (BBC)

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Contents

• History at CenterPoint

• Electromagnetic analysis

• Thermal analysis

• Event detection

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History

• Report by Robert Almonte, 1990 [1]

• Four transformers damaged

• Basic concepts of tank overheating

– Delta versus wye connection

– 3, 4, or 5-leg core design

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History

• Sequence components

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History

• Transformer secondary line-line voltages

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History

• Effect of voltage imbalance on motors

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History

• Report by David Gomes, 1998 [2]

• Two transformers damaged

• Measured voltages:

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~ ~

~

HL&P 2500 kVA 34500/19920V gnd-Y 12470/7200V gnd-Y

Customer’s TRF 12470/7200V ? 4160/2400V ?

VAB = 5000 VBC = 2500 VCA = 1800

History

• CKT-Y on August 23rd, 2016

• 8 transformers damaged

• BBC event from 2:09p to 4:00p (1 hr 51 min)

• Delta-connected transformer not damaged

• Feeder protection from microprocessor relays

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CKT-Y @ 1:59 pm

13 Feeder locked out

CKT-Y @ 2:09 pm

14 Feeder reclosed 10 minutes later, transformer inrush

CKT-Y @ 2:54 pm

15 Load or inrush

CKT-Y @ 2:58 pm

16 Line-to-line fault, protective device on the feeder clears within 1 cycle

History

• CKT-Z on August 24th, 2017

• T/C/T/C at 11:44 am

• Remote trip at 1:33 pm (1 hr 48 minutes)

• Remote close at 3:20 pm

• 5 out of 9 transformers on ckt damaged – 1 upstream of fault

– 1 recloser-protected

– 2 didn’t get too hot

• Feeder protection from electromechanical relays

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18

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20

21

22

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300 kVA 205 gallons of oil

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750 kVA 323 gallons of oil

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300 kVA 386 gallons of oil

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750 kVA 384 gallons of oil

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750 kVA 286 gallons of oil

History

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Almonte

Gomes

CKT-Y

CKT-Z

Electromagnetic Analysis

• Core Flux

• Magnetizing current

• Tank eddy current

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30

1.2 T

1.6 T

Main leg flux:

Main leg flux:

Ref. #[3]

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t = 0°

t = 60°

t = 120°

Ref. #[4]

kL/in2 tesla

38 0.6

76 1.2

114 1.8

152 2.4

Electromagnetic Analysis

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𝑖𝜑 = 0.20%

𝑖𝑐 = 0.17%

𝑖𝑚 = 0.10%

𝑖𝜑 → 𝑖𝑐 𝑖𝑚

→

→

𝑋𝑚

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Electromagnetic Analysis

• Simulated magnetizing current using Mork’s Hybrid Model in ATPDraw [5]

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Electromagnetic Analysis

ATPDraw Hybrid XFMR model of a 1500 kVA FLA = 25 amps, simulated amps = 15 amps -> 60%

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Electromagnetic Analysis

SEL-351R protecting a 1500 kVA, feeds office building FLA = 25 amps, measured amps = 15 amps -> 60%

Electromagnetic Analysis

• Agarwal described eddy current in solid iron (tank) and laminated iron (core) in 1959 [6]

• Laminated iron has lower losses because E & H fields are restricted in the time

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Electromagnetic Analysis

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Return leg and yoke flux exceed 2.0 T for roughly 3.5 ms per half-cycle (42%)

x 0.42

Electromagnetic Analysis

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Thermal Analysis

• No-load loss measured at GE in Shreveport

• Heat generation equations, time-to-failure

• CFD analysis in OpenFOAM

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Thermal Analysis [4]

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(500 kVA)

Thermal Analysis

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42

t = 90 minutes

t = 60 minutes

67 °C = 340 °K

110 °C = 383 °K

Event Detection

• Email notification from the EMS

• Negative-sequence overvoltage from smart-grid reclosers

• Voltage monitoring at pad-mounted transformer secondary

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Event Detection

• Email notification from the EMS:

– 3φ motor load drops offline

– 1φ load moved from one phase to another

– 90° lagging magnetizing current (extra KVAR) appears on the phase feeding both

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Event Detection

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Event Detection

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EVENTMAX

AMPS

CURRENT

UNBALANCE

MAX

KVAR

KVAR STEP-

INCREASE

CKT-W 320 50% 1400 2000

CKT-X 250 88% 1500 2100

CKT-Y 300 156% 3000 3000

CKT-Z 460 105% 2000 2000

Event Detection

• EMS checks each feeder every two minutes, sends email if:

– Max amps > 200

– Amps unbalance > 50%

– KVAR > 1000

– Step-increase in KVAR over 800

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VA

VB

VC

VB

VC

VB VA

References

• [1] Robert L. Almonte, “Effects of Single Phasing on Three Phase Pad Mount Transformers.” Houston Lighting and Power Company. Presented at the Power Distribution Conference in Austin, Texas on October 23rd, 1990.

• [2] David Gomes. “Pad Mounted Transformers Failures on Circuit Newport 44.” Houston Lighting and Power Company. January 1998.

• [3] T. D. Kefalas and A. G. Kladas, "Mixed Si-Fe Wound Cores Five Legged Transformer: Losses and Flux Distribution Analysis," in IEEE Transactions on Magnetics, vol. 48, no. 4, pp. 1609-1612, April 2012.

• [4] Will Elliott, Rich Lowe, Bud Stinson. “CenterPoint Jumper Failure Case Study 2-Phase 3-Coil Failure Analysis.” Presented by GE Shreveport to Major Underground on January 16th, 2018.

• [5] Hans K. Høidalen, Bruce A. Mork, Francisco Gonzalez-Molina, Dmitry Ishchenko, Nicola Chiesa. “Implementation of the Hybrid Transformer Model in ATPDraw.” URL: <www.elkraft.ntnu.no/~chiesa/Files/EEUG05_Hoidalen.pdf>.

• [6] Paul D. Agarwal, "Eddy-current losses in solid and laminated iron," in Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics, vol. 78, no. 2, pp. 169-181, May 1959.

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