TRNG-6 Potential Transformer

Doble Testing

Potential Transformers ( PT’s )

Bob Sarni

Doble Engineering Company

Doble Engineering Company 85 Walnut Street, Watertown, MA 02472

Doble Testing Potential Transformers 2

Potential Transformers (PTs)

H1

H0

Isurface

Secondary Windings

Not Shown

Cascade Type



H1

CH

CHX

CHY

X1

X2

X3

Y1

Y2

Y3

H0 Line To Ground PT


X1X2

X3

Y1

Y2Y3

CH1 CH2

CHY

CHX


H1 H2

Line To Line PT


Recommended Test Voltage

Liquid-Filled Line-to-Line PTs Rated Below 15 kV Insulation Class

PT Voltage Rating Test Voltage (kV)

7.2 to 8.7 5.0

4.2 to 5.0 2.5

2.4 2.0

*For Units Rated 15 kV and Above, Perform Tests at 10 kV


Liquid-Filled Line-to-Ground PTs

a) Perform Test At the Rated Voltage of the Neutral Bushing

b) Cross-Check Tests are Performed at 10 kV or at Rated Line-to-

Ground Voltage, Whichever is Lower



Dry-Type Line-to-Line PTs Rated Below 15 kV Insulation ClassPT Voltage Rating Test Voltage (kV)

Overall a. 2 kV

b. Line-to-Ground Voltage*

c. 10% to 25% Above Line-to-Ground

operating Voltage

Cross-Check a. 2 kV

b. Line-to-Ground Operating Voltage

Exciting Current a. Line-to-Ground Operating Voltage


*Note: Power Factor Tip Up Test



Dry-Type Line-to-Ground PTs Rated Below 15 kV Insulation Class

•Perform Test at a Voltage Not to Exceed the Neutral Bushing

Rating as Outlined in Previous Section

•Cross-Checks are Performed at 10 kV or Rated Line-to-Ground

Voltage, Whichever is Lower

•For Units Rated Above 15 kV, Perform Tests at 2 and 10 kV to

Determine Tip-Up Value


Test Procedure - Single Phase - P T

Test

No.

Test

Mode Energize Ground Guard UST Test Description

1 GST H1, H2 X1,Y1 - - Overall

2 GST H1 X1Y1 H2 - H1 Cross Check

3 GST H2 X1Y1 H1 - H2 Cross-Check

4 UST H1 X1Y1 - H2 Excitation H1 to H2

5 UST H2 X1Y1 - H1 Excitation H2 to H1

6 Supplementary Hot-Collar tests on the primary line-terminalbushings



X1X2X3

Y1Y2Y3

CH1 CH2

CHY

CHX

Test #1 : GST Ground - No LV Lead Required

Measure: CH1+CHX+CHY+CH2

H1 H0HV Lead



Test #2: GST-Guard (Cross-Check Test #1)

•Measure: CH1+ 1/2CHX + 1/2CHY

X1X2X3

Y1Y2Y3

H1 H0

CH1 CHX CH2

CHY

HV Lead LV Lead



Test #3: GST-Guard (Cross-Check Test #2)

•Measure: CH2+ 1/2CHX + 1/2CHY

X1X2X3

Y1Y2Y3

H1 H0

CH1 CHX CH2

CHY

HV LeadLV Lead



X1X2X3

Y1Y2Y3

H1 H0

CH1 CHX CH2

CHY

HV Lead LV Lead

Test #4: Ungrounded Specimen Test (UST)

Measure: Excitation Current



X1X2X3

Y1Y2Y3

H1 H0

CH1 CHX CH2

CHY

HV LeadLV Lead

Test #5: Ungrounded Specimen Test (UST)



Supplementary Tests - Single-Phase P T

Test No.

Test Mode

Energize

Ground

Guard

UST

Test Description

6 UST H1, H2 Yn - Xn CHX

7 UST H1, H2 Xn - Yn CHY

8 GST H1 - H2, Xn, Yn

- Bushing H1

9 GST H2 - H1, Xn, Yn

- Bushing H2

Note: n designates any winding connection, 1, 2 or 3


X1X2X3

Y1Y2Y3

CH1 CH2

CHY

CHX

Test #6 : UST

Measure: CHXH1 H0

HV Lead

LV Lead



X1X2X3

Y1Y2

Y3

CH1 CH2

CHY

CHX

Test #7 : UST

Measure: CHYH1 H0

HV Lead

LV Lead




X1X2X3

Y1Y2Y3

H1 H0

CH1 CHX CH2

CHY

HV Lead

LV Lead

Test #8: GST-Guard Red & Blue

Measure: CH1

LV Lead



X1X2X3

Y1Y2Y3

H1 H0

CH1 CHX CH2

CHY

HV Lead

LV Lead

Test #9: GST-Guard Red & Blue

Measure: CH2

LV Lead


Single Phase Internally Grounded PT

H1

CH

CHX

CHY

Neutral Shown Internally Grounded H0

X1

X2X3

Y1Y2Y3


Test No. Mode Energize Ground Guard UST

1 UST H1 HO ---- X1,Y1

2 GST H1 HO X1,Y1 ----

Test No. 1: Measures Line-End Interwinding Insulation

Test No. 2: Measure Excitation Current

Single Phase Internally Grounded PT

Test Procedure


Cascade Potential Transformers

H0

H1

Ground Lead



•Test Preparation - Single Phase Line-to-Ground PT

•One Side Of Each Secondary Winding Grounded

•Remove Ground Link From Neutral

•Short Circuit H1(Line) to H0 (Neutral)

•Test No. 1 Only

NOTE: Test Voltage for Tests 1, 3, 4, 5 Limited by

Rating of Neutral Bushing (5 kV)



Test #1: GST-Ground

H0

H1

Ground Lead

HV Cable

Test Voltage for Tests 1, 3, 4, 5 Limited by Rating of Neutral Bushing (5 kV)

Secondary Windings

Not Shown


H1


H0

HV Cable

Ground Lead

Test #2: GST-Guard

Measure: Cross-Check #1

LV Lead

Secondary Windings

Not Shown



Test #3: GST-Guard

Measure: Cross-Check #2

H0

HV CableGround Lead

H1

LV Lead


Secondary Windings

Not Shown



H0

HV Cable

Ground Lead

Test #4: UST


Secondary Windings

Not Shown



H0

HV CableGround Lead

H1

LV Lead

Test #5: UST



Secondary Windings

Not Shown



Test No. Test Mode Energize Ground Floating Test

Description

1A GST H0 X1, X2, X3 H1 Overall

Y1, Y2, Y3

Tests 2 Through 5 - Same as

Conventional PT

Alternative Method for Performing the Overall Test*

*Note: No Short Circuit Used Between H1 & H0



H0

HV CableGround Lead

H1

Test #1A: GST-Ground


No Short Circuit on Primary Winding

Secondary Windings

Not Shown


Use of Guard Collar on Cascade PT

H1

H0

Connect to Guard

Isurface

Secondary Windings

Not Shown


Test Procedure-Single-Phase PT - DTA


Analysis of Results: PTs

•Compare Overall Power Factor Test Values to

Previous Test Results

•Compare Overall Power Factor Values to Similar Units

(Doble Test-Data-Reference-Book)

•Compare Cross-Check Power Factors

•Most PTs Test Results are Similar



PT #1

Test No. mA Watts %PF

1 (Overall) 2.540 0.104 0.41

2 (H1 Cross-check) 1.256 0.056 0.45

3 (H2 Cross-check) 1.248 0.046 0.37

Examples:

PT #2

Test No. mA Watts %PF

1 (Overall) 2.650 0.354 1.32

2 (H1 Cross-check) 1.356 0.313 2.31

3 (H2 Cross-check) 1.248 0.055 0.44


What is Wrong With PT #2?


•Consider Performing Supplemental Tests to Locate

Cause of Problem

•Refer to Doble Test Equipment Instruction Manual


Test No. mA Watts % Power Factor

1 (Overall) 3.040 0.152 0.50

2 (H1 Cross-Check) 0.278 0.0312 1.12

3 (H0 Cross-Check) 2.810 0.128 0.46

Typical Good Unit




Test No. mA Watts % Power Factor

1 (Overall) 3.080 0.181 0.58

2 (H1 Cross-Check) 0.311 0.0682 2.19

3 (H0 Cross-Check) 2.810 0.128 0.46

Questionable Unit

Documents

TRNG-6 Potential Transformer