Feeder Neutral Potential Rise Due to GPR Transfer and ...apic/uploads/Forum/P2014_4.pdfObservation...

1

Confidential – all rights reserved

Feeder Neutral Potential Rise

Due to GPR Transfer and Induction

ATCO Electric

Presented by Alex Nassif at the

APIC Power & Energy Innovation Forum – 2014

University of Alberta, ETLC Solarium

5 November 2014

2 Confidential – all rights reserved

144 kV

Substation

MGN

25 kV Transmission side

Distribution side Customer side

gnR

nZ

nZnZnZnZ

subRgnR gnRgnRgnR

///

Equivalent circuit of Upstream ends

Problem Definition

Should the substation neutral be connected to the feeder neutral?

MGN ONLY


Problem Definition

For a long time ATCO and other utilities had no answer to the

question

1990’s – many questions raised

More prevalent in underground cables emanating out of the

substation

‒ Should the substation end of the cable be left disconnected?

‒ Should the riser pole end of the cable be left disconnected?

‒ Should both be connected?

‒ With current flowing through the underground cable neutrals will the

cable ampacity be reduced?

‒ Should an arrestor be used for bonding the neutral to the substation

ground?

144 kV

Substation

MGN

25 kV Transmission side

Distribution side Customer side

gnR

nZ

nZnZ

nZ

nZ

subRgnR gnRgnRgnR

///

Equivalent circuit of Upstream ends

U/G


Problem Definition

2012 – ATCO took a more active role in seeking out answers

Other AB utilities also had questions.

Fortis AB example:

‒ Prior to 2008 – Earth Return system in rural areas

‒ MGN required in urban areas – communication joint use of poles

‒ After the 2006 analysis by Dr. Xu, Fortis decided to introduce more MGN

systems (helps telephone interference too)


Problem Definition

Substation GPR can be transferred and cause NPR

‒ How bad is the resulting NPR?

What are the implications of not connecting?

‒ This represents a broken neutral configuration.


Problem Definition

This project was initiated to try and address these issues.

To ATCO Electric and other AB utilities safety is paramount.

Next slides show the developments, results and conclusions of the

APIC study.


GPR Transfer – Distribution or Transmission?

Equivalent voltage source of GPR Fault current flow pattern in substation

Fault inside substation Fault outside substation


Substation GPR Transfer

faultsubsubstation IRGPR

Propagation of GPR from the substation to the system:

substation

n

MGNn

MGNneutral GPR

ZZ

ZNPR

sRZZ gnMGN

Transfer ratio

0

2

4

6

8

10

12

14

16

0 0.5 1 1.5 2Neutral length [km]

Equiv

lent im

ped. [o

hm

]

Z’geq


Substation GPR Transfer - Results

GPR transfer ratio

substation

n

MGNn

MGNneutral GPR

ZZ

ZNPR

Transfer ratio



Observation GPR transfer ratio k

Max NPR per kA (V/kA)

Corresponding system condition

High end value 0.95 129 m 50S, 30 gnR

Typical value 0.91 124 m 001S, 15 gnR

Low end value 0.76 108 m 500S, 7 gnR

NPR – neutral potential rise = GPRneutral

Notice how high this can get



gnRgnRgnRgnR

nZ nnZnZ

nZ nZ nZ

maxNPR

GPRgnR

Customer #1 Customer #2

2NPR NNPR

Neutral voltage rise (NPR) at customer locations

1 maxNPR =NPR =k×GPR2 1 1NPR =k ×NPR i+1 i iNPR =k ×NPR

n

nNPR =k ×GPR

…..


Max NPR per kA

NPR per kA at 2 km (V/kA)

NPR per kA at 5 km (V/kA)

m 50S, 30 gnR 129 24 3.4

m 001S, 15 gnR 124 22 3.2

m 500S, 7 gnR 108 46.4 12

Noticeably lower as distance increases


Induction caused NPR

X N totalGPR I Z

|GPR|

Net GPR profile

Islanded neutral

I

I

Neutral V = Neutral GPR

Will the GPR close to the first pole become unacceptably high if the neutral is not connected?



Induced voltages in the neutral during a LG fault Conversion of voltage sources to current sources

Series connection of individual current sources Shunt current sources at locations of peak NPRs



Parallel Zone

NPR caused by

source at F

NPR caused by source at X

Net NPR profile

Neutral exposure

with fault current

X F

Neutr

al V

|N

eutr

al V

|

IF

znn

RgnRgn Rgn Rgn Rgn RgnRgn

znn znn znn znn znnen en en

+ - + - + -

X YF



A typical value of Ztransfer for distribution lines in AB is about 0.34 ohm (s = 1km).

For example if a fault current is 1kA, the maximum GPR is 340 V.

NPR profile due to two shunt current sources

1

2

mF nn gn F

nn

ZNPR Z R s I

Z

maxm

nn gn F

nn

ZNPR Z R s I

Z

Absolute value of NPR profile

Ztransfer Ztransfer



Case study

NPRmax for different fault locations NPR profile for fault at 6 km from substation


Substation GPR Transfer vs Induction

If neutrals are not connected:

Observation GPR transfer ratio k

Max NPR per kA (V/kA)


High end value 0.95 129 m 50S, 30 gnR

Typical value 0.91 124 m 001S, 15 gnR

Low end value 0.76 108 m 500S, 7 gnR

There may be a concern

(Note that the fault currents

are different)

NPRmax=0.124IF_1

NPRmax= Ztransfer IF_2 = 0.34 IF_2


Substation GPR Transfer vs Induction

GPR transfer & inductive coupling effects

Summary: more investigations are needed to find the best compromise


APIC Report Conclusions

The NPR is caused by substation GPR transfer and inductive coupling.

If the neutral is not connected:

‒ The substation GPR is not transferred to the MGN.

‒ Induction coupling effect is highest close to the substation.

If the neutral is connected:

‒ The GPR transfer is the worst and NPR is highest in the first few grounding

structures.

‒ Induction is greatly reduced close to the substation

Therefore, it can be said that the GPR transfer and induction have conflicting

requirements


ATCO’s Direction

Ground potential

No. of grounding points

from substation

Substation neutral

First grounding point outside fence

(neutral connection point)

600V threshold

Hazard zone

Should not happen in public area

i.e. if it happens in a public area,

NPR is too dangerous


ATCO’s Direction

Why 600V threshold?

V0

0.5m

Step Voltage

V0

V0 is the NPR

0.116body

s

It

0.1161000step

s

Vt

IEEE Std.80 (t=0.5s) Simplified (t=0.5s)

50 kg 726.0 V 453.8 V

70 kg 982.6 V 614.1 V

Rod=1.5m


ATCO’s Direction

Is the feeder O/H or U/G?

O/H

Is there a M.G.N on the riser pole?

Does the O/H line have a neutral leaving the

substation?

Is the pole located in an area frequently travelled

by the public?

Connecting the concentric neutral to the substation ground is recommended

Connect; Refer to isolation

manual

Not a Concern

U/G

Next Slide


ATCO’s Direction

Connecting the neutral to the substation ground is

recommended Fault level (amps)

Substation grounding resistance

Cable Leaving the Substation

System Neutral or U/G system Cable

Are the first X grounding points

(Ground Rods, Pull Boxes, etc.)

located in areas frequently

travelled by the public?

Connect; Refer to isolation

manual YES ….

Data for Calculation

Previous Slide


ATCO’s Direction

Some perceived additional benefits of connecting:

‒ Reduction in GPR at the fault point for a feeder fault (smaller equivalent

impedance).

‒ Reduction in interconnected ground impedance, helping protective devices

operate properly under high impedance fault conditions.

‒ Reduction of induced voltage closer to the substation due to feeder faults.

‒ Added safety to personnel by eliminating potential difference between MGN

and the concentric neutral at the riser pole where connection is made.

‒ Reduction in telephone interference (IT) with telephone buried service close to

the substation.

‒ Reduction in uncontrolled stray current traveling through earth return. These

can be harmful to both animals and humans.


ATCO’s Direction

Issues of connecting:

‒ Transferred GPR for a fault at the substation.

‒ Cable ampacity reduction, which may require de-rating.

‒ Increase in single-line to ground fault levels (positive or negative?).

‒ Increase in neutral voltages at customer sites during a fault.


ATCO’s Direction

Risk-benefit (HAZCOM) analysis:

‒ Faults inside the substation are rare and clearing time is very short.

Equipment failure and animal contact are the main reasons.

‒ The problem of NPR due to GPR transfer is not being introduced with

the connection. Only the likelihood is increased.

‒ What damage/safety concern can we expect from NPR?

‒ The likelihood of safety risks or damage is higher for faults outside the

substation and induction is the main concern. Lightning-related faults

are very common and the main reason for a fault on the line.

ATCO has established a process to connect the neutral to the

substation grounding grid and properly isolate dangerous

structures from the public, where needed.

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