VFT RELATED ISSUES IN HIGH VOLTAGE

GAS INSULATED SUBSTATIONS

Dr. M. Mohana Rao

BHEL Corporate R&D

Hyderabad

E-mail: mmrao@bhelrnd.co.in

by

CONTENTS

Introduction to VFT

Effect of VFT in GIS

VFTO / VFTC

Transient Enclosure Voltages (TEV)

Transient EMI / EMC

Induced Voltages in Control Circuitry

Measurement of Transient E-fields

Conclusions

What is VFT?

In a GIS, Very Fast Transient Over voltages (VFTO) are generated

mainly due to switching operations.

The voltage collapse across switching contacts takes place in 3 to

20 ns depending on breakdown voltage, electric field non-

uniformity and operating gas pressure.

The short-rise time pulse (i.e., voltage collapse) starts at the

switching contacts that propagate along the gas insulated bus

sections/components and take reflections at different

terminations. Because of superposition of the original pulse with

the reflected pulse, VFTO are developed.

The waveform of these transients depends on the configuration of

the GIS.

The VFTO levels are found to be on the higher side for the

following conditions of the switching configurations:

(1) Small length of bus sections on the load side of the switch.

(2). High surge capacitance components on source side of switch.

DS Operation

GENERATION OF VFTO

Opening Operation

Load

voltage

Source

voltage

Why VFT is a Problem?

SLD OF 245 kV GIS

VFTO LEVELS?

VFTO – SECONDARY BREAKDOWN

Characteristics of VFT

Frequency Components of VFTOs in GIS

VFTC LEVELS ?

Coupling Phenomena of VFT

Why TEV IS A CONCERN?

The very fast transient overvoltages and the associated

transient currents generated in gas insulated section

propagates partly to the overhead transmission line and

partly to the exterior surface of the bus section enclosure.

The most typical discontinuities in a GIS are SF6 gas-to-air

bushing and gas-to-cable termination. Out of these two,

gas-to-air bushing is the most significant one.

The transient voltages that appear on the exterior surface of

the enclosure during switching operations or earth faults is

known as Transient Enclosure voltages (TEV) or Transient

Ground Potential Rise (TGPR).

TRANSIENT ENCLOSURE VOLTAGES (TEV)

GIS enclosure is electrically continuous.

GIS enclosure is grounded at several points.

Induced magnetic field on the metallic structures and the

control circuits are reduced due to the flow of major

portion of return current in the GIS enclosure.

TEV Levels in 800 kV GIS MODEL

Application of ZnO arresters or surge

capacitors at the discontinuities of

electrically small insulated flanges, GIS

enclosures, GIS and connected

equipment enclosures.

Using low impedance copper strips for

shorting of the enclosures.

By using ground strips with large

perimeter (strips of rectangular bars)

to limit high frequency impedance.

By using multiple ground wires at the

discontinuities like gas-to-air bushing.

By decreasing the height of the

enclosure above the earth surface.

Generation of VFTO / VFTC

Radiated EM field Emission from GIS modules

Transient Voltages in control Circuit / Cables

Primary circuit malfunction

Sequence of events possible for a

Failure of Secondary Equipment

Why EMI is important in

GIS?(1). VFTO /VFTC generated EM fields are in the range of

a few MHz to a few hundreds of MHz.

(2). Compactness of the substation.

(3). Grounding of substations for AC not for transients.

(4). Control Panel is a few meter from GIS primary circuit.

(5). ICs are part of secondary equipment .

(6). Shielding of Control Cubicle enclosure is limited.

Radiated Emission through :

(1). SF6 Gas-to-Air Bushing.

(2). Cable/Air termination.

(3). Apertures like viewing ports, insulated flanges etc.

(4). Control Cable depends on its sheath characteristics.

(5). Control Circuitry.

Conducted Emission through :

(1). Voltage Transformer

(2). Current Transformer

(3). Ground Network.

Transient EMI - AIS

Porcelain Insulator

housing

Radiating Source

Transient EMI - GIS

Gas-to-Air Bushing Gas-to-Cable Connection

a). The malfunction of thyristors and protective relays

during Switching operation in GIS.

b). Flashover between

- flanges of gas insulated enclosure during switching

operations.

- gas insulated enclosure and ground strips.

- control cable sheath and ground objects.

c). Failure of Control Circuit connected to VTs and CTs.

EMI Problems in GIS

TRANSIENT EMI and EMC

Transient EMI :

- Momentary / Transient in nature.

- Generate during Switching, Lightning, Faults etc.

- Energy involved is very low.

-Control equipment is a victim.

Electromagnetic Compatibility (EMC) is the ability of a equipment

to function satisfactorily in its environment without causing

interference to other equipment and without suffering

interference from other sources.

Equivalent Disturbance Source has to be defined for each

substation to ensure reliable operation of the control equipment.

Following are some of the tests confirm the above:

Fast Transient Voltage test, 5-50 ns, 4 kV as per IEC 61000-4-4.

Damped oscillatory wave test, 1-1.5 MHz, 2.5 kV as per IEC

61000-4-12.

Rectangular wave test, 1 ns rise time, 1 s pulse width, 3 kV.

EMC test, 5 W/ 150 MHz, 400 MHz, 900 MHz noise.

High speed switching of an inductive circuit for 2 min.

SUSCEPTIBILITY / INTERFERENCE TESTING OF

THE CONTROL EQUIPMENT

Transient EM Fields in GIS

230 kV

500 kV 500 kV

230 kV

Transient Fields in Substations

Transient EMI and EMC

All the points of the grounding network are at different

potentials during switching. The electronic equipment

coupled to the grounding network.

Transient EMI - Coupling

1. Radiated / Inductive

2. Conductive

Radiated coupling is predominant due to high di/dt.

Impingement of radiated fields on shielded /

unshielded cables and control circuitry.

Conductive coupling is predominant due to high

dv/dt.

Stray capacitance between HV equipment and

control equipment.

Transient EMI - Coupling

Transient EMI - Coupling

Transfer Impedance Coupling

The transfer impedance is a characteristic parameter

of the control cable. Shielded cables have lower

transfer impedance. Depending on the transfer

impedance between the shield / sheath and the

central conductor of the cable, voltage/current is

induced at the control cable load circuit.

Pigtail Coupling

The pigtail coupling is an inductive coupling between

the metallic sheath termination (pigtail) and the load

circuit of the control cable.

Transient EMI – Induced voltages

Parameters affecting Transient EMI

Type of the substation (AIS / GIS / HIS).

Rated voltage of the substation.

Type of the switch being operated.

Operating speed of the switch.

Electrical characteristics of the bus being

excited.

Location of the excited high voltage bus with

respect to the EM field observation point.

Transient Voltages in Control Circuitry

Transient Voltages in Control Circuitry

-Height of the control cable above the ground plane.

-Type of grounding of the control cable sheath.

-Transfer Impedance of the cable.

-Physical distance between signal wiring.

-Terminal impedance of the electronic equipment.

-Transient EM field levels and their frequency

content.

-Type of Instrument transformer i.e., CT or PT.

-Secondary winding impedance of the Instrument

transformers.

FDTD Model of the 245 kV Gas-to-Air Bushing

Installation

0 100 200 300

-100

-50

0

50

100

Time (ns)

H -

F i

e l

d

(A

/ m

)

With Gas-to-Air Bushing

|A| = 112.6 A/m

0 100 200 300

-40

-20

0

20

40

Time (ns)

R a

d i

a l

E

- F

i e

l d

(

k V

/ m

)

With Gas-to-Air Bushing

|A| = 36.94 kV/m

100 MHz

Source

EM Field Emission from Bushing (Frequency

Spectrum).

1 10 100

0E+0

2E+4

4E+4

6E+4

8E+4

H-Field

h = 0.1 m

A m

p l

i t u

d e

(a.

u.)

Frequency (MHz)

300

1 10 100

0E+0

2E+3

4E+3

6E+3Radial E-Field

h = 0.1 m

A m

p l

i t u

d e

(a.

u.)

Frequency (MHz)

300 1 10 100

0E+0

1E+4

2E+4Radial E-Field

h = 2.0 m

A m

p l

i t u

d e

(a.

u.)

Frequency (MHz)

300

1 10 100

0E+0

1E+4

2E+4

3E+4

Axial E-Field

h = 0.1 m

A m

p l

i t u

d e

(a.

u.)

Frequency (MHz)

300

Transient E-Field Emission Measurement from the 245 kV

Rated Gas-to-Air Bushing Model.

Measurement using D-Dot

Sensor - Transient E-

fields

0 100 200 300 400 500

-40

0

40

80

120

160

Time (ns)

E -

F i

e l

d

(V/m

)

|A| = 148.4 V/m

R = 0.5 m

0 200 400 600 800 1000

-40

0

40

80

120

Time (ns)

E -

F i

e l

d

(V/m

)

|A| = 103.8 V/m

R = 1.5 m

0 200 400 600 800 1000

-40

-20

0

20

40

60

Time (ns)

E -

F i

e l

d

(V/m

)

|A| = 59.2 V/m

R = 2.0 m

The switching of disconnector and circuit breaker devices

inevitably give rise to transient voltages in the control

equipment connected to substations.

Transient EMI is a major concern:

-Digital electronic equipment is extremely sensitive to

transient voltages generated during switching.

-Conductive coupling through stray capacitance in the form

of CT, CCVT etc. Radiated coupling due to EM field

emission from the entire substation equipment.

The grounding network shall be designed from transients point

of view.

EMC of control equipment shall be confirmed before their

application in high voltage sub-station.

Conclusions