Electrical

EHV

Substations

Introduction

Sub Stations are important link between

Generating stations and Distribution networks.

They step down the EHV to HV as per the

requirement of the customers.

Stability of the system, reliability of power

depends on the type of layout chosen and

components used in the sub stations.

Hence great care has to be taken to select the

layout and critical equipment for the SS.

~

Transmission line Yard in Receiving

Station Out door yard in Generating Station

Out door yard in Generating station

Basic Requirements

Total land area & Ease of extension

Soil condition & Soil resistivity

Right of way for incoming and outgoing lines

Facility to move heavy Equipments

Load Centre

System Security

Operational Flexibility

Simplicity of protection arrangement

Ability to limit short circuit levels

Maintenance

Cost

Types of layouts

Single Busbar System

Single Busbar System sectionalising arrangement

Double Busbar system

Single Busbar with ring Bus

Double Busbar with Transfer Bus

One and Half Breaker arrangement

Mesh type arrangement

Standard clearances

VOLTAGE PHASE TO PHASE PHASE TO EARTH

400 KV 4.000 Mtrs 3.50 Mtrs

230 KV 3.350 Mtrs 1.650 Mtrs

110 KV 1.675 Mtrs 1.000 Mtrs

66 KV 1.220 Mtrs 0.750 Mtrs

33 KV 0.915 Mtrs 0.610 Mtrs

Standard clearances

VOLTAGE CLEARANCE

GROUND CLEARANCE

SECTIONAL CLEARANCE

400 KV 8.000 Mtrs 10.000 Mtrs

230 KV 5.500 Mtrs 4.270 Mtrs

110 KV 4.600 Mtrs 3.350 Mtrs

66 KV 4.600 Mtrs 3.050 Mtrs

33 KV 3.650 Mtrs 2.750 Mtrs

22 KV 3.650 Mtrs 2.750 Mtrs

11 KV 3.650 Mtrs 2.600 Mtrs

Standard Bay Width

VOLTAGE BAY WIDTH

400 KV 27.000 Mtrs

230 KV 15.900 Mtrs

110 KV 9.500 Mtrs

66 KV 7.000 Mtrs

33 KV 4.600 Mtrs

22 KV 3.800 Mtrs

11 KV 3.500 Mtrs

Single Busbar System

Single busbar with a sectionalizing

arrangement can be erected to give more

flexibility for operation and maintenance.

Single busbar with ring shaped bus can be

Constructed for connecting many feeders

from the substations.

Single Bus Scheme 1

Single Busbar System

This system consists of a single bus to which

various feeders are connected.

In case of a fault in bus / maintenance entire

bus bar has to be shut down.

Provides least flexibility.

Simple and most economical.

This type of layout is adopted for Small and

Medium size Substations and Power stations.

Double Bus Scheme 1

Double Busbar System

Double Busbar system consists of two buses to which all the

feeders emanating from a substations, are connected by

suitable switching arrangement.

Provides more flexibility, continuity of supply and periodical

maintenance on the bus is also possible without Total

shutdown.

In this arrangement the busbars are called Main and Reserve

Busbars.

A Breaker can be introduced between the buses to have

more operational flexibility.

A double busbar system with a Transfer bus are constructed

to have more reliability and easy maintenance of Equipment.

Main & Transfer Bus Scheme 1

Double Bus & Transfer Bus Scheme 1

One and a Half Breaker System 1

Double Busbar System

In this arrangement three circuit breakers are used to

control two feeders

Any circuit breaker can be switched off without difficulty for

maintenance works.

This system has high security against loss of supply.

Suitable for switch yards of large Generating stations

handling large quantity of Power in individual

circuits/feeders.

High cost but system security is very high.

Ring (Mesh) Bus Scheme 1

Ring (Mesh) Bus Scheme

In this arrangement three circuit breakers are used to

control two feeders

This type of layout is used to minimize the no of Breakers

used in a Substation.

The breakers are connected in the mesh formed by the

buses.The circuits are tapped from the node points of the

Mesh.

Four breakers are utilized to control eight circuits.

This arrangement is widely used in United Kingdom.

33/11 KV Substation (Regular) - SLD

1

33/11 KV Substation (Regular) - Plan

1

110/22 KV or 110/11 KV Substation - SLD

1

110/22 KV or 110/11 KV Substation - Plan

1

230/ 110 KV Substation – SLD (Single Bus)

1

230/ 110 KV Substation – Plan (Single Bus)

1

230/ 110 KV Substation – Plan (Double Bus)

1

230/ 110 KV SS – SLD (Main & Transfer Bus Scheme)

1

230/ 110 KV SS – Plan (Main & Transfer Bus Scheme)

1

230/ 110 KV SS – SLD (Double Bus Scheme)

1

230/ 110 KV SS – Elevation (Double Bus Scheme)

1

400/230 KV SS – SLD (One and a Half Breaker System)

1

400/230 KV SS – Plan (One and a Half Breaker System)

1

Standard clearances in outdoor sub-stations

1

Main Equipment in Substation

Power/Auto Transformers,

Circuit Breakers,

Instrument Transformers,

Surge Arresters,

Control and Relay Panels,

Station Batteries and Battery Chargers,

DC and AC Panels,

Control Cable,

Isolators with and without Earthing Device,

Capacitors, Reactors, Busbar materials,

Insulators, Connecters etc.

Main Equipment in Substation

Selection of right type of LAYOUT,

EQUIPMENTS of adequate ratings and

correct PROTECTION system will make the

Power system Safe, Secure and also give

operational Flexibility.

Best Maintenance

Practices/ Condition

Monitoring for Reliability

of EHV Substation

Equipments

Maintenance Practices

Preventive Maintenance

Breakdown Maintenance

Total Productive Maintenance (TPM)

Condition Based Maintenance (CBM)

Pro-Active Maintenance

Reliability Centered Maintenance (RCM)

Maintenance Prevention

Condition Based Maintenance

Equipment condition is assessed

Periodic Assessment

On-Line continuous assessment

Maintenance based on assessment

Maintenance based on criticality

Switchyard Equipment

Transformers/ Reactors

Circuit Breakers

Current Transformers

Capacitive Voltage Transformers

Surge Arresters

Dis-Connectors

Transformers

Core

Coil

Solid Insulation

Liquid Insulation

Cooling System

Bushings

Expected Problems in Transformers

Core/ Winding Movement

Bad joints

Wet Insulation

Winding Insulation Deterioration

Bushing Insulation Deterioration

Deterioration of oil quality

Blocked cooling system

Partial Discharge and Over Heating

Transformer Failure Statistics- PGCL

1

Winding 19%

Shields and Leads

6%

Bushing 69%

Tap Changer 6%

Core/ Winding Movement

Magnetic Balance Test

Excitation Current Measurement

Short Circuit Impedance

Measurement

Frequency Response Analysis

Magnetic Balance Test

This test is conducted only in three phase

transformers to identify imbalance in the magnetic

circuit.

After removing neutral from ground, apply single

phase 230V across one of the winding and

measure voltage in other two windings. Repeat

test for each winding.

Ensure V1 = V2+V3

Very negligible voltage induced to be investigated

Magnetizing current measurement

To detect shifting of winding, Core, failure

of turn to turn insulation.

Apply 3 phase voltage on HV/IV terminal

& measure current in each phase.

To be carried out before DC resistance

measurement.

Voltage to be equal and Current for each

set of measurement to be equal.

Interpretation

Test results on the outside legs should be

within 15% of each other.

Value for center leg should not be more

than either outside.

For 1Phase transformer value should not

be more than 10% variation.

Not more than 5% on the previous

reading.

Frequency Response Analysis (FRA)

Mechanical movement of winding may occur during

transportation shocks or Short Circuit forces or due

to ageing and may result in dielectric failure of

transformer.

Such winding displacement may not be detected by

DGA, winding resistance or C &Tan measurement.

Techniques available to detect such changes are FRA

or Vibration Measurement.

Principle OF FRA

Applied impulse signal and the corresponding winding

responses are recorded using a high performance digitizer.

Results transformed into frequency domain by Fast Fourier

Transform calculation.

A response function is obtained which is dependent almost

entirely on the test object and is independent of applied

signal and test circuit.

The changes in the test object can be more clearly and

consistently identified.

The technique used for FRA analysis is a sweep frequency

technique which is suitable for site use and has a superior

signal to noise performance at high frequencies.

FRA TEST SET-UP 1

Sinusoidal signal output of

approximately 2 V rms from

the Frequency Response

Analyser is applied.

One measuring input (R1) is

connected to the end of a

winding and the other

measuring input (T1) is

connected to the other end of

the winding. (Tank earthed)

The voltage transfer function

T1/R1 is measured for each

winding for five standard

frequency scans from 5 Hz to

10 MHz.

SFRA Variation with Tap position

Main winding

responses

unchanged

Tap winding responses show stepwise variation

FRA Interpretation

FRA : Change in original signature

213 kV LV Delta Winding

0 200 400 600 800 1000 1200 1400 1600 1800 2000

FREQUENCY

0

-10

-20

-30

-40

-50

-60

-70

-80

-90

-100

AMPLITUDE

AMP(dB) after failure AMP(db)before failure

FRA : Change in original signature

Advantages of FRA

FRA detects winding movement, which cannot be

ascertained by any other commonly known test

FRA test is sensitive and very repeatable

FRA is very powerful and effective tool and capable

of detecting a range of transformer faults, it is

nevertheless primarily a mechanical condition

assessment test and must be used in conjunction

with other diagnostic tests if a complete picture of

the condition of the transformer is to be obtained.

Winding Resistance Measurement to detect bad joints

Measured in the field in order to check for

abnormalities due to loose connections.

Interpretation of results is based on comparison with

previous results.

Within 5% deviation in result is accepted.

Measured values to be converted to the reference

temperature.

Winding Resistance

Winding Resistance is very sensitive to

temperature.

Hence temperature to be noted at the

time of measurement.

Resistance is converted to base

temperature for comparison.

R75 = Rm (75 + 235)

( Tm + 235)

Wet Insulation

Oil ppm

Recovery Voltage Measurement

Measurement of Relative Saturation

Winding Capacitance tanδ measurement

Typical Values of Water Content in Oil & Paper

Water Content at the Time of Commissioning

In Paper 0.5% 30 Liters

In oil 5 ppm 0.4 Liters

After 25 years of service

In Paper 2.5% 150 Liters

In Oil 20 ppm 1.6 Liters

Dependency between the water content in mineral oil and cellulose paper

Recovery Voltage

Measurement

(RVM)

Applying DC Voltage to the insulation, molecules are

polarized and turn in the direction of the Electric Field.

A short circuit is subsequently applied for a short

period of time and molecules are partially depolarized.

Opening the short circuit, a voltage due to remaining

charge will build up between the terminals of the

insulation. This is called Recovery Voltage.

The equivalent circuit which best describes the

phenomenon of the polarization is serial R-C circuit.

Physics behind RVM

Recovery Voltage Measurement

Test Setup

Recovery Voltage Measurement

2 kV DC is applied across the insulation for

a particular time

Insulation is shorted for half the time

Then recovery voltage is measured till the

maximum recovery voltage is reached

Maximum recovery voltage against time is

plotted

Method of Measurement

RVM Graphs Different ages of Transformers

Multiple peaks indicate non-

homogeneous dryness of winding.

More time constant indicates

healthiness of insulation.

Less time constant indicates wetness of

winding.

Method of Measurement

Before and after dry out

On-line

Moisture

Measurement

DOMINO test is used to determine the moisture

content of oil in Transformers and Reactors.

The sensor made of Thin Polymer film -

measures Capacitance.

The Capacitance changes proportional to the

change in relative saturation of water in oil.

On-line Moisture Measurement through Domino

Domino Principle

Relative Saturation (RS) is

RS = Wc * 100/S (%)

Where

Wc is concentration of water in oil &

S is Solubility of water in oil that can be held at a

given temperature

Solubility of water in oil (S) at a given temperature

is computed from,

logS = -1567/K + 7.0895

Where K is temperature in ºK

Relative Saturation

The instrument measures RS and ppm of

water

The output is locally displayed as well

downloaded to PC through RS-232 port.

Domino

Water in Oil at 60-70ºC Condition of Cellulose

insulation

< 15ppm Good

>15<20ppm Fair

>20<30ppm Probably wet

>30ppm Wet

Water content in Oil and Condition of Paper insulation

Deterioration of Winding Insulation

Capacitance and tanδ Measurement

Results are affected by external

interference

Interference suppression methods

Non-Repeatability of test results

Proper knowledge on connection and

measurement

Trend Analysis

Problems with the measurement

Basic Insulation

Insulation

Capacitance of Insulation

Effect of Medium

Dielectric Constants of different Mediums

Equivalent Circuit

HV Lead

Interference

Equivalent circuit for two winding transformers

CHL – CAPACITANCE BETWEEN HV

AND LV WINDINGS.

CH – CAPACITANCE BETWEEN HV

AND TANK.

CL – CAPACITANCE BETWEEN LV

AND TANK

Reliable instrument.

Jumpers to be disconnected for

Transformer/ Reactor winding capacitance

and Tan Delta measurement.

It should not be a rainy day.

Porcelain surface needs to be cleaned

thoroughly.

Precautions for Cap and Tan Delta Measurement

Transformer Windings

Shorting of Jumpers

UST Mode of Measurement

Earth will be connected to N Point. Hence only CHL will be measured

GST Mode of Measurement

Here measurement is made for the Parallel combination of CH and CHL.

GSTg Mode of Measurement

Here measurement is done for CHE as L is guarded

Construction of Bushings

Grading of insulation

C1 and C2 Tan delta measurement

Condition Monitoring of Bushings

CT Insulation

C1 and C2 Tan delta measurement

For C1: Apply 10kV between HV and test tap on UST mode. For C2: Apply 500 V between Test tap and Ground with HV Guarded on GSTg mode

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1

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1

Dissolved

Gas

Analysis

Summary of various interpretation technique for DGA data

Individual Fault Gases Acceptable Norms

Total Dissolved Combustible Gas (TDCG) limits

IEC 60599 Method

IEEE Standard C: 57.104/1995

Key Gas Method

Ratio Methods

Doernenberg Ratio Method

Rogers Ratio Method

Trend Analysis

Key to successful DGA interpretation

Well documented sampling procedure based on

IEC 60567.

Efficient gas extraction system (with more than

98% of gas extraction).

Accurate and precise gas chromatograph.

Well trained manpower to conduct the tests.

Documentation of DGA data and its

interpretation.

Sampling frequencies

Before commissioning

24 hours after 1st charging

15 days after 1st charging

One month after 1st charging

3 months after 1st charging

Thereafter every six months

Powergrid practice

Origin of fault gases

1. Corona or partial discharge

2. Pyrolysis or thermal heating

3. Arcing

These three causes differ mainly in the intensity of

energy dissipation per unit time per unit volume by

fault.

Most severe intensity of energy dissipation occurs

with arcing less with heating and least with corona.

Basics of DGA

Temperatures at which Gases evolve

METHANE (CH4) 1200 C

ETHANE (C2H6) 1200 C

ETHYLENE (C2H4) 1500 C

ACETYLENE (C2H2) 7000 C

Classified in 3 groups

1.HYDROCARBONS AND HYDROGEN

METHANE CH4 ETHANE C2H6

ETHYLENE C2H4 ACETYLENE C2H2

HYDROGEN H2

2.CARBON OXIDES

CARBON MONOXIDE CO

CARBON DIAOXIDE CO2

3.NON-FAULT GASES

NITROGEN N2 OXYGEN 02

FAULT GASES

1. CORONA

A) OIL H2

B) CELLULOSE H2, CO, CO2

2. PYROLYSIS LOW TEMP. HIGH TEMP.

A) OIL CH4, C2H6 ; C2H4,H2(CH4,C2H2)

B) CELLULOSE CO2 (CO) ; CO (CO2)

3. ARCING H2, C2H2 (CH4, C2H6, C2H4)

Fault gases vs Type of material involved and type of fault

Hydrogen 7.0% (least soluble)

Nitrogen 8.6%

Carbon monoxide 9.0%

Oxygen 16.0%

Methane 30.0%

Carbon dioxide 120.0%

Ethane 280.0%

Ethylene 280.0%

Acetylene 400.0% (most soluble)

Solubility of gases in transformer oil by volume

FURAN

ANALYSIS

Monitoring of Cellulose (paper) insulation is important for in-

service transformers to know the condition of paper.

Taking paper samples from charged transformer not practicable

or rather impossible.

Furans are soluble by product which is produced due to ageing

or cellulose decomposition/ overheating.

Experimental tests have shown that DP of paper insulation is

inversely proportional to FFA level and FFA level of 5 ppm.

corresponds to a DP of 250 which is reckoned to be the lower

limit for insulation strength.

Furan analysis in oil

The Furfuraldehyde in oil analysis is recognized as a very

significant diagnostic test for paper condition assessment

The FFA content of the oil from an aged equipment can be

measured using a High Performance Liquid Chromatograph

(HPLC).

Another method is using Spectrophotometer

The concentration of Furfuraldehyde in the oil and rate of

generation of furfural can be used for assessing the condition

and remaining life of the paper insulation in the transformer

Furan analysis in oil

Blocked Cooling System

Thermo scanning of Radiators

Differential Temperature

High oil and Winding Temperature