Kuliah Teknik Tegangan Tinggi (1)

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TKE

474

High Voltage Engineering

~High Voltage Engineering~

Overvoltage

O e o tage

and

Insulation

Coordination

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Overvoltage

‐‐‐‐‐‐

>

Insulation

coordination

Insulation

coordination

:

Insulation

coordination

:

1. Insulation coordination is the selection of the insulation strength

insulation strength.

2. The "selection of the insulation strength

consistent with the expected overvoltages to consistent with the expected overvoltages to obtain an acceptable risk of failure.

3. Insulation coordination is the "process of

bringing the insulation strengths of electrical equipment into the proper relationship with

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Types

of

Overvoltages

Overvoltages stressing

a

power

system

can

Overvoltages stressing

a

power

system

can

generally

be

classified

into

two

main

types

:

1. External overvoltages:

Generated by atmospheric disturbances i.e: li ht i t ik

lightning strike

2 Internal overvoltages: 2. Internal overvoltages:

Generated by changes in the operating

conditions of the network and can be divided into two types i.e:

(a) switching overvoltages and

(b) l

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The Lightning Discharge

ÖTypicallyyp y :

• straight hit to conductor

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The Lightning Voltage Surge

Ö Propagating overvoltage into both directions along the conductor

• Typically Z₀ = 250 – 500 Ω

Ö lt MV

Ö overvoltage ~ MV

1600

2200 m 1300 m

620 m 0 m

U

1200

2200 m 3

kV

U

400 800

t_/ μ_s

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Back

flashover

via

grounded

g

component

p

Ö

flashover

from

grounded

component

to

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Induced Overvoltages

lightning current cause a rapidly changing

i

magnetic field into the LC loops of the line inducing a voltage h h d d h Z i k

V_induced = ₀

k = considers propagation speed of discharge current. Typically 1.2 – 1.3 i = peak lightning current

h h i h f d h = height of conductor,

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Internal Overvoltages

busbar short circuit line fault busbar short circuit line fault

asynchronous network disruption of small inductive current

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Power System Overview (HV aspect)

Insulation

Over

‐

voltages

Electric insulation is a vital part of an

electrical power

system, a careful

analysis

of the

line insulation to

ensure

an

adequate line design is needed

(

l

)

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* Continuous Power Frequency Voltages

Th i l i h i h d l

The insulation has to withstand normal operating voltages. The voltage may

fluctuates caused changing the load. The fluctuates caused changing the load. The normal range of fluctuation is around

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Power System Overview (HV aspect) Power System Overview (HV aspect)

The voltage V_max is used for the selection of number of insulators for line application.

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• Basic

Switching Impulse Insulation Level

(BSL)

The

e

BSL

BS

is the electrical strength of insulation

s t e e ect ca st e gt o

su at o

expressed in terms of the

crest value of a

standard switching impulse. The statistical

sta da d s

tc

g

pu se.

e stat st ca

BSL,

insulation exhibits a 90% probability of

withstands a 10% probability

% p

y

of failure. In IEEE

Std 1313.1‐1996, the

BSL

is called the switching

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B i

Li h

i

I

l

•

Basic Lightning Impulse

Insulation Level (BIL)

(

)

The

BIL

or

basic

lightning

impulse

The

BIL

or

basic

lightning

impulse

insulation

level

is

the

electrical

strength

of

insulation

expressed

in

terms

of

the

crest

value

of

the

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I

l

I

l i

S

h

Impulse

Insulation Strength

Insulation strength is expressed in terms of

conventional or

statistical BILs and BSLs.

For

every application of an impulse having the

For

every application of an impulse having the

standard waveshape and whose

crest is equal

to the BIL

or BSL

the

probability

of a

to the BIL

or BSL, the

probability

of a

flashover

or failure is 10%.

The mean of this distribution or

characteristic

The mean of this distribution or

characteristic

is defined as the

critical flashover

voltage or

critical flash

over

(CFO)

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Impulse

Insulation Strength

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For the transmission_{Power System Overview (HV aspect)} line insulation strength is usually statistically described by a CFO voltage at

which the insulation exhibits a 50% probability of fl h d b t d d d i ti hi h i flashover and by a standard deviation

σ

which is

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• Switching ImpulsePower System Overview (HV aspect) Strength of Tower • Switching Impulse Strength of Tower

Conversion three to six‐phasep transmission lines will effect on the existing tower insulation i.e. striking

distance (clearance) and the insulator string length (number of insulator). The statistical withstand

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Clearance area is depending on horizontal and vertical distance between conductor and and vertical distance between conductor and tower.

Horizontal and vertical distance is 4.42 Horizontal and vertical distance is 4.42

meter and 1.78 meter. Relationship between the CFO and the strike distance can be

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S is the flash distance (meter)

k_g is gap factor. For a lattice steel t k i b t

tower, k_g is about 1.2

(IEEE Std 1313.2‐1999), h is conductor height, and

W is tower width (horizontal distance

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Power System Overview (HV aspect) Minimum clearancePower System Overview (HV aspect) based on lightning impulse Minimum clearance based on lightning impulse