115kV Sub-Station - Usual Transm & Distr 1

7/31/2019 115kV Sub-Station - Usual Transm & Distr 1

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Power System Planning Tehran 2008

2.6 Usual Transmission & Distribution Systems (115 kV & 22/11 kV)

Characteristics for 115 kV & 22/11 kV systems

OHL: UGC:

- rural areas, suburbs - urban and industrialised areas,

suburbs

- usually 2 or 4 circuits on one system;

often together with higher or lower

voltage levels

- security distance between cables in

the trench

- earthing conductor on top of the tower

(lightning protection)

- cable sheath grounded

- fibre optic or copper conductors,

integrated in earthing conductor as

telecommunication cable or separate

- telecommunication cable beside the

energy cable

- transmission of information on OHL

(Power Line Carrier)

- transmission on the cable sheath is

checked

- aluminium / steel 2A/mm2

560/50; 250/50

- copper, aluminium

- 1,2 conductors/phase - three-phase

- three single phase cables

- porcelain insulator - oil, gas pressure, XLPE

- conductor sag, depending on ice charge

or maximum temperature

- cable must have the chance to

move in the trench

- surge arrester at the end - surge arrester at the common point

OHL/ cable

- backward flash over, disturbing several

systems

- digging machines destroy cables

- conductor oscillations

- 100 km maximum length - 10...20 km maximum length

- Z = 380 Ohm (240/40 Al/St)

PN = 35 MW

St h= 125 MW

- Z = 40 Ohm (500 mm2 Al)

PN = 400 MW

Sth = 100 MVA

- fault localisation easy,

repairing time short

- electric-acoustical,

repairing time long

- short circuit current relative small

x' ~ 0,2 Ohm/km

- relative high,

x' ~ 0,14 Ohm/km

2. Network Planning - 2.6 Usual Transmission & Distribution Networks page1 / 13


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- transformer

harmonised with the capacity of the feeding network

long life time

losses as cost factor

long repair time

well protected and supervised (oil, tap changer, partly discharges,.. )

surge arrester

voltage oscillations (!)

overload cooling system

dimensioning of X, (uK)

transportability

spare parts

protection

two ore more transformers in each node substation 230/115 kV or 115/22 kV.



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- network configuration of subtransmission and distribution lines

meshed, ring, ring with connections, radial, T-connection

double busbar in important node substations

redundant, limited redundant or non redundant looping of substations in a line redundant connections by the lower voltage level lines in case of a failure

110 kV XLPE cable



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(115 kV)

110 kV double busbar air-insulated &

110 kV double busbar SF6 insulated



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- protection system

distance protection

overcurrent

short interruption (OHL)

differential protection (depends of network structure )

selectivity

- supervisory control / operation

switching telecontrolled

occupied substations (rare)

telephone (LV networks)

customers information

- maintenance

organisation and availability of staff and material maintenance data acquisition

preventive or event orientated

The electricity transmission + distribution is a SYSTEM

optimum, if all technical components of one supply level indicate a comparable

availability

operation and maintenance is done in an acceptable time

the feeding substations are harmonised by power capacity and protection with

the over-layed and underlayed networks



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- voltage dependent claims

only little increase of the voltage of the healthy conductors

use of surge arrester with low-rated voltage

avoidance of sequence faults avoidance of displacement potentials during the failure free operation

avoidance of overvoltages as consequence of ignition and extinguishing of

the arc

avoidance of ferro-resonance after clearing the earth fault and switching

measures

- operation and customer dependent claims

practically uninterrupted supply of all customers

economical solution, for later network extension too

automatic and selective fault localisation compatible with the industrial installation of the customers

It is not possible to fulfil all requirements; some of them are inconsistent with

others.

Network with insulated SP and compensated earth current

- during faults, the healthy conductors have an increased potential

- calculating the currents and the displacement potential with symmetrical

components,

- aggravation of repeating arc ignition by tuning the resonance circuit near to thenormal frequency (compensated system)

- repeating ignitions of arcs in insulated SP networks on from 10 A; transients!

arc currents of 50 A are normally stable; localisation



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Znyn5

11

Imax = 1.500 A



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Networks with low-impedance SP grounding

- calculating the operational frequency processes by symmetrical components:

one, two and zero system

- earth fault factor, the highest operational frequency conductor-to-earth voltageof a healthy phase during a fault with earth contact

is a scale for the insulation stress during the fault.

International Practice

- ideal

compensation to reduce the earth currents; arcs in air are extinguished

without switching measures; operating the meshed OHL or single conductor

cables during a longer time

over-voltage protection by surge arrester to avoid sequence faults

short low-impedance earthing to localise the fault by earth fault protection selective short circuit protection with short interruption to switch-off short

circuits with practically uninterrupting the customers

- reality

110 kV rather low-impedance grounding; earthing installations, earth ropes

and short circuit protection are dimensioned for the highest short circuit

current. (The earth rest current increases with network extensions,

especially by cables; the displacement potentials increase in OHL systems

with multiple circuits on one tower)

recommendation

* radial cable networks up to 22 kV : low-impedance 500 A...1000 A

* earth rest current up to 22 kV : maximum 60 A (comp. SP)

for 115 kV : maximum 130 A (comp. SP)

* OHL and UGC: short time SP grounding by resistances dimensioned

for 300 A

* 110 kV with insulated SP not allowed; ferro resonances

* insulated SP: only for networks with limited extension, economical reasons



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* earth current compensation

- reducing the damage proportions

- transformers Dy

* low-impedance grounding- reducing damage risk by quick and selective switching off

Both ways can be successful, as the technical risk can simplified described as

Risk the damage proportion x frequency

Consequences of faults

- low-impedance SP grounding

limited short circuit current in MV systems because of the reactance upto 1..2 kA; in 115 kV systems to...15 kA

self healing by short interruption in OHL networks in case of a

lightning stroke; customers without interruption of supply (only some

100 ms)

risk of damaged equipment is very small; short interruption time

- earth fault in compensated and insulated networks

capacitive current as fault current; 10 A...100 A

arc extinguishes by thermal prolongation of the arc

neglectable damage at the point of earth current risk of transient earth current because of slowly back coming voltage

self healing insulation, depends of energy at the failure point

not self healing; duration of accidental arc to ground; insulation stress

special behaviour of mixed OHL/UGC networks (statistics)

- double earth fault in compensated and insulated networks

two faulty points in a longer distance

double earth fault via ground: contact and influencing potential

relatively rare, grounding devices normally not dimensioned for this

- grounding system

thermal dimensioning

contact potential

earth current 10 A...hours; 10 kA...< 1 sec

minimum 50 mm2 iron (compensated SP grounding)

maximum 65 V contact voltage; 60 A rest current -> 4 Ohm

(meshed grounding system, 1-m-ring, 25% contact voltage)

low-impedance SP grounded



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- maximum short circuit current; grounding wires

100 mm2 iron

35 mm2 copper

influence to telecommunication lines

Basic grounding of a substation

- changing from compensated to low-impedance grounded SP

grounding system

ct

protection

surge arrester

- localisation of earth faults

insulated SP

active and reactive power flow of the 50 Hz oscillation to determine the

fault direction;

localisation relays; ct in Holmgreen position, cable encircling ct

compensated networks

power direction in the zero-system; high precision necessary, watt rest

current; increase by bypass switching (resistance parallel with

compensating inductivity; ohmic component; less compensation))

harmonics

5th harmonic; compensation not effective; minimum 0,5% Un



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transient earth relay (reactive power)

short time switching-off and on

short time SP grounding

short time phase grounding earth fault distance protection relay

central evaluation with computer support

2. Network Planning - 2.6 Usual Transmission & Distribution Networks page 13 / 13

115kV Sub-Station - Usual Transm & Distr 1

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