Power Distribution System Overview

• Overview of existing grid (transmission , generation, Distribution)• Distribution Transformer Station.• Distribution feeder and critical Equipment. • Important terms related DER , short-circuit capacity, thermal capacity,

islanding, anti islanding and temporary over voltages.

Traditional Power System This Photo by Unknown Author is licensed under CC BY

G

Volta

ge k

V

Transmission 400/230/115kV

Generation 12kVDistribution Voltage 27.6/25/4kV User level Voltages

120/240, 120/208, 600/347V

Step-Up transformerStep-Up transformer

Generator - 2Generator - 1

G2G1

Step-down Transformer

Distribution Transformer HV Fuse

Radial Distribution Feeder

Mesh Connected TX Lines

Circuit Breakers

GEN

ERAT

ION

TRAN

SMIS

SIO

N

DIST

RIBU

TIO

N

Distribution Transformer Station Typical Arrangement

115 kV Line # 1

115 kV Line # 2

T1 T2

F1

F3

F5 F6

F4

F2

Distribution feeders

115/27.6 kV Step-Down Transformers

NC – Normally CloseNO – Normally Open

NC NC

NC

NC

NC

NO

NO

NO

Remote OperableFeeder Tie 25M14 –MS-55M1Normally Open

Remote operableMid point Recloser25M25-RC1Normally close

Distribution feeder connectivity

Remote OperableFeeder Tie 25M14 –MS-55M1Normally Open(Close )Remote operable

Mid point Recloser25M25-RC1Normally close(Open)

Off Loading a Transformer Stations

Typical Feeder schematic Tr

ansf

orm

er S

tatio

n Bu

s (Normally Open)Remote Operable Switch

Normally closeMid point recloser

Load

Load Load

27.6 kV

G G

Load

FeederCircuitBreaker

Feeder Main trunk Conductor556 MCM

Load

Under Ground Switching Unit

Protective & Isolation Equipment in the Distribution feeder

34.5 kV, 1200 AFeeder Breaker

34.5 kV , 630 ARecloser

34.5 kV, 600 AMotorized Switch

27.6kV, 600AManual Switch

Dynamic behavior of the power system and Important terms related DER

• Frequency response to the load change

• Short circuit Capacity of a Transformer Station

• Thermal Capacity of a Transformer Station

• Islanding of a power system

• Remote trip requirement for Anti Islanding

• Temporary Over Voltage of distribution system

Generator – 2(Capacity - 100MW)

Generator – 1(Capacity - 100MW)

G2G1

Load100 MW

60.00 Hz

Generation = 50MW + Losses/2 Generation = 50MW + Losses/2

Syst

em F

requ

ency

Frequency response to the load change

Generator – 2(Capacity - 100MW)

Generator – 1(Capacity - 100MW)

G2G1

Load(100 + 15) MW

60.00 Hz

Generation = 50MW + Losses/2 Generation = 50MW + Losses/2Sy

stem

Fre

quen

cy

Load > Generation

Sudden Increase of 50MW Load

Frequency response to the load change

Generator – 2(Capacity - 100MW)

Generator – 1(Capacity - 100MW)

G2G1

Load(100 + 15) MW

60.00Hz

Generation = (50 + 7.5) MW + Losses/2 Generation = (50+ 7.5 )MW + Losses/2Sy

stem

Fre

quen

cy

Load = Generation

Sudden Increase of 15MW Load

Frequency response to the load change

Short circuit Capacity of a Transformer Station

115 kV Line # 1

115 kV Line # 2

T1 T2

F1

F3

F5 F6

F4

F2

Distribution feeders

115/27.6 kV Step-Down Transformers

NC – Normally CloseNO – Normally Open

NC NC

NC

NC

NC

NO

NO

NO

Supply Source

Distributed Generation

115 kV Line # 1

115 kV Line # 2

T1 T2

F1

F3

F5 F6

F4

F2

Distribution feeders

115/27.6 kV Step-Down Transformers

NC – Normally CloseNO – Normally Open

NC NC

NC

NC

NC

NO

NO

NO

Thermal Capacity of a Transformer Station

Islanding of a power systemTr

ansf

orm

er S

tatio

n Bu

s (Normally Open)Remote Operable Switch

Normally closeMid point recloser

Load 1

Load 4 Load 2

27.6 kV

G1 G2

Load 3

FeederCircuitBreaker

Feeder Main trunk Conductor556 MCM

Load 5

Under Ground Switching Unit

G1+G2 = Load 1 + Load 2 + Load 3 + Load 4 + Load 5

I = 0 Amp

Island Frequency is not equal to 60 Hz

Remote trip requirement for Anti Islanding Tr

ansf

orm

er S

tatio

n Bu

s (Normally Open)Remote Operable Switch

Normally closeMid point recloser

Load 1

Load 4 Load 2

27.6 kV

G1 G2

Load 3

FeederCircuitBreaker

Feeder Main trunk Conductor556 MCM

Load 5

Under Ground Switching Unit

Remote Trip Issued

Temporary Over Voltage of distribution systemTr

ansf

orm

er S

tatio

n Bu

s (Normally Open)Remote Operable Switch

Normally closeMid point recloser

Load

Load Load

27.6 kV

G G

Load

FeederCircuitBreaker

Load

Under Ground Switching Unit

Thank you