Typical Protection Schemes

2007-08Copyright Siemens AG 2007. All rights reserved.

PTD SE PTIAuthor: G. Ziegler

Typical Protection Schemes



Typical protection for a radial system

Most distant relay gets the shortest delay time

Definite-Time (51, 51N)System impedance (Infeed) high, no current selectivity

Inverse-Time (51, 51N)System impedance (Infeed) medium, current selectivity

Very Extremely Inverse-Time (51, 51N)

System impedance (Infeed) small, high current selectivity

1) Auto reclosing cycle only for overhead lines (79)

2) Negative-phase sequence system as back up protection (46, unbalance protection)



Typical protection of ring feeders

Switch off time of the relays has to be coordinated with down-stream fuses

Inverse-time characteristic curves (51, 51N)

Thermical overload (49) for cables

Negative-phase system protection (46) as sensitive back up protection

(52: Circuit-breaker)



Typical protection for a distribution feeder with reclosing

The relay settings and the auto reclosing cycle (79) must be coordinated with the down-streamed protection devices.



Feeder protection Distribution System



Feeder protection Transmission System



Undelayed I>>, IE>> (50, 50N)

Main protection OS

Time-delayed overcurrent-time (51)

Back-up protection OS

Buchholz (63)

Overload (49) Negative-phase

sequence system (46)

Limited earth-fault (87N)Main protection US

Time-delayed overcurrent I > (51G) (Earth path)

Back-up protection US

(OS: Overvoltage side)

(US: Undervoltage side)

>

Small transformers < 5 MVA



Dual infeed with single transformer



Important transformer infeeds

Differential (87T)Main protection

Overcurrent-time (50, 51, 51N)Back-up protection

Limited earth-fault (87N)

Neg.-phase sequence system (46)

Thermal (49)Overload protection

Buchholz (63)

Earth-fault current protection(51G)



Three-winding transformer



Autotransformer



Large autotransformer bank



Small and medium motors with nominal outputs < 1MW

1) Cable-type current transformer is necessary for sensitive earth-fault protection

b) In systems with high-resistance neutral earthing (IE IN Motor)

a) In systems with solid grounding or low-resistance neutral earthing (IE> IN Motor)

2) Directional protection, Earth-fault monitoring

(37) Undercurrent relay



Big high voltage motors > approx. 1 MW

1) Cable-type current transformers for the sensitive earth-fault detection

2) Sensitive directional earth-fault detection in operation with a Petersen-coil

3) Startup supervision

4) Temperature sensor

(27) Undervoltage-relay

Optional



Micro generators < 500 kW

b) Earthed via resistance

a) Solid grounding



Busbar protection Reverse interlocking principle

Applicable to distribution buses without considerable (< 0.25 In) back feeding from the outgoing feeders



Low impedance - Busbar protection

Especially used in multiple busbars for which an isolator replica is necessary

Current transformers can be different

Feeder protection can be connected to the same current transformer core

(87): Differential protection

(86): Tripping relay

(BF): Breaker failure protection



High impedance busbar protection



1-1/2-breaker arrangement



Typical protection for parallel service without back-feed

Directional overcurrent - time protection relays (67, 67N) switch off fault on the protected line without delay

Non-directional relays (51, 51N) must be delayed about 1 grading zone



Overcurrent-time Phase/Earth directional (67, 67N) non-directional (50, 51, 51N)

Main protection

Overcurrent-time Phase/Earth (51, 51N)

Back-up protection

Thermal (49)

Overload protection

Buchholz (63)

Limited earth-fault (51G)

Negative-phase sequence system (46)

Dual transformer infeed



Cable / short overhead line < 20km with infeed of both sides

Differential (87)Main protection

Overcurrent-time (51, 51N)Back-up protection

ThermalOverload protection

1) Automatic reclosing only in overhead lines (79)

2) Overload protection (49)mainly in cables



Overhead lines or long cables with infeed of from sides

Distance (21, 21N)Main protection

2) Directional Overcurrent-time (67N)

Back-up protection

3) Auto reclosing (79)(single or multiple) only in overhead lines

1) Signal comparison (85)

Documents

Typical Protection Schemes