Power System Power System ProtectionProtection

Dr. Ibrahim El-AminDr. Ibrahim El-Amin

Chapter FourChapter Four

Non-pilot Overcurrent Protection of Transmission Lines

Techniques for Line Techniques for Line ProtectionProtection

1. Instantaneous Overcurrent2. Time Overcurrent3. Directional instantaneous and/or time4. Step time overcurrent5. Inverse time distance6. Zone distance7. Pilot relaying

Selecting a Protective Selecting a Protective SystemSystem

1. Type of circuit

2. Line function and importance

3. Coordination and matching requirements

4. Economics

4.1 Introduction4.1 Introduction• Lines are subjected to phase-phase

or phase-ground faults.• Wide range of fault and load

currents.• System configuration and the issue

of directionality of fault current.• Line length has an impact on relay

setting.

• A Relay is set to protect a line and provide a backup for other line sections.

• Relays can not differentiate between faults at end of a line section and beginning of an adjacent section.

Short vs Long linesShort vs Long lines• Short line has lower

impedance than system’s

• Little difference of fault currents at ends of line.

• Relay will overreach in other sections.

• Long lines will not overreach but the fault and load currents may be closer.

Available Protective DevicesAvailable Protective Devices• Fuses• Sectionalizer and Reclosures• Instantaneous Overcurrent• Inverse time-delay overcurrent• Directional overcurrent• Distance• Pilot

4.2 Fuses, Sectionalizers, 4.2 Fuses, Sectionalizers, ReclosuresReclosures

• These devices are used in distribution system.

• A distribution system is made of mains(3-phase) & laterals (single phase).

• It is mostly radial.• Most common protection device is

the current-limiting fuse.

• Fuse characteristics are defined by minimum melt-time and total clearing time.

• Minimum melt-time is the time between the instant the element commences to melt & when arcing occurs. (pre-arcing time)

• Total Clearing Time (TCT)=melt-time +arcing time.

• Fuses are defined by1. Continuous load current is the maximum current to be carried without fuse melting.

2. Hot-load current can be carried, interrupted and re-energized without damage.

3. Cold-load current follows a 30min. Outage is the high current after service is restored.

• Sectionalizer 1. Can not interrupt a fault

2. It counts the number of times, it “sees” the fault current.

3. It opens after a preset number while circuit is de-energized.

• Reclosurer 1. It senses & interrupts faults. 2. Has limited fault-interrupting

capability 3. Recloses automatically in a

programmed sequence.

• Fault at A cleared by branch fuse.

• Fault at B cleared by recloser & sectionalizer.

• The sectionalizer. “sees” the fault and registers one count.

• The recloser sees the faults and trips.

• Sectionalizer opens and allows recloser to restore service.

• Faults at C cleared by recloser.

• Distribution systems are now more complex because of Independent Power Produces .

• Short circuits vary because of the Non-utility generators (NUG).

• Fuses and Reclosers must recognize this.

• The tie switch S is open.

• Fault F1 recloser will open automatically.

• Open down stream breaker and close S to supply load.

• Fault at F2 both breakers 1 & 2 will work.

• Load will be supplied from transformer 2

4.3 Inverse Time-Delay 4.3 Inverse Time-Delay Overcurrent RelaysOvercurrent Relays

• Applicationa. Used mostly on radial

systems.b. Two phase and one ground relays.

c. A third phase relay provides backup & redundancy.

c. Used in some industrial systems

ProtectionProtection SystemSystem for Phase Faults for Phase Faults

Time overcurrent 51

Instantaneous & time overcurrent 50/51

Directional time overcurrent 67

Instantaneous & directional time over current 50/67

Directional Instantaneous overcurrent 67

Step time overcurrent 51

Directional Instantaneous and directional 67

Zone Distance 21

Protection SystemProtection System for Ground Faults for Ground Faults

Time Overcurrent 51N

Instantaneous & Time Overcurrent 50N/51N

Product Overcurrent 67N

Instantaneous and Product Overcurrent 67N/50N

Directional time overcurrent 67N

Instantaneous and directional time overcurrent 67N

Directional Instantaneous Overcurrent 67N

Three-zone distance system 21N

Setting RulesSetting Rules

• Pickup Setting a. It should be above normal currents &

below minimum fault currents.

b. If possible, it may provide a backup role.

c. The setting is calculated using max load current and minimum fault current.

• Time-Delay Settinga. A time dial provides relative positions between the moving and fixed contacts.

b. Dial setting from ½ (fastest) to 10 (slowest)

• The operating speed is determined by the operating current.

• The operating time is determined by the distance it has to travel.

• This an inverse time-current characteristics.

• Time delay allows coordination between relays.

• A family of curves must be provided so that relays seeing same fault current can operate at different operating times.

• Addition of time delay will convert a single characteristics into a set of curves

• Examples 4.2 and 4.3

• Setting of Phase Relays

Relay setting is between 2* maximum load current and 1/3 of

minimum fault current.

The minimum fault current is taken as the phase-phase fault current.

• Closer to maximum load current: Increase dependability and reduce security.

• Closer to minimum fault current: decrease dependability and increase

security.

• Setting of Ground Relays: must see all phase-to-ground faults.

No need to consider load current.

Relay setting is between 2* normal ground current and 1/3 of

minimum ground fault current.

The normal ground current is taken as the 10% of load current.

• Example 4.4

Relay co-ordinationRelay co-ordination• For fault at F1, Rd

operates first.• Rcd has higher

time lever including co-ordination time S to provide a backup

• Rab has the longest time delay

• For faults Between 3-4, Rd will not see the current & will not operate.

• Rcd will trip first before Rbc

• Fault should be cleared by Rd and CB4.

• Rcd sees the fault and start to close its contacts.

• Rcd will reset after some over travel.

• Rcd must be set longer than: -Rd operating time U - CB4 clearing time V - Safety factor X (including over travel W) Usually 0.3-0.5

Example 4.5Example 4.5

• In a network, relay co-ordination is complicated by the problems of infeed or outfeed.

• There might be different current in downstream relays than the setting.

Example 4.6Example 4.6

4.4 Instantaneous Overcurrent4.4 Instantaneous Overcurrent Relays Relays

• Application -Used when short circuit current

reduces substantially as fault moves away from the source.

• The closer fault to the source, the higher is the current.

• If 51 Relays used, yet the

longer the time• Relay 50 can be set to see

faults almost up to the end of line but not including next bus.

• Setting Rules Instantaneous Relays as follows: Set at about 125-135 % above

maximum current for which Relay should not work.

And 90 % of the minimum value for which

it should operate.

• Example 4.7

4.5 Directional Overcurrent 4.5 Directional Overcurrent (67) Relays(67) Relays

• Application-Used in multiple source

circuits.

- Require two inputs; an operating current and a reference or polarizing quantity

(Voltage/current)

• If X is open, breakers 4 & 5 receives no current for fault F1.

• If X is closed, Breaker 4 can not be set above B5 to be selective for F2 and still maintain coordination for fault F1.

• Use 67 Relays if ratio between forward & backward current is 4:1

• Example 4.8

• There are two method to provide directionality:

1. Directional Control

2. Directional Overcurrent

1. Directional Control• Overcurrent element will not

operate till the directional element has worked.

• Directional contact in series with overcurrent contact

2. Directional Overcurrent• Has Independent contacts

in series

• Both contact must close for relay operation.

• Directional Control is more secure.

• Consider directional overcurrent below.

• If fault occurs , overcurrent element of breaker 4 picks up, but the directional contact will not close.

• Assume breaker 2 has opened.

• • Current direction

changed.

• There will be a race between overcurrent & directional elements.

• If Directional element of 4 closes, that is a false trip.

• This case can not happen in directional control design.

• Directional contact at 3 will close, but the overcurrent will not close as it has to co-ordinate with 2.

• But it is difficult to apply the directional control relay

4.6 Polarization4.6 Polarization• Directional relay works by comparing

an operating quantity (fault current) and a constant parameter (system voltage) .

• The constant parameter is referred to as Polarizing Quantity.

• Power Directional Relays : 1.Relays work for balanced V, I &

high Power factor. 2. Vectors are almost in phase. 3. Relay will pick for power in one

direction and reset for opposite direction

4. An auxiliary Transformer is needed.

• Fault Directional Relays 1. System voltage collapse under

faults. 2. Polarizing must not include faulted

phase. 3. Power factor is very low.

Potential PolarizingPotential Polarizing

• For ground faults, The current is obtained fro residual circuits of the CTs (In=3I0).

• Use polarizing voltage (3E0). It has same direction regardless of fault location.

• The mag. of 3E0 depends on fault location, ground impedance, zero sequence impedance.