Switchgear (busbar) protection [Industrial Electrical Power Systems]

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Why use busbars

Metal-Enclosed BuswayBusbars are the most important component in a distribution network. They can be open busbars in an outdoor switch yard, up to several hundred volts, or inside a metal clad cubicle restricted within a limited enclosure with minimum phase-to-phase and phase-to ground clearances. We come across busbars, which are insulated as well as those, which are open and are normally in small length sections interconnected by hardware.

They form an electrical 'node' where many circuits come together, feeding in and sending out power.

++++ Schematic area of busbar zone.

From the above diagram, it’s very clear that for any reason the busbars fails, it could lead to shutdown of all distribution loads connected through them, even if the power generation is normal and the feeders are normal.

I-Line Busway--Available in both plug-in and feeder busway types, maximum ratings are 5000 A Copper and 4000 A Aluminum (600V).

The important issues of switchgear protection can be summarized as:

  1. • Loss very serious and sometimes catastrophic
  2. • Switchgear damaged beyond repair
  3. • Multi-panel boards not available 'off-the-shelf'
  4. • Numerous joints
  5. • Air enclosure
  6. • Dust build-up
  7. • Insect nesting
  8. • Ageing of insulation
  9. • Frequency of stress impulses
  10. • Long ground fault protection tripping times.

Busbar protection

Busbars are frequently left without protection because:

  • Low susceptibility to faults - especially metal clad switchgear
  • Rely on system back-up protection
  • Too expensive and expensive CT's
  • Problems with accidental operation - greater than infrequent busbar faults
  • Majority of faults are ground faults -- limited ground fault current - fast protection not required.
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However, busbar faults do occur.

The requirements for good protection

The successful protection can be achieved subject to compliance with the following:

• Speed

- Limit damage at fault point

- Limit effect on fault stability

• Selectivity

- Trip only the faulted equipment

- Important for busbars divided into zones

• Stability

- Not to operate for faults outside the zone

- Most important for busbars

- Stability must be guaranteed

• Reasons for loss of stability

- Interruption of CT circuits - imbalance

- Accidental operation during testing

• Tripping can be arranged 'two-out-of-two'

- Zone and check relays.

Busbar protection types

  1. • Frame leakage
  2. • High-impedance differential
  3. • Medium-impedance biased differential
  4. • Low-impedance biased differential
  5. • Busbar blocking.

Frame leakage protection

This involves measurement of fault current from switchgear frame to the ground. It consists of a current transformer connected between frames to ground points and energizes an instantaneous ground fault relay to trip the switchgear. It generally trips all the breakers connected to the busbars.

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Care must be taken to insulate all the metal parts of the switchgear from the ground to avoid spurious currents being circulated. A nominal insulation of 10 ohm to ground shall be sufficient. The recommended minimum setting for this protection is about 30% of the minimum ground fault current of the system.

CT Insulation

Note: Foundation bolts, secondary cables and vent pipes, etc. must also be insulated from ground.

Low Voltage Switchgear with Masterpact NW and NT Circuit Breakers---Low voltage metal-enclosed, drawout switchgear designed to provide superior electrical distribution, protection, and power quality management for the entire facility.
above: Low Voltage Switchgear

++++ Requirements of frame leakage BB protection; Electrode ground resistance; Common system and frame ground Feeders; Feeders 11 kV busbar; Metal-clad switchgear enclosure (frame); Frame leakage current relay 88 kV; Incoming feeders

++++ Schematic connections for frame leakage protection

Differential protection

This requires sectionalizing the busbars into different zones.

++++ Zoned busbar (switchgear) protection --- Check zone Note:

This arrangement is repeated for each phase.

VDR = Voltage dependent resistor

RS = Stabilizing resistor

DR Differential relay (Type SPAE or RADHA) =

++++ Single line diagram high-impedance busbar protection

++++ BMID single-phase circuit

++++ BMID single-phase circuit - external fault

++++ -- BMID single phase circuit - internal

++++ -- Stability characteristic busbar protection -- 1.0 2.0 IT3 Restraint area Operating area; Restraint

High-impedance bus zone


• Relays relatively cheap -- offset by expensive CTs

• Simple and well proven

• Fast - 15-45 ms

• Stability and sensitivity calculations - easy providing data is available

• Stability can be guaranteed.


• Very dependent on CT performance

• CT saturation could give false tripping on through faults

• Sensitivity must be decreased

• DC offset of CTs unequal - use filters

• Expensive class X CTs - same ratio - Vknp = 2 times relay setting

• Primary effective setting (30-50%)

• Limited by number of circuits

• Z- grounded system difficult for ground fault

• Duplicate systems - decreased reliability

• Require exact CT data

• Vknp, R_sec, imag, V_setting

• High voltages in CT circuits (±2.8 kV) limited by volt-dependent resistors.


• Additional CTs six per circuit

• Space problems on metal clad switchgear

• Long shutdowns

• CT performance important

• Class X

• Vknp = 2 times setting

• R_sec must be low

• Limit on number of circuits

• CT polarity checks required

• Primary injection tests required

• Compete switchboard

• Separate relay cubicle

• Differential relays

• Auxiliary relays

• CT cabling

• Busbar tripping cabling.

Biased medium-impedance differential


• High speed 8-13 ms

• Fault sensitivity ±20%

• Excellent stability for external faults

• Normal CTs can be used with minimal requirements

• Other protection can be connected to same CTs

• No limit to number of circuits

• Secondary voltages low (medium impedance)

• Well proven 10 000 systems worldwide

• Any busbar configuration

• No need for duplicate systems

• Retrofitting easy

• No work on primary CTs

• Biasing may prevent possibility of achieving a sensitive enough ground fault setting of Z- grounded systems.


• Relays relatively expensive

• Offset by minimal CT requirements

• Relays with auxiliary CTs require a separate panel.

Low-impedance busbar protection

Principle: Merz-price circulating current biased differential CT saturation detector circuits (inhibit pulses). On through-fault, one CT may saturate - does not provide balancing current for other CT. Spill current (i1 - i2), then flows through the operating coil.

Electronics detect CT saturation - shorts out differential path. Inhibit circuit only allows narrow spikes in differential coil. Relay stable.

S1 S2

External fault--Inhibit pulses--With inhibit

++++ Low-impedance busbar protection

For an internal fault, differential current in phase with saturated CT current,

• Inhibit pulses remove insignificant portion of differential current. Relay operates

• Setting range: 20-200%

• Operating time: Less than 20 ms

• CT supervision: Alarms and blocks or trips after 3s for CT open cct (). Internal fault

S1 S2 Inhibit pulses -- With inhibit

++++ Low-impedance busbar protection

Saturation detectors:

The CTs feed the differential circuit via auxiliary transformers, which in turn feed a typical saturation detector circuit shown:

• A voltage Vc is developed across the resistor R

• Capacitor C is charged to the peak value of that voltage

• A comparator compares voltage with 0.5 V stored in capacitor

• On saturation, V drops below 0.5 V capacitor voltage

• Comparator then turns on electronic switch across buswires

• Pulse width increases with optimum philosophy.

Electronic switch; Input CT 0.5Vc

++++ Saturation detectors

Busbar blocking system


• Very low or no cost system

• Simple

• Faster than faults cleared by back-up relays

• Covers phase and ground faults

• Adequate sensitivity - independent of no. of circuits

• No additional CTs

• Commissioning is simple -- no primary current stability tests.


• Only suitable for simple busbars

• Additional relays and control wiring for complex busbars

• Beware motor in-feeds to busbar faults

• Sensitivity limited by load current.


• Easy -- if starting contacts available, if not they need to be added

• Modern microprocessor relays have starters

• No need to work on CTs

• Most work is done with system operational

• Final commissioning requires very short shutdown. Injection to prove stability between up- and downstream relays.

I1>, I2>, I3> = Outgoing feeder overcurrent relays IT> = Busbar incomer overcurrent relay Rb = Blocking relay Tb = Blocking timer M = Master trip relay (incomer)

++++ Busbar blocking scheme.

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