HiZ vs LoZ Busbar Protection

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Busbar protection can be realised in different ways - notably the "reverse blocking" schemes have become an effective means of using general over-current relays on the incomers and outgoers.

Busbar differential protection can be achieved using High Impedance (Merz-Price circulating current connection CTs) relays or Low Impedance relays.  The selection of either is very much application dependant.  Here are a few key aspects to be considered.

HiZ schemes:LoZ schemes

Two types:

  • Voltage setting relay
  • Current setting relay

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Two types

  • Centralised scheme
  • Decentralised scheme

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The CT connections shown above are for one of the three phases. 
CTs for each location would typically be connected in a standard Holmgren connection.

Requires all CTs to be the same WINDING(TURNS) ratio.

Can compensate for different ratio CTs at different locations

CTs must be IEC 61869 class PX (IEEE C57.13 class X) – the specification requirements of PX ensure the CTs are physically identical and hence have identical dynamic response

Possible to use IEC 61869 class P (IEEE C57.13 class C), although PX (X) is still better.  Bias settings can cater for potentially different excitation curve performance of P class CTs

All CTs must sufficient VKP according to the formula:
Vkp > 2 x Ifmax x (Rct + Rloop)

This also means it may be better to specify only Ifmax and Rloop and request the CT manufacturer choose Vkp and Rct to suit the formula

Vkp can be lower if the relay has CT saturation stabilisation - remembering that stabilisation usually means blocking/delay/biasing of relay operation in some way.

Dedicated CT cores.  Not recommended to put any other devices in the loop (as that further increases the Vkp requirement)

Does not necessarily require dedicated CT cores, potentially using existing CT cores.  Other relays can be in the loop as CT saturation is "tolerable" (and hence making retrofit of busbar prot a bit easier to existing substations if no dedicated differential PX cores available), but remember to consider all elements, including the existing protection performance when CT does saturate.

Need to be very careful if you have schemes involving CT secondary switching (e.g. double bus topology) where CTs may be inadvertently open circuited or incorrectly switched by the mechanical contacts on the isolator/circuit breaker

Far more flexible for systems with ‘varying’ bus zone topology as the zone changes are not involving CT switching directly, however there is still reliance on correct operation of the mechanical contacts on the isolator/circuit breaker.

Generally involves only one simple relay box, maybe a stabilising resistor if it is a current pick up type device (voltage setting devices are generally inherently high impedance) and possibly a metrosil

Generally involves more expensive microprocessor relays and may involve multiple boxes/multiple input cards associated with each CT location

Can develop large voltages on CT wiring needing metrosil voltage limitation

Less likely to need metrosils

Only one relay setting (plus in some cases the setting of the external stabilising resistor value)

May have lots of settings for ratio, bias .... on a per CT basis

Exceptionally simple, reliable and fast - particularly the well proven (many decades) electromechanical relays.

The trend to use electronic/microprocessor relays needs careful understanding of relay pick up performance with heavily saturated waveforms such as occur in the required critical tripping for internal faults

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