HiZ vs LoZ Busbar Protection

© Copyright Rod Hughes Consulting Pty Ltd
Rod Hughes Consulting
General Web Site
 
Applications
Home
Innovations and
Solutions Home

A bit about
Rod Hughes
 Link to this page...

The URL in the browser address bar is volatile and may be broken at any time.

To obtain a link to this page, click the <<Share>> button top-right of the screen.

  

Note - if the navigation pane on the left of this window is not visible, click the 2-pane icon on the top bar



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

click to enlarge
click to enlarge

Two types

  • Centralised scheme
  • Decentralised scheme

click to enlarge
click to enlarge

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



Share a permanent link to this page: https://rhconsult.tiny.us/32knx8hz


Contact Me

Skype: (ping even if showing offline)

Email Me

A phone call is nearly always welcome depending on the time of night wherever I am in the world.
Based in Adelaide UTC +9:30 hours e.g.

April-SeptemberNoon UK = 2030 Adelaide
October-March:Noon UK = 2230 Adelaide

  Office + 61 8 7127 6357
  Mobile + 61 419 845 253



Extra Notes:

Disclaimer
No Liability:
Rod Hughes Consulting Pty Ltd accepts no direct nor consequential liability in any manner whatsoever to any party whosoever who may rely on or reference the information contained in these pages.  Information contained in these pages is provided as general reference only without any specific relevance to any particular intended or actual reference to or use of this information. Any person or organisation making reference to or use of this information is at their sole responsibility under their own skill and judgement.

No Waiver, No Licence:
This page is protected by Copyright ©
Beyond referring to the web link of the material and w
hilst the information herein is accessible "via the web", Rod Hughes Consulting Pty Ltd grants no waiver of Copyright nor grants any licence to any extent  to any party in relation to this information for use, copy, storing or redistribution of this material in any form in whole or in part without written consent of Rod Hughes Consulting Pty Ltd.