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Integrity or continuity of the stabilising resistor for high impedance busbar protection scheme is of paramount importance.I have seen two cases where the slider ring of the variable stabilising resistor was broken probably due to excessive tightening of the slider screw. Has any one else...

Thanking all the contributors. I agree with dpc and Stevenal regarding degradation of reliability by add third party external DC/DC convertor. As part of my research, I did find that Siemens compact relay type 7SX800 does have truly universal DC range i.e. DC 24 V through DC 125 V

In our system few old Substations run with 32 V DC power supply. We would like to standardise on 125 V DC rated relays which are used in majority of the substations. In that context do you see any issue if we power 125 V rated relay by an external 32/125 volt step up convertor? Regards,

With hv side ungrounded(star ungrounded) and LV side star grounded arrangement- the ground fault current on lv side is seen as line to line fault on hv side due to the effect of tank tertiary (aka phantom tertiary). Zero sequence impedance of such transformer is quite high in order of 50-100%

What is the general practice of grounding the sheath of control cables especially in HV substation/ switch yard. Do you ground the sheath or screen of the cable only at one end or at both the sends of the control cable?

Thanks for your valuable comments and yes CT magnetisation/demagnetisation is another issue. Could you please share the test equipment details which can do contact resistance measurement with both side grounded as mentioned by thermionic1?

Thanks Raghu. 100 mA is at the rated Ek of 500 V but in practice, depending on connected burden, the CT will only develop fraction of rated Ek at rated current of 1000 A and hence the excitation current will be very low.

I agree 100% with mLp85.There seems to be a confusion regarding correct notation of Trf vector group where HV winding is star unearthed and LV side is star earthed. In my view we can use YNyn0 symbol to represent this vector group. Earthing a trf neutral or not earthing is not governed by the...

Capacitive reactance is inversely proportional to the frequency hence Xc@250 hz will be 1/5 of Xc@50. Additional current due to prevailing fifth harmonic voltage on the network will therefore be V@250 divided by Xc@250

Transformation of phase to ground fault current in YNYno vector group transformer is one of the least understood concept. As mentioned in my previous post, the balancing zero sequence flux flows through the tank of the transformer in case of three limb core type transformer.Zero seq impedance of...

YNYn0 transformer will always transform line to ground fault on LV side even though the neutral HV winding is isolated. Depending on Trf construction (like 3 limb) core ; the zero seq balancing flux flows via the tank of the transformer. It called phantom tertiary. Zero seq impedance of such...

Hi Raghu I agree with you view point. In this case other two healthy phase CTs had remanent flux from previous high DC offset fault and hence were quickly driven into deep saturation without corresponding high primary current. The relay in question is low impedance bus protection scheme which...

Thank you all for great suggestion and comment. In the mean time i have come across a case wherein very high fault on A phase( 250A secondary current on 5 A relay) caused sufficient voltage across healthy CT secondary of remaining two health phase to drive them into saturation and caused...

Hi Davidbeach- Why is that? I mean why the burden should be lower in case of 4 wire ?

Is there a specific reason for not forming the CT star point in the yard especially when dealing with UHV substations?

Raghu - In our case Primary side is not isolated as its hard on connection to Air Insulated Busbar (outdoor switchyard)

Voltage tester/sensors may be alright for medium voltages but can these be used for voltages 220kV and above?

Most of us know that during planned outage of primary plant/equipment its customary to isolate all possible points/sources of energisation and secondary of voltage transformer being one of them. Our issue is that we also check for the status of PT/VT mcb as one of the condition required to close...

Does any member has experience or know of the possibility of distance relay's inability to perform as designed due to suppression of negative sequence current or limited positive sequence current contribution from voltage source converters commonly applied at HVDC stations? I have heard that...

I was talking about the minimum fault level at the bus in a substation(say 1000A). For example an over current relay with high set element set at 900A may under some condition, will fail to pickup