Negative Seq. Relay setting for Xfmr
Negative Seq. Relay setting for Xfmr
(OP)
I have 69/13.8KV Xfmr Delta(HV)/Way(LV)
I want to set the –ve seq. relay to coordinate it with OC protection in the LV side
If the L-G fault is 4500 A. What is the value of the –ve sequence in the HV side
I got the result from Aspen Oneliner but I could not get the same result by hand.
So, what is the the formula shall I use?






RE: Negative Seq. Relay setting for Xfmr
1. The fault current expressed in symmetrical components has a positive sequence component, a negative sequence component and a zero sequence component.
2. The neutral may have a system-grounding resistor limiting the L-G fault current.
RE: Negative Seq. Relay setting for Xfmr
Yes in the LV side i have the +ve , -ve and Zero sequence but in the HV side i have +ve and -ve only, there is no zero component.
Now, I want to get the -ve seq. in the HV side.
What i did is that:
I got the (-ve seq. current LV side)*(Xfmr ratio)=-ve sequence in the HV side.
Unfortunately, I got different value by using the program.
Is my formula correct?
RE: Negative Seq. Relay setting for Xfmr
Does your transformer have a phase shift eg Dy11, etc, or is it a Yy0 for example which has no shift. Is this the source of your error?
Scotty.
RE: Negative Seq. Relay setting for Xfmr
SMB1 — Negative-sequence protection is typically applied to rotating machines. Can you please explain its purpose in a transformer-protection application?
RE: Negative Seq. Relay setting for Xfmr
ScottyUK: My Xfmr is Dyn1
busbar: During L-G fault in 13.8KV outgoing feeders, as you know, there will be a -ve seq. component in the delta side(HV). So, I can coordinate the GOC relay at LV side's feeders with the -ve seq. relay in the HV side.
I hope it is clear
(All numerical O/C relays having -ve seq. setting feature)and I want to make use of it.
RE: Negative Seq. Relay setting for Xfmr
If you are off by 1.732, you need to account for the delta-wye connection. The actual turns ratio of the two windings is not the same as the effective turns ratio at the transformer terminals.
busbar,
With advent of digital relays, negative sequence current OC relaying is essentially "free". It can be used to some advantage, especially for protection against line-line faults, since these will have neg. seq current while 3-phase faults do not. Because the neg.seq element is insensitive to balanced load current, it can be set below transformer full load current, unlike the phase OC elements.
And the neg seq on the primary of the delta-wye transformer will also detect SLG faults on the secondary. The benefit of this is questionable, since the same relay will also provide residual OC protection against ground faults from the secondary CTs.
On the other hand, I've never actually put the neg seq elements into service on any of the relays I've set. Generally, you want some other OC element to trip first if the fault is outside the transformer differential zone, so you end up needing to coordinate with phase OC elements anyway.
I think there is a tendency to think that we must use every feature on these relays because the features are there. But the more trip functions that are active, the greater the risk of a false operation.
RE: Negative Seq. Relay setting for Xfmr
dpc, I appreciate your comments. A concern would be that a 69kV negative-sequence overcurrent function would suitably coordinate with various overcurrent functions on the 13.8kV side. A 69kV breaker operation would have to be secure from 13.8kV faults in industrial or utility settings where (mis-)coordination with feeder fuses might be involved.
Timely restoration efforts following power-transformer outages initiated by primary protection seem to bring about added fear and loathing [hopefully without career-changing consequences] in the often tense “discovery/evaluation” period.
I suppose that I had too narrow a view that primary-side negative-sequence overcurrent protection could erroneously be interpreted as general transformer or {potentially conflicting with} lowside-feeder protection.
I wonder if Professor Schweitzer in his basement workshop imagined the vast new aspects {or can of worms to some} in system protection he was creating,
RE: Negative Seq. Relay setting for Xfmr
you must use L-N voltage for 13.8kv
for line to ground fault the ratio=(LL)/(LN)
RE: Negative Seq. Relay setting for Xfmr
RE: Negative Seq. Relay setting for Xfmr
RE: Negative Seq. Relay setting for Xfmr
http://doc.newage-avkseg.com/doku.pdf/pl/xs2/xs2_e.pdf
for a negative sequence relay that has a universal application.
Its characteristics show that it will be applicable for time setting above one second. When it comes to the negative sequence current on the primary side, this relay might serve as a “second? third? ..” line of defense in the transformer protection.
IEEE Std 242 Buff Book does not address the negative sequence relay for the transformer protection. There will be other much faster relays that will be in operation first.
RE: Negative Seq. Relay setting for Xfmr
DEPU: I used same ratio as you wrote
jbartos: We have a solodily grounding system
4500A is the end of line fault
the colse in fault is about 9800A
RE: Negative Seq. Relay setting for Xfmr
Don't forget that your line to ground fault of 4500 amps has equal parts +, -, and 0 sequence. Your - sequence component then is 4500/3. How's 300 amps primary sound? (confirmed by Aspen V2001G.
Don't forget that some relays (SEL) input negative sequence quantities as 3I2, while others (ABB) use inputs of I2. Make sure you're using the right quantity in Aspen.
RE: Negative Seq. Relay setting for Xfmr
As stevenal says, I was referring to current that would be measured in a line CT at the transformer terminals, not in the delta.
So you just go by the nameplate voltage ratio for the transformer.
RE: Negative Seq. Relay setting for Xfmr
If you set it lower and grade it with overcurrent protection, what is the point, wouldn't HV side overcurrent do the job.
This setting will not be very sensitive, but could clear a fault quicker than the back-up overcurrent protection on the HV side.
But I am not sure if it is worth the trouble
RE: Negative Seq. Relay setting for Xfmr
Beaside havin the same question as busbar,I thought -ve sequence relays are usually set to some % of the rated currents with time delays in ‘Several’ seconds, while the secondary side LV GOC protection would be set at low currents and more importantly to trip within ‘0.5’ seconds.
This by itself should discriminate the primary from the secondary side trip on GF.
A very sensitive –ve sequence setting on primary will result in nuisance trips. What is the type of relay you have?
RE: Negative Seq. Relay setting for Xfmr
RE: Negative Seq. Relay setting for Xfmr
//I'll disagree with DEPU. The negative sequence quantities are a balanced set. Unless you are looking for the negative sequence currents inside the delta,(an unlikely CT location) use the effective turns ratio from the line to line quantities, not the actual ratio.//
96/13.8KV Delt(HV)/Y(LV)
1. During LG fault in the LV side we will have L-N voltage not L-L. As a result, the Xfmr ratio will be LL/LN
2. Also, during the LG fault
(-ve seq. outside the delta = -ve seq. inside the delta)
this should be clear.
To prove this:
any LG fault in Y side will behave as LL fault in the Delta side.
The fault current will flow on one winding (Y side). After applying the (AmpsXTurns) concept the current in the Delta side will flow also on one winding.
So, in the (HV)Delta side during the LG fault in(LV)Y side
the line current= the phase current
i.e,
(-ve seq. outside the delta = -ve seq. inside the delta)
RE: Negative Seq. Relay setting for Xfmr
Again, negative sequence currents are a balanced set, just like your positive sequence load currents. For this case, use effective turns ratio just as you do for load current.
Use actual turns ratio for phase quantities. Do this for a sanity check. 4500 Amps > 520 amps primary line to line. If the other two line to line currents are zero, then your line to line quantity is equal to the adjacent phase quantities. Answer 520 amps primary on the two affected phases.
Back to sequence quantities: Positive and negative primary phase currents are both 300 amps. Zero sequence is zero outside the delta. Add them up. One non-zero quantity is lagging the secondary current by 30 degrees, the other leading. The resultant is 300*2*cos(30)=300*sqrt(3)=520 amps primary. Sanity check complete.
RE: Negative Seq. Relay setting for Xfmr
RE: Negative Seq. Relay setting for Xfmr
RE: Negative Seq. Relay setting for Xfmr
By far, the most common grounding system used in US utility MV distribution systems is solid grounding. 9800A would not be considered excessive.
SMB1,
What values do you get by hand calculation and from Aspen? Seems like failure to divide line-to-ground fault current by 3 as noted in stevenal's Sep 2 post is a likely source of error. Also, the relay setting current may not be the negative sequence current. Seems to me I remember SEL using 2 or 3 (I forget which) times the negative sequence current as the setting value.
RE: Negative Seq. Relay setting for Xfmr
Check SMB1's Sept 3 statement that "(-ve seq. outside the delta = -ve seq. inside the delta)" His error is that he believes negative sequence current exists only in the faulted phase, and doesn't understand that symmetrical components are actually symmetrical and present in all phases. His error is by a factor of sqrt(3) as dpc first indicated.
RE: Negative Seq. Relay setting for Xfmr
Please refere to my post dated Sep 3, 2003, and let me know where is the error in the prove.
I got the correct furmula from the (Symmetrical Components)book,
But still I believe that during LG fault in the Y side:
(-ve seq. outside the delta = -ve seq. inside the delta)in the HV side.
Thanks and Regards
RE: Negative Seq. Relay setting for Xfmr
Negative sequence current in each one of the delta connected windings is of equal magnitude and 120 degrees apart. To find the negative sequence current outside the delta, on A phase for example, do node analysis at the A terminal. The negative sequence leaving the A bushing is something like I2A=I2BA-I2AC (check me on this, cause this is the hard way). An easier way is to remember that the line currents in a balanced system have sqrt(3) times the magnitude of the line to line currents and are shifted by 30 degrees. An even easier way is to use the effective turns ratio rather than actual turns ratio as it already includes the sqrt(3) factor, leaving just the phase shift to deal with. Now if you know which way the positive sequence currents shift across the delta wye, just shift the negative sequence currents 30 degrees the other way. Have I earned my star yet? Down below there to the left where it says "Mark this post as a helpful/expert post!"
RE: Negative Seq. Relay setting for Xfmr
I'll give you a star. The key thing that SMB1 has to understand is that sequence currents are balanced three phase currents by definition.
RE: Negative Seq. Relay setting for Xfmr
RE: Negative Seq. Relay setting for Xfmr
any LG fault in Y side will behave as LL fault in the Delta side.
The fault current will flow on one winding (Y side). After applying the (AmpsXTurns) concept the current in the Delta side will flow also on one winding.
So, in the (HV)Delta side during the LG fault in(LV)Y side
The fallacy is in treating sequence currents like fault currents. Even though, during a low side line-to-ground fault, there is fault current in only one low side phase and two high side phases, there is negative sequence current in all three phases of both the low side and the high side. The positive and negative sequence currents are present in all three phases and are equal.
If you had 4500A <0° in secondary Phase A, then on the secondary side:
Voltage ratio=69000/13800=5
Turns ratio=8.66
Ia0=Ia1=Ia2=1500<0°
Ia=4500<0°, Ib=0, Ic=0
on the primary side:
IAC=519.6<0°, IBA=0, ICB=0
IA=IAC-IBA=519.6<0°, IB=IBA-ICB=0, IC=ICB-IAC=519.6<180°
IA0=0, IA1=300<30°, IA2=300<-30°
IA1 and IA2 both flow in Phase B, but from the definition of symmetrical components:
IB=IA0+a²·IA1+a·IA2 where a=1<120°, a²=1<240°
IB=0+300<270°+300<90°
IB=0
RE: Negative Seq. Relay setting for Xfmr
Negative-sequence overcurrent relaying is applicable on transformer primaries, based on ANSI/IEEE C37.91-1985 Protective Relay Applications to Power Transformers. §5.6.2 “Since these relays do not respond to balanced load or three-phase faults, negative-sequence overcurrent relays may provide the desired overcurrent protection. This is particularly applicable to ∆-Y grounded transformers where only 58% of the secondary per-unit phase-to-ground fault current appears in any one primary-phase conductor.”