In high voltage lines, for dead zone protection, where a fault occurs between the CT and open CB, DTT will be sent to the other side relay, for fast tripping.
My question is that, if the fault occurs between CT and closed CB, distance relay senses it in its reverse zone and could not operate. Just the busbar protection will operate, but the fault should also be obviated by the other side distance relay. In this case the trip time could not be less than the time of second zone of the distance relay.
However, this time, i.e., 500ms is too high for 400kv.
What is the actual solution in real cases?
Good day Farhad,
The time delay for Zone 2 element is normally set at 250ms provided that the microprocessor based relays are employed.
However, there are various options to consider as solutions when it comes to dead zone protection. One option to look at is the ZONE EXTENSION SCHEME.
- In principle, the zone extension functions in conjunction with an Auto Reclose scheme. In this scheme, the zone 1 has two settings. The first setting is set such that zone 1 covers up to 120% of the protected line and the second zone 1 setting covers 80% of the protected line. The second setting will be initiated after the ARC has operated from the first setting.
Thank you,
yes it is 250 ms; however, it does not make much difference; because both are infinite times where the fault should be tripped less than 50ms by the relay.
Thank you very much for your response. But, what is the practical solution where AR is not used? In this condition you have to set the first zone of distance up to 80% of the protected line. Therefore, the fault occurred between closed CB and CT of the other side can only be obviated by the second zone of distance relay in 250ms which is TOO high. What can they do here in practice?
There are two other options as well,
- Enable the zone 1 reverse element.
- Use a Weak in feed scheme which will operate in 55ms.
Otherwise, make use of the bus zone scheme.
In our experience, we use differential relay for bus bar protection. So the relay will work very fast. We have used this term in some factories.
If the fault is between the bus-side CT and an open CB of the line, then that fault is external to the line zone of protection. No DTT shall be sent to the other end of the line. That line is separated from the fault by the open CB and shall stay energized, it may feed tapped loads or may be a multi-terminal line. Normally, line relay is blocked by the 52A signal and will not "see" this fault. The bus relay uses the line-side CT and shall and will see this fault. You may be thinking about "stub bus" protection - a portion of the line zone between closed CB(s) and open disconnect switch. This is achieved by an IOC (50P) element enabled by the open disconnect switch signal (89). This protection trips the CBs, but it does not send DTT either because the line isolated from the fault by the open DS. When it comes to line protection under these circumstances (open CB or DS), you have logic called open breaker key or open breaker echo (return the permissive signal if the CB is open to allow the remote-end POTT to trip). That is for permissive schemes. Blocking schemes work inherently under the open CB scenario. You can find more about pilot protection in this paper: Permissive or Blocking Pilot Protection Schemes? How to Have It Both Ways.
This type of problems can be overcome by the ,"Simulation and Logic based Protection of Six Phase System" , proposed by me and my Research Scholar, Dr.G. Chandrasekhar in his Ph.D.Thesis.
This scheme is Equally applicable to Three Phrase Systems as well as the Six Phase System is just a Two Mutually Coupled Three Phase Systems with mutual coupling present only between the two Zero Sequence Components.
In this Scheme the present Voltage or Currently Waveform is compared with a Delayed prefault Waveform , delayed only by a few Cycles. and only the Comparator sends the Fault Signal for tripping.
The full Thesis is also
available in Research Gate , under my profile .
Dr.P.S.Subramanyam
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