THE PERFECT PROTECTION RELAY

[521AB]

I will repost this thread as the first one was clearly misunderstood. I have then to be more explicit, and this unfortunately will probably limit the discussion field.
The question is only technical.

"People" believe that the relay is a perfect equipment, from the power system protection point of view. The fact that a relay may not trip for a fault inside its protected area or that it may trip unwanted, is commonly non accepted, even among the technician.

Some considerations now:

Line Distance Protectiom measures local currents and voltages. Even if the measurement errors would be perfect (in terms of accuracy and response time), it cannot be "perfect" because the relay does not physically see a large part of the system it is supposed to protect. Remote current infeed (not measured by the relay) for instance makes the relay to underreach, we can try to do our best maybe using different setting groups (digital technology helps in this), but we are always trying to guess something that we do not measure, so that we do not see. As we cannot make perfect settings, the possibility to fail should be considered.

 

[davidbeach]

Properly applied differential relays are about as close to perfect protection as you are going to get, if money is no object and you are not concerned about overload conditions. Protection is always a series of compromises regarding cost vs. benefit and dependability vs. security.

 

[DiscoP]

I agreed with davidbeach.

Feeder Differential is as close to ideal as possible, especially if you are using a moden numerical relay.

The disadvantages of traditional electromechanical feeder diff is that a pilot failure can cause a trip and no back-up or overload protection is available.

Both these issues are no problem to a numeric relay.

 

[521AB]

Line differential protection is completely dependent on teh communication system. If it fails (technical failure), there is no protection anymore. Communication system can remain out of service for days. This is a quite strong drawback for line differential protection idea.
It also cannot do any back-up trip, so it can just protect its own area, and nothing else.

I liked very much this sentence:
"Protection is always a series of compromises regarding cost vs. benefit and dependability vs. security."
Are non-relay engineers aware of that?

 

[DTR2011]

Most line diff relays have a back up distance protection in case of comm failure. A properly designed system will also have alarms wired to SCADA on a comm fail, therefore the amount of time the comm channel is out would be limited.

I think davidebeach was refering to feeder or buss diff, where there would be no comm channels to fail. A simple and defined zone of protection that poses no coordination problems up/ or downstream. 87B, either high or low Z does this. Then there is the cost. The 87B Low Z (IED) can now accomodate multiple CT ratios and be in series with other devices. DB is probably correct in his statement.

 

[davidbeach]

Note:

if money is no object

Comm outages can be eliminated under the same conditions. And yes there should always be backup protection. The question was about the perfect relay, not the most practical. Machine diff, transformer diff, line diff, bus diff, and you have the whole system covered. No overload protection, but very fast and totally selective fault isolation. The perfect relay system. Will that ever happen though? No, because it isn't practical or affordable. We may put line diff on transmission circuits, but line diff on a typical utility distribution feeder would be neigh unto impossible.

 

[oldfieldguy]

I personally am awaiting the perfection of the "fault anticipator" circuit for use in system protection. I surely exists because I've had managers and clients tell me that we should have seen this fault before the cable blew and took down the whole site...

old field guy

 

[DiscoP]

Just to add to smallgreek, we quite often have 2 channels over different routes. If one channel fails, it raises an alarm, but the differential protection is still available.

521AB - a modern relay also includes Distance or O/C to offer the back-up trip - either permanent or on channel fail.

Any scheme you can conjure up will have down sides.

Non-protection people are usually not aware of the cost/speed/reliability/discrimination balancing act that Protection people need to perform.

It is our job to not only perform this act, but to point out the benefits and drawabacks of the scheme to non-protection people. Usually the person who has to approve the proposal is from the second group.

 

[Skogsgurra]

521,

Why should it be perfect?

I don't think that I have ever needed anything "perfect" in my rather long technical life. Well designed, correctly applied and maintained is what does it.

Perfect would be overkill. We don't have it elsewhere - why in protection? Do you have a perfect solution?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[slavag]

Hi.
Before several days I recieved some material from protection relay devoloped department and it's remaind me about two topics "perfect relay " and "two distance protection" I removed all relay name from document and send to you . I think it's good document

Bennefits and drawbacks with distance and differential protection
The following general benefits and drawbacks can be noted for distance and differential protections.
1 Distance protection
1.1 Benefits
• The measuring is based on local criteria, no need for Teleprotection equipment for operation
• Well known technique with more than 50 years of experience
• Easy to obtain remote back up protection
1.2 Drawbacks
• Have limited possibilities to detect high resistive faults, especially when there exists fault infeed from remote end. Due to this distance protection should always be complemented with earth fault protection
• The measurement will be influenced by the mutual coupling in applications with parallel lines (automatic compensation or setting consideration needed)
• Dependent on system conditions, the measurement might be influenced by power swings, which might cause unwanted operations (special function needed to detect power swing and block selected zones)
• Special function might be needed to release trip if fault occurs during power swing
• Have difficulties to operate secure if the protected line is short or very short due to the small voltage distribution that might occur for close in and remote end faults (risk for overreaching)
• If there is a requirement that faults through ought the hole line shall be tripped instantaneously, then distance protection needs to be complemented with Teleprotection i.e scheme communication
• If Teleprotection is used and there is a risk that the apparent power at one side can be so low that the distance protection will not operate, special weak infeed function will be needed to obtain instantaneous trip at the strong side and to trip the weak side, if selected
• The distance protection can mal operate in applications with series compensation due to voltage inversion and sub harmonic oscillations if not designed to handle those issues
2 Line differential protection
2.1 Benefits
• Have very high sensitivity for both phase to phase as phase to earth fault
• Are not influenced by power swings
• Are not influenced by mutual coupling between parallel lines
• Low apparent power at one end is no problems
• Superior choice for short and very short lines since it performs high reliability
• Outstanding performance in series compensated applications (i.e not influenced by sub harmonic oscillations, nor voltage and current inversion)
• Few setting parameter (easy to operate)
2.2 Drawbacks
• Needs a digital communication channel which increases the investment cost compare to distance protection with Teleprotection due to the need of higher band with
• Is influenced by the charging current that occurs when the line is charged. If the line is long or consist of considerable among of cables, the charging current might be considerable which lower the sensitivity. However in our relay , the charging current is compensated, although in different ways in the two products.
• If 1½ breaker is used and external summation of the CT is needed due to the design of the differential protection, the summation of the currents might not be 100% and the difference will appear as a differential current. The reason to this error is mostly non equal remanence in the two CT´s due to earlier faults. However this leads to the effect that the slope for differential protection might be increased with lower sensitivity as result. This is a general phenomena when the IED can only handle inputs from one set of CT. However modern differential protections, such as our , the IED can do the summation of the two CT groups internally with secure operation and increased sensitivity as result
• The differential protection is a unit protection, which means that it operates only for faults between the local and remote CT. It can not be used as back up protection for other protections. Faults on remote bus bar must be taken care by other protections.
3 Weighting of distance and differential protection against two distance protections
The set up with distance protection and differential protection is the best choice with respect to reliability. However one must put availability requirements on the communication scheme and especially the communication channel. It is preferable or our recommendation to use optical fibre as communication media since that is not influenced by interference problems and atmospheric disturbances. Radio links have the disadvantage that it can be out of service some time during the year due to fading (atmospheric disturbances) and cupper wires are influenced by interference problems and attenuation.
The use of different algorithms as the case with distance and differential protection increases the dependability.
It is not unusually that refurbishment in the network might lead to three terminal configurations with a tapped transformer on the existing line. With modern differential protections this tapped line included the transformer can be protected by differential protections . This means that the breaker on the high voltage side of the transformer can be excluded with reduced investment cost as a benefit. This might be something that should be considered if differential protection solutions is to be selected.
Distance protection is a well proven technique and very reliable. However some attention must be taken when doing the setting calculation to overcome the application difficulties mention above for distance protection.
4 Some issues for parallel line application
When distance protection is used in application with parallel lines, the selected Teleprotection scheme should be Blocking scheme or Permissive overreach transfer trip (POTT) scheme. If POTT scheme is selected, it is a must that the scheme communication is provided with current reversal to avoid the risk for unwanted operation of the non faulty line due to current reversal when the parallel faulty line is disconnected at one side and still not jet opened at the other side.
If it is a requirement to issue single phase auto reclosing at simultaneous multi phase fault (for example L1 fault on protected line and L2 fault on parallel line) the Teleprotection scheme must be phase segregated.
Line differential protection ( do not have any problems to perform single line trip and auto reclosure due to that the protection design is phase segregated.

 

[521AB]

Gunnar (skogsgurra),

why "perfect"? because this is what "non relay people believe they are", and I wanted to open a discussion (quite interesting actually) on that.