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.
