Case Study on Unit Protection Scheme
This is one of the interesting technical finding we had while reviewing the unit protection scheme of Power TR as a part of Power System Study analysis and Relay co-ordination. Unit protection, as the name says is the protection scheme responsible for the fault in particular unit / equipment only and ensures the fast fault clearance, at the same time it should resist tripping in case of the out zone fault i.e., external network fault.
The following case gives insight, How important is selection of Protection scheme and Relays during design engineering or retrofitting job in existing network !
Case:
Power TR of rating 25 MVA, 132/3.3 kV, YNyn0 vector group is installed at site. The star point of the 132 kV side of TR is solidly grounded, while the 3.3 kV side of TR is grounded through 100 A, 19.05 ohms NGR. The Power TR is running in parallel with 8 MW, 3.3 kV WHRS at 3.3 kV voltage level. The star point of WHRS generator is grounded through 100A, 19.05 ohms NGR. At time only One NGR i,e., Power TR is kept in line.
Power TR is protected with Siemens make 7UT612 numerical relay. In relay all the required protections such as Differential protection (87T), LV REF (Low impedance REF) protection (64R) and LV SEF protection (51NS) are enabled. The detail scheme implemented for Power TR is shown in below figure.
Observation:
The earth fault current of 3.3 kV voltage level is limited to 100 Amp. The low impedance REF protection has been implemented for 3.3 kV side of Power TR. The sensitivity of the REF protection in 7UT612 relay is determined by IREF> setting and its depends on the rated current of the protected object.
The limited minimum possible setting offered by the 7UT612 relay IREF> is 0.05 I/InS.
Where, I/InS is the rated current of the winding to be protected of the main protected object.
So, by converting it to primary side, the pick up is coming around 220 A
While Earth fault current of 3.3 kV side of Power TR is limited to 100 A only.
With the calculations above, it is evident that, total protected zone is -1.2%. Hence, there is no protected zone in LV side of Power TR.
Conclusion :
For the above discussed case, any genuine ground fault in the 3.3 kV side of Power TR, LV REF remains inoperative until the fault becomes severe or observed by any other enabled protections, and by the time any other protections operate damage has already occurred in the TR. This leads to significant Financial loss to an organization with cumulative effect of loss of their main asset/equipment, Production loss due to downtime and man power sitting idle.
Reference:
- Relay manual of siemens make 7UT6x Numerical relay
- Siemens application note on 7UT6x relays
- IEEE C37.91 - Guide for protective relay application to Power Transformers
Planning/scheduling Lead (AMEA)
3 年I believe the setting is not on the higher side its in the border line of the operating characteristics.. Its always better to use voltage operated( high Impedanc) REF relays.. everyone know the reason behind it.. Also when you convert current from Sec to primary u shall use CT ratio not the rated current I primary = 200A not 220A.. so bascially setting is on the border.. Also NGR need to be protected from damage so an over current element shall be implemented on the 100/1A CT with time delayed isolation.. it would really interesting to hear how you solved the problem based on this case study
Power Systems Entrepreneur | Energy Consultant | Product Strategist and Researcher
3 年(3/3) TLDR: A completely wrong perspective... Low Impedance works by comparing the neutral CT current with your phase CT current and trips for the measured neutral CT current. In essence, low impedance REF is like a phase differential protection, but only on one side of the transformer. This also has slope characteristics like phase differential, meaning it is very sensitive to internal faults and highly stable against external faults. Though fault current is limited to 100A, the neutral CT ratio is 100/1 A, thus making measured fault current referred to on the secondary side as 1.0A
Power Systems Entrepreneur | Energy Consultant | Product Strategist and Researcher
3 年(2/3) For REF, we essentially have a setting less than phase differential. So, phase differential has a setting of 0.2p.u., REF has a setting of close to 0.1 p.u. The base value of the current is the full load current of your 3.3kV side. Towards the end of the article, you have just referred to as “Earth Fault” not “Restricted Earth Fault”. Even in the case of normal earth fault protection, the CT ratio shall be considered as 100/1A, not 4400/1A.
Power Systems Entrepreneur | Energy Consultant | Product Strategist and Researcher
3 年(1/3) Low Impedance REF is used a UNIT PROTECTION of one side of the transformer, meaning it only operates for faults on one winding (star side) and restraints for faults outside this. Low Impedance REF works on a comparison principle – it compares the current through the Neutral CT giving you the measured value of 3I0’ (neutral current) AND the sum of the phase currents giving you the calculated 3I0”. But tripping effect is a result of only the 3I0’ quantity, i.e. Ifrom = |3I0’| and the restraining current Istab = k.(|3I0’-3I0”|- |3I0’+3I0”|)During healthy conditions – both 3I0’ and 3I0” are zero (considering a balanced system). The second diagram you have shows an internal fault, essentially at the 3.3kV winding. In case of an internal fault fed only from the starpoint, as shown, then 3I0’ = 100A or 1A in the secondary side of the neutral CT and the stabilising current Istab = 0, resulting in a trip. For an internal fault fed from starpoint AND the system, 3I0’ = 100A / 1A referred to the secondary of the neutral CT and Istab = -2.|3I0”| ~ 0A, resulting again in a trip. Please note I have assumed the k factor to be 1.