VARIOUS CASE STUDIES ON OPERATION AND CONTROL SCHEMES FOR GRID SUB-STATION Contd….

14 Nov 2024

1. Introduction:

For the last few months, the response of the readers to the case studies on various incidents is overwhelming. Hence this month we are again choosing the write up on similar kind of studies for developing the synchronization of practical observation to the theoretical concepts. The analysis of each incident being supported by actual observations had been described during the situation to add awareness amongst the operation, testing and commissioning engineers to know the cause of problems and be helpful for easy rectification of the problems. This can also help to develop economic schemes for the smooth running of the operation and control system in the Grid Sub-Station.

PLEASE NOTE: From next issue, NEW CASE STUDY MODULE shall be published. In this CASE STUDY, important Electrical concepts shall be described with practical RELEVANT CASE STUDIES.

In this CASE STUDY we are repeating few important published cases for better understanding of the subject by the valued readers.

2.1. Tripping of Transformer on Auxiliary Relays:-

Tripping of 40 MVA, 220/33 KV Power transformer, on Mechanical Relay Protection Relays ( WTI, OTI, BUCHOLZ, PRV etc..) for the condition of heavy fault on the 33 KV feeders going out from the 33 KV Bus of the Sub-Station.

Observation on Checking:

1. The detail circuit of the Mechanical Protection relays related to the DC leakage was checked but found with no any discrepancies. Moreover this kind of apprehension could be overruled because if it would have the reason, then all the time during the fluctuation/ Change of DC supply this could have been occurred. But such occurrence of the incident was only resulting during the time of Close in fault on the 33 KV feeders.

2. This was found with use of ARMOURED CABLE on the Protection Control circuit from MK (Marshall Kiosk) to Relay Panel in the Control Room.

3. This cable was in damaged condition and black insulation cover over the armouring portion being damaged was kept in touch to the LT Neutral conductor.

Analysis:-

In fact the occurrence of the fault, specifically on the outgoing feeder, has the source of its origin is the winding of the transformer. LT side fault results the current flow path through the LT neutral conductor buried to the sub-station earth mat. During close fault in the system, the fault current becoming very high results rise of potential on the ground conductor. In this situation, the ARMOURING PART being in touch to the LT Neutral conductor results momentary rise of potential on it. At the same time the reflection of this rising voltage on the return wires in the cable, meant for mechanical protective relays, causes the unwanted voltage appearance on the relay and tripping of the scheme. 

Actions Taken:-

1. The armoured cable was replaced by normal cable and re-routed away from the LT neutral conductor of the transformer.

2. The LT neutral conductor buried to earth was checked and strengthens by use of another connection to the Sub-Station Earth mat.

3. Similarly the earthing connections for HT Neutral were checked and strengthen. Then the occurrence of fault was monitored. This time the occurrence of unwanted tripping was reduced.

2.2. Case of Intermittent Tripping of Transformer on Differential Relay: -

One of the 33/11 KV, 5 MVA transformers was tripping on differential relay intermittently during fault on the 11 KV side outgoing feeders. 

Actions taken:-

For each situation, the tripping was occurring only on differential relay with no any mechanical trouble relay of the transformer, for which this can be confirmed that the transformer has no any objectionable problem. The problem may be due to the network circuitry of the differential relay or relay might have resulted mal-operation. So the transformer was taken shut down for detail checking.

1. The LV test on the transformer was done and results were found in order.

2. The CT circuitry for both 33 KV and 11 KV were also checked on the injection of current to the involved CTs and the results were also found satisfactory.

3. The stability check was done with supply on the 33 KV side and shorting of the primary 11 KV side for the transformer.

a. This time certain spill current was observed on the R phase operating coil.

b. This abnormality was not getting sticky to the value permanently.

c. The value such obtained sometimes becoming OK and sometimes not.

d. This reason of the fluctuation was indicating on the problem on the connecting terminals.

e.The TBs used on 11 KV CT secondary side were checked thoroughly and found with sparking black spot on R phase differential core TBs during checking of tightness.

f. This was also found with very less gap with CT secondary terminals and mounting plate on which 11 KV indoor CTs were installed.

4. So during actual fault occurrence, the return current instead going directly to relay circuitry, certain current passes through the mounting plate. The magnitude of this current diversion depends upon the severity and magnitude of fault current, Hence unbalance current spills on the operating coil and results the tripping of the transformer.

5. This fluctuation some conditions may be within the limit of operating current and do not cause the tripping of the transformer.

Rectification:- The TBs were totally replaced on 11 KV side for this core. The CT mounting plate was also changed with sufficient clearance of CT secondary terminals.

2.3. Tripping of a particular Transformer:

In a 132/33 KV Grid Sub-Station, three power transformers were connected in parallel. It was observed with tripping of a particular transformer during the close in fault and subsequent tripping of other transformers on over loading situation.

Observations:

1.The situation as mentioned does not operate always. Sometimes for the condition with heavy close in fault, one of the particular transformers was tripping earlier followed by other two on over loading situation.

Investigation and analysis:

1. The settings adopted on the relay was checked and found OK.

2. The percentage impedance of the transformer was checked and found with lower in per unit value as compared to the other two.

3. So during the fault occurrence, this transformer will take more fault current as compared to the other two, for which this one was tripping earlier to the other two.

4. After tripping of the transformer, due to over loading situation, the other two are tripping subsequently.

Action Taken:

1. The settings of the transformers were reviewed as per the load sharing during normal condition.

2. According to the percentage impedance, the settings were changed.

3. The gradation of the relay was done as per requirement.

4. Then it was kept under observation and situation became OK.

2.4. Operation of Power transformer with lower IR value:

One of the 40 MVA transformer, Y-y connected 132/33 KV was in operation and taken shut down for condition monitoring. The results were found on IR measurement for HT-Body being 1 Mohm with abnormal value of the tan delta value also.

Investigation and Analysis:

The test results were reviewed and found with normal value for other terminals (HT-LV, LV-body). But it was observed with approximate equal value for HT-LV, LVbody. So from this observation it was confirmed that the HT neutral path either have been connected to the body or maintained with very less clearance to body. So the oil was drained for physical verification of the HT neutral lead and found with no such abnormality.

Now the tap changer system was suspected as because of this path is associated with the HT winding. The diverter switch was lifted and found with debris and sludge on the lugs. The tap position connecting rod was also deposition of foreign materials. The same was cleaned and rinsed thoroughly with transformer oil with overhauling of the total diverter switch portion. Then the results were taken and found with normal value. 

The transformer was charged and loaded successfully.

Note: So it can be noted here that for the case of abnormal IR value between HT-Body for any transformer, the common system i.e TAP CHANGER portion has to be overhauled before concluding the problem in the transformer.

2.5. Tripping of 132/33 KV Transformer on differential Relay:

During running condition of a 20 MVA 132/33 KV, two winding Transformer, it was found with abnormal tripping on differential relay.

Action Taken:

1. As the tripping was only on differential relay with no mechanical relay actuation, it was confirmed regarding the problem on the electrical circuit.

2. This problem may be on the CT circuitry or DC protection circuitry or due to the problem on the differential relay in the system.

3. The detail circuit was checked, load stability was done, DC tripping circuitry was also checked and found in order.

4. Finally the relay was checked and found with the problem on Static electronics circuit of one unit i.e R phase.

5. This unit was changed with the correct one and transformer was charged and stood OK.

NOTE: It may be noted that for any tripping of transformer, only on electrical relays with no any mechanical relay, then the problem may be due to external electrical CT circuit and in the relay circuits.

2.6. Tripping of Auto Transformer on PRV switch:

One of the 160 MVA, 220/132/33 KV Auto Transformer was reported tripping on PRV relay during summer season. Observations with Analysis:

a. The transformer was taken shutdown for physical verification.

b. It was observed with no spilling of oil from the PRV( Pressure Release Valve) with non-operation of the plunger attached to the switch.

c. But continuance of the tripping pulse to the relay was indicating of the mal-operation of the PRV switch.

d. So the switch was cleaned with its contacts for the occurrence of the first incident around 2.30 PM (during May month).

e. The transformer was charged and stood OK.

f. The repeated incident again resulted during the same at night 9.30 PM.

g. This time, the switch was opened for the verification of the operation of the micro switch and its operating lever inside the internal cabinet.

h. The micro switch with its accessories was replaced by a good one.

i. The reason of such occurrence and mal-operation of the switch was investigated and found with the followings.

» The operating lever is basically housed in the plastic-rubber cabinet and functionality of such lever depends upon spring loaded latch switch that moves in and out during operation. Normally the lever remains in OUT position, keeping the microswitch OPEN against spring loaded latch switch for which the electrical contacts remain OPEN and non-operation of the relay.

» Sometimes the distance of the latch, upon the micro switch gets disturbed due to un-even expansion of the spring and plastic-rubber cabinet due to temperature and stiffness of the spring load. If this happens, then with non-operation of the plunger, the lever may press upon the micro switch and result mal-operation of the relay. This was resulting for this case as described both in day time and also night time during pick load condition on the transformer.

Note:- For any transformer, if any MECHANICAL RELAY actuates with no electrical relay support, then the situation could be either the case of mal-operation or alarm of the change of mechanical parameter. So the grid personnel should not be panic to handle the situation.

2.7. Actuation of Differential Relay after Shutdown of 160 MVA Auto Transformer:

One 160 MVA 220/132KV Auto transformer was availed with shutdown. After completion of shutdown work, when this transformer was loaded, the differential relay actuated and resulted tripping of the transformer.

Similar attempt was done two times and in each case, similar tripping on differential relay resulted.

Actual Observation:

a. This transformer was in loading condition and catering load successfully with the available another 3 Nos of transformers in the network being connected in parallel.

b. After taking shut down for maintenance checkup, this transformer was idle charged and stood OK.

c. When 132KV system was connected with loads, the current on 132 KV R phase became less as compared to the other phases.

d. Now by raising the limit of differential current setting to 50 %, the transformer was charged once again.

e. This time differential current becomes 19%, and rest of the core for the R phase CT secondary currents were checked on 132 KV side and observed with similar pattern of reduced current as compared to the other phases.

f. So it was confirmed with the problem of primary side of the CT.

Action Taken:

1. Now this CT was decided for the replacement.

2. After replacement, the transformer was charged successfully.

3. The faulty CT was opened and found with reduction of the Oil level and sparking on internal terminal of the primary conductor ( Photo is attached for reference)

Analysis:

a. Due to reduction of oil level, the required clearance between the primary conductor and top tank got reduced.

b. So certain current started to flow through the metal tank of the CT, with other current through the actual primary conductor.

c. Now due to flow of part current, the secondary current accordingly became reduced as like it was observed.

d. So due to this, the differential current was getting available and causing the tripping of the transformer.

2.8. Tripping of a power Transformer with no connected load.

One of the 132/33KV Power transformer was charged idle with station transformer (33/0.4Kv). This one tripped on Differential and LV back up relay with fault current being HV side (R-170A, Y =170 Amp, B=500A and N=250A) and LV side current being ( r =1A, y=1A, b= 2696A and n=2696A).

Observations:-

1. The testing of the transformer was done and found in order.

2. The physical condition of the transformer was also checked and found OK.

3. The LV side B phase LA was checked and found faulty.

4. Required faulty LA was replaced and transformer was charged OK.

Analysis:-

a. With reference to the figure 2.8, it is seen that the LV side LA comes within the protection zone of the transformer, as the LV side covers this zone.

b. So during the problem of this LA, this might have been punctured with arching in the system inside.

c. So due to result of BN fault in the system, the fault current has raised to higher value as mentioned under observation.

d. So due to internal zone, differential relay has been tripped.

e. The earth fault feature being set with high set instantaneous value, this has also tripped.

–Author– Er. P.K.Pattanaik

Er. P.K.Pattanaik, is presently working with OPTCL as General Manager, EHT (O&M) Circle Bhubaneswar - Odisha and associated with the Protection and Control schemes of Electrical systems. Having 29 years of technical experience on various HT and EHT voltage level in the field of transmission sector. Specialization on the development techno-economical design of protection control schemes for system development and planning. At present involved with various on-going projects on GIS, SAS and updated Remote SCADA control stations of OPTCL.