Current Transformer Preventive Checking (Residual Life Checking)

"Kindly advise in this issue to avoid such failure"

Call someone who can actually see the equipment and knows how to fix it.

Or, hire a sober electrician to fix it.

You could always just stare at it real hard, and hope that it fixes itself.

That never works for me.

[PDF]

Transformers: Basics, Maintenance, and Diagnostics - Bureau

Sensible transformer maintenance. | content content from Electrical ...

anandchy:

Perhaps you have an answer here.

http://wiki.answers.com/Q/What_are_the_procedures_to_synchronize_generators_in_parallel

"If the generators are tied together out of phase two things could happen. The least problematic would be the generators breaker will trip. The second and most dangerous is if the lag generator is just a little bit out of phase when the switch is thrown, then the generator will be jerked into phase lock as momentary heavy current flows between the two generators. The sudden loading of the generator has been known to break crankshafts of the engine driving it."

Hi,

I assume the CT failure was due to insulation breakdown.

The following two tests may be done on isolated CTs and may indicate when the insulation is deteriorating and failure is imminent. The tests are better if original manufacturer tests of the same values are available for comparison.

Tan Delta or dissipation factor measurements of the CT.

Partial Discharge measurements of the CT.

After going through various suggestions, I have a comment and few more suggestions to make

When Insulation levels of various equipment in HV electrical system are designed called INSULATION CO-ORDINATION, the insulation level of CTs and PTs is greater than most of equipment so that in case the system is subjected to a surge or overvoltage, CTs shall be last to fail as these are to protect the system.

Hence failure of CT due to Dielectric Breakdown is very remote.

Other common reason of failure of electrical equipment is thermal breakdown of insulation. That is remote as for maximum burden of CT of 20 VA (very common), and is ruled out.

I have seen some time failure of CT due to heating - but this is indirect heating, due to bad connection (Resistance at joints) between CT and Bus Bars. Example: Star Point CTs of generator. Solution to avoid the failure due to indirect heating is:

1. Use Flexible links in place of solid bus bars as shorting links to form Star point.

2. Measure periodically (during annual maintenance) Joint resistance with Ductor keep record - clean joints if necessary.

3. Check tightness of joints with torque wrench. Use 8.8 Grade or better rolled thread fasteners.

4. As a routine - once a fortnight but not more than a month, under operating conditions, from Remote, with Infra Red Temperature Detector check for hot spots on joints. If abnormal temperature or discolouration is noticed - attend at the earliest opportunity.

The Cause of CT Failure in this occurrence in particular could have happened in following sequence.

1. Mechanical failure due to high magnetic field created by fault current - resulting in crack in insulation. This can further worsen if there was out of step synchronizing between STG and Grid. It is something like subjecting a mechanically week part to another hit with a hammer. If the protections relays are of Numerical Type - one can check from peak current recorded and stored in the relay and confirm this happened.

2. After the STG was started and voltage developed- it took 1 hour to slowly flash over through cracked insulation.

If the CT which failed is in same phase which feed the system fault, then above is further justified.

To prevent above:

1. The fault level of station shall be checked. May be there is growth in the Grid Capacity and Maximum Fault Level at concerned Power Station has gone up. If it is gone up - Fault Withstand Capability of all equipment shall be checked and replaced where it is marginal or less that fault levels.

2. Check that free length (Unsupported Length) of bus bar between Primary Terminals of CT and next immediate support insulator is not more than 300mm. Greater is the free length more is the movement of Bus Bars during fault and resultant strain on Terminals - resulting in damage to insulation.

3. Any power cable connected to CT Primary Terminals shall be supported independently to ensure that its weight is not hanging on CT and causing strain on Terminals of CT. To check this just disconnect the cable, see that no movement in position of cable and it can be easily placed back on Terminal studs.

4. It is preferred that last piece of link between Solid bus bars and CT shall be a flexible link so that any growth or shrinking of bus bars with variation in load (Thermal Cycling) does not mechanically strain the CT or any other insulators such as support insulators, Bushing of Transformer or Generator etc.

Suggest that based on suggestions received from various readers, the concerned Maintenance Engineer of plant shall make check list to suit his installation to carry out first Quality and Reliability Audit of Installation and Preventive Maintenance Check List.

He can also consider other factors, such as

1. Risk of any rodent or reptiles, even spider webs etc. causing breakdowns.

2. Environmental conditions such as industrial pollution and humidity which are another cause of insulation failures.

3. Type of insulating materials, some or these, such as epoxy and resin cast insulators are prone to surface tracking - thus frequent cleaning of surface and coating with anti-tracking varnish is required - frequency depending up factors in 2 above.

4. Carry out study on protection relay settings to confirm proper co-ordination and fastest tripping so that equipment feed or carries fault current for minimum possible time. In a power system, generators are the last to trip, as a result if primary protection is not adequate, tripping on back up protection subjects generator and its other accessories to more than desirable length of fault current.

Best regards,

Ramesh Kapur

IR value of Generator Stator of 100M Ohm at 5 kV is acceptable value - but now a days insulation systems gives IR in Giga Ohms. Additionally PI (Polarising Index) which is Ratio of IR values at 10 min. and 1 min shall be taken of stator windings and kept as reference.

Preferred value of PI is more than 2 but shall never be less than 1. If it is less than 2 means, contamination and moisture in winding - which means surfaces of insulation needs cleaning and stator winding to be heated to dry out.

Regarding CT: We can meger CT between Primary and secondary with secondary terminals shorted and grounded - disconnect all protection relay and meters etc. so that in case of failure of CT during testing the fault current does not flow through these devices.

Also if doubt, CT can be tested at 18kV DC for 1 min. and leakage current recorded. (But not PT or LA hence these shall be disconnected). Before carrying out HV DC Test must take IR and PI values - IR shall be comparable with earlier test reports and PI shall be more than 2 before carrying out HV Test.

Carry out above tests (IR, PI and 18kV HV DC) on cleaned and dry CT and Stator winding and keep record of values for comparison in future.

It is important to note temperature of winding when testing. The Insulations have negative temperature co-efficient and value of k at different temperatures is available from supplier of equipment or can be searched on web for particular insulation system. Always convert the IR value at test temperature to that of temperature at which Reference values of IR value of clean, dry and healthy windings/ insulation of CT were taken for proper comparison.

Pl note that PI is not effected by change in winding temperature and is one of good tool with Electrical Engineers to know health of machine.

similarly leakage current after 1 minuet at 18kV DC (for 11kV System) is function of Insulation Resistance and thus temperature of windings.

You were getting low value, probably you might have megered the PT Primary along with CT. Please note that we never subject Potential Transformers to High Voltage test. Reason being that insulation of PT is not uniform over entire winding with respect to earth. The insulation of Primary winding close to secondary or core is as low as LV winding and is gradually increased as the layers of winding increases which is mostly achieved by increasing distance between LT and HT. Hence if we subject PT to HV it is very likely that its turn near neutral flash over to ground resulting in reduction in PT ratio and thus damage to PT.

Safer way to test whether PT is healthy is to back charge it by applying LV ac supply at its secondary through a dimerstat increasing slowly to rated secondary voltage.

Further to avoid such occurrence, it is better to visually inspect Bus Insulator Supports, CTs and LAs for any cracks. It is possible that while feeding system fault, or power swing, CT has developed crack due to heavy Electro- magnetic forces created by heavy fault current. The CT failed in this case would have been of phase which fed the fault current.

We shall also physically inspect the tying cords and surge rings supporting stator winding for intactness after such heavy short circuit currents are fed by an alternator. Result of heavy fault current is that adjacent coils in over hang portion of stator winding will repel each other (Like poles repel each other - these poles are formed by fault current flowing through the coils of same phase). if any tying cord has come off, same shall be repaired before taking STG in service.

Best regards,

Ramesh

I detect the smell of Spam.

Tony, I Could not follow how you could rate my comments and suggestions as SPAM

Regards,

Ramesh

I would like to (again) make a comment about current transformers. Under normal circumstances a CT is not doing any real work to speak of and should not wear out, heat up, or become damaged. However all CTs have load resistors. If the load resistor fails, the CT's secondary voltage will rise to high levels and the CT will certainly be badly damaged. In this case it sounds like a load resistor failed. I'd think that after a high-current event the CT load resistors would be the first thing to check before restarting.

The question arises of how to choose the load resistor's power rating. Your 1900A event certainly created an abnormally-high CT load-resistor voltage, and an excessively-high load-resistor power dissipation, by the square of the current. The situation cries out for a high peak-power rating for the load resistor, a very high rating. Even so high as to be impractical. But I have never been happy with this scenario.

I'm an electronics engineer, not an electrical engineer, and have no special expertise relevant to power plants like yours. But the unnecessary CT-damage issue has long bothered me as amenable to a few simple solutions.

Normally the "full-scale" voltage across the CT load resistor is quite small, e.g., 100mV. So it seems to me one solution is to add a pair of back-to-back Schottky power diodes across the CT load resistor. Under normal conditions the diodes would play no role. But in an over-current event the diodes would limit the CT's high-current fault secondary voltage to about 1V, no matter how high the fault-current event. Furthermore, these diodes wold conduct the high CT secondary current rather than the CT load resistor. But despite this they would not be as severely stressed as the load resistor would have been, because first, their low "fixed" forward voltage means they don't suffer from the "current-squared" power-dissipation effect, and second, the large semiconductor and metal structure some diodes employ means that they can handle a power surge better than a common resistor (the part pictured can handle 15kW). Moreover, with a little engineering effort a protective MOSFET clamp circuit could be devised that would be even better.

A second approach to the problem is to not use a conventional CT at all, but to use a Closed-Loop Hall-Effect current transducer, such as those made by LEM (although I don't see any products in their industrial catalog meant for continuous operation in 15kV systems). In the event of an excessive current event well over their full-scale range, these do not suffer high internal voltages, and are therefore more robust against failure.

Hah, a third approach could be to terminate your conventional high-voltage CT's secondary with a shorted wire, rather than a load resistor. A nice heavy wire isn't going to fail! This wire would be threaded through a second more-sensitive current sensor, such as a LEM type.

In the electrical power engineering application of CTs, you generally do not see load resistors used. The burden of the secondary wiring and the protective relays, meters, or whatever is the connected device is the load on the CT. I have not seen any situations in my (limited, but over 20-year) experience where there was a separate load resistor.

Without some forensic investigation into the OP's specific situation, we cannot tell what caused the CT to fail. From having dealt with many apparatus failures over the years, it could be caused by many things: excessive transients related to the fault condition that was experienced; physical contamination (dirt/dust/oil/etc.); moisture ingress into the insulation; thermal degradation of insulation; corona damage to insulation; etc.... Many of these can be prevented by a regular program of preventive maintenance.

I would not encourage the OP to add load resistors or any other components to the CT circuit that could deleteriously affect the operation of any protective devices that the CT feeds, unless they were specifically designed into the complete scheme. Who knows what its affect would be on the input of the newer, electronic relays, for example? But I do appreciate your concern - the damage done by an open-circuited CT secondary can be quite significant, and definitely something to be avoided. I know - I've had to clean up the mess afterwards.

Yes, good point, but a disturbing one. There is a load resistor someplace, even if it's contained within a meter, or other device. In my designs using CTs (industrial equipment, non-power-plant) I've made a habit of wiring (soldering) the appropriate load resistor right at the CT, making it part of the CT assembly. It just seems far too dangerous to have connectors, miscellaneous wiring and whatever between the CT's secondary and its all-important load resistor. Hmm, at least I'd add my protective diode clamp or similar scheme as part of the CT assembly. I know most electrical engineers don't think like electronics engineers when considering adding complex/sophisticated electronics to a system, but this seems a place where it could help reduce the failure rate of CTs.

Win,

With open secondary CT, high voltage is induced at secondary terminals but power is not enough to flash over a CT. I have seen electrician by mistake opening secondary of CT of 110kV system, he just got severe shivering and screw driver fell of his hand - not fettle or any harm. He was just normal and continue with his work. The effect is similar to high voltage shock from ignition circuit of automobile engine. One experiences a shock without any damage as power is low.

By low power, I mean, as soon as burden increases, high voltage disappears (that is why with secondary of CT loaded, no high voltage). Same thing will happen - for CT, that means under open circuit conditions, voltage is present - and the moment insulation of secondary circuit punctures, resistance across CT secondary drops to Zero (other terminal of secondary of CT is always earthed) and High Voltage disappears. Hence CT can not burst if there is open circuit at its secondary terminals.

Hence an Open circuit across secondary of CT, at the most insulation of low voltage (secondary circuit) can flash over at the week point. Again in description of Occurrence, no failure of CT secondary is reported.

Regards,

Ramesh Kapur