Saturday, April 14, 2007

A PROLOGUE TO FAULT CALCULATIONS (Part III)

A PROLOGUE TO FAULT CALCULATIONS
(Third of a Series)



TRUE-TO-LIFE STORY

In an effort to highlight system fault protection, it is deemed worthwhile to share a true-to-life story - the lessons learned from which, are worthy to recall:

In one afternoon of July 1999, four explosions in succession were heard over the whole plant area, the last of which was the loudest. One of the four XLPE 500 MCM phase “X1” (Phase “A”) main secondary conductors of the 5 MVA, 69KV/4.16KV Substation “B” developed an single-line-to-ground fault with the steel structure supporting the cables & the AVR by-pass switch. All the four events of faults did not trip the 69 KV SF6 primary circuit breaker that could have isolated the fault. The fourth fault eventually melted & cut-off the faulted cable while cutting a portion of the 8” steel channel member of the structure. In other words, the fault cleared itself when the faulted cable was cut-off - likened to the operation of a 500 MCM ‘fuse link’. Power from the Utility remained on line until plant personnel shut off the primary breaker. Utility System did not trip off because to the eyes of the utility it was only a throughfault. The nearest Utility breaker is some 25 kilometers away.

Each substation system was provided with GEC Alsthom top-of-the-line KCGG 140 phase overcurrent with residual ground relay at the primary circuit. Likewise, the transformer was provided with GEC Alsthom MBCH differential relay coupled with a lockout relay. The lock-out relay also accommodates signals on transformer over-temperature and over-pressure parameters that will trigger trippings of both the primary and secondary breakers. There was also a back-up ground fault protective relay 51G at the transformer neutral circuit. The system therefore was provided with millions of peso worth of protection equipment & devices the books could offer.

After the smoke cleared, subsequent investigation revealed the following:

 "The GEC Alsthom KCGG 140 Relay (50-51/50N-51N) for the primary circuit was found not ‘armed’ or ‘set up’. Parameters found were factory default values and therefore not “enabled”. Likewise, the tripping contact circuit was erroneously wired such that even if the relay worked, no breaker tripping could be realized.

 "The GEC Alsthom MBCH differential relay (87T) for transformer protection was tested and found OK, but the lockout relay (86T) did not trigger tripping because the lock-out relay dc power supply was incompletely wired. When the lock-out relay was finally made activated and simulated, no trippings of the 69 KV primary breaker and the main 5 KV secondary breaker happened. Again, the wirings were found erroneous.

 "The back-up ground protection (51G) was not set up as it was not correctly wired. If correctly wired and set up, this relay could have tripped the primary breaker and could have isolated the transformer should all other relays failed.

The subject plant is located somewhere in the southern part of the country. Built in nice quality aesthetics with recognition as ISO 9002 & ISO 14000 certified, the plant was made operational in 1995. However, it took four (4) years to discover the uselessness of the installed protective devices because of a simple reason, i. e., failing to arm the protective system into putting it to work when the need comes. A check with the other 5 MVA Substation "A" revealed the same condition as the faulted Substation "B".
The rest is now history…

By sheer luck, there was a good thing that did not happen in the above story: the transformer was not burned, at least for the moment. Otherwise the transformer could have joined the archives of electrical disasters.

According to IEEE publications, damage to transformers brought by faults is cumulative. It means that in this case, there were already four faults that the transformer had been subjected to! Thanks, it did not give way in the fourth fault! But luck will surely run out. The engineer must not rely on luck, because the possibility of a disaster is always there - like a Sword of Damocles hanging over his neck.

In such a ground fault, how much destructive current was involved? Among others, this is what this paper seeks to answer.

POWER SYSTEM STUDY FOR EXISTING PLANTS

For existing installations where there are no visible fault calculation records, a Power System Study is necessary to establish the integrity of the power system. The value of Power System Study confirming correct ratings of circuit breakers leading to effective protective relaying can not be appreciated until a major fault occurred, after all - the protection system only comes to work during these abnormal times. Beyond a nice looking system, any industrial plant must review and see to it that the protection scheme is properly commissioned, set to predetermined performance levels, tested, simulated and live up to predictable expectations. The new Distribution Code and the new Grid Code of the Philippines require these commissioning chores.

Moreover, it seems that budgets for power system studies & commissioning most of the time, are forgotten or even not considered as necessary when it is in fact, the most important phase of a power system project. The truth of the matter is that commissioning the power system of a plant is never easy. It requires expertise & confidence acquired over long years of experience & practice. It also requires a battery of tests and test equipments that are not cheap, and the activity itself is NOT CHEAP.

In fact it needs a Power System Study to correctly set or arm the system protection devices. What happens next is that systems are energized with protective equipment & devices not commissioned to respond to abnormalities. Again, as long as when the system is energized, that’s it. Why mess with success? With lots of luck, the system operates for a few years, until an explosion is heard. Then, the electrical engineer will be brought to the center stage – grilled! Electrical Engineering is perceived by many as an easy job! No! Electrical Engineering is never easy! And nobody can substitute electrical engineers in his playing field.

(to be continued...)

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