MYSTIQUES IN SYSTEM PROTECTION - PART 1

MYSTIQUES IN SYSTEM PROTECTION
By Doods A. Amora, PEE

[PART 1 OF A SERIES OF 3]

PROLOGUE

One sunny morning in what could have been a promising smooth routine; the Diesel Power Plant of an Independent Power Producer (IPP) suddenly lost its 50 MW power flow to an Industrial Economic Zone. The audible thuds at the switchyard somehow announced that the circuit breakers at the primary and secondary sides of the twin - 40 MVA Power Transformers T1 and T2 tripped off simultaneously! From the looks of it, the Transformer Lock-Out Relays (86T’s) must have done it! But, why...?


It didn’t occur just once - it already happened a number of times. And it would certainly happen again.

In each of such eventualities, the IPP was brought into total power interruption –isolating entirely its valued customers.

The management of the Power Plant didn’t desire it. And the hundreds of companies in the Industrial Zone didn’t like it, either. So much productivity had been lost. Something’s needed to be done - necessitating that the power plant’s operating limits, protection system and its vulnerabilities re-revisited.

Subsequent investigation revealed that a Single Line-to-Ground Fault occurred at a 69 kV Feeder far and kilometres away from the Power Plant. But why the triggering of the Lock-Out Relays when there are circuit breakers in series closer to the fault? They must have failed doing their function when they shouldn’t...!



THE NEED FOR PROTECTION

“System Protection is the very heart of power generation & distribution systems. Faulty or inadequate protection can bring about the loss of the entire facility— or, worse, it can cause needless deaths or injuries to personnel.” Quoted from an e-book, those words are still true today as every application of power system results in the need for protection.

As one GE Publication said, “The designer of the system must face the reality that no matter how much redundancy he builds into the system, and no matter how much he pays for premium quality components, he simply cannot build a system which will never fail. This is where system protection becomes important. If component failure is inevitable, then it is necessary to provide a means of detecting these failures. Better and faster protection affords a number of desirable attributes, all of which ultimately result in saving the owner of the system money through cost avoidance.”

And faults are for real! Even with the best design possible; materials and equipment deteriorate, and the likelihood of faults increases with age.


Indeed, a serious uncleared fault can put at risk utility operation and can result in area outages that would affect numerous customers, notwithstanding the damage on pieces of equipment which are in some cases, irreparable. Every system then is subject to short circuits and ground faults that should be removed quickly - and the awareness of the magnitudes and effects of faults is necessary to arm suitable system protection.

Failures or breakdowns of various components of a power system are either man-made, accidental or by natural causes such as those brought about by lightning, hurricane or from mere deterioration. Thus, protection in an electric system is a form of insurance. It pays nothing as long as there is no fault or other emergency, but when a fault occurs, it can be credited with reducing the extent and duration of interruption, the hazards of property damage and personnel injury. A certain number of faults can be tolerated during the life of the system provided they are immediately isolated before they cause damage or cause the loss of system stability. As reliability experts say, “should the system fail, it must fail well”.


Losses associated with a service interruption vary widely in different types of industries. For example, a service interruption in a machining operation may mean only a delay in production, while a similar interruption in a chemical production plant can cause loss of material and production, costly clean-up operations and possible damage to production equipment. Other industries such as semi-conductor plants, refineries, paper mills, textile mills, breweries, cement plants, steel mills and food processing plants are affected similarly, but in varying degrees. For some type of loads involving complex automation, a momentary voltage dip can be as serious as a complete interruption. Others can tolerate a momentary interruption, but not a sustained one. Thus, the character of industrial operation has a major influence on the type of fault protection applied to the electric system.


THE ART OF SYSTEM PROTECTION

While it is being said that it would neither be practical nor economical to build a fault-proof power system, the application of protective relays is often referred to as more of an “art” than a “science”. Relaying is an art because there is judgment involved in the selection of protective devices and its subsequent parameterization. The selection of protective relays requires compromises between conflicting objectives, such as: maximum protection vs. minimum protection, reliable protection vs. high-speed operation, high sensitivity to faults but insensitive to fleeting overloads, selectivity in isolating only a small faulty part in the system but capable of operating properly for several system operating conditions.


System Protection of a plant must thus be armed to respond to the various modes of faults at any magnitudes of over-currents at any parts of the system. At the same time, faster and more sensitive detection of problems means that the cause of the problem can be corrected while it is still a minor problem, and before it escalates into a major catastrophe.

So then, abnormal conditions are always associated with electrical faults and a well-planned fault control system from the medium voltage level down to the last low voltage circuit is one where only the faulted circuit is isolated without disturbing any ‘unfaulted’ parts of the system. Although electrical systems are designed by responsible system designers to be free from short circuits as possible, even with these precautions, the plant cannot escape from faults because short circuits will surely occur – perhaps not today but in the near future.


THE SYSTEM PROTECTION & COORDINATION STUDY

Proper coordination of circuit interrupting devices is an essential but frequently overlooked phase of industrial power system design. To many, this is a mystery. For those in the utility companies assigned in system protection for years, protective relaying is a special competency.

SYSTEM PROTECTION & COORDINATION STUDY is sought to address the problems and to virtually arrive at a desirable tripping sequences within the identified problematic zones of protection of an electric system. The study covers two major areas, as follows:

a) FAULT CALCULATIONS & SIMULATIONS
b) PROTECTIVE RELAY SETTINGS & COORDINATION

The end state of this Study is to have armed or set-up the installed protective devices ready to operate and respond to various modes of faults at any magnitudes of over-currents at any parts of the system in a predetermined sequence to satisfy Selective Coordination requirements.

So then, a system protection & coordination study aimed to re-arm the protective relays became obligatory as in the case of the IPP’s predicament referred to above. If other relays appeared to have displayed no problems, an attempt to re-set some relays to address specific problems would lead to a domino effect the discrimination of the operation of other relays with respect to each other. To ensure total coordination of protective devices in the system, all other relays in the system (from the load ends to the generating end) must likewise be probed and re-set whenever found necessary.

But, as system protection deals on faults, the Study must include Fault Calculations & Simulations to establish the levels of fault duties brought about by the configuration of the system, among others.

To be continued...
DAA 3/4/2009