Industrial Power-System Protection--Faults, types and effects

The development of simple distribution systems

When a consumer requests electrical power from a supply authority, ideally all that is required is a cable and a transformer.

T2, T1, T3, Consumer 2, Consumer 1, Consumer 3, Power station

++++ A simple distribution system

This is called a radial system and can be shown schematically in the following manner. T1 -- T2 -- T3

++++ A radial distribution system

Advantages:

If a fault occurs at T2 then only the protection on one leg connecting T2 is called into operation to isolate this leg. The other consumers are not affected.

Disadvantages:

If the conductor to T2 fails, then supply to this particular consumer is lost completely and cannot be restored until the conductor is replaced/repaired.

This disadvantage can be overcome by introducing additional/parallel feeders connecting each of the consumers radially. However, this requires more cabling and is not always economical. The fault current also tends to increase due to use of two cables. T1, T2, T3

++++ Radial distribution system with parallel feeders

The Ring main system, which is the most favored, then came into being. Here each consumer has two feeders but connected in different paths to ensure continuity of power, in case of conductor failure in any section.

T1, T2, T3

++++ A ring main distribution system

Advantages:

Essentially, meets the requirements of two alternative feeds to give 100% continuity of supply, whilst saving in cabling/copper compared to parallel feeders.

Disadvantages:

For faults at T1 fault current is fed into fault via two parallel paths effectively reducing the impedance from the source to the fault location, and hence the fault current is much higher compared to a radial path. The fault currents in particular could vary depending on the exact location of the fault.

Protection must therefore be fast and discriminate correctly, so that other consumers are not interrupted.

The above case basically covers feeder failure, since cable tend to be the most vulnerable component in the network. Not only are they likely to be hit by a pick or alternatively dug-up, or crushed by heavy machinery, but their joints are notoriously weak, being susceptible to moisture, ingress, etc., amongst other things.

Transformer faults are not so frequent, however they do occur as windings are often strained when carrying through-fault current. Also, their insulation lifespan is very often reduced due to temporary or extended overloading leading to eventual failure. Hence interruption or restriction in the power being distributed cannot be avoided in case of transformer failures. As it takes a few months to manufacture a power transformer, it’s a normal practice to install two units at a substation with sufficient spare capacity to provide continuity of supply in case of transformer failure.

Busbars on the other hand, are considered to be the most vital component on a distribution system. They form an electrical 'node' where many circuits come together, feeding in and sending out power.

On E.H.V. systems where mainly outdoor switchgear is used, it’s relatively easy and economical to install duplicate busbar system to provide alternate power paths. But on medium-voltage (11 kV and 6.6 kV) and low-voltage (3.3 kV, 1000 V and 500 V) systems, where indoor metal clad switchgear is extensively used, it’s not practical or economical to provide standby or parallel switchboards. Further, duplicate busbar switchgear is not immune to the ravages of a busbar fault.

The loss of a busbar in a network can in fact be a catastrophic situation, and it’s recommended that this component be given careful consideration from a protection viewpoint when designing network, particularly for continuous process plants such as mineral processing.

Fault types and their effects

It’s not practical to design and build electrical equipment or networks to eliminate the possibility of failure in service. It’s therefore an everyday fact that different types of faults occur on electrical systems, however infrequently, and at random locations.

Faults can be broadly classified into two main areas, which have been designated 'active' and 'passive'.

Active faults

The 'active' fault is when actual current flows from one phase conductor to another (phase-to-phase), or alternatively from one phase conductor to ground (phase-to- ground). This type of fault can also be further classified into two areas, namely the 'solid' fault and the 'incipient' fault.

The solid fault occurs as a result of an immediate complete breakdown of insulation as would happen if, say, a pick struck an underground cable, bridging conductors, etc. or the cable was dug up by a bulldozer. In mining, a rockfall could crush a cable, as would a shuttle car. In these circumstances the fault current would be very high resulting in an electrical explosion.

This type of fault must be cleared as quickly as possible, otherwise there will be:

• Increased damage at fault location. Fault energy I^2 f = I Rt ×× , where t is time in seconds.

• Danger to operating personnel (flashes due to high fault energy sustaining for a long time).

• Danger of igniting combustible gas in hazardous areas, such as methane in coal mines which could cause horrendous disaster.

• Increased probability of ground faults spreading to healthy phases.

• Higher mechanical and thermal stressing of all items of plant carrying the fault current, particularly transformers whose windings suffer progressive and cumulative deterioration because of the enormous electromechanical forces caused by multi-phase faults proportional to the square of the fault current.

• Sustained voltage dips resulting in motor (and generator) instability leading to extensive shutdown at the plant concerned and possibly other nearby plants connected to the system.

The 'incipient' fault, on the other hand, is a fault that starts as a small thing and gets developed into catastrophic failure. E.g., some partial discharge (excessive discharge activity often referred to as Corona) in a void in the insulation over an extended period can burn away adjacent insulation, eventually spreading further and developing into a 'solid' fault.

Other causes can typically be a high-resistance joint or contact, alternatively pollution of insulators causing tracking across their surface. Once tracking occurs, any surrounding air will ionize which then behaves like a solid conductor consequently creating a 'solid' fault.

Passive faults

Passive faults are not real faults in the true sense of the word, but are rather conditions that are stressing the system beyond its design capacity, so that ultimately active faults will occur. Typical examples are:

• Overloading leading to over heating of insulation (deteriorating quality, reduced life and ultimate failure).

• Overvoltage: Stressing the insulation beyond its withstand capacities.

• Under frequency: Causing plant to behave incorrectly.

• Power swings: Generators going out-of-step or out-of-synchronism with each other.

It’s therefore very necessary to monitor these conditions to protect the system against these conditions.

Types of faults on a three-phase system

Largely, the power distribution is globally a three-phase distribution especially from power sources. The types of faults that can occur on a three-phase AC system are shown.

It will be noted that for a phase-to-phase fault, the currents will be high, because the fault current is only limited by the inherent (natural) series impedance of the power system up to the point of fault (Ohm's law). By design, this inherent series impedance in a power system is purposely chosen to be as low as possible in order to get maximum power transfer to the consumer so that unnecessary losses in the network are limited thereby increasing the distribution efficiency. Hence, the fault current cannot be decreased without a compromise on the distribution efficiency, and further reduction cannot be substantial.

On the other hand, the magnitude of ground fault currents will be determined by the manner in which the system neutral is grounded. It’s worth noting at this juncture that it’s possible to control the level of ground fault current that can flow by the judicious choice of grounding arrangements for the neutral. Solid neutral grounding means high ground fault currents, being limited by the inherent ground fault (zero sequence) impedance of the system, whereas additional impedance introduced between neutral and ground can result in comparatively lower ground fault currents.

In other words, by the use of resistance or impedance in the neutral of the system, ground fault currents can be engineered to be at whatever level desired and are therefore controllable. This cannot be achieved for phase faults.

++++ Types of faults on a three-phase system: (A) Phase-to- ground fault; (B) Phase-to-phase fault; (C) Phase-to phase-to- ground fault; (D) Three-phase fault; (E) Three-phase-to- ground fault; (F) Phase-to-pilot fault*; (G) Pilot-to- ground fault*

*In underground mining applications only.

Transient and permanent faults

Transient faults are faults, which don’t damage the insulation permanently and allow the circuit to be safely re-energized after a short period.

A typical example would be an insulator flashover following a lightning strike, which would be successfully cleared on opening of the circuit breaker, which could then be automatically closed. Transient faults occur mainly on outdoor equipment where air is the main insulating medium. Permanent faults, as the name implies, are the result of permanent damage to the insulation. In this case, the equipment has to be repaired and recharging must not be entertained before repair/restoration.

Symmetrical and asymmetrical faults

A symmetrical fault is a balanced fault with the sinusoidal waves being equal about their axes, and represents a steady-state condition.

An asymmetrical fault displays a DC offset, transient in nature and decaying to the steady state of the symmetrical fault after a period of time.

++++ An asymmetrical fault. Asymmetrical peak, Offset, Steady state

Total asymmetry factor -- Power factor, Practical max

++++ Total asymmetry factor chart: The amount of offset depends on the X/R (power factor) of the power system and the first peak can be as high as 2.55 times the steady-state level.