CHAPTER 1 - INTRODUCTION TO FINAL YEAR PROJECT 2

1.1 INTRODUCTION

Power system protection is the most important requirement in the industrial or domestic electrical to prevent equipment from damage due to fault. It is the fact of life 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 are designated as Active and Passive.

        1.1.1 Active Fault

The Active fault is when actual current flows from one phase conductor to another (phase-to-phase) or alternatively from one phase conductor to earth (phase-to-earth). 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, or the cable was dug up by a bulldozer. In mining, a rock fall 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 greatly increased of damage at the fault location. This later will cause danger to operating personnel (Flash products), danger of igniting combustible gas such as methane in hazardous areas, giving rise to a disaster of horrendous proportions, increased probability of earth faults spreading to other phases, higher mechanical and thermal stressing of all items of plant carrying the current fault. Particularly transformers windings will suffer progressive and cumulative deterioration because of the enormous electromechanical forces caused by multi-phase faults proportional to the current squared, sustained voltage dips resulting in motor (and generator), instability that could leading to extensive shut-down at the plant concerned and possibly other nearby plants.

The incipient fault, on the other hand, is a fault that starts from very small beginnings, from say some partial discharge (excessive electronic activity often referred to as Corona) in a void of the insulation. It will increase and develop over an extended period, until such time as it burns away adjacent insulation and eventually running away and develop 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.

        1.1.2 Passive Fault

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.

Figure 1 : Type of fault

Typical examples are overloading - leading to overheating of insulation (deteriorating quality, reduced life and ultimate failure), overvoltage - stressing the insulation beyond its limits, under frequency - causing plant to behave incorrectly and power swings - generators going out of step or synchronism with each other. It is therefore very necessary to also protect against these conditions. Types of Faults on a Three Phase System that can occur on a three phase A.C is shown in Figure 1.

As refer to Figure 1, the types of Faults on a Three Phase System are:

(A) Phase-to-earth fault

(B) Phase-to-phase fault

(C) Phase-to-phase-to-earth fault

(D) Three phase fault

(E) Three phase-to-earth fault

(F) Phase-to-pilot fault *

(G) Pilot-to-earth fault *

* In underground mining applications only

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 faulty (refer Ohms 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 and limit unnecessary losses in the network itself in the interests of efficiency. On the other hand, the magnitude of earth faults currents will be determined by the manner in which the system neutral is earthed. Solid neutral earthing means high earth fault currents as this is only limited by the inherent earth fault (zero sequence) impedance of the system. It is worth noting at this juncture that it is possible to control the level of earth fault current that can flow by the judicious choice of earthing arrangements for the neutral. In other words, by the use of Resistance or Impedance in the neutral of the system, earth fault currents can be engineered to be at whatever level is desired and are therefore controllable. This cannot be achieved for phase faults.

    

   1.1.3 Transient & Permanent Faults

Transient faults are faults which do not damage the insulation permanently and allow the circuit to be safely re-energized after a short period of time. A typical example would be an insulator flashover following a lightning strike, which would be successfully cleared on opening of the circuit breaker and later could automatically re-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 reclosing must not be entertained.

   1.1.4 Symmetrical & 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 d.c. offset, transient in nature and decaying to the steady state of the symmetrical fault after a period of time:

Figure 2 : Steady State Graph

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. Figure 2 shows the steady state graph.

1.1.5 Electrical Faults

A fault is any abnormal situation in an electrical system in which the electrical current may or may not flow through the intended parts. Equipment failure also attributable to some defect in the circuit such as loose connection, insulation failure or short circuit etc. The type of faults in a distribution network that is detected by an ELCB is:

i. Over-Current Fault

ii. Short-Circuit Fault

iii. Lightning Fault

1.1.6  Over-current Fault

The National Electrical Code defines over current as any current in excess of the rated current of equipment or the amp city of a conductor. It may result from overload, short circuit, or ground fault. Current flow in a conductor always generates heat. The greater the current flow, the hotter the conductor. Excess heat is damaging to electrical components. For that reason, conductors have a rated continuous current carrying capacity or amp city. Over current protection devices are used to protect conductors from excessive current flow. These protective devices are designed to keep the flow of current in a circuit at a safe level to prevent the circuit conductors from overheating. In term of over-current fault, the fuse or wire may melt or damage the other elements of the circuit when a current greater than that which a circuit or a fuse is designed to carry [2].

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1.1.7 Short-Circuit Fault

A short circuit in an electrical circuit is one that allows a current to travel along different path from the one originally intended. The electrical opposite of a short circuit is an “open circuit”, which is an infinite resistance between two nodes. It is an abnormal low-resistance connection between two nodes of an electrical circuit that are meant to be at different voltages. This results in an excessive electric current (over-current) and potentially causes circuit damage, overheating, fire or explosion [2].

Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example for the purpose of voltage-sensing crowbar circuit protectors. In circuit analysis, the term short circuit is used by analogy to design at a zero-impedance connection between two nodes. This forces the two nodes to be at the same voltage. In an ideal short circuit, this means there is no resistance and no voltage drop a cross the short. In a simple circuit analysis, wires are considered to be shorts. In real circuits, the result is a connection of nearly zero impedance, and almost no resistance [2].

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1.1.8 Lightning Fault

Lightning is the visible discharge of static electricity within a cloud, between clouds, or between the earth and a cloud. Scientists still do not fully understand what causes lightning, but most experts believe that lightning is caused due to different kinds of ice interact in a cloud. Updraft in the clouds separate charges, so that positive charges flow towards the top of the cloud and the negative charges flow to the bottom of the cloud. When the negative charges moves downwards, a “stepped leader” is created. The leader rushes toward the earth in 150-foot discrete steps, producing an ionized path in air. The major part of the lightning discharges current is carried in the return stroke, which flows along the ionized path. One of the temporary faults is cause by direct lightning phenomena. The example of permanent fault can mostly been seen on electrical equipment [2].

1.2  Operation of ELCB Trip Situation

There are two types of fault normally detected by ELCB, which are permanent fault and temporary fault [2].

1.2.1 Permanent Failure or Permanent Damage

It is usually trip when there is any leakage current in the circuit to earth or ground. For permanent failure, the damaged must to repair first or remove the damage from current before ELCB can be triggered again. If the damage has not been repaired or removed from the circuit, it will trip again when the ELCB is automatically triggered. If this happen many times, it will damage the ELCB [2].

1.2.2 Temporary Failure or Temporary Damage

It can automatically trigger ELCB without need to repair or remove the damage from supply circuit. If usually lightning and over loading occurs in resident or industrial, it can give more problems to user to automatically trigger by itself. For example is lightning occurrence [2].

1.2.3 Earth Leakage Circuit Breaker (ELCB)

An Earth Leakage Circuit Breaker (ELCB) is a device used to directly detect currents leaking to earth from an installation and cut the power. It was mainly used in TT earthing systems. In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator. The big advantage of the TT earthing system is the fact that it is clear of high and low frequency noises that come through the neutral wire from various electrical equipment connected to it. This is why TT has always been preferable for special applications like telecommunication sites where the benefit from the interference-free earthing can be obtained. Also, TT does not have the risk of a broken neutral. In locations where power is distributed overhead and TT is used, installation of earth conductors are not at risk if there is any overhead distribution conductor be fractured by, say, a fallen tree or branch [3].

Earth Leakage Circuit Breaker (ELCB) operates by measuring the current balance between two conductors using a differential current transformer. The device will open its contacts when it detects any difference in current between the line conductor and the neutral conductor. The supply and return current must sum to zero, otherwise there is a leakage of current to somewhere else (to earth/ground, or to another circuit, etc.). ELCB is designed to prevent electrocution by detecting the leakage current, which can be far smaller (typically 5-30 mill amperes) than the currents needed to operate conventional circuit breakers or fuses (several amperes). RCD (Residential Current Device) are intended to operate within 25-40 milliseconds before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock [2].

In the United States, the National Electrical Code requires GFCI (Ground Fault circuit Interrupter) devices to protect people by interrupt the circuit if the leakage current exceeds a range of 4-6 mA of current (the trip setting is typically 5 mA) within 25 milliseconds. ELCB devices which protect equipment (not people) are allowed to trip as high as 30 mA of current. In Europe, the commonly used RCD have trip currents of 10-300 mA. Residual current detection is complementary to over-current detection. Residual current detection cannot provide protection for overload or short-circuit currents [2].

ELCB with trip currents as high as 500 mA are sometimes deployed in environments (such as computing centers) where a lower threshold would carry unacceptable risk of accidental trips. These high-current ELCB serve more as an additional fire-safety protection than as an effective protection against the risks of electrical shocks. For many years, the voltage operated ELCB and the differential current operated ELCB were both referred to as ELCB because it was a simpler name to remember. However, the use of a common name for two different devices gave rise to considerable confusion in the electrical industry. If the wrong type was used on an installation, the level of protection given could be substantially less than that intended. The ELCB is an important equipment to install at each of house, hospital, factory, or every place that need the power supply [2].

In pre-RCD (residual Current device) era, the TT earthing system was unattractive for general use because of its worse capability of accepting high currents in case of a live-to-PE (Protective earth) short circuit (in comparison with TN systems). But as residual current devices mitigate this disadvantage, the TT earthing system becomes attractive for premises where all AC power circuits are RCD-protected. The RCD is an electrical wiring device that disconnects a circuit whenever it detects the electric current is not balanced between the phase conductor and the neutral conductor. Such an imbalance is sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit [2].

ELCB has become one of the home safety systems in our life today. ELCB has reset button which is to reclosed circuit breaker when the tripping occur. Today, many of people busy with work and usually not at home. The problem will occur during the over current, short circuit or current leakage at live conductor, it can trip the circuit breaker “OFF” and cut off the whole power supply. This situation can make certain important component or equipment cannot be operated.  In most household, ELCB need to be reclosed manually during tripping, hence it is a troublesome thing for user who is not at home and it might take a long time to reset back the button of the circuit breaker. The main mechanism in operation is tripping coil which is it can operate either in live or off condition. This ELCB will operate when current is exceeding the rating of the current ELCB. These high currents will not flows into equipment after ELCB tripped. It will flow directly into ground by using ground rod. This ground rod must have the lower resistance to ensure an easy flowing of high current. There are two types of ELCB [2]:

i. Voltage Earth Leakage Circuit Breaker (vELCB)

ii. Current Earth Leakage Circuit Breaker (iELCB)

1.2.3.1 Voltage Earth Leakage Circuit Breaker (vELCB)

vELCB is stand for the voltage operated circuit breaker. The device will function when the current passes through the ELCB. vELCB contains relay loop which it being connected to the metallic load body at one end and it is also connected to ground wire at the other end. If the voltage of the load body is rise which could cause the difference between earth and load body voltage, the danger of electric shock will occur. This voltage difference will produce an electric current from the load metallic body that passes through the relay loop and to earth. When voltage on the load metallic body raised to the danger level which exceed to 50Volt, the flowing current through relay loop could move the relay contact by disconnecting the supply current in order to avoid from any electric shock danger [2].

1.2.3.2 Current Earth Leakage Circuit Breaker (iELCB)

iELCB is stand for the current operated circuit breaker. Current-operated ELCBs are generally known today as RCD (residual current device). iELCB also will protect the load against earth leakage, even though the details and method of operation are different. The device will be functioning when the current passes through ELCB. This current admitted to current transform device and on the load. Current from the load also admitted again to transform device. In normal state, total current applied to the load is equal with the total current out of the load. Due to the in and out of current are in balance, it does not affect the current transform device. If there is any earth current leakage caused by earth damage, then the in and out current will no longer in balance. This unbalance current phenomenon will generate the current and if the current exceeded the prescribed rate, the ELCB will jerked and cut off the supply. The device also being called RCD, Residual Current Device in IEC or RCCB, Residual Current Circuit Breaker [2].

1.3 Scope of Project

In Malaysia it is common, The faults might be LG (Line to Ground), LL (Line to Line), 3L (Three lines) in the supply systems and these faults in three phase supply system can affect the power system. To overcome this problem a system is built, which can sense these faults and automatically disconnects the supply to avoid large scale damage to the control gears in the grid sub-stations. [2]

An Earth Leakage Circuit Breaker (ELCB) is a safety device used in electrical installations with high earth impedance to prevent shock. It detects small stray voltages on the metal enclosures of electrical equipment, and interrupts the circuit if any dangerous voltage is detected.

The ELCB is a specialized type of latching relay that has a building's incoming mains power connected through its switching contacts so that the ELCB disconnects the power in an earth leakage (unsafe) condition. The ELCB detects fault currents from live to the earth (ground) wire within the installation it protects. If sufficient voltage appears across the ELCB's sense coil, it will switch off the power and remain off until manually reset. A voltage-sensing ELCB does not sense fault currents from live to any other earthed body.

This device has one advantage, they are less sensitive to fault conditions, and therefore have fewer nuisance trips. This does not mean it always do, as practical performance depends on installation details and the discrimination could enhance the filtering in the ELCB. Therefore, by electrically separating cable armor from the cable circuit protective conductor, an ELCB can be arranged to protect against cable damage only, and not trip on faults in down line installations.

Even though ELCB is good but it has some limitations such as they do not detect faults that do not pass current through the CPC (Circuit Protective Conductor) to the earth rod. They do not allow a single building system to be easily split into multiple sections with independent fault protection due to the facts that earthing systems are usually bonded to pipework.[1]

1.4 Objectives

The objectives of this project are:

i.              To develop an automatic re-closed Earth Leakage Circuit Breaker for fault in three phase system.

ii.            To develop the prototype automatic tripping mechanism of permanent and temporary fault in three phase system

1.5 Problem Statement

Earth Leakage Circuit Breaker (ELCB) is one type of electrical equipment that used as a protection device. The main purpose of this type of equipment is to cut off the power when the problem occurred. The main problem is, if there is any fault occurred and there is no human that can switch on the device again due to many reasons. The device that been used is mechanical switch that must be activated manually after the ELCB been tripped. The ELCB will stay off until the user push it back to ON condition although the problem that occurred is temporary fault and it occurred only in one millisecond. So, it is not working as automatic device that can operate intelligently.

Automatic Earth Leakage Circuit Breaker (Auto-ELCB) is the solution for the temporary trip since it is the main cause for tripping in Malaysia. After the temporary fault such as lightning happened, the situation will be back to normal as the installation of Auto-ELCB will automatically re-closed the breaker and hence the power will be supplied continuously. However, the Auto-ELCB will be permanently trip if the permanent fault occurred. The owner needs to re-closed the Auto-ELCB manually by using this system.