Feeder Protection

Feeder Protection

What is a Feeder? Overhead lines or cables which are used to distribute the load to the customers. They interconnect the distribution substations This is an electrical supply line, either overhead or underground, which runs from the substation, through various paths, ending with the transformers. It is a distribution circuit, usually less than 69,000 volts, which carries power from the substation. with the loads.

Why Protection Is Important? The modern age has come to depend heavily upon continuous and reliable availability 0f electricity and a high quality of electricity too. Computer and telecommunication networks, railway networks, banking and continuous power industries are a few applications that just cannot function without highly reliable power source. No power system cannot be designed in such a way that they would never fail. So, protection is required for proper working.

Basic Requirements of Protection A protection apparatus has three main functions: 1. Safeguard the entire system to maintain continuity of supply 2. Minimize damage and repair costs where it senses fault 3. Ensure safety of personnel Protection must be reliable which means it must be: 1. Dependable: It must trip when called upon to do so. 2. Secure: It must not trip when it is not supposed to.

Basic Requirements of Protection These requirements are necessary for early detection and localization of faults and for prompt removal of faulty equipment from service. Selectivity: To detect and isolate the faulty item only. Stability: To leave all healthy circuits intact to ensure continuity or supply. Sensitivity: To detect even the smallest fault, current or system abnormalities and operate correctly at its setting before the fault causes irreparable damage. Speed: To operate speedily when it is called upon to do so, thereby minimizing damage to the surroundings and ensuring safety to personnel.

What Is Fault? A fault is defined as defect in electrical systems due to which current is directed away from its intended path. It is not practical to design and build electrical equipment or networks to eliminate the possibility of failure in service. It is therefore an everyday fact that different types of faults occur on electrical systems, however infrequently, and at random locations.

Classification of faults Faults can be broadly classified into two main areas which have been designated as Active faults Passive faults

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 earth. This type of fault can also be further classified into two areas Solid Fault Incipient Fault

Solid Faults The solid fault occurs as a result of an immediate complete breakdown of insulation as would happen. 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 – Danger of igniting combustible gas in hazardous areas – Increased probability of faults spreading to healthy phases

Incipient Fault The incipient fault is a fault that starts as a small thing and gets developed into catastrophic failure. Some partial discharge in a void in the insulation over an extended period can burn away adjacent insulation, eventually spreading further and developing into 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 Overvoltage Under frequency Power swings

Transient and 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. 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.

Symmetric and Asymmetric Faults A symmetrical fault is a balanced fault with the sinusoidal waves being equal about their axes, and represents a steadystate 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.

Basic Fault Clearing Mechanism

The main requirement of line protection is 1. In the event of short circuit, the circuit breaker near to fault should open and all other circuit breakers remain in closed position. 2. If the circuit near to fault fail to trip, back up protection should be provided by the adjacent circuit breaker. 3. The relay operating should be the smallest possible in order to preserve system stability without unnecessary tripping of circuits.

Types of protection The need to analyze protection schemes has resulted in the development of protection coordination programs. Protection schemes can be divided into two major groupings: Unit schemes Non-unit schemes

Unit Type Protection Unit type schemes protect a specific area of the system, i.e., a transformer, transmission line, generator or busbar. The most obvious example of unit protection schemes is based on Kerchief’s current law – the sum of the currents entering an area of the system must be zero. Any deviation from this must indicate an abnormal current path. In these schemes, the effects of any disturbance or operating condition outside the area of interest are totally ignored and the protection must be designed to be stable above the maximum possible fault current that could flow through the protected area.

Non unit type protection The non-unit schemes, while also intended to protect specific areas, have no fixed boundaries. As well as protecting their own designated areas, the protective zones can overlap into other areas. While this can be very beneficial for backup purposes, there can be a tendency for too great an area to be isolated if a fault is detected by different non unit schemes. The most simple of these schemes measures current and incorporates an inverse time characteristic into the protection operation to allow protection nearer to the fault to operate first.

Non unit type protection

Non unit type protection The non unit type protection system includes following schemes: – Time graded over current protection – Current graded over current protection – Distance or Impedance Protection

Over current protection This is the simplest of the ways to protect a line and therefore widely used. It owes its application from the fact that in the event of fault the current would increase to a value several times greater than maximum load current. It has a limitation that it can be applied only to simple and non costly equipments.

Earth fault protection The general practice is to employ a set of two or three over current relays and a separate over current relay for single line to ground fault. Separate earth fault relay provided makes earth fault protection faster and more sensitive. Earth fault current is always less than phase fault current in magnitude. Therefore, relay connected for earth fault protection is different from those for phase to phase fault protection.

Earth fault protection

Time graded protection This is a scheme of over current protection is one in which time discrimination is incorporated. In other words, the time setting of the relays is so graded that minimum possible part of system is isolated in the event of fault. We are to discuss the application of the time graded protection on – Radial feeder – Parallel feeder – Ring feeder

Protection of radial feeder The main characteristic of the radial feeder is that power can flow in one direction only from generator to supply end of the load line. In radial feeder number of feeders can be connected in series and it is desired that smallest part of the system should be off in the event of fault. This is achieved by time graded protection. In this system time setting time setting of a relay is so adjusted that farther the relay from the generating system lesser the time of operation.

Drawbacks of time graded protection on radial feeder The drawbacks of graded time lag over current protection are given below: – The continuity in the supply cannot be maintained at the load end in the event of fault. – Time lag is provided which is not desirable in on short circuits. – It is difficult to co-ordinate and requires changes with the addition of load. – It is not suitable for long distance transmission lines where rapid fault clearance is necessary for stability.

Protection of parallel feeder For important installations continuity of supply is a matter of vital importance and at least two lines are used and connected parallel so as to share load. In the event of fault occurring the protecting device will select the faulty feeder and isolate it while other instantly assumes increased load. The simplest method of obtaining such protection is providing time graded over relays with inverse time characteristics at one end and reverse power directional relay at the other end.

Protection of ring main feeder The ring main is a system of inter connection between a series of power stations by an alternate route. The direction of power flow can be changes at will.

IDMT Relay In time graded protections IDMT (Inverse definite minimum time) relays are used. As the name implies, it is a relay monitoring the current, and has inverse characteristics with respect to the currents being monitored. This relay is without doubt one of the most popular relays used on medium- and lowvoltage systems for many years, and modern digital relays’ characteristics are still mainly based on the torque characteristic of this type of relay.

IDMT relay

Block diagram of IDTM Relay

It can be seen that the operating time of an IDMTL relay is inversely proportional to function of current, i.e. it has a long operating time at low multiples of setting current and a relatively short operating time at high multiples of setting current.

Current graded protection It is an alternative to time graded protection and is used when the impedance between two substations is sufficient. It is based on the fact that short circuit current along the length of protected length of the circuit decreases with increase in distance between the supply end and the fault point. If the relays are set to operate at a progressively higher current towards the supply end of the line then the drawback of the long time delays occurring in the graded time lag system can be partially overcome.

DISTANCE OR IMPEDANCE PROTECTION A distance relay, as its name implies, has the ability to detect a fault within a pre-set distance along a transmission line or power cable from its location. BASIC PRINCIPLE The basic principle of distance protection involves the division of the voltage at the relaying point by the measured current. The apparent impedance so calculated is compared with the reach point impedance. If the measured impedance is less than the reach point impedance, it is assumed that a fault exists on the line between the relay and the reach point.

BASIC PRINCIPLE OPERATION OF IMPEDANCE RELAY

BALANCED BEAM PRINCIPLE OF IMPEDANCE RELAY The voltage is fed onto one coil to provide restraining torque, whilst the current is fed to the other coil to provide the operating torque. Under healthy conditions, the voltage will be high (i.e. at full-rated level), whilst the current will be low thereby balancing the beam, and restraining it so that the contacts remain open. Under fault conditions, the voltage collapses and the current increase dramatically, causing the beam to unbalance and close the contacts.

Three stepped distance protection Zone 1 First step of distance protection is set to reach up to 80 to 90% of the length of the line section. This is instantaneous protection i.e. there is no intentional delay . Zone 2 second zone is requires in order to provide primary protection to remaining 10 to 20% of the line and a cover up to 50% of the next line section. The operating time of this zone is delayed so as to be selective with zone 1.

Three stepped distance protection Zone 3 The third zone is provided with an intention to give full back up to adjoining line section. It covers the line of the section, 100% of the next line section and reaches farther into the system. The motivation behind the extended reach of this step is to provide full back up to the next line section. Its operating time is slightly more than that of zone 2.

Main or Unit Protection

Main or Unit Protection The graded over current systems described earlier do not meet the protection requirements of a power system. The grading is not possible to be achieved in long and thin networks and also it can be noticed that grading of settings may lead to longer tripping times closer to the sources, which are not always desired. These problems have given way to the concept of ‘unit protection’ where the circuits are divided into discrete sections without reference to the other sections. The power system is divided into discrete zones. Each zone is provided with relays and circuit breakers to allow for the detection and isolation of its own internal faults.

Back-up Protection It is necessary to provide additional protection to ensure isolation of the fault when the main protection fails to function correctly. This additional protection is referred to as ‘back-up’ protection. The fault is outside the zones of the main protection and can only be cleared by the separate back-up protection. Back-up protection must be time delayed to allow for the selective isolation of the fault by the main or unit protection.

Types of Main Protection Following types of main or unit protections are used in feeder networks – Differential protection – Carrier current protection using phase comparison – Translay Y protection system

Methods of obtaining selectivity The most positive and effective method of obtaining selectivity is the use of differential protection. For less important installations, selectivity may be obtained, at the expense of speed of operation, with time-graded protection. The principle of unit protection was initially established by Merz and Price who were the creators of the fundamental differential protection scheme.

Differential protection Differential protection, as its name implies, compares the currents entering and leaving the protected zone and operates when the differential between these currents exceeds a pre-determined magnitude. This type of protection can be divided into two types, namely – Balanced current – Balanced voltage

Balanced current Protection The CTs are connected in series and the secondary current circulates between them. The relay is connected across the midpoint thus the voltage across the relay is theoretically nil, therefore no current through the relay and hence no operation for any faults outside the protected zone. Similarly under normal conditions the currents, leaving zone A and B are equal, making the relay to be inactive by the current balance.

Differential protection using current balance scheme (external fault conditions)

Differential protection and internal fault conditions

Balanced current Protection The current transformers are assumed identical and are assumed to share the burden equally between the two ends. However, it is not always possible to have identical CTs and to have the relay at a location equidistant from the two end CTs. It is a normal practice to add a resistor in series with the relay to balance the unbalance created by the unequal nature of burden between the two end circuits. This resistor is named as ‘stabilizing resistance’.

McColl circulating current protection for single phase systems

Balanced voltage system As the name implies, it is necessary to create a balanced voltage across the relays in end A and end B under healthy and out-of-zone fault conditions. In this arrangement, the CTs are connected to oppose each other . Voltages produced by the secondary currents are equal and opposite; thus no currents flow in the pilots or relays, hence stable on through-fault conditions. Under internal fault conditions relays will operate.

Balanced voltage system – external fault (stable)

Balanced voltage system, internal fault (operate)

Translay Y Protection system The system can be employed for the protection of single phase or 3-phase feeders, transformer feeders and parallel feeders against both earth and phase faults. It works on the principle that current entering one end of the feeder at any instant equals the current leaving the feeder.

Translay Y Protection system

Advantages of Translay system The capacitance currents do not effect the operation much. Only two pilot wires needed. The current transformers of normal designs are employed i.e. air core type The pilot resistance do not effect the operation as the major part of power is obtained from CTs for operation.

Carrier current protection using phase comparison In this type of relay we exploit the phase shift undergone by the current at the end by which is nearest to the fault. The end which is far from the fault cannot discern any changes in the phase of fault current and the closer end sees a sharp, almost 180 change in the phase current. Under normal conditions, load currents and external fault currents can be arranged to be exactly out of phase but in case of internal faults the currents become in phase.

Time taken to clear faults With the inherently selective forms of protection, apart from ensuring that the relays do not operate incorrectly due to initial transients, no time delay is necessary. Operating times for the protection, excluding the breaker tripping/clearing time are generally of the following order: – – – – Machine differential – few cycles Transformer differential – 10 cycles Switchgear (busbar) differential – 4 cycles Feeder differential – few cycles These operating times are practically independent of the magnitude of fault current.

Advantages of unit protection Fast and selective Unit protection is fast and selective. It will only trip the faulty item of plant, thereby ensuring the elimination of any network disruptions. No time constraints Time constraints imposed by the supply authorities do not become a major problem anymore. Future expansion relatively easy Any future expansion that may require another in-feed point can be handled with relative ease without any change to the existing protection