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POWER SYSTEM PROTECTION IEEE Press Hoes Lane, P.O. Box 6 5 4 3 To Ginny BOOKS IN THE IEEE PRESS POWER ENGINEERING SERIES. power system protection and switchgear by badri ram, dn vishwakarma pdf Books in the IEEE Press Series on Power Engineering. Principles of Electric. Practical Power Systems Protection Other titles in the series Practical Data Acquisition for Instrumentation and Cont.


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What is System. Protection? System protection is the art and science of detecting problems with power system components and isolating these components. Power System Protection-PM Anderson - Free ebook download as PDF File .pdf) or read book online for free. Wiley also publishes its books in a variety of electronic formats. .. This third edition takes the problem of power system protection an additional step forward.

This method is not reliable for timing or pick-up tests. The book features an introduction covering the need for protection, fault types and their effects, simple calculations of short circuit currents and system earthing. This is necessary because floating voltage and equalizing voltage levels critically affect battery performance and life expectancy. Since then, the development of relays has been related to the general development of electronics. Bhide Book Free Download. Because the dynamic range of the signal levels to be measured is quite high, the 8-bit conversion is as such not enough to give a good accuracy over the whole current or voltage span. The main conductor and parallel fuse have to be replaced after each operation.

The main tasks of instrument transformers are: Instrument transformers are special versions of transformers in respect of measurement of current and voltages. The theories for instrument transformers are the same as those for transformers in general. A transformer comprises of two windings viz. When the primary winding is connected to a source and the secondary circuit is left open, the transformer acts as an inductor with minimum current being drawn from the source.

At the same time, a voltage will be produced in the secondary open-circuit winding due to the magnetic coupling. When a load is connected across the secondary terminals, the current 46 Practical Power Systems Protection will start flowing in the secondary, which will be decided by the load impedance and the open-circuit secondary voltage.

Switchgear and Power System Protection - PDF Drive

A proportionate current is drawn in the primary winding depending upon the turns ratio between primary and secondary. This principle of transformer operation is used in transfer of voltage and current in a circuit to the required values for the purpose of standardization. A voltage transformer is an open-circuited transformer whose primary winding is connected across the main electrical system voltage being monitored. A convenient proportionate voltage is generated in the secondary for monitoring.

The most common voltage produced by voltage transformers is — V as per local country standards for primary voltages from V to kV or more. However, the current transformer is having its primary winding directly connected in series with the main circuit carrying the full operating current of the system. An equivalent current is produced in its secondary, which is made to flow through the relay coil to get the equivalent measure of the main system current. The standard currents are invariably 1 A and 5 A universally.

Electromagnetic type commonly referred to as a VT 2. Capacitor type referred to as a CVT. The electromagnetic type is a step down transformer whose primary HV and secondary LV windings are connected as below see Figure 6. HV LV Figure 6. The above diagram is a single-phase VT. In the three-phase system it is necessary to use three VTs at one per phase and they being connected in star or delta depending on the method of connection of the main power source being monitored. For still higher voltages, it is common to adopt the second type namely the capacitor voltage transformer CVT.

Figure 6. Here the primary portion consists of capacitors connected in series to split the primary voltage to convenient values. The magnetic voltage transformer is similar to a power transformer and differs only so far as a different emphasis is placed on cooling, insulating and mechanical aspects.

The primary winding has larger number of turns and is connected across the line voltage; either phase-to-phase or phase-to-neutral. The secondary has lesser turns however, the volts per turn on both primary and secondary remains same. Hence, in EHV national grid networks of utilities, the CVTs are commonly used for both protection and communication purposes. It is possible to have one common primary winding and two or more secondary windings in one unit. The voltage transformers having this kind of arrangement are referred to as two core or three core VT depending on the number of secondary windings.

The primary parameters are suffixed with p while the secondary parameters have suffix s. It is to be noted that the vector diagram for a three-phase connection will be identical, except for the phase shift introduced in each phase in relation to the other phases Figure 6.

The capacity of a voltage transformer is normally represented in VA rating, which indicates the maximum load that can be connected across its secondary. Output burdens of VA per phase are common. However, the close accuracy is more relevant for metering purposes, while for protection purposes the margin of accuracy can be comparatively less.

Permissible errors vary depending on the burden and purpose of use and typical values as per IEC are as follows see Table 6. Thus, it is common to select voltage transformers based on the loads choosing appropriate rated burden. However, capacitor voltage transformers can only be connected phase-to-earth.

Voltage transformers are commonly used in three-phase groups, generally in star—star configuration. Typical connection is as per Figure 6. With this arrangement, the Instrument transformers 49 secondary voltages provide a complete replica of the primary voltages as shown below and any voltage phase-to-phase or phase-to-earth may be selected for monitoring at the secondary see Figure 6.

The residual voltage neutral displacement voltage, polarizing voltage for earth fault relays can be obtained from a VT between neutral and earth, for instance at a power transformer neutral. It can also be obtained from a three-phase set of VTs, which have their primary winding connected phase to earth and one of the secondary windings connected in a broken delta. Therefore a VT secondary normal voltage of: A residual voltage of V is obtained in both the cases.

VTs with two secondary windings, one for connection in Y and the other in broken delta can then have the ratio: U n V V 3 3 3 U n V V 3 3 and for high-impedance and effective earthed systems respectively. Other nominal voltages than V e. Ferro-resonance in magnetic voltage transformer When the ferro-resonance in a CVT is an internal oscillation between the capacitor and the magnetic IVT, the ferro-resonance in a magnetic voltage transformer is an oscillation between the magnetic voltage transformer and the network.

The oscillation can only occur in a network having an insulated neutral. The oscillation can be triggered by a sudden change in the network voltage.

It depends on the design of the transformer. The corresponding value for cable is: S Un2 km 52 Practical Power Systems Protection Damping of ferro-resonance The magnetic voltage transformer will be protected from ferro-resonance oscillation by connecting a resistor across the open delta point in the three-phase secondary winding.

Short-circuit on the secondary winding gives only a few amperes in the primary winding and is not sufficient to rupture a high-voltage fuse. Hence high-voltage fuses on the primary side do not protect the transformers, they protect only the network in case of any short-circuit on the primary side. The voltage drop in the secondary fuses and long connection wires can change the accuracy of the measurement.

It is especially important for revenue metering windings of high accuracy class 0. The total voltage drops in this circuit must not be more than 0. Typical values of resistance in fuses: Instrument transformers 53 The voltage drop in the leads from the VT to the associated equipment must be considered as this, in practice, can be alarming mainly in case of measuring circuits. This is the one that separates the metering circuits with low burden from protective circuits with higher burdens.

Refer Figure 6. Earthing should be made at only one point of a VT secondary circuit or galvanically interconnected circuits. A VT with the primary connected phase-to-earth shall have the secondary earthed at terminal n.

Windings not under use shall also be earthed see Figure 6. Alternatively, it is often a common practice to earth the white phase as shown. This practice stems from metering where the two wattmeter method requires two CTs and two line voltages.

With this arrangement the red and blue phases now at line potential to the white and it saves the expense and bother of running a neutral conductor throughout the panels see Figure 6. They must therefore withstand the networks short-circuit current. There are two types of current transformers: Wound primary type 2. Bar primary type.

Wound type CT is shown in Figure 6. The wound primary is used for the smaller currents, but it can only be applied on low fault level installations due to thermal limitations as well as structural requirements due to high magnetic forces. For currents greater than A, the bar primary type is used as shown in Figure 6. If the secondary winding is evenly distributed around the complete iron core, its leakage reactance is eliminated see Figure 6.

Instrument transformers Primary Secondary Figure 6. The standard symbol used to depict current transformers is shown in Figure 6. The exciting current is not being transformed and is therefore the cause of transformer errors. The amount of exciting current drawn by a CT depends upon the core material and the amount of flux that must be developed in the core to satisfy the output requirements of the CT. This can be explained vectorally in Figure 6. It is a graph of the amount of magnetizing current required to generate an open-circuit voltage at the terminals of the unit.

Due to the non-linearity of the core iron, it follows the B-H loop characteristic and comprises three regions, namely the initial region, unsaturated region and saturated region see Figure 6. This transition characteristic makes a CT not to produce equivalent primary current beyond certain point. For most applications, it means that current transformers can be considered as approximately linear up to this point.

Hence, it is common to have metering CTs with a very sharp knee-point voltage. A special nickel-alloy metal having a very low magnetizing current is used in order to achieve the accuracy. Following curve shows the magnetization curve of metering CT see Figure 6.

On the other hand these are concerned with a wide range of currents from acceptable fault settings to maximum fault currents many times normal rating. Larger errors may be permitted and it is important that saturation is avoided wherever possible to ensure positive operation of the relays mainly when the currents are many times the normal current see Figure 6.

Polarity is very important when connecting relays, as this will determine correct operation or not depending on the types of relays. BS states that at the instant when current is flowing from P1 to P2 in primary, then current, in secondary must flow from S1 to S2 through the external circuit.

Connect battery —ve terminal to the current transformer P2 primary terminal. If the polarities are correct, a momentary current will flow from S1 to S2. Core is then made of very good metal to give small magnetizing current.

On open-circuit, secondary impedance now becomes infinite and the core saturates. This induces a very high voltage in the primaryup to approximately system volts and the corresponding volts in the secondary will depend on the number of turns, multiplying up by the ratio i. Since CT normally has much more turns in secondary compared to the primary, the voltage generated on the open-circuited CT will be much more than the system volts, leading to flashovers.

This should always be kept as low as possible. In the relevant BSS the various accuracy classes are in accordance with the following tables see Tables 6.

The meanings of these figures are as below: For this type of CT an exact point on the magnetization curve is specified, e. Rated primary current Turns ratio Rated knee-point emf at maximum secondary turns Maximum exciting current at rated knee-point emf Maximum resistance of secondary winding.

The secondary currents obtainable with this connection are the three individual phase currents and the residual or neutral current. The residual current is the vector sum of the threephase currents, which under healthy conditions would be zero.

Under earth fault conditions, this would be the secondary equivalent of the earth fault current in the primary circuit. The reasons for adopting this connection are one or more of the following: All terminals that are marked P1, S1 and C1 should have the same polarity. Connect either the S1 terminal or the S2 terminal to earth. For protective relays, earth the terminal that is nearest to the protected objects. For meters and instruments, earth the terminal that is nearest to the consumer.

When metering instruments and protective relays are on the same winding, the protective relay determines the point to be earthed. If there are taps on the secondary winding, which are not used, then they must be left open. If two or more current transformers are galvanic connected together they shall be earthed at one point only e.

If the cores are not used in a current transformer, they must be short-circuited between the highest ratio taps and should be earthed. It is dangerous to open the secondary circuit when the CT is in operation.

High voltage will be induced. Current transformer connections Figures 6. Additional test windings can be provided to make such tests easier. These windings are normally rated at 10 A and when injected with this value of current produce the same output as the rated primary current passed through the primary winding. It should be noted that when energizing the test winding, the normal primary winding should be open circuited, otherwise the CT will summate the effects of the primary and test currents.

Conversely, in normal operation the test winding should be left opencircuited. Test windings do, however, occupy an appreciable amount of additional space and therefore increase the cost.

Alternatively, for given dimensions they will restrict the size and hence the performance of the main current transformer. However, it is difficult to use the same for higher current circuits. In order to overcome the limitations as experienced by series trip coils, current transformers are used so that the high primary currents are transformed down to manageable levels that can be handled comfortably by protection equipment. A typical example would be fused AC trip coils. These use current transformers, which must be employed above certain limits i.

Some basic schemes are: Under normal conditions, the fuses carry the maximum secondary current of the CT due to the low-impedance path.

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Under fault conditions, Isec having reached the value at which the fuse blows and operates, trip coil TC to trip the circuit breaker. Characteristic of the fuse is inverse to the current, so a limited degree of grading is achieved. The CTs are connected in the same way as seen in the earlier pictures.

Figures 6. Instrument transformers 6. Circuit breakers are the main making and breaking devices in an electrical circuit to allow or disallow flow of power from source to the load.

These carry the load currents continuously and are expected to be switched ON with loads making capacity. Under fault conditions, the breakers should be able to open by instructions from monitoring devices like relays. The relay contacts are used in the making and breaking control circuits of a circuit breaker, to prevent breakers getting closed or to trip breaker under fault conditions as well as for some other interlocks.

The circuit breaker functions under control of the relay, to open the circuit when required. A closed circuit breaker has sufficient energy to open its contacts stored in one form or another generally a charged spring. When a protective relay signals to open the circuit, the store energy is released causing the circuit breaker to open.

Except in special cases where the protective relays are mounted on the breaker, the connection between the relay and circuit breaker is by hard wiring. Figure 7. From the protection point of view, the important parts of the circuit breaker are the trip coil, latching mechanism, main contacts and auxiliary contacts. The roles played by these components in the tripping process is clear from Figure 7. Circuit breakers are normally fitted with a number of auxiliary contacts, which are used in a variety of ways in control and protection circuits e.

The important characteristics from a protection point of view are: Modern high-speed circuit breakers have tripping times between three and eight cycles.

The tripping or total clearing or break time is made up as follows: The time between instant of application of tripping power to the instant of separation of the main contacts. The time between the instant of separation of the main circuit breaker contacts to the instant of arc extinction of short-circuit current. The sum of the above see Figure 7.

In the earlier chapters we have studied simple examples of calculating the fault currents expected in a system. These simple calculations are applied with standard ratings of transformers, etc.

Behavior under fault conditions Before the instant of short-circuit, load current will be flowing through the switch and this can be regarded as zero when compared to the level of fault current that would flow see Figure 7.

IASYM 2. Cathode end —ve: There is approximately 30—50 V drop due to emission of electrons. Arc column: Ionized gas, which has a diameter proportional to current. Volt drop 10—20 V. When short-circuit occurs, fault current flows, corresponding to the network parameters. The breaker trips and the current is interrupted at the next natural current zero.

The network reacts by transient oscillations, which gives rise to the transient recovery voltage TRV across the circuit breaker main contacts. All breaking principles involve the separation of contacts, which initially are bridged by a hot, highly conductive arcing column. After interruption at current zero, the arcing zone has to be cooled to such an extent that the TRV is overcome and it cannot cause a voltage breakdown across the open gap.

Three critical phases are distinguished during arc interruption, each characterized by its own physical processes and interaction between system and breaker. Proper contact design prevents the existence of metal vapor in the critical arc region. This thermal phase is characterized by a race between the cooling of the rest of the plasma and the reheating caused by the rapidly rising voltage.

Due to the temperature and velocity difference between the cool, relatively slow axial flow of the surrounding gas and the rapid flow in the hot plasma core, vigorous turbulence occurs downstream of the throat, resulting in effective cooling of the arc. This turbulence is the dominant mechanism, which determines thermal re-ignition or interruption. However, due to marginal ion-conductivity, local distortion of the electrical field distribution is caused by the TRV appearing across the open break.

This effect strongly influences the dielectric strength of the break and has to be taken into account when designing the geometry of the contact arrangement. The medium could be oil, air, vacuum or SF6. The further classification is single break and double break. In a single break type only the busbar end is isolated but in a Circuit breakers 75 double break type, both busbar source and cable load ends are broken. However, the double break is the most common and accepted type in modern installations.

The arc control devices, otherwise known as turbulator or explosion pot achieves this: Turbulence caused by arc bubble. Magnetic forces tend to force main contacts apart and movement causes oil to be sucked in through ports and squirted past gap. When arc extinguished at current zero , ionized gases get swept away and prevents restriking of the arc see Figure 7. Fixed contact Ports Turbulator Moving contact Figure 7.

However there are many installations, which still employ these breakers where replacements are found to be a costly proposition. In this design, the main contacts are immersed in oil and the oil acts as the ionizing medium between the contacts. The oil is mineral type, with high dielectric strength to withstand the voltage across the contacts under normal conditions.

Arc is in a bubble of gas surrounded by oil. In the initial stages, the use of high-volume bulk oil circuit breakers was more common. In this type, the whole breaker unit is immersed in the oil. This type had the disadvantage of production of higher hydrogen quantities during arcing and higher maintenance requirements.

Subsequently these were replaced with low oil minimum oil types, where the arc and the bubble are confined into a smaller chamber, minimizing the size of the unit. Circuit breakers 7. Arc is chopped into a number of small arcs by the Arc-shute as it rises due to heat and magnetic forces. For medium- and low-voltage installations, the SF6 circuit breaker remains constructionally the same as that for oil and air circuit breakers mentioned above, except for the arc interrupting chamber which is of a special design, filled with SF6.

To interrupt an arc drawn when contacts of the circuit breaker separate, a gas flow is required to cool the arcing zone at current interruption i.

The resulting gas expansion is directed through nozzles to provide the required gas flow. The pressure of the SF6 gas is generally maintained above atmospheric; so good sealing of the gas chambers is vitally important. Leaks will cause loss of insulating medium and clearances are not designed for use in air. A circuit breaker is designed for high through-fault and interrupting capacity and as a result has a low mechanical life.

Vacuum breakers are also similar in construction like the other types of breakers, except that the breaking medium is vacuum and the medium sealed to ensure vacuum. Figures 7. It has pure oxygen-free copper main connections, stainless steel bellows and has composite weld-resistant main contact materials. A typical contact material comprises a tungsten matrix impregnated with a copper and antimony alloy to provide a low melting point material to ensure continuation of the arc until nearly current zero.

Because it is virtually impossible for electricity to flow in a vacuum, the early designs displayed the ability of current chopping i. This sudden instantaneous collapse of the current generated extremely high-voltage spikes and surges into the system, causing failure of equipment. Another phenomenon was pre-strike at switch on.

Due to their superior rate of dielectric recovery, a characteristic of all vacuum switches was the production of a train of pulses during the closing operation.

Although of modest magnitude, the high rate of rise of voltage in pre-strike transients can, under certain conditions produce high-insulation stresses in motor line end coils. Unfortunately, this led to many instances of contacts welding on closing.

Restrike transients produced under conditions of stalled motor switch off was also a problem. When switching off a stalled induction motor, or one rotating at only a fraction of synchronous speed, there is little or no machine back emf, and a high voltage appears across the gap of the contactor immediately after extinction.

Modern designs have all but overcome these problems. In vacuum contactors, higher operating speeds coupled with switch contact material are chosen to ensure high gap breakdown strength, produce significantly shorter trains of pulses. In vacuum circuit breakers, operating speeds are also much higher which, together with contact materials that ensure high dielectric strength at a small gap, have ensured that prestrike transients have ceased to become a significant phenomenon.

These have led to the use of vacuum breakers more common in modern installations. Following are the types of mechanisms employed. Hand operated: Cheap but losing popularity. Speed depends entirely on operator. Very limited use in modern installations that too for low-voltage applications only. Hand operated spring assisted: Hand movement compresses spring over top deadcentre. Spring takes over and closes the breaker.

Circuit breakers 81 3. Quick make: Spring charged-up by hand, then released to operate mechanism. Motor wound spring: Motor charges spring, instead of manual. Mainly useful when remote operations are employed, which are common in modern installations because of computer applications. As name implies. Used at 66 kV and above. Convenient when drying air is required. Dashpots prevent this. They also prevent unnecessary physical damage to the contacts on impact.

Their use of course depends on the design. Moving contacts normally have a special tip Elkonite to prevent burning from arcing. Batteries provide this power and hence they form an important role in protection circuits. It is therefore necessary to ensure that batteries and chargers are regularly inspected and maintained at the highest possible level of efficiency at all times to enable correct operation of relays at the correct time.

A battery is an assembly of cells. Whether it is used to make a call using mobile phone or to trip a circuit breaker, every cell has three things in common — positive and negative electrodes and an electrolyte.

Whereas some of the dry cell batteries drain out their energy and are to be discarded, a stationary or storage battery used in the switchgear protection has the capability to be recharged.

There are two types of batteries used in an electrical control system: Lead acid type 2. Nickel cadmium type. Both the above types can be classified further into flooded type and sealed maintenance free type.

The flooded cell construction basically refer to the electrodes of the cell in the electrolyte medium, which can be topped up with distilled water as the electrolyte gets diluted due to charging and discharging cycles. The higher discharge of H2 in lead acid cells have resulted in the manufacture of sealed maintenance free or valve-regulated lead acid VRLA batteries. Here the H2 discharge is restricted to be below the hazardous limit.

In addition, the hydrogen discharge in a nickel cadmium cell is comparatively less. Hence, for conventional switchgear protection applications, sealed nickel cadmium batteries are not required. As such, the sealed nickel cadmium cells are only used for small battery cells used in modern electronic gadgets.

The cell contains a pure lead Pb positive plate, a lead oxide PbO2 negative plate, and an electrolyte of dilute sulphuric acid. The nickel cadmium cell has an electrical voltage of 1. The following table briefly gives the advantages and disadvantages of nickel cadmium batteries over lead acid type, the most common types being used for protection application see Table 8. Advantages Better mechanical strength Easy maintenance Long life Space and weight low Low H2 discharge and no spill over issues Disadvantages Lower cell voltage 1.

A similar phenomenon occurs with nickel cadmium cell. The result is the dilution and weakening of the electrolyte. The cell is recharged by injecting a direct current in the opposite direction using another source to restore its plates and electrolyte to their original state.

Application guide see Table 8. Figure 8. The negative plate is of the pasted grid type made by forcing lead oxide paste into a cast lead alloy grid. The positive and negative plates are interleaved and insulated from each other to prevent short circuits, and are mounted in transparent plastic containers to allow visual checking of the acid level and general condition. Because of the high initial cost of Plante cells, specially designed flat plate cells have been developed to provide a cheaper but shorter-lived alternative source of standby power.

Although this is the basis of the modern car battery, it is totally unsuitable for switch-tripping duty because it has been designed to give a high current for a short time as when starting a car engine. Cells with tubular positive plates are also available but these are normally used to power electric trucks, etc.

On discharge, the recommended final voltage at which the discharge should be terminated depends on the discharge rate. This is shown in discharge curves, as shown in Figure 8.

The capacity of a battery is defined in terms of amperehour AH related to 5 h or 10 h duty. It refers to the capacity of the battery to supply a load current over a period, until it reaches its pre-defined final cell voltage. After this time, the cell has to be recharged to again feed a load. For example, in case of lead acid batteries, the acceptable final cell voltage could be as low as 1.

But it is common to define the capacity of the lead acid batteries for different cell voltages like 1. Accordingly, the discharge curves of a battery vary showing comparatively higher time to reach the lowest acceptable cell voltage. Table 8. The cell will reach 1. Hence, while designing the capacity of the cell, proper margins should be taken into account based on the nature of loads and the likely currents to be drawn over a cycle.

Capacity is also affected by ambient temperature. The lower the ambient temperature, the capacity will be comparatively higher.

Hence, the batteries are normally kept on charge continuously by a battery charger. The charger is a rectifier, which produces a slightly higher voltage compared to the nominal cell voltage of a battery.

The main power source is derived from the normally available AC source, which is rectified by the charger. Typical connection is as seen in Figure 8. The correct trickle charge current is that which does not allow the cell to discharge gas and does not allow the specific gravity to fall over a period. The cell voltage will be approximately 2.

This 88 Practical Power Systems Protection method is usually adopted in conjunction with supplying continuous and variable DC loads from the charging equipment, as would typically happen for a substation battery. The loads in a substation normally comprise of small continuous load consisting of pilot lamps, relays, etc.

Since the charger, battery and load are all connected in parallel as per Figure 8. Any load that exceeds the charger capacity will lower its voltage slightly, to the point where the battery discharges to supply the remainder.

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If there should be a complete power failure the battery will supply the entire load for a period depending on the AH capacity and the load, until AC power is restored and then automatically starts being recharged. A fully charged cell will have a specific gravity reading of between 1.

On recharge, the voltage increases and reaches a saturation value as the charge proceeds. The highest voltage reached with the finishing rate of charge flowing is 2. It is possible to recharge a cell by limiting the voltage of the charging equipment to a much lower value than 2. This will result in an extended recharge period, as the battery will automatically limit the charge current irrespective of the charger output.

It also has to handle the electrical load. Chargers presently used in stationary applications are normally of the constant voltage type. Voltage adjustments can be made with precision to 0. This is necessary because floating voltage and equalizing voltage levels critically affect battery performance and life expectancy. Voltage level specifications are normally expressed to two decimal places, i. Proper specifications and correct adjustments of the battery charger are the most important factors affecting the satisfactory performance and life of the battery cells.

Voltage levels from the charger also usually serve the electrical load, so changes in charger voltage output affect the load. Tripping batteries 89 Chargers are normally equipped to accommodate normal float voltages and the higher voltages for equalizing charges when required. As the charger DC output is rectified from AC there will be a ripple on the output unless smoothing techniques are employed.

Failure to do this will result in a phenomenon called AC corrosion, where the negative peak of the AC component reverses the direction of the charging current, leading to corrosion and ultimate destruction of the plates in the cells. Use distilled water wherever possible.

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Impurities in normal tap water such as chlorine and iron tend to increase internal losses. Frequent topping up of electrolyte means excessive gassing brought about by overcharging.

This normally starts when lead acid cell voltage reaches 2. At full charge, most energy goes into gas, oxygen being liberated at the positive plate and hydrogen at the negative.

Four percent hydrogen in the air may be hazardous. The room employing lead acid batteries must be well ventilated. However, the nickel cadmium batteries discharge very low hydrogen. This is a build-up of a sponge like layer of lead on the negative plates, which can accumulate to such an extent as to bridge over or around the separators and cause a short circuit to the adjacent positive plate.

This condition is usually an indication of overcharging. If this increases to the point where it reaches the bottom of the plates, it will short them out to cause failure. Overcharging can also accelerate the accumulation of sediment and shorten the useful life of the battery. A short boost charge is beneficial on a regular basis to prevent stratification, to freshen the electrolyte and to equalize the cells.

Ensure that the charger is working properly and that it is operating in accordance with the manufacturers recommended settings. Cells reaching end voltage after disconnection of charger comparatively faster than the other cells need to be replaced completely, as there is no question of changing the electrolyte. The nickel cadmium batteries on the other hand do not pose such major hazards though they are quite costlier compared to lead acid type.

Two batteries and chargers should then be installed to ensure the integrity of each tripping system, DC fail relays being installed on each panel to monitor the continuity of each supply see Figure 8. For breakers fitted with only one trip coil, a single battery and charger should be installed and trip coil supervision relays fitted to monitor each circuit see Figure 8. Charger no. One popular method is to earth the negative rail see Figure 8. It will be noted that in this case a solid link is used instead of a fuse on the Tripping batteries 91 negative side.

This ensures that the negative is never lost. The drawback with this system is that the first earth fault on the wiring could possibly create a short on the battery. An earth fault on either the positive or negative rail will cause a very small current to flow up the neutral 0 V connection.

The supply will however remain on but this condition should be attended to before a second earth fault occurs. A test button is also provided to check the relay is functional at any time, by offsetting this away from the center zero point.

There are two possible types a supervision only when the breaker is in closed in service condition; b supervision irrespective of the breaker status. These are achieved by using breaker auxiliary contacts in the DC trip circuits as shown in Figures 8. Open-circuited DC shunt trip coil 2.

Burnt out DC shunt trip coil 5. Failure by open circuit of control wiring or defective relay contact 6. Breaker mechanically jammed: Trip circuit supervision TCS provides continuous monitoring and gives immediate warning of conditions 1 to 5 before breaker is called upon to trip.

Corrective action can then be taken before the event, to prevent breaker failure occurring. Breaker fail BF protection covers all conditions but only highlights problem after a system primary fault occurs and the breaker has failed to clear.

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TCS can be regarded as the fence at the top of the cliff whereas BF is the ambulance at the bottom, only operating after the event! Failures to open could arise from: They can also be used as an alternative tripping supply for installations, which employ two-shunt trip coils per circuit breaker, one connected to the tripping battery and the other to the capacitor storage unit.

The unit comprises two capacitors to provide short-time storage of auxiliary energy, one to operate a protective relay whilst the other operates the circuit breaker trip coil.

The device has three inputs, which may be fed from DC or AC sources. A test button is provided for checking the degree of capacitor charge, whilst an in-built signaling relay with a normally closed contact offers remote indication of discharged capacitors. They should be designed to saturate at about V rms.

The recommended interposing CTs may well pose a problem in this regard. Relays 96 9 Relays 9. The basic parameters of the three-phase electrical system are voltage, current, frequency and power. Any shift from this normal behavior could be the result of a fault condition either at the source end or at the load end.

The relays are devices, which monitor various parameters in various ways and this chapter gives a brief outline of their principles of operation.

The types of relays can be broadly classified as: The electromechanical relays had been dominating the electrical protection field until the use of silicon semiconductor devices, becoming more common.

The use of static relays in the early stages were more due to the advantages like lower weight, non-moving mechanical parts, reduced wear and tear, etc. Further, the reliability of electronic components in the initial stages had been unsatisfactory due to the quality issues and their ability or inability to withstand source fluctuations and ambient temperature conditions.

However, the reliability of electronic components improved subsequently, and the advent of digital electronics technology and microprocessor developments gave a completely different picture to the use of static relays. The use of static analog relays is not so common. Hence, it is worthwhile studying the operation of this relay in detail to understand the characteristics adopted in the digital relays see Figure 9.

Figure 9. Fluxes A and B are out of phase thus producing a torque in the disk causing it to rotate. Now, speed is proportional to braking torque, and is proportional to driving torque. The characteristic curve is defined by BS and is shown in Figure 9. Two adjustments are possible on the relay, namely: The current pick-up or plug setting: This adjusts the setting current by means of a plug bridge, which varies the effective turns on the upper electromagnet.

The time multiplier setting: This adjusts the operating time at a given multiple of setting current, by altering by means of the torsion head, the distance that the disk has to travel before contact is made.

In this context, the plug setting is that current at which the operating and restraining torques are in a state of balance. In practice, BS requires that the relay should definitely not operate at the setting, and to ensure this, a relay may display a slight tendency to reset at the normal setting. Relays 99 Figure 9. Earth fault A: This has the effect of moving the inverse curve down the axis as shown in Figure 9.

Its characteristic shows an operating time of 3 s at 10 times the current plug setting i. The time of operation of the relay is chosen by collectively selecting the current and time plug settings. There is another popular version, which has an operating time of 1.

It is possible to manufacture relays with different characteristics, but the principle of operation remains the same.

Other characteristic curves popular are very and extremely inverse. These are represented in logarithmic graphs due to the exponential nature. For electromechanical relays, this is normally stated as 3 VA nominal. The modern electronic relays offer a much lower figure, which is one of their virtues. However, for the electromechanical type, the selection of the plug setting does have an effect on the burden.

As there is a required minimum amp-turns of magnetic flux to get the relay to pick-up, Relays the lower the current the more turns are necessary. The lower the setting therefore results in higher the burden on the CTs. This is often not the case in low ratio CTs.

A mistaken impression is created that the relay is at its most sensitive setting when it is set on its lowest tap. However, the fact is that the CTs may saturate under these conditions due to the higher burden, causing the electromechanical relay to respond more slowly, if at all it picks up. However, the modern digital relays do not exhibit such behavior and have constant burden through out its operating range.

Table 9. Impedance varies according to plug setting Many modern relays are of the draw out type so that, the relay can be removed from its case even when the CT circuits are alive. This is possible as the associated CT terminals in the case are short circuited just before the relay contacts break whilst the relay is being withdrawn. Certain models also have catches, which hold the relay in its case. When these catches are unlatched, the tripping circuit is opened so that accidental closing of the trip contacts will not trip the associated circuit breaker.

This feature must not always be relied upon to prevent tripping as it does not necessarily isolate all tripping circuits and the feature is not present in all relays. However, dirt and magnetic particles are the biggest cause of problems in electromechanical relays. Hence, when this type of relay is removed for testing, it should be covered, while not being actually tested. It should preferably be kept in a spare case or a plastic bag if stored or transported.

For operational tests, a load transformer and variac can be used to supply current, while a timer will indicate when the tripping contacts close. Typical connections are shown in Figure 9. This method can be used to check that the relay operates, that the flag drops correctly just as the contacts are made with slow disk operation and that the contacts do make positively with good pressure. This method is not reliable for timing or pick-up tests. A proper relay current test set is necessary for accurate tests as with this simple set up, distorted non-sinusoidal currents result because of the non-linear magnetic circuit of the relay.

When testing from the normal V supply, we have a pure voltage source. Hence, the current now becomes distorted and non-sinusoidal, giving the relay false parameters on which to operate.

Special test sets are on the market, which are designed to inject sinusoidal currents into the relays so that accurate timing and pick-up currents can be recorded. If the relay timing is found to be outside the tolerance limits, do not attempt to rectify this by adjusting the spiral hairspring at the top of the disk shaft, as this could upset the whole characteristic. The following table gives the margin of errors in the test results based on the testing source.

Calculate the plug setting and time multiplier setting for an IDMTL relay on the following network so that it will trip in 2. Therefore, current into relay as a multiple of plug setting during fault: Referring to characteristc curves below, read time multiplier setting where 10 times plug setting current and 2. The graph shows that eight times plug setting to operate in 2. This technique is fine if the required setting falls exactly on the TM curve.

The following procedure is therefore recommended see Figure 9. Then divide the desired time setting by this figure. This will give the exact time multiplier setting: Relays 9. Load conditions: Must not trip for healthy conditions, i. Load current redistribution after tripping 3.

Fault currents: High fault currents can cause saturation of CTs. Choice of CT ratio is important 4. CT performance: Magnetization curve. Its internal resistance 5. Relay burden: Increases at lower taps on electromechanical relays 6. Relay accuracy: Better at top end of curve. Attempt to use in tight grading applications. Some of these early designs have been improved over the years. One of the most successful types of electromechanical protection relays has been the previously discussed inverse definite minimum time IDMT overcurrent relay based on the induction disk.

With the introduction of electronic devices such as the transistor in the s, electronic protection relays were introduced in the s and s. Since then, the development of relays has been related to the general development of electronics.

By the late s, extensive experience in the use of electronics in simple protection systems enabled the development of many quite advanced protection schemes and the first high-voltage substations were equipped with static protective relays. Over a period, these have been extended to cover other equipments such as transmission lines, motors, capacitors and generators.

New measuring techniques have been introduced, measurements that are more accurate can be performed and high overall quality, reliability and performance of the protection system for high-voltage power systems have been reached.

Developments in the s concentrated on improving reliability through improved design of printed circuit boards leading to integrated circuits and general improvements in substation designs, particularly earthing. In general, most static protective relays of that time were designed to match or improve on the basic electromechanical performance features. Improvements introduced included low-current transformer burden, improved setting accuracy and repeatability as well as improved speed.

Also, during this period, experiments were conducted in Europe, Japan and the USA to test computer-based protection systems based on the availability of digital electronics. This is particularly true with IDMT overcurrent relays, where it was both difficult and expensive to provide the inverse time characteristics by means of analog electronic circuits.

The overcurrent relay is undoubtedly the most common type of protection relay used by electricity supply authorities for protection on distribution systems. This chapter concentrates on the various features of modern static overcurrent protection relays in relation to the older electromechanical relays, which are still commonly used on Practical Power Systems Protection distribution systems today.

The purpose is to clarify some of the arguments for and against static protection relays, particularly for medium-voltage applications.

What is a static protection relay? Static relays are those in which the designed response is developed by electronic or magnetic means without mechanical motion. The analog relays refer to electronic circuits with discrete devices like transistors, diodes, etc. However, the digital designs incorporate integrated chips, microprocessors, etc. In recent years, very few relays of the analog type are being developed or introduced for the first time.

Most modern overcurrent relays are of the digital type. There are many reasons for this, the main ones being associated with cost, accuracy, flexibility, reliability, size, auxiliary power drain, etc. Many of these reasons will become evident during the course of this chapter, which will concentrate on relays of the digital type.

Microprocessor relays are of the digital type. The main objective of using static relays is to improve the sensitivity, speed and reliability of a protection system by removing the delicate mechanical parts that can be subject to wear due to vibration, dust and corrosion. During the early development of static relays, the use of static components were particularly attractive for the more complicated relays such as impedance relays, directional relays, voltage regulating relays, etc.

On the other hand, the early static IDMT overcurrent relays were expensive because it was difficult to match the inverse time characteristic using analog protection circuits. The battery drain associated with these static IDMT relays was too high and this discouraged the use of this type of relay for medium-voltage applications.

The general developments in the field of electronics and the introduction of digital circuits have overcome many of the above problems. Using modern microprocessor relays, almost any characteristic is possible and economical, even for the simplest applications such as, overcurrent relays and motor protection relays.

What is a microprocessor relay? For example the formula used to derive the inverse time characteristics in an overcurrent relay that comply with IEC and BS is mathematically defined as follows: Introduction to the numerical relay The measurement principle is based on sampling of the energized currents or voltages, analog to digital conversion and numerical handling, where all settings are made in direct numerical form in a non-volatile memory.

In addition, the operation of the self-supervision is described see Figures 9. Here we can recognize the input signal path with the signal processing parts, the output circuits for trip and signal, and the self-supervision circuits.

A digital relay comprises of sensitive devices and hence it is necessary that they do not fail because of the input changes. This is taken care by the isolating transformer and the limiter used in the relay. The energizing currents or voltages are brought into the relay using the internal matching transformer 1.

After the transformer there is a voltage limiter 2 , which cuts the voltages entering the relay at a safe level, should there be an extremely high-current or voltage input. The reasons for this limiting are only to protect the internal circuits of the relay from being destroyed by too high an input voltage.

Together with the limiting circuit, a filter can also often be implemented. In this case, the reason is that the harmonics contents of the energizing signal is not wanted and is filtered out.

Switchgear and Power System Protection

A typical filtering level here is that the third harmonics are attenuated by a factor of 10 and the fifth harmonics or higher are attenuated by factors of and more. His research interest is applying techniques of Artificial Intelligence to power system protection schemes.

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