VePi NewslettersThe Electrical Power Systems Division The Protection & Control section Number: 1 Introduction:
In this coverage of power systems protection, the subject is divided into three major parts. The first part will cover an overview of power systems protection including an introduction to the other major components, beside the relays, that constitute an operative protection system. The second part will cover the hardware aspect of the subject and the third will cover the calculations, testing, setting of the protective relays. Also, it will cover the sizing of breaking devices and the typical protection schemes of transformers (power and distribution), feeders, bus and motors. The presentation coverage will end by a few numerical examples to emphasize the previously given protective principles.
Protective relays are mostly installed in switchgear assemblies or may be found in relay panels. The assembly will be divided into compartments to accommodate the different equipment and devices. The switchgear assembly will contain the circuit breakers and/or load break switches plus power/current limiting fuses and/or disconnect switches plus the instrument transformers (current and voltage feeding the relays and meters) and control power transformer. These assemblies will contain also the following accessories (to complete the operative system): the test switches (which come in a variety of designs to suit the different applications) to isolate the relays from their transformers, assist in testing the relays and provide the necessary protection (shortening) of the current transformers terminals, terminal blocks that come in different designs, current/voltage ratings, materials of the insulation and screws/clamps to suit the diversified requirements, the control wires, control circuit protection fuses or molded case (or miniature) circuit breakers and power supply (d.c. batteries or control power transformers) to operate the breaking devices, provide indications locally, and supply the auxilliary (supply) voltage to solid state/microprocessor based relays.
Circuit breakers:
If circuit breakers are used in low voltage installations, the protective devices will be an integral part of it; if switches are used, then the protective devices will be separate and protection is provided by fuses. Low voltage circuit breakers can be divided broadly into molded case and power (air magnetic). In molded case breakers, the protective device against overcurrent and short circuit is provided by thermal-magnetic or electronic (solid state) inegral protective device. With power breakers, the most commonly used configurations will have the protective device as integral one with the breaker. Some designs will have current transformers (sensors) and relays separate from the breakers. The functions of the protective devices vary according to the user requirements and protective philosophy. In medium voltage installations, the most common configurations will have separate protective devices (relays or fuses) from circuit breakers or switches, though some designs of circuit breakers may have integral protective devices (sensors and relays). For integral protection from undervoltage conditions, the breakers may be supplied with under voltage releases. The standards that govern the design and testing of circuit breakers are either the ANSI C37 or the IEC 56 series. The ANSI C37.04 and 06 gives the ratings structure and preferred ratings for AC MV CBs, respectively. Standard C37.09 gives the test procedure for AC MV CBs, rated on a symmetrical current basis. Standard C37.010 gives an overview of the application procedures for AC MV CBs. Standard C37.16 covers the preferred ratings, related requirements and application recommendations for LV power CBs. In general, the ratings of the circuit breakers are the limits of the operating conditions of the following parameters, for which the breakers are designed (without causing damage to itself or the surroundings for the projected lifetime of such device):
rated operating voltage range (eg. minimum, nominal and maximum).
rated frequency (eg. 400 c/s or 50/60 c/s).
rated maximum continuous current (eg. 1200, 2000,...).
rated interrupting currents under the different operating voltages of the breakers.
rated dielectric strength limit or maximum voltage withstandability (eg. for power frequency voltages, impulse voltage wave shapes and switching voltage impulses).
duty cycle to which the breakers are designed and tested.
rated operating times.
equipment to which the breakers will be suitable to apply and use safely.
mechanical durability under no load, full load and short circuit currents.
control circuit supply (operating) requirements.
Load break switches: Medium voltage load break switches in series with power or current limiting fuses in medium voltage installations is a common configuration. Though this design is much simpler in design and maintenance than circuit breakers, it has its disadvantages. The construction is of the fixed type (vs. draw out with circuit breakers). If the fuse is blown, the cell door has to be opened and the fuses replaced (vs. reclosing of breakers after fault removal) - a relative safer operation. The breaker construction is more compact than the switch (as clearances have to be met in air). Low voltage switches can be of the molded case type or the safety switch construction. Both types wil require fuses to provide the protection to the system and connected equipment.
Current transformers: Instruments transformers are covered in ANSI standard C57.13 and CSA CAN3 - C.13. Current transformers come in a few forms to provide for the space limitations in certain designs and the required high accuracy for others. Current transformers can be classified broadly into bar, window, bushing and wound (for higher accuracy). Current transformers for protection application (rather than metering) do not require a high accuracy at rated current as much as require their operation without saturation at high fault currents. For eaxample a C.T rated 2.5 L 200 will have a 2.5 % error at 20 times its rated secondary current times the burden connected to the secondary. If the rated secondary current is 5 then the maximum burden to be connected is 2 ohm, if rated secondary is 1 then the burden is 10. The 200 is the knee voltage on the secondary exciting voltage vs. excitaton current curve of C.T. The other important pieces of information that are required about the C.T. are: the secondary resistance, cross section area of the core and the saturation flux density for the silicone iron grade used for the core so that the C.T. can be checked for its proper operation under the given conditions. Current transformers are subjected to heavy primary currrent for short periods of time during faults. Mechanical damage may be caused by magnetic forces in the windings due to the first cycle (and subsequent ones) peak fault current and is proprtional to square the peak current. Thermal damage to the transformer insulation (and maybe its copper) may occur due to heating of the winding due to the integrated effective value of the fault current over the time period until the fault removal. (Note: the same discussion is applicable when sizing cables and cable supports). Partial dicharge in instrument transformers are obtained by testing at the factory and the acceptable level varies depending on the requirement of the end user, though the limit (a maximum value) is given as 50 picocoulomb. A measure of the condition of the isulating material is the dielectric loss which is given by the dissipation or power factor. An ideal insulation material should have a very low power factor close to 0 ( = cos = tan ). Polarity of instrument transformers is important with differential protection schemes. Polarity markings designate the relative instantaneous directions of current in the transformer leads. A high and a low voltage (or current) leads have the same polarity at a given instant if the current enters the high voltage lead and leaves the low voltage lead at this instant (or vice versa), giving the effect as though the two leads belong to the same continuous circuit. The polarity of transformers can be additive (H1 & X1 are diagonally opposite) or subtractive (H1 & X1 are adjacent). A.C. or D.C. polarity tests can be performed at site to determine the polarity of unmarked transformers.
Potential (voltage) transformers: This type of instrument transformers provide an isolation between the high voltage circuit and the control/metering circuit. It also provides a standard low voltage signal irrelevant of the voltage class of the high voltage system. In relay circuits, P.T. is used in voltage restrained, voltage and distance relays. The burden of the loads to the secondary of the P.T. winding should not exceed the maximum designated to the P.T. Each P.T. is rated for thermal withstandability at different ambient temperatures. According to the standards there are three groups of potential transformers, they define the rated voltage of the P.T. and what should be the system voltage to which this P.T. shoud be applied to. They, also, define the rated overvoltage factor and the duration of the overvoltage for each group. Definition of PT groups.
Test switches: They come in many configurations and designs, some are of the open style and others covered (opaque or transparent). They can be front or back connected to the other devices. The switch can come with different number of poles up to ten with any combination of current type or potential type poles. Broadly, the pole type can be classified accordingly: blank space, single pole potential, ganged two or more potential poles, through bar, fuse mounting clip (max. 30 amp.), single pole short circuiting assembly, single pole test jack / short circuiting current assembly and three pole short circuiting current assembly.
Terminal blocks: Terminal blocks come in many shapes and forms to suit the specific needs of the user, standard feed through terminals, C.T. double clamp, sliding link, knife disconnect terminals, ground, neutral/disconnect, miniature feed-through terminals, just to list a few types. They are rated up to 750 volt, up to about 130 amp. Certain designs come up to 3-tier for dense wiring applications. The material of the insulating material can be black phenolic, melamine or polyamide.
Control wires: The three most commonly used types have as insulating material TBS, SIS or teflon. The size varies to provide the minimum possible burden on the C.T. or P.T. that it is connected to. For C.T., sies AWG 8, 10 and 12 are common. For P.T. sizes, AWG 12 and 14 arw not unusual. The ratings for TBS vs. SIS are as follows: temperature rating: 90 C vs. 90 C, voltage rating 600 v vs. 600 V, solid or stranded for both from #14 to .2, insulaqtion thickness for both is the same though the base material of TBS is PVC (with cotton braid) and for SIS is XLPE (filled). Wires with teflon insulation have higher temperature ratings.
Stationary batteries: The stationary battery is designed to serve as an auxilliary /standby source of power to all devices connected to it. The battery is normally mounted on racks and is continously charged except for intermittent discharging periods of varying times and power. Battery voltage gradually declines during discharge and should not be permitted to drop below the minimum tolerated by the load plus the line drop. To protect the battery against over discharge, a low voltage relay (d.c.) can be used as part of the installation. The rate at which the voltage declines depends upon: the demand current of the load, duration of the discharge, chemical design and type of cells, number and size of plates in each cell, battery state of charge at beginning of discharge, age of battery cells and temperature of cells. The capacity of the battery is basically its ability to supply a given current for a given period of time at a given cell temperature without going below a minimum voltage (batteries are rated in ampere-hour at a given discharge rate). Stationary batteries are usually rated for 8-hour, 3-hour, 1-hour, 1- minute discharge. The ampere-hour rating is simply the product of the discharge in amperes multiplied by the given dicharge time period. For the lead acid battery the positive plates are: the pasted (Faure) plate which comprises of a latticework metallic grid with the opennings filled with lead oxide paste. The grid may be made up of lead antimony or lead calcium. The second type is the multitubular plates, which use porous plates to contain the lead oxide. The grid (lead antimony) is a row of spines extending from the top bar to the bottom cap bar. Porous tubes filled with lead oxide (powedered) with the grid forms the positive plate. This design provides more AH of capacity per cubic foot of battery volume at moderate rates of discharge. The third and last type is the plante type which is considered to have the longest life expectancy of all lead acid stationary battery designs. The positive plate consists of a grid (lead antimony) of large area with thin layers of lead oxide. Such plates have complex designs with circular opennings where corrugated lead ribbons are rolled into spiral ribbons. The negative plates irrelevant of the type of the positve plates are built with pasted plate design. Metallic sponge lead is used on the negative plates. The negative grid for the multitubular and plante (positive plates) is made of lead antimony, with the pasted plate it is either lead calcium or lead antimony. The grid of alloys antimony or calcium serves both purposes gives physical support and strength to the soft lead and acts as an electric conductor. The grid achieves and retains a physical shape and conducts the current to all parts of material.
For the nickel-alkaline batteries there are two types of plates, the pocket type and the sintered type. The pocket type is used for both positive and negative plates. The active material (nickel hydrate - positive and cadmium sponge - negative plus addittives to help conductivity) is sandwiched between two perforated strips (nickel plated steel). The strips are crimped together and this assembly is placed in a U- shape frame. After intermeshing the positive and negative the insulator pins are put in place, through the frame and plates. These elements are then put in a container and the cell cover (with vent cap and appropriate hole for terminal poles) is installed. There are three common ratings: high (discharge shorter than 1 hour), medium rate (discharge shorter than 4 hrs) and low (the battery will supposedly carry loads for up to 20 hrs).
For lead acid batteries, the electrolyte is a solution of diluted sulphuric acid. When the battery is fully charged, the positive plate is lead peroxide and the negative one is sponge lead. The specific gravity of the electrolyte is maximum at start of discharge and the specific gravity gradually decreases as dicharge occurs. Specific gravity for stationary batteries used for switchgear applications, control and emergency lighting is approximately 1.210. To determine the state of charge of the battery, the gravity reading is compared with the full charge value and to the specific gravity drop of a particular cell size at a specific discharge rate. The reading has to be corrected to the ambient temperature at time of measurement, if other than 25 deg C. For the Ni-Cad the electrolyte is a solution of potassium hydroxide diluted in water with normal specific gravity of 1.16 to 1.19 at 25 deg C. When the battery is fully charged the positive plate, nickel hydrate, is highly oxidized and the negative plate is sponge metallic cadmium. After discharge takes place the positive plate reduces to lower oxide while the metallic cadmium in the negative plate oxidizes. The specific gravity of the electrolyte can not be used to indicate the state of charge of the battery. The specific gravity readings will vary from normal rating when the electrolyte temperature is lower or higher than 25 deg C, when the solution level drops below the normal or the battery has been in service for long time.
The charger is a static rectifier (scr), its function is to change the single phase or three phase input (120, 208, 240, 480, 600V- 60HZ) to a d.c. output suitable for charging the battery and maintainning a constant voltage throughout the battery's load range. To prevent the self discharge phenomena (standing loss) the charger maintains a float charge that continuously monitors and corrects for these internal losses.
Relays: Every system is subject to short circuits and ground faults, that should be removed quickly. The most common relay for S.C. protection is the O/C relay. A short circuit on an electric system is always accompanied by a corresponding voltage dip (an overload will cause a moderate voltage drop). A voltage restrained or voltage-controlled O/C relay is able to distinguish between O/L and fault conditions.
Directional overcurrent: consists of a typical O/C unit and a directional unit, which are combined to operate jointly, for a pre-determined phase angle and magnitude of current. Such a relay operates only for current flow to a fault in one direction and will be insensitive to current flow in the opposite direction.
Directional power relays: comes in single or three phase versions and they work on the watt-meter principle. The contacts (in electromechanical construction), movable and fixed, get in contact at a pre-determined value of power. It could be used for directional overpower, to operate if excess energy flows out of an industrial plant into the utility. It can also be used to sense an under-power condition and separate two sources operating in parallel.
Differential relays: the basic principle of operation for such relays is the continuous comparison of two or more current quantities. When a fault occurs, the resulting differential current will cause the relay to operate. Differential protection schemes for generators, motors, two winding transformer banks and buses are common in industrial plants. In subtransmission and distribution levels, differential protection is used with power transformers and in bus protection schemes. It protects against abnormalities within a zone and should be insensitive to faults outside this zone (through faults), overexcitation or during energization or starting conditions.
Ground fault relaying can be any of the following configurations: residually connected, direct sensing or zero sequence (vectorial summation).
A single window-type current transformer is mounted in such a way to encircle all three phase conductors of incoming or outgoing circuits. For 3-phase, 4 wire circuits, the neutral is also run through the sensor and the secondary of this sensor is connected to an O/C relay. For neutral relaying (direct sensing), a current transformer is in the neutral grounding circuit and connected to the O/C relay. The synchro-check relay is used to verify, when two alternating current circuits are within the desired limits of frequency, voltage and phase angle, to permit them to operate in parallel.
The synchronizing relay monitors two separate systems that are to be paralleled, initiating switching when the following three conditions are met: the voltage difference of the two systems and the frequency difference are within the pre-determined range. The phase angle between the two systems voltage is zero, taking into consideration the operating time of the switching devices.
Pilot wire relays, operate on the principle of comparing the conditions at the terminals of the protected line. The relays will operate if the comparison indicates a fault internally on the line, they are insensitive to external faults. This scheme is used when tie lines have to be protected, either between the industrial system and the utility system or between major load centres within the industrial plant.
Mi>Voltage relays can be classified according to their reason of operation (i.e., overvoltage, undervoltage or both, voltage unbalance, reverse phase voltage or excessive negative sequence voltage).
Under/overvoltage relays are found in the following circuits: capacitor switching control, a.c. & d.c. overvoltage protection for generators, automatic transfer of power supplies, load shedding on U/V and U/V protection for motors.
Voltage unbalance (comparing two sources), an example for the application of such a relay is with the voltage restrained relays, when the P.T. fuse blows. This is seen as a fault by the voltage restraint relay. The use of balance relays can block the operation of the restrained relay.
Reverse phase voltage relays are used to detect reverse connections in three phase circuits, feeding motors, generators or transformers.
Negative sequence voltage relays are used to detect single phase conditions, as long as the sensing P.T. is on the load side of the opened phase.
Negative sequence overcurrent relays are used for single phase protection. The location of the current transformer with respect to the opened point is insignificant.
Distance relays come in the following types: the MHO type, impedance, reactance, MHO or admittance, OHM or angle impedance, offset MHO, modified impedance, complex characteristics type, elliptical characteristics and quadrilateral type. They measure voltage, current and the ratio is expressed in terms of impedance. The impedance can represent the equivalent impedance of a generator or large synchronous motor or a transmission line. The MHO relay is used to detect the loss of field of synchronous generators and motors.
Frequency relays sense under or over frequency conditions during system disturbances. The usual application of these relays is: to selectively drop the load, based on the frequency, in order to restore normal system stability, splitting up a grid by opening tie lines to prevent complete system collapse, for the generators and auxiliaries protection, when frequency supervision can prevent turbines and drive damages and for isolating small systems having their own generation from the main system.
Temperature sensitive relays usually operate in conjunction with temperature detecting devices. These devices can be classified into RTD (resistance temperature detectors) and thermocouples. They are located in the equipment to be protected (embedded in the stator winding or the bearings of the motor or generator). The temperature detectors can have 10, 100 or 120 ohm and is connected in a bridge configuration, with the temperature sensitive relay connected diagonally across the bridge. Replica-type temperature relays have their operating characteristics closely matching the heating curve of the general purpose motor curves (in the light and medium overload zones), thus they are used for overload protection of motors in the medium voltage range.
Multi function relays: These relays are microprocessor based and provide more than one protection function and even some indications and metered data. They can be classified inrto feeder protection units, induction motors protection and synchronous motors protection units. They have the provision of being interrogated and adjusted remotely through their communication ports and the local network they are connected to.

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