RELAY CO-ORDINATION full report
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PROJECT REPORT ON
Done at FACT, COCHIN DIVISION
The purpose of the protective relays and protective relaying systems is to operate the correct circuit breakers so as to disconnect only the faulty equipment from the system as quickly as possible thus minimizing the trouble and damage caused by faults when they do occur. Relays have revolutionized protection control metering and communication in power systems. Relay co-ordination will help to isolate the faulty region in the power system so that the fault at a particular region will not affect the other region in the system. Relay co-ordination can be done by selecting proper PSM and tsm of the relay and co-ordination can be checked graphically . Relay coÃ‚Â¬ordination in a power system will help to prevent the great loss and hazards caused due to the faulty conditions. Relays are also used to protect the transformer and motors also. Calibration of such relays are also done and also checked whether the relays are co-ordinated in each plant in FACT
SREERAJ V SUBEESHKS SHY AMKUMAR RB TED XA VIER ANTONY
INTRODUCTION ABOUT FACT
1. FUNDAMENTAL REQUIREMENTS
2. OPERATING PRINCIPLE
3. BASIC RELAY
4. RELAY TIMING
BASIC CONSTRUCTION OF STATIC RELAYS
COMPARISON BETWEEN STATIC AND ELECTROMAGNETIC RELAY
MAINTENANCE AND FIELD TESTING OF RELAYS
DIGITAL / NUMERIC RELAYS
MAIN AND BACK UP PROTECTION
CDG 1 1 RELAY 1. CALIBRATION
RELAY CO-ORDINATION IN FACT
LAY OUT OF POWER SYSTEM
TSM/ PSM SETTING
SINGLE LINE DIAGRAM
In a power system consisting of generators, transformers and distribution circuits, it is inevitable that sooner or later some failure will occur somewhere in the system. When a failure occurs on any part of the system, it must be quickly detected and disconnected from the system. There are two principle reasons for it. Firstly, if the fault is not cleared quickly, it may cause unnecessary interruption of service to the customers. Secondly, rapid disconnection of faulted apparatus limits the amount of damage to it and prevents the effects of fault from spreading into the system.
The detection of a fault and disconnection of a faulty section or apparatus can be achieved by using fuses or relays in conjunction with circuit breakers. A fuse performs both detection and interruption functions automatically but its use is limited for the protection of low-voltage circuits only. For high voltages (say above 3.3kv), relays and circuit breakers are employed to serve the desired function of automatic protective gear. The relays detect the fault and supply information to the circuit breaker which performs the function of circuit interruption.
Fertilizers And Chemicals Travancore Limited is the largest public sector undertaking in Kerala. It is located in Eloor, 14 Kms from the costal city of Kochi.
1947 - The very first plant of the factory - the only one in the world to use the wood gasification on a commercial scale to produce ammonia.
It is The Fertilisers And Chemicals Travancore Limited., popularly known as FACT, which set up the first large scale Nitrogenous factory in the country, as early as 1944, on the banks of Periyar at Udyogamandal, near the Cochin Port.
From a single product fertiliser plant of the forties, FACT has through the years grown into a large multi-product, multi-divisional corporation today - a legend of our times and triumph of the public sector.
FACT'S two fertiliser manufacturing divisions at Udyogamandal and Cochin together have so far produced and distributed millions of tonnes of fertiliser nutrients, which has helped farmers to produce over 50 million tonnes of food grains.
FACT's marketting division has a well-organised sales network, which ensures that even the fanner in the remotest village is fully benefitted through its agronomy and rural development services.
The rich fund of expertise, experience and skills gained over the years in manufacturing units of FACT were pooled together in the mid sixties to form two separate engineering divisions, FACT Engineering & Design Organisation(FEDO) & FACT Engineering Works(FEW). These two divisions between them cover the entire spectrum of consultancy and engineering services and have contributed a great deal to attain self-reliance in fertiliser and chemical technology in the country.
In 1990,FACT further diversified into the field of petrochemicals by setting up a Carprolactam unit. Today, FACT is on the threshold of further diversification and backward integration.
Starting commercial production in 1947 with a single ammonium sulphate plant having a production capacity of 10,000 tonnes of nitrogen, Udyogamandal division today comprises a variety of medium capacity plants using different process routes to produce a wide variety of fertilisers and chemicals - ammonia,sulphunc acid,phosphoric acid,ammonium sulphate,FACTAMFOS 20 - 20.
Set up in the late sixties at Ambalamedu, Cochin, this division has the distinction of being on of the largest fertilizer factories using sophisticated process technology totally designed and engineered by our own technocrats keeping foreign technical assistance to the minimum.
Commissioned in 1990, as a major diversification from our traditional field of fertilizers and allied chemicals, this division adds to FACT'S product mix, a versatile petrochemical, Caprolactum.
FACT has been a pace-setter in marketting, evolving a continuous and comprehensive package of communication and promotional programmers to increase the fertiliser consciousness among our fanners.
FACT Engineering & Design Organisation (FEDO) was established in 1965 to meet the emerging need for indigenous capabilities in vital areas of engineering, design and consultancy for establishing large, modern fertiliser plants.
FEDO has the unique advantage of having the back up support of the three manufacturing divisions of FACT. It offers multidimensional services from project and implimentation identification and evaluation to plant design, procurement, project and implimentation management, site supervision, commissioning and operating new plants, as well as revamping and modernising old ones.
Established in 1966, FACT Engineering Works was originally conceived as a unit to fabricate and erect equipment for fertiliser plants. Over the years, it developed capabilities in the manufacture of Class-I pressure vessels, heat exchangers, spiral-guided wet type multi lift gas holders, rail mounted LPG tank wagons
Research & Development
FACT R&D has won the Indian Chemical Manufacturers' Association Award twice for
1.Development of technology 2. Export of technology.
FACT has already invested Rs. 500 million in specially setting up pollution control plants at Udyogamandal, Cochin and Petrochemical Divisions, with a recurring expenditure of Rs. 120 million annually.
The management Development Centre equips FACT'S managers to meet the challenges of the future.Training schools at Udyogamandal and Ambalamedu conduct special programmes for employees with the help of inhouse as well as outside expe
Mother Industry Role
The setting up of FACT in Udyogamandal with adequate infrastructure facilities built up along with it, in course of years attracted a number of other chemical industries to its neighbourhood. These include the Hindustan Insecticides Ltd., The Indian Rare Earths Ltd., Travancore Cochin Chemicals Ltd
Take off to the 21st century
Over four decades ago, FACT was in the forefront of the drive to modernise Indian agriculture & industry. Today, FACT is all set for take off into the 21st century. A century of great opportunities. Which is as things shouldbe. Or else, what are pioneers for
A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.
The relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. Electrical quantities which may change under fault conditions are voltage, current, frequency and phase angle. Through the changes in one or more of these quantities, the faults signal their presence, type and location to the protective relays. Having detected the fault, the relay operates to close the trip circuit of the breaker. This results in the opening of the breaker and disconnection of the faulty circuit.
The relay circuit connections can be divided into three parts viz.
1. First part is the primary winding of a current transformer (C.T) which is
connected in series with the line to be protected.
2. Second part consists of secondary winding of C.T and the relay
3. Third part is the tripping circuit which may be either AC or DC. It consists of a
source of supply, the trip coil of the- circuit breaker and the relay stationary contacts.
When a short circuits occurs at point on the transmission line, the current flowing in the line increases to an enormous value. This results in a heavy current flow through the relay coil, causing the relay to operate by closing its contacts. This in turn closes the trip circuit of the breaker, making the circuit breaker open and isolating the faulty section from the rest of the system. In this way, the relay ensures the safety of the circuit equipment from damage and normal working of the healthy portion of the system.
FUNDAMENTAL REQUIREMENTS OF PROTECTIVE RELAYING
The principle function of protective relaying is to cause the prompt removal from service of any element of the power system when it starts to operate in abnormal manner or interface with the effective operation of the rest of the system. In order the protective relay system may perform this function satisfactorily; it should have the following qualities:
(i) Selectivity: It is the ability of the protective system to select correctly that part of system in trouble and disconnect the faulty part without disturbing the rest of the system.
In order to provide selectivity to the system, it is a usual practice to divide the entire system into several protection zones. When a fault occurs in a given zone, then only the circuit breakers within that zone will be operated. This will isolate only the faulty circuit or apparatus, leaving the healthy circuits intact.
f he Single Line Diagram of a I'ortion of a lypical I'OWL'I: System
The system can be divided into the following protection zones:
(b) Low-tension switchgear
(d) High-tension switchgear
(e) Transmission lines
(ii) Speed : The relay system should disconnect the faulty section as fast as possible for the following reasons :
(a) Electrical apparatus may be damaged if they are made to carry the fault currents for a long time.
(b) A failure on the system leads to a great reduction in the system voltage. If the faulty section is not disconnected quickly, then the low voltage created by the fault may shutdown consumers' motors and the generators on the system may become unstable.
© The high speed relay system decreases the possibility of development of one type of fault into the other more severe type.
fiii) Sensitivity: It is the ability of the relay system to operate with low value of actuating quantity.
Sensitivity of a relay is the function of the volt-amperes input to the coil of the relay necessary to cause its operation. The smaller the volt- ampere input required to cause relay operation, the more sensitive is the relay. Thus, a 1 VA relay is more sensitive than a 3 VA relay. It is desirable that relay system should be sensitive so that it operates with low values of volt- ampere input.
(iv) Reliability: It is the ability of the relay system to operate under the pre-determined conditions. Without reliability, the protection would be rendered largely ineffective and could even become a liability.
(v) Simplicity: The relaying system should be simple so that it can be easily maintained. Reliability is closely related to simplicity. The simpler the protection scheme, the greater will be its reliability
(vi) Economy: The most important factor in the choice of a particular protection scheme is the economic aspect. Sometimes it is economically unjustified to use an ideal scheme of protection and a compromise method has to be adopted. As a rule, the protective gear should not cost more than 5% of total cost. However, when the apparatus to be protected is of utmost importance (e.g. generator, main transmission line etc.),
economic considerations are often subordinated to reliability.
OPERATING PRINCIPLE OF INDUCTION RELAYS
The two ac fluxes 0i and 02 differing in phase by an angle a induce emf s' in the disc and cause the circulation of eddy currents i2 and h respectively. These current lag behind their respective fluxes by 90 .
Let 0!= d>lmax sinoot
Where <t>i and 4>2 are the instantaneous values of fluxes and <t>2 leads <t>i by an angle a. Assuming that the paths in which the rotor currents flow have negligible self-inductance, the rotor currents will be in phase with their voltages.
ha dOi/dt a d/dt(0lm3X sincot)
ct ^lmax cosoot and l2 a d02 /dt a 02m,lx cos(wt+ a) Now, F i a 0i i2 and F2 a 02'i
Net force F at the instant considered is FaF2- Ft
a 02h - Qih
a 02max sin(wt+a) 0lmax cosoot - 4>lm3X sintot 4>2m3X cos(urt+ a)
a 0] ct>2 sina
Where and O2 are the r.m.s. values of the fluxes. The following points may be noted from exp.(i):
(a) The greater the phase angle between the fluxes, the
greater is the net force applied to the disc. Obviously, the
maximum force will be produced when the two fluxes are
90 out of phase.
(b) The net force is the same at every instant.
© The direction of net force and hence the direction of
motion of the disc depends upon which flux is leading.
I BASIC RELAYS
Most of the relays in service on electrical power system today Ã‚Â¦re of electro-mechanical type. They work on the following two operating principles:
(i) Electromagnetic attraction
(ii) Electromagnetic induction
ELECTROMAGNETIC ATTRACTION RELAYS
Electromagnetic attraction relays operate by virtue of an armature being attracted to the poles of an electromagnet or a plunger being drawn into a solenoid. Such relays may be actuated by DC or AC quantities. The important types of electromagnetic attraction relays are:
(i) Attracted armature type relay: It consists of a laminated electromagnet M carrying a coil C and a pivoted laminated armature. The armature is balanced by a counterweight and carries a pair of spring contact fingers at its free end. Under normal operating conditions, the current through the relay coil C is counterweight
(ii) Solenoid type relay: It consists of a solenoid and movable iron plunger.
Under normal operating conditions, the current through the relay coil C is such that it holds the plunger by gravity or spring. However on occurrence of a fault, the current through the relay coil becomes more than the pickup value, causing the plunger to be attracted to thesolenoid. The upward movement of the plunger closes the trip circuit, thus opening the circuit breaker and disconnecting the faulty circuit.
(iii) Balanced beam type relay: It consists of an iron armature fastened
to a balance beam. Under normal operating conditions, the current through the relay coil is such that the beam is held in the horizontal position by the spring. However, when a fault occurs, the current through the relay coil becomes greater than the pickup value and the beam is attracted to close the trip circuit. This causes the opening of the circuit breaker to isolate the faulty circuit.
Electromagnetic induction relays operate on the principle of induction motor and are widely used for protecting relaying purposes involving AC quantities. They are not used with DC quantities owing to the principle of operation. An induction relay essentially consists of a pivoted aluminum disc placed in two alternating magnetic fields of the same frequency but displaced in time and space. The torque is produced in the disc by the interaction of one of the magnetic fields with the currents induced in the disc by the other.
The following three types of structures are commonly used for obtaining the phase difference in the fluxes and hence the operating torque in induction relays:
1] Shaded-pole structure
2) Watt-hour-meter or double winding structure
3) Induction cup structure
(i) Shaded pole structure: It consists of a pivoted aluminium disc free to
rotate in the air gap of an electromagnet. One half of each pole of the
magnet is surrounded by a copper band
known as shading ring.The alternating _Å¾Å¾^__|
flux Os in the shaded portion of the I
poles will, owing to the reaction of the
current induced in the ring, lag behind w : # \i->**-^ '* the flux Ou in the unshaded portion by an angle a. These two AC flux differing in phase will produce the necessary torque to rotate the disc.
(ii)Watt-hour-meter structure: It consists of a pivoted aluminium disc arranged to rotate freely between the poles of the electromagnets. The upper electromagnet carries two windings; the primary and the secondary. The primary winding carries the current h while the secondary winding is connected to the winding of the lower magnet. The primary current induces e.m.f in the secondary and so circulates a current h in it. The flux O2 induced in the lower magnet by the current in the secondary winding of the upper magnet will lag behind <t>i by an angle a. The two fluxes <Pi and <P> differing in phase by a will produce a driving torque on the disc proportional to Oi02 sin a.
^/0ZtJ To (rip tiro*
(iii) Induction cup structure: It most closely resembles an induction motor, except that the rotor iron is stationary,only the rotor conductor portion being free to rotate. The moving element is a hollow cylindrical rotor which turns on it axis. The rotating field is produced by two pairs of coils wound on four poles. The rotating field induces currents in the cup to provide the necessary driving torque. If <t>i and <t>2 represent the fluxes produced by the respective pairs of poles, then torque produced is proportional to 0i<t>2 sin a where a is the phase difference between two fluxes. A control spring and the back stop
for closing of the contacts carried on an arm are attached to the spindle of the cup to prevent the continuous rotation.
Induction cup structures are more efficient torque producers than the shaded-pole or the watt-hour-meter structures. Therefore, this type of relay has very high speed and may have an operating time less than 0.1 second.
An important characteristic of relay is its time of operation. By 'the time of operation' is meant length of the time from the instant when the actuating element is energized to the instant when the relay contacts are closed. Sometimes it is desirable and necessary to control the operating time of a relay.
(i) Instantaneous relay: An instantaneous relay is one in which no
intentional time delay is provided. In this case,the relay contacts
are closed immediately after current in the relay coil exceeds
the minimumcalibrated value.
The fig shows an instantaneous Q â€â€ \
solenoid type of relay. Although there will be a short time interval between the instant of pickup and the closing of relay contacts, no intentional time delay has been added. The instantaneous relays have operating time less than 0.1 second.
(ii)Inverse-time relay: An inverse-time relay is one in which the
operating time is approximately inversely proportional to the magnitude of the actuating quantity. At values of current less than pickup, the relay never operates. At higher values, the time of operation of the relay decreases steadily with the increase of current.
(iii) Definite time lag relay: In this type of relay, there is a definite
time elapse between the instant of pickup and closing of relay contacts. This particular time setting is independent of the amount of current through the relay coil; being the same for all values of current in excess of the pickup value.
1. Pick-up current: It is the minimum current in the relay coil at
which the relay starts to operate. So long as the current in the relay is less
than the pick-up value, the relay does not operate and the breaker
controlled by it remains in the closed position. However, when the relay
coil current is equal to or greater than the pickup value, the relay
operates to energize the trip coil which opens the circuit breaker.
2. Current setting: It is often desirable to adjust the pick-up
current to any required value. This is known as current setting and is
usually achieved by the use of tapings on the relay operating coil. The
taps are brought out to a plug bridge. The plug bridge permits to alter the
number of turns on the relay coil. This changes the torque on the disc and
hence the time of operation of the relay. The values assigned to each tap
are expressed in terms of percentage full-load rating of C.T with which the
relay is associated and represents the value above which the disc
commences to rotate and finally closes the trip circuit.
Pick-up current = Rated secondary current of C.T x Current
For example, suppose that an over current relay having setting of 125% is connected to a supply circuit through a current transformer of 400/5. The rated secondary current of C.T is 5 amperes. Therefore, the pick-up value will be 25% more than 5 A i.e. 5 xl.25 =6.25 A. It means that with above current setting, the relay will actually operate for a relay coil current equal to or greater than 6.25 A.
The current plug setting usually range from 50% to 200% in steps of 25% for over current relays and 10% to 70% in steps of 10% for earth leakage relays. The desired current setting is obtained by inserting a plug between the jaws of a bridge type socket at the tap value required
3. Plug-setting multiplier (P.S.M) : It is the ratio of fault current in relay coil to the pick-up current i.e.
P.S.M = Fault current in the relay coil Pick-up current
= Fault current in the relay coil
Rated secondary current of C.T x Current setting
For example, suppose that a relay is connected to a 400/5 current transformer and set at 150% with a primary fault current of 2400 A, the plug-setting multiplier can be calculated as under:
Pick-up value = Rated secondary current of C.T x Current setting
= 5x1.5 = 7.5 A
Fault current in relay coil = 2400 x 5/400 =30 A
P.S.M =30/7.5 = 4
4. Time-setting multiplier: A relay is generally provided with control to adjust the time of operation. This adjustment is known as time setting multiplier. The time- setting dial is calibrated from 0 to 1 in steps of 0.5 sec. These figures are multipliers to be used to convert the time derived from time/P.S.M curve into the actual operatingtime. Thus if the time setting is 0.1 and the time obtained from the time/P.S.M curve is 3 seconds,tfien the actual relay operating time = 3 x 0.1 = 0.3 second. For instance, in an induction relay, the time of operation is controlled by adjusting the amount of travel of the disc from its reset position to its pickup position. This is achieved by the adjustment of the position of a movable V backstop which controls the travel of the disc and thereby varies the time in which the relay will close its contacts for given "\ values of fault current. The actual time of operation is calculated by multiplying the time setting multiplier with the time obtained from time/P.S.M curve of the relay.
TSM /PSM CURVE
3 SEC RELAY
Basic construction of static protective relay
Protective relays are analogue-binary signal converters with measuring functions. The variable such as current, voltage, phase angle or frequency are derived values obtained by differentiation, integration or the arithmetic operations, appears always as analogue signals at the input of the measuring unit. The output will always have binary signal, i.e. either an open (or OFF) signal if the relay is not to trip or a close (or NO) signal if the relay is to trip these output signals can therefore be easily evaluated by subsequent control elements requiring very little technical effort. Each protective relay is build up of individual elements in accordance with the basic block diagram shown in the fig.
1. Measuring Circuit
2. Measuring Signals
3. Converter Element
4. Measuring Element
5. Output Element 6.Output Signal
7.Control led Element
8. Feed Element
9. Aux Voltage Source
i O. Measuring Circuit Supply
The signal which occur in the analogue and therefore in the continuously variable form from the measuring circuit (C.T and/or V.T) are first fed to the converter unit in the protective relay. This converts the measured signals so that they can be processed by measuring element will be operated when the input signal reaches a certain value-providing a close signal at its output. The output element amplifies this binary but weak signal and transfers it to one or more controlled elements. The controlled element carry out the final switching function as opening of circuit breakers, etc. Power is obtained either from an auxiliary voltage source or from the measuring circuit itself.
This element contains chiefly the matching transformers to obtain the required signal level. The rest of the construction depends on whether one or two or more inputs are to be handled by the relay.
Relays for one quantity are supplied with only one electrical quantity, e.g. current or voltage. After suitable transformation by the tnatching transformers, this quantity is fed to diode bridges at whose output it appears as a d.c variable with ripple. Through setting network consisting of fixed and variable resistors, clipping diodes, etc. the measured value of the quantity is fed into a harmonic filter since the subsequent measuring element deals only with d.c variables. Sometimes smoothing filters are used to eliminate ripples, but in high speed relays such filters cannot be used.
This is an analogue-binary signal converter with measuring functions. In the simplest form it consists of the Schmitt trigger circuit as the basic circuit. The Schmitt trigger circuit can be compared to an extremely fast polarized d.c relay and act as a level detector. Transistors are used in common emitter connection giving high input resistance and large current gain. The level detector gives a step output when the input voltage exceeds a specific level.
This element amplifies the output signal from the ^measuring element, multiplies it, may combine it with certain other signals and also introduce a delay if necessary. Since it has to process only binary signals, this Ineed not be a precision element. It may thus take the form of an auxiliary relays or contactors. These provide potential separation between controlling and Controlled circuit. It may also take the form of a bistable or monostable bnultivibrator circuit and if requited modulated by logic circuits like AND, OR, NOR pr timing element. Where large power are involved, e.g., operating trip coils of fcircuit breakers, silicon controlled rectifiers (SCR) are used after the logic element.
The function of this element is to supply the power is lo obtained either from a built-in auxiliary supply (nickel cadmium rechargeable tells) or from station battery. In many cases it is derived from the measuring circuit itself .In all cases, the feed element should supply a stabilized voltage to ihe static circuit, so that the measuring accuracy is not impaired. In the initial stages of development, nickel cadmium rechargeable cells(commonly known as specially in the U.K., but experience has shown that their reliability is poor. They !are being given up at present and in their place station batteries with suitable laps at the appropriate are being preferred. In the case of several type of relays the supply is derived from the current and voltage transformers themselves as rnentioned above with the refinement that the power supply to the relay is Ã‚Â¦witched on only in the case of a fault being detected by the suitable fault Betector.
COMPARISON OF STATIC RELAYS WITH
The conventional electromagnetic relays are robust and quite reliable but
are required to work under differential forces under fault conditions. This leads to
delicate setting small contact gaps, special bearing systems, special plugs
assemblies and several manufacturing difficulties. These require current and
voltage transformers with high burden and are bulky in size also.
The advantages of static relays are
1. The moving parts and contacts are greatly reduced: the only
moving parts are those of the actual tripping circuits.
2. The volt-ampere burden on the instrument transformer is
reduced to a very low value which permits the use of linear
coupler in place of current transformer, making it cheaper as
well as solving the difficulty of the DC component of the fault
3. A high degree of accuracy.
4. A high speed of operation.
5. Low power consumption.
6. Re-setting time and over shoots can be reduced.
7. Static relays are very compact.
8. Static relays have superior characteristics and accuracy.
9. Simplified testing and servicing is possible.
10 Several functions can be accommodated in a static relay.
l. Auxiliary DC supply is needed constituting a constant loss of
2. Reliability is unpredictable.
3. Susceptible to voltage transients.
i 4- It is not very robust in construction.
5. Easily affected by surrounding interference.
Applications of static relays
Static relays are extensively used in ultra high speed protection schemes rf EHV AC lines utilizing distance protection. Also the main element of the [differential relay in equipment protection is also, in some cases, a static relay. Static relays are also used in over current and earth fault protection schemes.
DIFFERENT STATIC RELAYS
1. STATIC OVER CURRENT RELAY
2. STATIC TIME OVERCURRENT REtAY
3. STATIC INSTANTANEOUS OVERCURRENT REtAY
4. DIRECTIONAL STATIC OVERCURRENT RELAY
5. STATIC DIFFERENTIAL RELAY
6. STATIC DISTANCE RELAY
ELECTRONIC CIRCUITS COMMENLY USED IN STATIC
1. Auxiliary DC voltage supply
Usually a DC to DC converter is used if rating 200 v DC to 50 v DC. The
converters of sufficient rating to supply the DC power requirement of several
2. Full wave bridge rectifier.
A 4 diode bridge is usually used for filtering the input relaying quantity
from AC to DC ripples are made very low by adding smoothing circuits.
3. Smoothing circuits
This comprises of RC or RL circuits in order to smoothen the output of the
4. Voltage stabilization circuitry
Usually Zener diode is used for tins purpose . it stabilizes the output
voltage of the rectifier over a wide range oOf current.
5. Time delay circuits .
These circuits are used for introducing very short delay in the protection
systems. Mostly RC circuits are used for this purpose.
6. Frequency filters
Band pass or band stop filters are used for either passing or stopping a
certain frequency band. Resonating circuits are commonly used for this
purpose. In costly relays operational amplifiers are also used as frequency
7. Component filters
Sequence current filters may be used in the static relays to filter the
respective sequence filtering quantities. For a negative phase sequence
static relay a negative phase sequence filter is used in order to filter the
negative phase sequence component of current from the input signal.
8. Saturable reactors
The new magnetic material under the general name saturable magnetic core has square loop B-H characteristics. These materials have saturable flux density and low coercive force. The additional advantage of the saturable reactor is that they do not require external power supply for operation like transistors.
9. Output device
The actual tripping of a static relay is achieved with the help of an SCR the trip coil is connected in series with the SCR. The charge on capacitor C is discharged through the resistor R3 into the SCR gate when the transistor is made conductive by the negative pulse.
10. Phase discrimination using transistors.
The phases of 2 electrical quantities can be compared by direct coincidence method. This method makes use of the fact that when the two voltages X and Y differ in phase by an angle o, they are simultaneously positive for an angle (180-$) this angle is compared with a constant angle a and a change in output is produced when 180-0 >=a
11. Amplitude comparator
The main purpose of amplitude comparator is to provide direction and distance protection. It may work on either voltage balance or circulating current principle.
12. Phase comparator
It is circulating current bridge whose output current is equal to the smaller of the two current inputs.
13. Level detectors
For providing overload protection a certain level must be fixed at which the relay should be set to operate. When ever the bias crosses a certain limit, high current flow in the relay causing it to trip.
Maintenance and Field Testing of Relays
Personnel- In the handling of static relays, unlike electro magnetic relays, the operating personnel should be trained in the basic knowledge of electro magnetic components and circuits used in static relays. Repair of circuits and replacement of defective discrete components should be avoided-instead the complete module found defective should be Ã‚Â¦replaced by a new one like the replacements in digital computers.
Since solid state components are susceptible to temperature changes, air conditioning of the rooms is very essential. Also these personnel who are required to maintain static relays should see that their hands are dry. And any traces of perspiration should not be allowed go into the static relay cases. Personnel should be trained in the colour coding of resistors and capacitors so that they are able to read the correct values in the circuit
Static relays require very little maintenance compared to e.m. type relays.
Tests : Installation and Commissioning
Installation and commissioning on relays are necessary to ensure that the protective relays are in proper condition subsequent to mechanical handling during transport, creation etc., that the electrical circuits associated with the relays are in healthy condition for the application and the relay action results in the operation of the relevant circuit breakers, alarms etc. Some of the tests are to be repeated at periodic intervals to ensure continued and proper functioning of relays and circuits.
Preliminary Examinations and Checks
Relays should be examined for the following:
a) Damage like dents, broken glass covers, loose parts, etc.
b) Iron filings in the air gaps of magnets.
c) Moving parts getting jammed, etc.
d) Rating, range, auxiliary supply voltages as per drawings or specifications.
e) Mounting as per manufacturers specifications.
f) Contact surfaces of output relay contacts should not be handled carelessly.
Before relays are mounted and wired up, acceptance tests are to be performed to check
For correctness of relay range, pick-up values of characteristics, etc. against the specification. This will also eliminate any manufacturing defects like short circuits, open circuits, etc. The tests are conveniently made in the test laboratory. After acceptance tests, a few commissioning checks are necessary to establish healthiness of associated Electrical circuits.
List of electrical tests:
a) Insulation resistance
b) Pick-up value
c) Drop-out value
e) Polarity check
f) Directional sensitivity
B) Stray operation check
h) Slope characteristic
i) Closed characteristic
J) Flag indication and auxiliary contactor
Precommissioning tests- The following tests may be supplemented or modified as per Â¢manufactures instruction manuals. The following are some of the relays ad may not include all types:
lest on Relay Type
In addition to tests on individual's relays, it is essential to check out the complete protection circuits
Fest before Energization
'The aim of these tests is to check continuity of circuits, loose terminal connections, open J circuits,, connection to wrong phase/incorrect panels/ incorrect circuits, continuity at all disconnecting links or junction boxes. After these tests are completed, no further disconnection of wiring is to be prepared.
a) Primary injection- The test involves application of current to primary terminals of protected equipments. Current should be applied up to pick-up levels of the relays-this is usually possible for current transformers. For application of voltage to voltage transformers sufficient safety precautions are to be taken.
b) Secondary injection- When primary injection is not possible, this is the alternative. Currents/voltages are applied to the secondary terminals of current/voltage transformers, they should be segregated and care exercised to see that they are not overexcited from the secondary side.
Tor efficient working, these tests should be done with proper communication between the person nearthe protected equipment and the person nearthe relay location. One fchase at a time, where possible, may be checked.
Ã‚Â¦ ests after Energizing of Primary Equipment
Currents circuits of relay, specially differential relays for generators-transformer units can be checked by running the generator with low excitation and with a short circuit placed suitably to stimulate either a through fault or an internal fault. The through and spill -fever currents are measured at the relay terminals.
Test when equipments is energized and in lord service Test when equipments is energized and in lord service
a) Currents, voltage and phase angle measurement are made at relay terminals as a final check on the correctness of all circuits.
b) The out put relay position for a directional phase relay should conform with the direction of power flow in the primary circuit (provide sufficient current is available to actuate the relay).
c) Tests on directional ground relays.
Staged fault tests
The protection circuits can also be tested by closing a breaker on to a fault. Adequate oscillographic recording equipment should be available to study the specific features of interest. Such tests are however undertaken very occasionally only when a relatively new (type of protection is being tried out or when there is a special investigation on relay ^behaviour to be carried out.
Periodic check of relays at its actual settings is needed to ensure the following :
a) Healthiness of relays.
b) Operation of relay flags or visual indicators.
c) Tripping of associated breakers.
d) Operation of annunciators.
Frequency of Tests
This depends on various factors including the following:
a) Environment, i.e. temperature, humidity and pollution in the relay room and, whether or not, the relay room is air conditioned.
b) Whether it is primary or back-up protection.
c) Importance and size of the equipment being protected.
d) Effect of maloperation of the protective circuits.
As a rough guide, when relay are operating in clean dry surroundings a frequency of once ii year is sufficient. Polluted area may require checks twice a year where as back-up protection on less important circuits could be tested once in two years-this, however, is modified as per manufactures' advice.
Secondary injection tests are made at existing settings. The tests are the same as electrical tests except for polarity check which can be omitted and closed characteristics Checked at few points on the characteristics.
When an outage of the circuit is not possible the relay characteristics can be checked by using the drawout facility to replace the relay module by a spare tested one and to test all the modules separately. However, since this method does not check the control circuits connected to the relay, the full check mentioned above by primary secondary injection from current/voltage transformer terminals to trip the breaker is to be preferred where possible.
To ensure continuity of circuits, measurement of current or voltage burdens, V.T. Voltage, D.C. Voltage, all at relay terminals may be carried out.
In addition to periodic tests, it is usually advisable to make a simple operational check for testing the healthiness of d.c. flag indications, operation of associated trip relays and breakers, without any attempt to check the characteristics.
These are simple checks which can be made by the operating staff at the station and do not require special instruments.
Frequency of Checks: Once in three months is adequate. Where the circuit cannot be tripped as in the case of thermal units, which run for extended periods without outage, the frequency will be less. In such cases a more thorough check is to be done when the outage is possible.
Tests: Secondary injection test on relays from the panel ends-120% of the tap value of operating quantity is applied to operate the relay output contacts. Operations of Â¢such contacts by hand is not advised as it may damage the same. Where there are o output contacts, the operation of the output circuit should be checked by energization as Ibove.
Relays are delicate instruments and require careful handling and maintenance if reliable service is expected of them. Maintenance should include general inspection of the physical condition of all parts at regular intervals.
a) Relays are provided with dust proof covers and before a cover is removed, the case should be carefully dusted.
b) Relay interior should be free from dust, dirt, iron particles, etc. Dust and dirt should be carefully wiped off by a soft squirrel hair brush; mechanical blowing or blowing by mouth is not recommended.
c) The internal wiring, coil ends, printed circuits, integrated circuits (IC) should be examined for any sulphation or corrosion. Excessive heat may age insulation.
d) Contact surfaces should never be touched by hand. Polluted atmosphere usually causes black discolouration of contact surface although it may not affect contact operation. A special contact burnishing tool (fine flexible steel strip etched on both sides with fine lines to serve as a super fine file) is used to clean the contact surfaces. Both contacts are pressed together and the file passed in between. The file surface is then cleaned with a clean piece of paper using again. Abrasives should not be used as the grit may remain embedded on the contact surfaces. Contact alignment and gaps should be checked against specifications.
e) In case of electromagnetic type of relays, disc, cup unit axles should have slight play against the bearings. Bearings (guide and support) should be taken out carefully and jewel surface examined with a fine needle for cracks. For all such relays no lubricant is to be used for moving parts. The pivots, jewels should be wiped clean with a watch repairer's cleaning stick (soft weed stick).
f) Relay flags/targets should operate freely without friction and also be reset freely.
g) The finger contacts on draw out modules must be carefully examined for signs of sulphation and tracking between terminals and should be cleaned. The maintenance should be done every time the tests are done on the relays.
1) Pick-up and drop out values (tests b and c) Instantaneous relays:
Current/voltage is applied suddenly in increasing steps, till the relay operates-this [gives pick-up value. Then the operating quantity is increased by 20% and reduced sharply kill relay resets-this gives drop-out value. Near the pick-up value, the relay operation ishould not be oscillatory.
Time delay relays:
Current/voltage is applied in steps and the operating value noted. Reset value is also [noted. In each case sufficient time is to be allowed to see if the relay operates.
2) Timing tests: (Test d).
Time of operation is noted by a digital timer-started at the same time as the operating quantity and stopped by the relay output circuit operation.
3) Polarity check and directional sensitivity: (Tests e and f).
For directional relays the correct relative polarity of current and voltage should give operating torque. One of the quantities should be reversed to check non-operation or restraint.
In-phase values of current and voltage are applied and minimum value to cause [operation checked.
Using a phase-shifting transformer, the characteristic angle (or angle of maximum torque in electromagnetic relays) and the minimum value of current required for operation should be noted. This gives the directional sensitivity on directional relays. During this tests, the operating and reset zones are also noted.
L 4) Stray operation test: (Test g).
a) Current alone is applied up to 10-15 times tap value and voltage terminals kept shorted.
b) Rated voltage is then applied and current terminals shorted.
The relay should not operate in the above cases. Manufacturer's recommendations Ii respect of actual values of currents and voltages should be followed.
5) Slope characteristics: (Test h).
Restraint current of two to four times rated tap is applied suddenly and the operating current applied in increasing steps till the relay operates. For every reading of "slope", (two readings are taken with restraint terminals connected one way and also .interchanged to check for any stray effects. The two readings of slope should be identical.
6) Closed characteristics: (Test j).
Voltage is applied through a phase shifter and readings of volts and amperes taken at various phase angles and impedance values computed therefrom. These values are checked with manufacturer's characteristics.
7) Primary injection tests:
These are intended to prove the correctness of C.T. connections and behaviour. Precautions necessary:
a) A one line diagram showing the C.T. location and the test lead connection for each test should be made.
b) Primary equipment must be dead, i.e. isolated from the H.V. system.
c) Tripping of adjacent circuit due to flow of current through C.T.'s others than those under test should be prevented.
d) Earth fault relays and auxiliary resistors are usually short-time rated and suitable precautions taken against this.
e) Current should be switched on at a low value and increased slowly to see if any C.T.'s are open circuited. If test results are different from those expected, the current should be reduced, switched off and the matter investigated.
f) Test connections should be good electrically and load sizes should be reduced to keep circuit impedance as low as possible as high currents are to be passed at low voltage.
| 7.1) Injection of cur-rent into a C. T. to test a relay on phase A:
I If sufficient primary current to operate relay is not available, the current magnitude [obtained is measured by ammeter and recorded.
I 7.2) Injection of current into a set of C.T.'s forming a circulating scheme:
I Current is injected through the current transformers on phase A of the feeders-there will be no current in the relay under the test.
f 7.3) Restricted E/F protection:
f 7.4) Primary voltage injection:
I H.V. test site should be cordoned off. V.T. rating should not be exceeded. Magnitude of the H.V. applied should be available from the test set.
8) Secondary injection tests:
Test current source should have as low L/R ratio as possible to keep time constant of test circuit low. Current magnitude is measured nearest to the relay. Current is injected at the C.T. secondary studs. Sufficient current to operate the relay ca be passed-all auxiliary d.c. circuits of the relay should be in service. Flag and C.B. operations should be Initiated and noted.
9) Tests after energizing primary eguipment:
They are basically primary injection tests. Generator itself is used as a test source for proving transformer, generator differential protection schemes.
9.1) Three phase shorts are put at location 1 and generator run up without excitation. By carefully controlling the excitation, generator current is built up gradually. At this stage, generator differential relay currents are checked. There should be no spill currents. The generator current is increased in steps, up to rated value. During the test the unit differential relay should be kept inoperative.
[ The set is shut down, short removed from location 1 and put on location 2. Test when repeated A voltmeter is connected across the relay and the generator open circuit [voltage slowly built up to reach the pick-up value of the generator ground relay. The voltage is then brought down and the set shut down.
10) Tests when equipment is in load service:
a)Phase angle measurement:-A phase angle meter is used to check (i) phase
difference between relay currents and a fixed reference potential, say, the bus voltage transformer and (ii) phase difference between relay current and potentials applied to the relay, say, for directional relays.
It is advisable to do the check on all newly commissioned relay circuits and the results recorded-relay current is also measured by an ammeter
b) Directional ground relays:-Fuse on the A phase voltage transformer is removed and primary links shorted. C phase C.T. is shorted and disconnected. When the feeder is taken in, service current phases in the relay and a residual voltage gets applied. Depending on the phase angle of the load circuit, the directional relay will operate or restrain. For this test, a phase angle meter is also required so that the actual vector position of load current cold be obtained. In this case the directional relay will operate for load current phase angles between 60Ã‚Â° lead and 90Ã‚Â° lag.
A DIGITAL PROTECTIVE RELAY utilizes a microcontroller with software based protection algorithms for the detection of electrical faults.
DESCRIPTION AND DEFINITION
The digital protective relay, also called a numeric relay by some manufacturers and resources, refers to a protective relay that uses an advanced microprocessor to analyze power system voltages and currents for the purpose of detection of faults in an electric power system. There are gray areas on what constitutes a digital/numeric relay, but most engineers will recognize the design as having the Â¢majority of these attributes:
The relay applies A/D (analog/digital) conversion processes to the ^incoming voltages and currents.
The relay analyzes the A/D converter output to extract, as a minimum, magnitude of the incoming quantity, most commonly using Fourier transform concepts (RMS and some form of averaging are used in basic products). Further, the Fourier Transform is commonly used to extract the signal's phase angle relative to some reference, except in the most basic applications.
The relay is capable of applying advanced logic. It is capable of analyzing whether the relay should trip or restrain from tripping based on current and/or voltage magnitude (and angle in some applications), complex parameters set by the user, relay contact inputs, and in some applicatons, the timing and order of event sequences.
The logic is user-configurable at a level well beyond simply changing front panel switches or moving of jumpers on a circuit board.
The relay has some form of advanced event recording. The event recording would include some means for the user to see the timing of key logic decisions, belay I/O (input/output) changes, and see in an oscillographic fashion at least the fundamental frequency component of the incoming AC waveform.
The relay has an extensive collection of settings, beyond what can be 'entered via front panel knobs and dials, and these settings are transferred to the relay via an interface with a PC (personal computer), and this same PC interface is used to collect event reports from the relay.
As a point of comparison, an electromechanical relay converts the [voltages and currents to magnetic and electric forces and torques that press against spring tensions in the relay. The tension of the spring and taps on the electromagnetic coils in the relay are the main processes by which a user sets such a relay. In a solid state j relay, the incoming voltage and current waveforms stay within analog circuits that use transformers, resistor, capacitors, inductors, transistors, op amps, comparators, etc. The Incoming waveform is not recorded or sent into an A/D circuit. The analog values are (compared to settings made by the user via potentiometers in the relay, and in some case, taps on transformers.
In some solid state relays, a relatively simple microprocessor does some of the relay logic, but the logic is relatively fixed and simple. For instance, in some time jbvercurrent solid state relays, the incoming AC current is first converted into a small signal AC value, then the AC is fed into a rectifier and filter that converts the AC to a DC value proportionate to the AC waveform. An op-amp and comparator is used to create a DC that rises when a tripping point is reached. Then a relatively simple microprocessor does a slow speed A/D conversion of the DC signal, integrates the results to create the time-overcurrent curve response, and trips when the integration rises above a setpoint. iThough this relay has a microprocessor, it lacks the attributes of a digital/numeric relay, knd hence the term "microprocessor relay" is not a clear term.
The digital/numeric relay was introduced in the early 1980's, with S.E.L. making some of the early market advances in the arena, but the arena has become [crowded today with many manufacturers. In transmission line and generator protection, by the mid 1990's the digital relay had nearly replaced the solid state and electromechanical relay in new construction. In distribution applications, the replacement by the digital relay proceeded a bit more slowly. While the great majority of feeder relays in new applications are digital, the solid state relay still sees some use in distribution systems where simplicity of the application allows for simpler relays, and allows one to avoid the complexity digital relays, which is of benefit to some users.
Low voltage and low current signals (i.e., at the secondary of a VT and CT) are brought into a low pass filter that removes frequency content above about L./3 of the sampling frequency (a relay A/D converter needs to sample faster than 2x per cycle of the highest frequency that it is to monitor). The AC signal is then sampled by the relay's analog to digital converter at anywhere from about 4 to 64 (varies by relay) jsamples per power system cycle. In some relays, the entire sampled data is kept for Ã‚Â¦oscillographic records, but in the relay, only the fundamental component is needed for rnost protection algorithms, unless a high speed algorithm is used that uses subcycle data to monitor for fast changing issues. The sampled data is then passed through a low pass pjter that numerically removes the frequency content that is above the fundamental frequency of interest (i.e., nominal system frequency), and uses Fourier transform algorithms to extract the fundamental frequency magnitude and angle. Next the microprocessor passes the data into a set of protection algorithms, which are a set of logic equations in part designed by the protection engineer, and in part designed by the relay manufacturer, that monitor for abnormal conditions that indicate a fault. If a fault condition is detected, output contacts operate to trip the associated circuit breaker(s).
PROTECTIVE ELEMENT TYPE
Protective Elements refer to the overall logic surrounding the electrical condition that is being monitored. For instance, a differential element refers to the logic required to monitor two (or more) currents, find their difference, and trip if the difference is beyond certain parameters. The term element and function are quite interchangeable in many instances.
For simplicity on one-lines, the element/function is usually identified by what is referred to as an ANSI device number, and hence there are three terms (element, function, device number) in use for approximately the same concept. In the era of electromechanical and solid state relays,
any one relay could implement only one or two protective elements/functions, so a complete protection system may have many relays on its panel. In a numeric relay, many functions/elements are implemented by the microprocessor programming. Any one numeric relay may implement one or all of these device numbers/functions/elements.
A relatively complete listing of device numbers is found at the site ANSI Device Numbers. A summary of some common device numbers seen in digital relays is:
11 - Impedance (21G implies ground impedance)
27 - Under Voltage (27LL = line to line, 27LN = line to neutral/ground)
32 - Directional Power Element
46 - Negative sequence current
17 - Negative sequence voltage
50 - Instantaneous OverCurrent (subscript N or G implies Ground)
51 - Inverse Time Overcurrent (subscript N or G implies Ground)
59 - Over Voltage (597LL = line to line, 59LN = line to neutral/ground) 67 - Directional Over Current (typically controls a 50/51 element) 81 - Under/Over Frequency
Ã‚Â§7 - Current Differential (87L=transmission line diff; 87T=transformer diff; 87G=generator diff)
There are many more than listed here. This especially becomes true when one includes relays manufactured for niche or regional markets, and manufactures that offer relays in part hidden and buried within a larger product mix.
A DIGITAL RELAY
CALCULATION OF PLUG SETTINGS
phe choice of plug setting is made by considering
1. Normal full load current and permissible over load
2. CT primary Current
For each relay location K = Full load current x CTSC/CTPC
CTSC-CT Secondary Current CTPC - CT Primary Current
FThe plug setting will be the plug position nearest to the value of K on higher side.
JCALCULATION OF RELAY OPERATING TIME
In order to calculate the actual relay operating time, the following things must be known.
a) Time/PSM Curve
b) Current Setting
c) Time Setting
d) Fault Current
e) Current Transformer Ratio
[The procedure for calculating the actual relay operating time is as follows.
i) Convert the fault current into the relay coil current by using the current
ii) Express the relay current as a multiple of current setting, i.e calculate the
iii) From the Time/PSM curve of the relay, read off the time of operation for the
iv) Determine the actual time of operation by multiplying the above time of the
relay by time-setting multiplier in use.
Type references are built up in the followin g manner:
First letter-operating quantity
A-phase angle comparison O-oil pressure
B-balanced current P-poly phase VA
C-current (amperes) R-reactive VA
D-diffe rentia 1 S-slip frequency
F-frequency V-potential (volts)
l-directional current W-watts (power)
K-rate of rise of current X-reactance
M-manual Y-admittance. Z-lmpedence
A-attracted armature P-plug
C-induction cup S-synchronous motor
D-induction disc T-transistor
G-galvanometer (moving coil) W-weight (gravity)
l-transactor J-mixed types
A-auxillary M-semaphoreor motor
Ã‚Â¦B-testing N-negative sequence
C-carrier or counting O-out of step
D-directional P-potential failure
[E-earth (ground) Q-alarm
[F-flag and alarm indicate R-reclosing
G-general or generator S-synchronising
| H-harmonic restraint T-timer or transformer
l-interlocked or industrial U-definite time
|J-tripping V-voltage restraint
[jE-tripping (elec-reset) W-pilot wire
JH-tripping (hand reset) WA-interposing
i JS-tripping(self reset) WJ-inter tripping
JC-control X-super visory
K-check alarm Y-flashback (back fire)
L-load limiting Z-special application ZS-zero sequence
Fourth letter: M-special variations
First number: indicates the number of units in the relay essential to its operation-not including seal-in auxiliary units. Second number indicates a particular character of one of a group of similar relay.eg: CDE 11, CDG 12, similar character but different curve.
MAIN AND BACKUP PROTECTION
I Primary protection is essential protection provided for protecting an
Ã‚Â¦equivalent machine. As a precautionary measure an additional protection is [generally provided and is called backup protection. The primary protection is first lo act and backup protection is net in the line of defense, if primary protection fails ,the backup protection comes into action and removes faulty part from the ihealthy system.
Backup protection is provided for the following reason. If due to some (reason ,the main protection fails ,the backup protection serves the purpose of protection. Main protection can fail due to failure of one of the components in the protective system such as relay auxiliary relay, CT, PT, trip circuit, circuit breaker etc. If the primary protection fails there must be an additional protection otherwise the fault may remain un cleared resulting in disaster .When main protection is made inoperative for the purpose of maintenance Testing etc, the backup protection acts like main protection.As a measure of economy ,back up protection is given against short circuit protection and generally not for other abnormal conditions.The extent to which back up protection is provided depends upon economic and technical consideration.
PROJECT REPORT 2008
Over current And Earth fault Relay
Â¢ Identical time/current characteristics on all taps.
Â¢ Self-powered, no necessity for separate auxiliary supply.
Â¢ High torque, ensuring consistent timing even under adverse conditions.
Â¢ Very low overshoot.
Â¢ Simple construction, easily accessible.
Â¢ Comprehensive range of high-set unit ratings.
Â¢ Dustproof drawout case and tropicalised finish.
Selective phase and earthfault protection, in time graded systems for AC ^machines, transformers, feeders etc.
A non-directional heavily damped induction disc relay which has an adjustable inverse time/current characteristic with a definite minimum time. The relay has a high torque movement combined with low burden and low overshoot. The relay disc is so shaped that as it rotates the driving torque increases and offsets the changing restraining torque of the control spring. This feature combined with the high torque of the relay ensures good contact pressure even at currents near pick-up. Damping of the disc movement is by a removable high retentivity permanent magnet.
The unique method of winding the operating coil ensures that the time/current characteristics are identical on each of the seven current taps. Selection of the required current setting is by means of a plug setting bridge which has a single insulated plug. The maximum current tap is automatically connected when the plug is withdrawn from the bridge, allowing the setting to be changed under load without risk of open circuiting the current transformers.
The IDMT relay has an auxiliary unit which is powered by a secondary winding on the electromagnet through rectifier and as such a separate auxiliary supply is not required. The disc unit operates and closes its contacts, the auxiliary element connected across the secondary winding on the electromagnet operates, one normally open contact of the auxiliary element reinforces the disc contact. Two other contacts of the auxiliary element are brought out to the terminals of the relay
The relay operating time can be adjusted by movement of the disc backstop which is controlled by rotating a knurled moulded disc at the base of the graduated time multiplier scale.
A high-set instantaneous over current / earth fault unit, type CAG 17 can be fitted in the same case to provide instantaneous protection under maximum short circuit conditions and to improve discrimination on time graded protective systems.
Type CDG 21 relay is a single pole type CDG 11 relay with a high-set instantaneous unit. Type CDG 31 is a triple pole version the type CDG 11 with three over current units or two over current units and one earth fault unit in the centre. Type CDG 61 relay is a triple pole version of type CDG 21 relay.
CALIBRATION OF CDG RELAY