DISCUSSION 01. Discuss about the three stage Over Current Characteristics of the KCGG Relay
KCGG relay has three stages and this relay is used for time graded over current and earth fault protection. These stages have threshold limits and time delays which are independent from each other Furthermore, for phase faults and earth faults separate settings have defined (for phase faults - I>, earth faults - Io>). Relay operates when the power frequency component of the current exceeds the set threshold. These elements may be set as a low set overload protection and may be expected to have relatively long associated time delays. In the first stage several inverse time curves and definite time characteristics are available. In addition adjustable reset timer also provided for intermittent faults. Stage 2 and 3 facilitates for instantaneous tripping using definite time setting. Therefore according to user requirement, suitable characteristic can be obtained setting those parameters appropriately in three stages. Stage 2 & 3 proved definite time settings for instantaneous operation which can be disabled if not required. Therefore this relay can be used for providing overcurrent protection in various s ituations. Three stages over current protection can be applied t o, 1. Phase Fault Protection 2. Thermal over Load Protection 3. Earth Fault Protection 02. What are the advantages of having various time/ current Characteristics?
The way of using a relay protection in a power system may vary from place to place depending on the applications. Rapid operation of relays must be needed in place where very expensive equipment ’s are installed. Same time slow operation is needed in order to regain the stability of the system after a fault occurred. Further to have proper grading among relays, it is mandatory to have different time/ current characteristics. Therefore having different time/current curve characteristics, proper discrimination among equipment’s can be established. STI30XDT – short time inverse curve (becomes constant time at 30x setting).For use where relatively short operation times are required to preserve system stability and where grading with downstream relays is not required. SI30XDT – standard (moderately) inverse curve (becomes constant time at 30x setting).The standard characteristic for co-ordination of protection on systems where the amount generation varies and for providing back-up protection on transmission systems. I30XDT – inverse curve (becomes constant time at 30x setting).Slightly more inverse than the SI30XDT curve, but also for co-ordination of protection on distribution systems and for back-up protection for relays on other circuits. This curve will allow closer grading with some relays that do not have an IEC standard curve. VI30XDT – very inverse curve (becomes constant time at 30x setting).Being very inverse it allows for longer operation times at low currents, or shorter times at high currents, than the previous two curves. This characteristic is especially useful when co-ordinating a number of relays and where there is great variation in fault current between relay locations.
EI20XDT – extremely inverse curve (becomes constant time at 20x setting).Main use is for grading with downstream fuses and upstream overcurrent relays on distribution and industrial systems. The relay maintains it’s very inverse characteristic to higher current levels than electro-mechanical relays and therefore, provides for closer grading with the fuse characteristic. LT30XDT – long time inverse curve (becomes constant time at 30x setting). Provides protection for starting surges and overloads of short duration, but the cold load start/pick-up feature would be considered the preferable solution for these applications. Most useful for grading with low voltage system protection devices or for protecting neutral earthing resistors. DT – definite time characteristic Useful for achieving constant time grading steps over a wide current range and for offering faster tripping times than inverse time protection at low multiples of current setting. This characteristic is often used where there is no significant variation in fault level between relay locations eg. In the case of earth fault protection of a resistance earthed system, or in the case of low circuit impedance compared to source impedance. 03. Discuss the operation of IDMT Relay coordinate system.
The over load inverse time relay . It consists of an upper electromagnet that has been provided with two windings one primary and the other secondary. Primary is connected to a current transformer in the line which is under protection and is provided with eight tappings. These tappings are connected to a plug setting bridge by which the number of turns to be used can be adjusted in order to have the desired current setting. The second winding called secondary is energized by the induction effect and is wound over the central limb of the upper magnet as well as it is spread over the two limbs of the lower magnet. By this method, the leakage flux from the upper magnet entering the disc has been displaced in phase from the flux entering the disc from the lower magnet. The deflecting torque is produced on the disc in the fashion as already explained. The spindle of the disc carries a moving contact which bridges two fixed contacts after the disc has rotated through a certain angle which has been set before. Any setting for this angle is possible varying from 0 to 360°. The variation of this angle imparts to the relay, various time settings. The speed of rotation of the disc is dependent upon the torque which in turn is dependent on the current setting, when the load current increases from this setting it will increase the speed of rotation of the disc resulting into decrease of operation time. Thus the time current characteristics of the relay observe inverse-Square law. The definite minimum time characteristics of the relay are obtained by the use of a saturated upper magnet. This ensures that there is no further increase in f1ux when the current has reached a certain value and any further increase of current will not affect the relay operation. This results in a flattened current time characteristic and the relay obtains its name as Inverse definite minimum time lag (I.D.M.T.) relay. The current time characteristics of the relay It represents the time required to close the trip contacts for different values of over current. Its horizontal scale is marked in terms of current-setting multipliers i.e. number of times the relay current is in excess of current setting
