19-03-2012, 04:52 PM
Study of Numerical Overcurrent Relay
Indika_NumericalRelay.docx (Size: 74.26 KB / Downloads: 135)
DISCUSSION
• Three stage over current Characteristics of the KCGG relay
Three stages of KCGG relay is shown above 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.
• Advantages of having various Time/Current characteristics.
In power systems there are many power protection methods and elements. These relays are used not only as main protection elements, but also backup protections such as after fuses. So relays must coordinate with the thermal characteristics of fuses and coordinate and grade with other protection schemes and devices easily. Usually numerical relays are having various time/current characteristics because of the above reasons. Relay is provided with seven inverse time/current characteristics to cover the range of applications to which the relay may be applied.
• Discuss the operation of IDMT Relay coordination system
In earlier most of the overcurrent relay works on the induction principle. The moving system consists of an aluminum disc fixed on a vertical shaft and rotating on two jeweled bearings between the poles of an electromagnet and a damping magnet. The winding of the electromagnet is provided with seven taps generally, which are brought on the front panel and the required tap is selected by a push-in -type plug. The pick-upcurrent setting can thus be varied by the use of such plug multiplier setting. Thepick-up current values of earth fault relays are normally quite low.The operating time of all overcurrent relays tends to become asymptotic to a definiteminimum value with increase in the value of current. This is an inherent property ofthe electromagnetic relays due to saturation of the magnetic circuit. By varying thepoint of saturation, different characteristics can be obtained and these are
1. Definite time
2. Inverse Definite Minimum Time (IDMT)
3. Very Inverse
4. Extremely Inverse