http://www.scribd.com/doc/6993791/Protection-of-Electrical-Networks-C http://www.scribd.com/doc/45996284/Power-System-Protection-Coordination http://www.scribd.com/doc/66635579/Chap-5 http://www.electrowaveps.co.uk/services/protection_&_discrimination.htm
Protection, Discrimination and Restricted Earth Fault (REF) Studies The primary function of the protection device system is to protect power distribution system which need to supply electrical power with the required voltage level and energy demand the power distribution system and its components can be protected by various types of fuses, relays, low voltage circuit breakers, trip units, etc. Three basic considerations during a protection device system design:
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The system has to be safe for the personnel. The system has to be capable of fully protecting electrical power equipment such power cables, transformer, motors, etc.
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The system has to be selective (co-ordinated).
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Short circuit study should be performed to provide short circuit fault levels on each bus in the system. The interrupting ratings of all of the breakers and fuses should be greater than the calculated fault levels available at the point of application. Objective of coordination study is to coordinate system so that the protective device closest to the fault on the power-source side has the first chance to operate, but each preceding protective device upstream should be capable, within its designed settings of current and time, to effect the isolation.
If the fault persists the settings of the protection devices must be so selected to first, satisfactorily protect electrical equipment from overloads (<250% full load)
and second, interrupt short circuit (500%-2000% full load) as rapidly as possible.
Data needed to obtain coordination study:
System single line diagram Short circuit fault levels the electrical equipment ratings and withstand damage characteristic. Protective devices ratings, settings and timecurrent characteristic.
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**Engineering Note** Be VERY careful when using a hybrid of BS88 Fuses and ACB/MCCB’s as they are VERY difficult to Co-ordinate.
That is not guarantee that the system will remain coordinated forever. System changes and additions plus power source changes frequently modify the protection requirements, leaving some of the protective devices unable to properly interrupt the available fault currents consequently,. Periodic study of protective-device settings is as important in preventing power outages as is periodic maintenance of the distribution system . Many large insurance Companies are insisting, and applying the following conditions;
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“The primary purpose of a coordination study is to determine satisfactory ratings and settings for the electric system protection devices. The protection devices should be chosen so that pickup currents and operating times are short but sufficient to override system transient overloads such as inrush current experienced when energizing transformers or starting motors. Further, the devices should be coordinated so that the circuit interrupter closest to the fault opens before other devices.” “Determining the ratings and settings for protective devices requires familiarity with 16th Edition requirements for the protection of cables, motors, and transformers, also for transformer magnetizing inrush current and transformer thermal and magnetic stress damage limits.”
Short-Circuit Fault Current Calculation Analysis.
A Short Circuit and Coordination Study is one of the most important steps you may ever take to protect your personnel and electrical distribution system. When an electrical fault or overload exceeds the interrupting rating of the protective device, the consequences can be devastating including injury, damaged electrical equipment, and costly downtime. Electrowave Power Solutions Ltd will perform a Short Circuit Study using a the latest AMTECH Power Software™ computer models that determines the 'worst case' fault level at the Electrical Distribution Equipment. The fault levels are compared to the existing device interrupting ratings to determine if the protective devices are rated in accordance with the manufacturer's published data and applicable standards. If required, new ratings will be determined. For more details on Short Circuit Conditions "Click this link" Restricted Earth Fault (REF) Current Calculation Analysis. 1
REF Scope
The REF relay, is used with a stabilising resistor, and is designed for applications where sensitive settings with stability on heavy through faults are required, and is recommended for balanced and restricted earth fault, bus-zone and certain forms of differential protection for generators, auto-transformers, reactors and motors. The relay operates as a high impedance unit protection scheme.
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REF Application
When circulating current protection schemes are subjected to heavy through faults, the sudden, and often asymmetrical growth in the system current can cause the protective current transformers to approach or even reach saturation level. This may result in a high unbalancing current due to the variations in the magnetising characteristics of the current transformers. To ensure stability under these conditions, it is common practice to use a high impedance relay, set to operate at a voltage slightly higher than that developed by the current transformers under maximum external fault conditions. Excel CT Calculations required.
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Calculate the CT “Sensitivity Calc (Iprim)”.
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Calculate the CT “Stabilising Voltage Calc”.
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Calculate the CT “Knee Point Voltage Calc”.
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Calculate the CT “Excitation Current Calc (Ie)”.
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The “Supply Transformer” details.
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MV Fault Level used in CT calculations.
Apart from the REF calculations listed above there are also a few engineering procedures to be followed eg; Amtech Calculation Graphs Discrimination of protection devices required.
a) If the LV ACB is fitted with “Earth protection” then the discrimination curve to be checked against the REF calculation? b) The Network Supplier (DNO) normally has “Earth Protection” on the 11kV (VIP Relay), this curve has to discriminate with both the LV downstream devices. All the above calculation results and settings will be contained in a composite report
