Only questions 1 and 5 please ltle: Primary and Backup Protection Score:/100 Stu
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Question
Only questions 1 and 5 please ltle: Primary and Backup Protection Score:/100 Student name: Scope: Simulink Modelling Primary and Backup Date: Protection Student #: Answer the following questions in your pre-lab report: 1) Explain the attributes of a profection relay: Speed, redundancy, selectivity, reliability and sensitivity 3) What are the requirements of a backup protection? 4) Explain the working of the primary and backup protections implemented in the following Figure. Relay B Relay A operating time Selective time interval (STI) ??? Source Fault Load relay Rg Load relay RA Load 5) Explain the mal-operation risks in backup protection?Explanation / Answer
Attributes of protective relay
Speed
To maximize safety, and minimize equipment damage and system instability, a fault should be cleared as quickly as possible. This implies that relay should quickly arrive at a decision and circuit breaker operation should be fast enough. Typically, a fast circuit breaker would operate in about two cycles. A reasonable time estimate for ascertaining presence of fault is one cycle. This implies approximately three cycle fault clearing time for primary protection. On the other hand, if five cycle circuit breaker is used, fault clearing time increases to six cycles.
Sensitivity
Sensitivity of the protective system implies that it must detect smallest possible value of actuating quantity whether current or voltage.
For simplicity, consider the case of overcurrent protection. It must have ability to detect the smallest possible fault current. The smaller the current that it can detect, the more sensitive it is.
Selectivity
Like sensitivity, selectivity also implies an ability to discriminate. A relay should not confuse some peculiarities of an apparatus with a fault. For example, transformer when energized can draw up to 20 times rated current (inrush current) which can confuse, both overcurrent and transformer differential protection. Typically, inrush currents are characterized by large second harmonic content.
This discrimination is used to inhibit relay operation during inrush, thereby, improving selectivity in transformer protection. Also, a relay should be smart enough, not just to identify a fault but also be able to decide whether fault is in it's jurisdiction or not. For example, a relay for a feeder should be able to discriminate a fault on it's own feeder from faults on adjacent feeders. This implies that it should detect first existence of fault in it's vicinity in the system and then take a decision whether it is in it's jurisdiction. Recall that directional overcurrent relay was introduced to improve selectivity of overcurrent relay.
Reliability
A relaying system has to be reliable. Reliability can be achieved by redundancy i.e. duplicating the relaying system. Obviously redundancy can be a costly proposition. Another way to improve reliability is to ask an existing relay say, protecting an apparatus A to backup protection of apparatus B. Both the approaches are used (simultaneously) in practice. However, it is important to realize that back-up protection must be provided for safe operation of relaying system. Redundancy in protection also depends upon the criticality of the power apparatus.
For example, a 400 kV transmission line will have independent (duplicated) protection using same or a different philosophy; on the other hand, a distribution system will not have such local back-up. A quantitative measure for reliability is defined as follows:
%Reliability = {no of correct trips / (no of desired trips + no of incorrect trips)} * 100
Mal-operation risks of backup protection
Load Encroachment and Power Swings
In swing conditions, amplitude and phase angle of voltage and current oscillate, which may caused mal-operation particularly in the backup relays. Distance protection relays are equipped with power swing detection modules,e.g. Concentric characteristics method.
Concentric Characteristic method is the simplest way to measure speed of impedance oscillations. It monitors the impedance trajectory between two concentric areas which is defined in the protection relays. The advantage of this method is that the power swing is detected before impedance entrance into protection zones. One of the limitations of this method is that the concentric characteristics are barriers for outer characteristics to extend the load area. These factors lead to serious restrictions for the long lines and heavy loads.
Voltage Instability
The main cause of voltage instability is shortage of reactive power in power system. Mal-operation of distance relay in terms of voltage instability can occur by a relay which having a maximum area of protection as backup protection.
The worst case of voltage instability occurs when the dynamic load tries beyond the capability of transmission line or generation to be used.
Various methods for fault detection from voltage instability are presented.
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