Objectives
Introduction to power system protection
Understand the need for the protection of electric equipment and their protection schemes
Understand Generator Protection, Transformer Protection, Transmission line protection
Understand the operations of various types of circuit breakers and their ratings
Contribute to the system reliability improvement
Why Protection is needed?
To prevent severe damage to sub-station equipment during faults or power system disturbances
The power system must maintain acceptable operation 24 hours a day
–Voltage and frequency must stay within certain limits
•Small disturbances
–The control system can handle these
–Example: variation in transformer or generator load
•Severe disturbances require a protection system
–They can jeopardize the entire power system
–They cannot be overcome by a control system
Principles and elements of the protection system
We need to balance reliability and cost in designing a power system. While it is impossible to avoid the occurrence of faults and other abnormal operating conditions that produce large power system disturbances, a protection system is intended to take preventive or corrective actions in such cases.
The first line of defense is the protection of power system elements. The function of this type of protection is to detect faults and abnormal conditions and to disconnect the faulted element in order to prevent further damage in the element or a system disturbance. Modern power systems operate near the security limits. The system also needs protection functions at the system level that can include low frequency or low voltage load shedding among others.
Protection operation disconnects system elements. It is then important to provide automatic restoration functions. Among these functions, we might mention the automatic reclosing of transmission lines, automatic transfer to alternate power supplies, and automatic synchronization.
Protection System
A series of devices whose main purpose is to protect persons and primary electric power equipment from the effects of faults
Elements of Protection System
When any abnormal condition develops, the main function of a protective relay is to isolate the faulty section with the least interruption to the service by controlling or operating the circuit breaker.
The relay may be designed to detect and measure abnormal conditions and close the contacts of the tripping circuit.
It can make or break a circuit either manually or automatically under all conditions (no load, full load and fault) i.e.
a) It can make or break a circuit manually or by remotely under normal conditions
b) Break a circuit automatically under fault condition
c) Make a circuit either manually or by remote under fault condition
•Current and voltage transducers
High-magnitude primary currents are reduced to a value suitable for relay operation with the help of current transformers (CTs). (Then CTs provide current in the relay which are proportional to those in primary.) The primary winding of the CTs is connected in series whit the load and carries the actual power system current (normal or faulty). The secondary is connected to the measuring circuit or the relay.
It is not possible to connect the voltage coils of the protective device directly to the system in the case of high-voltage systems. So it is necessary to step down the voltage, also to insulate the protective equipment from the primary circuit. This is achieved by using a voltage transformer. Also known as a potential transformer (PT) which is similar to a power transformer. The voltage transformer is rated in terms of the maximum burden (VA) output it
delivers without exceeding specified limits of errors. Whereas the power transformer is rated by the secondary output it delivers without exceeding a specified temperature rise. The output of PTs is usually limited to a few hundred volt-amperes and the secondary voltage is usually 110V between phases.
For protective relay and automatic control scheme in power system use two kinds of auxiliary supplies: DC and AC
DC auxiliary power supply is provided by batteries which are maintained and continuously charged. The advantages of storage batteries are their high reliability and independence of power circuit conditions and of the existence of faults.
Usually, the voltage of the auxiliary supplies is maintained at 220 V DC.
Mainly the auxiliary supplies power to protective relays, automatic control and the circuit breakers tripping circuit. Separate buses may also be provided for supplying power to relays, CB and other indicating circuits such as alarm and warning signals.
Performance Requirements of Protection System
Discriminate between load (normal) and fault (abnormal) conditions
Not be confused by non-damaging transient conditions
Be selective – coordinate with other protection systems
Fast enough to prevent damage and hazards – but not too fast
Have no “blind spots” i.e. unprotected zones
A high degree of reliability and availability
Secure against incorrect operation (security)
Should not restrict rating of primary plant and equipment
Should be affordable
