The operation of a power system is affected by disturbances that could be due to natural occurrences such as lightning, wind, trees, animals, and human errors or accidents. These disturbances could lead to abnormal system conditions such as short circuits, overloads, and open circuits.
Short circuits, which are also referred to as faults, are of the greatest concern because they could lead to damage to equipment or system elements and other operating problems including voltage drops, decrease in frequency, loss of synchronism, and complete system collapse. There is, therefore, a need for a device or a group of devices that is capable of recognizing a disturbance and acting automatically to alleviate any ill effects on the system element or on the operator. Such capability is provided by the protection system.
The protection system is designed to disconnect the faulted system element automatically when the short circuit currents are high enough to present a direct danger to the element or to the system as a whole.
When the fault results in overloads or short-circuits currents that do not present any immediate danger, the protection system will initiate an alarm so that measures can be implemented to remedy the situation.
Key Components of Protection System
There are three principal components of a protection system:
- Protective relay
- Circuit breaker
These components are described briefly in the following paragraphs.
The transducer serves as a sensor to detect abnormal system conditions and to transform the high values of short-circuit current and voltage to lower levels. The main sensors used are the current transformer (CT) and the potential transformer (PT).
The current transformer is designed to provide a standard continuous secondary current of 5 A. Standard CT ratios available include 50/5, 100/5, 150/5, 200/5, 250/5, 300/5, 400/5, 500/5, 600/5, 800/5, 900/5, 1000/5, 1200/5, 1500/5, and 2000/5. During fault conditions, the short-circuit currents could reach over 10 times normal for short periods of time without damaging the CT windings.
The current transformer has a primary winding that usually consists of one turn and a secondary winding of several turns. It is, therefore, unsafe to open- circuit the secondary of a CT whose primary is energized.
The potential transformer is designed to operate at a constant standard secondary voltage of 120 V. For low-voltage applications, the PT is just like any other two-winding voltage transformer. For primary voltages in the HV and EHV levels, a capacitor voltage-divider circuit is used together with the PT. The primary voltage is impressed across the series-connected capacitors. The PT is used to measure the voltage of a few kilovolts across the capacitor of the smaller capacitance value.
A protective relay is a device that processes the signals provided by the transducers, which may be in the form of a current, a voltage, or a combination of current and voltage. These signals arise as a result of a faulted condition such as a short circuit, defective equipment or lines, lightning strikes, or surges.
The protective relay can initiate or permit the opening of various interrupting devices or sound an alarm. There are two main classifications of protective relays based on their construction: electromechanical and solid state.
The electromechanical relay develops an electromagnetic force or torque from the signal provided by the transducer; this force or torque is used to physically open, or close, a set of contacts to permit or initiate the tripping of circuit breakers or actuate an alarm.
The solid-state, or static, relay is energized by the same signals as in an electromechanical relay. However, there is no physical opening, or closing, of the relay contacts. Instead, the switching of the relay contacts is simulated by causing a solid-state device to change its status from conducting (closed position) to non-conducting (open position).
Electromechanical relays predate the solid-state relays. A majority of power system installations still use electromechanical relays. The improved reliability, versatility, and faster response (as low as one-quarter cycle) of solid-state relays have made them more attractive.
Some electromechanical relays have been replaced by solid-state relays, and in newer installations, a mixture of both types would usually be found.
A circuit breaker is a mechanical device used to energize and interrupt an electric circuit. It should be able to open and close quickly, maybe in the order of a few milliseconds.
It should be able to carry the rated current continuously at the nominal voltage, and it must be able to withstand the large short-circuit current (called its momentary rating) that flows during the first cycle after a fault occurs.
The circuit breaker must be able to interrupt a large short-circuit current called its interrupting rating. The momentary rating is about 1.6 times the interrupting rating because the former includes the effect of the DC component of the transient short-circuit current.
The actual value of current interrupted by the circuit breaker depends on its speed, which could be 1/2, 3, 5, or 8 cycles.
When the current-carrying contacts of the circuit breaker are opened, an electric field appears across the contacts that ionize the medium between them, and an arc is established between the contacts. The circuit breaker must be able to extinguish, or interrupt, this arc as quickly as possible.
The arc is made to take an elongated path, cooled, and finally extinguished when the AC current feeding the arc passes through its zero value. Sometimes, the arc is extinguished in the air, oil, sulfur hexachloride (SF6), or a vacuum.