Electric Power, System Protection, Control, and Monitoring of
ELECTRIC POWER, SYSTEM PROTECTION, CONTROL, AND MONITORING OF
Protection is the branch of electric power engineering concerned with the principles of design and operation of equipment (called "relays" or "protective relays") which detect abnormal power system conditions and initiate corrective action as quickly as possible in order to return the power system to its normal state. The quickness of response is an essential element of protective relaying systems—response times of the order of a few milliseconds are often required. Consequently, human intervention in the protection of system operation is not possible. The response must be automatic, quick, and should cause a minimum amount of disruption to the power system.
THE NATURE OF PROTECTION
In general, relays do not prevent damage to equipment; they operate after some detectable damage has already occurred. Their purpose is to limit, to the extent possible, further damage to equipment, to minimize danger to people, to reduce stress on other equipment, and above all, to remove the faulted equipment from the power system as quickly as possible so the integrity and stability of the remaining system is maintained. There is a control aspect inherent in relaying systems which complements the detection of faults and helps return the power system to an acceptable configuration as soon as possible so that service to customers can be restored. There is also a vital need to constantly monitor the power and the protective systems to analyze operations for correct performance and to rectify errors in design, application, or settings.
Reliability, Dependability, and Security
Reliability is generally understood to measure the degree of certainty that a piece of equipment will perform as intended. Relays, in contrast with most other equipment, have two alternative ways in which they can be unreliable. They may fail to operate when they are expected to, or they may operate when they are not expected to. This leads to the two-pronged definition of "dependability," the measure of certainty that the relays will operate correctly for all faults for which they are designed to operate and "security," the measure of certainty that the relays will not operate incorrectly for any fault.
Zones of Protection
Relays have inputs from several current transformers (CTs) and the zone of protection is bounded by these CTs. While the CTs provide the ability to detect a fault inside the zone, circuit breakers (CBs) provide the ability to isolate the fault by disconnecting all of the power equipment within the zone. Thus, a zone boundary is usually defined by a CT and a CB. When the CT is part of the CB it becomes a natural zone boundary. When the CT is not an integral part of the CB, special attention must be paid to the fault detection and fault interruption logic. The CT still defines the zone of protection, but communication channels must be used to implement the tripping function. Figure 1 shows the zones of protection in a typical system.
It is, of course, desirable to remove a fault from the power system as quickly as possible. However, the relay must make its decision based upon voltage
and current waveforms which are severely distorted due to transient phenomena which follow the occurrence of a fault. The relay must separate the meaningful and significant information contained in these waveforms upon which a secure relaying decision must be based. These considerations demand that the relay take a certain amount of time to arrive at a decision with the necessary degree of certainty. The relationship between the relay response time and its degree of certainty is an inverse one and is one of the most basic properties of all protection systems.
Although the operating time of relays often varies between wide limits, relays are generally classified by their speed of operation as follows:
- These relays operate as soon as a secure decision is made. No intentional time delay is introduced to slow down the relay response.
- An intentional time delay is inserted between the relay decision time and the initiation of the trip action.
- A relay that operates in less than a specified time. The specified time in present practice is 50 milliseconds (3 cycles on a 60 Hz system)
- Ultra high-speed—
- This term is not included in the present relay standards but is commonly considered to be operation in 4 milliseconds or less.
Primary and Backup Protection
The main protection system for a given zone of protection is called the primary protection system. It operates in the fastest time possible and removes the least amount of equipment from service. On extra-high-voltage
systems (230 kV and above) it is common to use duplicate primary protection systems in case any element in one primary protection chain fails to operate. This duplication is therefore intended to cover the failure of the relays themselves. One may use relays from a different manufacturer, or relays based on a different principle of operation, to avoid common-mode failures. The operating time and the tripping logic of both the primary and its duplicate system are the same.
It is not always practical to duplicate every element of the protection chain. Particularly on lower voltage systems, backup relaying is used. Backup relays are slower than the primary relays and, generally, remove more system elements than may be necessary to clear a fault. They may be installed locally, that is, in the same substation as the primary relays, or remotely.
RELAY OPERATING PRINCIPLES
In general, as faults (short circuits) occur, currents are increased and voltages decrease. Besides these magnitude changes, other changes may occur. Relay operating principles are based upon detecting these changes.
This is the simplest of all relay operating principles. Any current above, or voltage below, a set level may be taken to mean that a fault or some other abnormal condition exists inside the zone of protection. Figure 2 shows a definite time and an inverse time overcurrent relay.
This operating principle is based upon the comparison of one or more operating quantities. The relay will operate when the phasor division between the two or more circuits differs beyond the normal operating parameters. In Figure 3, IA and IB may be equal or at a fixed ratio to each other.
This is one of the most sensitive and effective methods of providing protection against faults and is shown in Figure 4. The algebraic sum of all currents entering and leaving the protected zone will be close to zero if no fault exists within the zone and will be the sum of I1 and I2 if a fault exists within the zone. A level detector can be used to detect the magnitude of this comparison or a special relay such as a percentage differential or harmonic restrained relay is applicable. This is the most common protective device used for generators, motors, buses, reactors, capacitors, etc. Its only drawback is that it requires currents from the extremities of a zone of protection which may require excessive cable lengths or a communication system.
Phase Angle Comparison
This type of relay compares the relative phase angle between two alternating-current quantities. It is commonly used to determine the direction of a current with respect to a reference quantity. Normal power flow in a given direction will result in the phase angle between the voltage and the current varying around the power factor angle (e.g., 30°) while power in the reverse direction will differ by 180°. Under fault conditions, since the impedance is primarily the inductance of the line, the phase angle of the current with respect to the voltage will be close to 90°.
This type of relay compares the local current with the local voltage. This is, in effect, a measurement of
the impedance as seen by the relay. An impedance relay depends on the fact that the length of the line (i.e., its distance) for a given conductor diameter and spacing determines its impedance. This is the most commonly used relay for the protection of high voltage transmission lines. As shown in Figure 5, zones can be identified as "zone one" which provides instantaneous protection to less than 100 percent of the associated line segment, and zones two and three which cover more than the line involved but must be delayed to provide coordination.
Currents and voltages in a power system usually have a sinusoidal waveform of the fundamental power system frequency plus other normal harmonics
(e.g., the third harmonic produced by generators). Abnormal or fault conditions can be detected by sensing any abnormal harmonics that accompany such conditions.
Normal power system operation is at 50 or 60 Hz depending upon the country. Any deviation from these values indicates that a problem exists or is imminent.
The following discussion covers a very small sample of the possible designs. Specific details must be obtained from the manufacturers.
The fuse is a level detector and is both the sensor and the interrupting device. It is installed in series with the equipment being protected, and it operates by melting a fusible element in response to the current flow.
The actuating forces are created by a combination of input signals, stored energy in springs, and dash-pots. The plunger type relay consists of a moving plunger inside a stationary electromagnet. It is typically applied as an instantaneous level detector. The induction-type relay is similar to the operation of a single-phase ac motor in that it requires the interaction of two fluxes across a disc or cup. The fluxes can be produced by two separate inputs or by one input electrically separated into two components. Depending on the treatment of the inputs (i.e. one current separated into two fluxes, two currents, or a current and a voltage), this design can be used for a time-delay overcurrent relay, a directional relay, or a distance relay.
All of the functions and characteristics of electro-mechanical relays can be performed by solid-state devices, either as discrete components or as integrated circuits. They use low-power components, either analog circuits for fault-sensing or measuring circuits as a digital logic circuit for operation. There are performance and economic advantages associated with the flexibility and reduced size of solid-state devices. Their settings are more repeatable and hold closer tolerances. Their characteristics can be shaped by adjusting logic elements as opposed to the fixed characteristics of induction discs or cups.
The observation has often been made that a relay is an analog computer. It accepts inputs, processes them, electromechanically or electronically, to develop a torque or a logic output resulting in a contact closure or output signal. With the advent of rugged, high performance microprocessors, it is obvious that a digital computer can perform the same function. Since the usual inputs consist of power system voltages and currents, it is necessary to obtain a digital representation of these parameters. This is done by sampling the analog signals and using an appropriate computer algorithm to create suitable digital representations of the signals.
Individual types of electrical apparatus, of course, require protective schemes that are specifically applicable to the problem at hand. There are, however, common detection principles, relaying designs and devices that apply to all.
Transmission Line Protection
Transmission lines utilize the widest variety of schemes and equipment. In ascending order of cost and complexity they are fuses, instantaneous overcurrent relays, time delay overcurrent relays, directional overcurrent relays, distance relays, and pilot protection. Fuses are used primarily on distribution systems. Instantaneous overcurrent relays provide a first zone protection on low-voltage systems. Time delay overcurrent relays provide a backup protection on low-voltage systems. Directional overcurrent relays are required in loop systems where fault current can flow in either direction. Distance relays provide a blocking and tripping function for pilot relaying and first, second, and third zone backup protection on high-voltage and extra-high-voltage systems. Pilot protection provides primary protection for 100 percent of the line segment by transmitting information at each terminal to all other terminals. It requires a communication channel such as power line carrier, fiber optics, microwave, or wire pilot.
The dominant protection scheme for generators and motors is the differential relay. Access to all entry points of the protected zone is usually readily available, no coordination with the protection of other connected apparatus is required, and the faulted zone is quickly identified. Motor protection also includes instantaneous and time delay overcurrent relays for backup.
Differential relaying is the universal bus and transformer protection scheme. The inrush current associated with power transformers requires a special differential relay utilizing filters to provide harmonic restraint to differentiate between energizing current and fault current.
Instantaneous and time delay overcurrent relays are the most common protective devices used on shunt reactors, capacitors and station service equipment.
Transmission line faults are predominantly temporary, and automatic reclosing is a necessary complement to the protective relaying function. The reclose time must be greater than the time required to dissipate the arc products associated with the fault. This varies with the system voltage and ranges from 15–20 cycles at 138 kV to 30 cycles for the 800 kV systems. Automatic reclosing requires that proper safety and operating interlocks are provided.
Rotating equipment, transformers, and cables do not, in general, have temporary faults, and automatic reclosing is not provided.
The importance of monitoring the performance of power system and equipment has steadily increased over the years.
Oscillographs and other fault recorders such as sequence of events are, by nature, automatic devices. The time frame involved in recognizing and recording system parameters during a fault precludes any operator intervention. The most common initiating values are currents and voltages associated with the fault itself. Phase currents increase, phase voltages decrease, and there is normally very little ground current, so all of these are natural candidates for trigger mechanisms. There are transient components superimposed on the 60 Hz waveform that accompany faults and other switching events. They are revealed in the oscillographic records and are an essential element in analyzing performance. Figure 6 is a typical record of a
single phase to ground fault and unsuccessful high-speed reclose.
With the advent of digital relays, the situation changed dramatically. Not only could the relays record the fault current and voltage and calculate the fault location, they could also report this information to a central location for analysis. Some digital devices are used exclusively as fault recorders.
Stanley H. Horowitz
See also: Electric Power, Generation of; Electric Power, System Reliability and; Electric Power Transmission and Distribution Systems.
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