ENGINEERING ARTICLES

Existing protection/control systems may be improved and new protection/control systems may be developed to better adapt to prevailing system conditions during system-wide disturbance. While improvements in the existing systems are mostly achieved through advancement in local measurements and development of better algorithms, improvements in new systems are based on remote communications. However, even if communication links exist, conventional systems that utilize only local information may still need improvement since they are supposed to serve as fallback positions. The increased functions and communication ability in today’s SCADA systems provide the opportunity for an intelligent and adaptive control and protection system for system-wide disturbance. This, in turn, can make possible full utilization of the network, which will be less vulnerable to a major disturbance. Out-of-step relays have to be fast and reliable. The present technology of out-of-s...

Increasingly popular over the past several years are the so-called special protection systems, sometimes also referred to as remedial action schemes. Depending on the power system in question, it is sometimes possible to identify the contingencies or combinations of operating conditions that may lead to transients with extremely disastrous consequences. Such problems include, but are not limited to, transmission line faults, the outages of lines and possible cascading that such an initial contingency may cause, outages of the generators, rapid changes of the load level, problems with HVDC or FACTS equipment, or any combination of those events. Among the many varieties of special protection schemes, several names have been used to describe the general category: special stability controls, dynamic security controls, contingency arming schemes, remedial action schemes, adaptive protection schemes, corrective action schemes, security enhancement schemes, etc. In the strict sense of protec...

Voltage stability is defined by the System Dynamic Performance Subcommittee of the IEEE Power System Engineering Committee as being the ability of a system to maintain voltage such that when load admittance is increased, load power will increase, and so that both power and voltage are controllable. Also, voltage collapse is defined as being the process by which voltage instability leads to a very low voltage profile in a significant part of the system. It is accepted that this instability is caused by the load characteristics, as opposed to the angular instability that is caused by the rotor dynamics of generators. The risk of voltage instability increases as the transmission system becomes more heavily loaded. The typical scenario of these instabilities starts with a high system loading, followed by a relay action due to either a fault, a line overload, or hitting an excitation limit. Voltage instability can be alleviated by a combination of the following remedial measures: adding re...

Outage of one or more power system components due to the overload may result in overload of other elements in the system. If the overload is not alleviated in time, the process of power system cascading may start, leading to power system separation. When a power system separates, islands with an imbalance between generation and load are formed. One consequence of the imbalance is deviation of frequency from the nominal value. If the generators cannot handle the imbalance, load or generation shedding is necessary. A special protection system or out-of-step relaying can also start the separation. A quick, simple, and reliable way to reestablish active power balance is to shed load by under-frequency relays. The load shedding is often designed as a multistep action, and the frequency settings and blocks of load to be shed are carefully selected to maximize the reliability and dependability of the action. There are a large variety of practices in designing load shedding schemes based on t...

Every time a fault or a topological change affects the power balance in the system, the instantaneous power imbalance creates oscillations between the machines. Stable oscillations lead to transition from one (pre-fault) to another (post-fault) equilibrium point, whereas unstable ones allow machines to oscillate beyond the acceptable range. If the oscillations are large, the stations’ auxiliary supplies may undergo severe voltage fluctuations, and eventually trip. Should that happen, the subsequent resynchronization of the machines might take a long time. It is, therefore, desirable to trip the machine(s) exposed to transient unstable oscillations while the plant auxiliaries remain energized. The frequency of the transient oscillations is usually between 0.5 and 2 Hz. Since the fault imposes almost instantaneous changes on the system, the slow speed of the transient disturbances can be used to distinguish between the two. For the sake of illustration, let us assume that a power system...

As can be seen from Fig. 9.33, step distance protection does not offer instantaneous clearing of faults over 100% of the line segment. In most cases this is unacceptable due to system stability considerations. To cover the 10–20% of the line not covered by Zone 1, the information regarding the location of the fault is transmitted from each terminal to the other terminal(s). A communication channel is used for this transmission. These pilot channels can be over power line carrier, microwave, fiber-optic, or wire pilot. Although the underlying principles are the same regardless of the pilot channel, there are specific design details that are imposed by this choice. Power line carrier uses the protected line itself as the channel, superimposing a high frequency signal on top of the 60 Hz power frequency. Since the line being protected is also the medium used to actuate the protective devices, a blocking signal is used. This means that a trip will occur at both ends of the line unless a s...

A) RELIABILITY Reliability, in system protection parlance, has special definitions which differ from the usual planning or operating usage. A relay can miss-operate in two ways: it can fail to operate when it is required to do so, or it can operate when it is not required or desirable for it to do so. To cover both situations, there are two components in defining reliability: DEPENDABILITY: This refers to the certainty that a relay will respond correctly for all faults for which it is designed and applied to operate; and SECURITY: This is the measure that a relay will not operate incorrectly for any fault. Most relays and relay schemes are designed to be dependable since the system itself is robust enough to withstand an incorrect trip-out (loss of security), whereas a failure to trip (loss of dependability) may be catastrophic in terms of system performance. B) ZONES OF PROTECTION The property of security is defined in terms of regions of a power system, called zones o...

The study of transmission line protection presents many fundamental relaying considerations that apply, in one degree or another, to the protection of other types of power system protection. Each electrical element, of course, will have problems unique to itself, but the concepts of reliability, selectivity, local and remote backup, zones of protection, coordination and speed which may be present in the protection of one or more other electrical apparatus are all present in the considerations surrounding transmission line protection. Since transmission lines are also the links to adjacent lines or connected equipment, transmission line protection must be compatible with the protection of all of these other elements. This requires coordination of settings, operating times and characteristics. The purpose of power system protection is to detect faults or abnormal operating conditions and to initiate corrective action. Relays must be able to evaluate a wide variety of parameters to estab...

The latest technological leap in generator protection has been the release of digital multifunction relays by various manufacturers. With more sophisticated characteristics being available through software algorithms, generator protective function characteristics can be improved. Therefore, multifunction relays have many advantages, most of which stem from the technology on which they are based. IMPROVEMENTS IN SIGNAL PROCESSING Most multifunction relays use a full-cycle Discrete Fourier Transform (DFT) algorithm for acquisition of the fundamental component of the current and voltage phasors. Consequently, they will benefit from the inherent filtering properties provided by the algorithms, such as: • Immunity from DC component and good suppression of exponentially decaying offset due to the large value of X/R time constants in generators; • Immunity to harmonics; • Nominal response time of one cycle for the protective functions requiring fast response. Since sequence quantities are ...

A number of methods for isolating a generator once a fault has been detected are commonly being implemented. They fall into four groups: • Simultaneous tripping involves simultaneously shutting the prime mover down by closing its valves and opening the field and generator breakers. This technique is highly recommended for severe internal generator faults. • Generator tripping involves simultaneously opening both the field and generator breakers. • Unit separation involves opening the generator breaker only. • Sequential tripping is applicable to steam turbines and involves first tripping the turbine valves in order to prevent any over-speeding of the unit. Then, the field and generator breakers are opened. Figure 9.25 represents a possible logical scheme for the implementation of a sequential tripping function. If the following three conditions are met, (1) The real power is below a negative pre-set threshold SET_1, (2) The steam valve or a differential pressure switch is closed (eith...

A number of catastrophic failures have occurred in the past when synchronous generators have been accidentally energized while at standstill. Among the causes for such incidents were human errors, breaker flashover, or control circuitry malfunction. A number of protection schemes have been devised to protect the generator against inadvertent energization. The basic principle is to monitor the out-of-service condition and to detect an accidental energizing immediately following that state. As an example, Fig. 9.23 shows an application using an over-frequency relay supervising three single phase instantaneous overcurrent elements. When the generator is put out of service or the overfrequency element drops out, the timer will pick up. If inadvertent energizing occurs, the overfrequency element will pick up, but because of the timer dropout delay, the instantaneous overcurrent elements will have the time to initiate the generator breakers opening. The supervision could also be implemented...

In an apparatus protection perspective, generators constitute a special class of power network equipment because faults are very rare but can be highly destructive and therefore very costly when they occur. If for most utilities, generation integrity must be preserved by avoiding erroneous tripping, removing a generator in case of a serious fault is also a primary if not an absolute requirement. Furthermore, protection has to be provided for out-of-range operation normally not found in other types of equipment such as overvoltage, over-excitation, limited frequency or speed range, etc. It should be borne in mind that, similar to all protective schemes, there is to a certain extent a “philosophical approach” to generator protection and all utilities and all protective engineers do not have the same approach. For instance, some functions like over-excitation, backup impedance elements, loss-of- synchronism, and even protection against inadvertent energization may not be applied by some ...

Any numbers of conditions have been the reason for an electrical transformer failure. Statistics show that winding failures most frequently cause transformer faults (ANSI/IEEE, 1985). Insulation deterioration, often the result of moisture, overheating, vibration, voltage surges, and mechanical stress created during transformer through faults, is the major reason for winding failure. Voltage regulating load tap changers, when supplied, rank as the second most likely cause of a transformer fault. Tap changer failures can be caused by a malfunction of the mechanical switching mechanism, high resistance load contacts, insulation tracking, overheating, or contamination of the insulating oil. Transformer bushings are the third most likely cause of failure. General aging, contamination, cracking, internal moisture, and loss of oil can all cause a bushing to fail. Two other possible reasons are vandalism and animals that externally flash over the bushing. Transformer core problems have been a...

Switching of shunt reactors (and other devices characterized as having small inductive currents such as transformer magnetizing currents, motor starting currents, etc.) can generate high phase-to-ground over-voltages as well as severe recovery voltages, especially on lower voltage equipment such as reactors applied on the tertiary of transformers. Energizing the devices seldom generates high overvoltages, but overvoltages generated during de-energizing, as a result of current chopping by the switching device when interrupting the small inductive currents, can be significant. Neglecting damping, the phase-to-ground overvoltage magnitude can be estimated by: Where i is the magnitude of the chopped current (0 to perhaps as high as 10 A or more), L is the reactor’s inductance, and C is the capacitance of the reactor (on the order of a few thousand picofarads). When C is small, especially likely with dry-type reactors often used on transformer tertiaries, the surge impedance term can be ...

SERIES CAPACITOR BANK APPLICATIONS Installation of a series capacitor bank in a transmission line (standard or thyristor controlled) has the potential for increasing the magnitude of phase-to-ground and phase-to-phase switching surge over-voltages due to the trapped charges that can be present on the bank at the instant of line reclosing. In general, surge arresters limit the phase-to-ground and phase-to-phase over-voltages to acceptable levels; however, one problem that can be serious is the recovery voltage experienced by circuit breakers when clearing faults on a series compensated line. Depending the bank’s characteristics and on fault location with respect to the bank’s location, a charge can be trapped on the bank, and this trapped charge can add to the surges already being generated during the fault clearing operation. The first circuit breaker to clear is sometimes exposed to excessive recovery voltages under such conditions. FIGURE 10.25 Voltage magnification circuit. ...

Surges associated with switching transmission lines (overhead, SF6, or cable) include those that are generated by line energizing, reclosing (three phase and single phase operations), fault initiation, line dropping (de-energizing), fault clearing, etc. During an energizing operation, for example, closing a circuit breaker at the instant of crest system voltage results in a 1 pu surge traveling down the transmission line and being reflected at the remote, open terminal. The reflection interacts with the incoming wave on the phase under consideration as well as with the traveling waves on adjacent phases. At the same time, the waves are being attenuated and modified by losses. Consequently, it is difficult to accurately predict the resultant wave-shapes without employing sophisticated simulation tools such as a transient network analyzer (TNA) or digital programs such as the Electromagnetic Transients Program (EMTP). Transmission line over-voltages can also be influenced by the presenc...

Switching surges occur on power systems as a result of instantaneous changes in the electrical configuration of the system, and such changes are mainly associated with switching operations and fault events. These over-voltages generally have crest magnitudes which range from about 1 per unit to 3 pu for phase-to- ground surges and from about 2.0 to 4 pu for phase-to-phase surges (in pu on the phase to ground crest voltage base) with higher values sometimes encountered as a result of a system resonant condition. Wave-shapes vary considerably with rise times ranging from 50 μs to thousands of μs and times to half-value in the range of hundreds of μs to thousands of μs. For insulation testing purposes, a wave-shape having a time to crest of 250 μs with a time to half-value of 2000 μs is often used.

A direct stroke is defined as a lightning stroke when it hits either a shield wire, tower, or a phase conductor. An insulator string is stressed by very high voltages caused by a direct stroke. An insulator string can also be stressed by high transient voltages when a lightning stroke hits the nearby ground. An indirect stroke is illustrated in Fig. 10.12. FIGURE 10.12 Illustration of direct and indirect lightning strokes. The voltage induced on a line by an indirect lightning stroke has four components: 1. The charged cloud above the line induces bound charges on the line while the line itself is held electro-statically at ground potential by the neutrals of connected transformers and by leakage over the insulators. When the cloud is partially or fully discharged, these bound charges are released and travel in both directions on the line giving rise to the traveling voltage and current waves. 2. The charges lowered by the stepped leader further induce charges on the l...

The level reached by VFT overvoltages originated by disconnector switching or line-to-ground faults inside a GIS is below the BIL of substation and external equipment. However, aging of the insulation of external equipment due to frequent VFT must be considered. TEV is a low energy phenomenon and it is not considered dangerous to humans; the main concern is in the danger of the surprise-shock effect. External transients can cause interference with or even damage to the substation control, protection, and other secondary equipment. The main effects caused by VFT to equipment and the techniques that can be used to mitigate these effects are summarized below. FIGURE 10.33 Measurement and simulation of overvoltages in a 420 kV GIS at closing a switch. (Copyright 1999 SF6 INSULATION: Breakdown caused by VFT overvoltages is improbable in a well-designed GIS insulation system during normal operations. The breakdown probability increases with the frequency of the oscillations. In additi...

A) FUSES The most commonly used protective device in a distribution circuit is the fuse. Fuse characteristics vary considerably from one manufacturer to another and the specifics must be obtained from their appropriate literature. Figure 9.28 shows the time-current characteristics which consist of the minimum melt and total clearing curves. FIGURE 9.28 Fuse time-current characteristic. Minimum melt is the time between initiation of a current large enough to cause the current responsive element to melt and the instant when arcing occurs. Total Clearing Time (TCT) is the total time elapsing from the beginning of an over-current to the final circuit interruption; i.e., TCT = minimum melt plus arcing time. In addition to the different melting curves, fuses have different load-carrying capabilities. Manufacturer’s application tables show three load-current values: continuous, hot-load pickup, and cold-load pickup. Continuous load is the maximum current that is expected fo...

As can be seen from Fig. 9.33, step distance protection does not offer instantaneous clearing of faults over 100% of the line segment. In most cases this is unacceptable due to system stability considerations. To cover the 10–20% of the line not covered by Zone 1, the information regarding the location of the fault is transmitted from each terminal to the other terminal(s). A communication channel is used for this transmission. These pilot channels can be over power line carrier, microwave, fiber-optic, or wire pilot. Although the underlying principles are the same regardless of the pilot channel, there are specific design details that are imposed by this choice. Power line carrier uses the protected line itself as the channel, superimposing a high frequency signal on top of the 60 Hz power frequency. Since the line being protected is also the medium used to actuate the protective devices, a blocking signal is used. This means that a trip will occur at both ends of the line unless a s...