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...