ENGINEERING ARTICLES

The basic requirement for security analysis is to assess the impact of any possible contingency on system performance. For the purpose of setting planning and operating rules that will enable the system to be operated in a secure manner, it is necessary to consider all credible contingencies, different network configurations, and different operating points for given performance criteria. Hence, in the deterministic approach, these assessments may involve a large number of computer simulations even if there is a selection process at each stage of the analysis. The decision in that case is founded on the requirement that each outage event in a specified list, the contingency set, results in system performance that satisfies the criteria of the chosen performance evaluation. To handle these assessments for all possible situations by an exhaustive study is generally not reasonable. Since the resulting security rules may lead to the settlement and schedule of investment needs as well as op...

A genetic algorithm (GA) is a search algorithm often used in nonlinear discrete optimization problems. The development of GAs was inspired by the biological notion of evolution. Initially described by John Holland, they were popularized by David Goldberg who described the basic genetic algorithm very well. In a GA, data, initialized randomly in a data structure appropriate for the solution to the problem, evolves over time and becomes a suitable answer to the problem. An entire population of candidate solutions (data structures with a form suitable for solving for the problem being studied) is “randomly” initialized and evolves according to GA rules. The data structures often consist of strings of binary numbers that are mapped onto the solution space for evaluation. Each solution (often termed a creature) is assigned a fitness; a heuristic measure of its quality. During the evolutionary process, those creatures having higher fitness are favored in the parent selection process and are...

Since the introduction of electricity supply to the public in the late 1800s, people in many parts of the world have grown to expect an inexpensive reliable source of electricity. Providing that electric energy economically and efficiently requires the generation company to carefully control their generating units, and to consider many factors that may affect the performance, cost, and profitability of their operation. The unit commitment and economic dispatch algorithms play an important part in deciding how to operate the electric generating units around the world. The introduction of competition has changed many of the factors considered in solving these problems. Furthermore, advancements in solution techniques offer a continuum of candidate algorithms, each having its own advantages and disadvantages. Research continues to push these algorithms further. This chapter has provided the reader with an introduction to the problems of determining optimal unit commitment schedules and e...

Training simulators were originally created as generic systems for introducing operators to the electrical and dynamic behavior of power systems. Today, they model actual power systems with reasonable fidelity and are integrated with EMS to provide a realistic environment for operators and dispatchers to practice normal, every-day operating tasks and procedures as well as experience emergency operating situations. The various training activities can be safely and conveniently practiced with the simulator responding in a manner similar to the actual power system. An operator training simulator (OTS) can be used in an investigatory manner to recreate past actual operational scenarios and to formulate system restoration procedures. Scenarios can be created, saved, and reused. The OTS can be used to evaluate the functionality and performance of new real-time EMS functions and also for tuning AGC in an off-line, secure environment. The OTS has three main subsystems (Fig. 12.4). ENERGY CON...

Power systems are designed to survive all probable contingencies. A contingency is defined as an event that causes one or more important components such as transmission lines, generators, and transformers to be unexpectedly removed from service. Survival means the system stabilizes and continues to operate at acceptable voltage and frequency levels without loss of load. Operations must deal with a vast number of possible conditions experienced by the system, many of which are not anticipated in planning. Instead of dealing with the impossible task of analyzing all possible system states, security control starts with a specific state: the current state if executing the real-time network sequence; a postulated state if executing a study sequence. Sequence means sequential execution of programs that perform the following steps: 1. Determine the state of the system based on either current or postulated conditions. 2. Process a list of contingencies to determine the consequences of each co...

Generation control and ED minimize the current cost of energy production and transmission within the range of available controls. Energy management is a supervisory layer responsible for economically scheduling production and transmission on a global basis and over time intervals consistent with cost optimization. For example, water stored in reservoirs of hydro plants is a resource that may be more valuable in the future and should, therefore, not be used now even though the cost of hydro energy is currently lower than thermal generation. The global consideration arises from the ability to buy and sell energy through the interconnected power system; it may be more economical to buy than to produce from plants under direct control. Energy accounting processes transaction information and energy measurements recorded during actual operation as the basis of payment for energy sales and purchases. Energy management includes the following functions: • System load forecast: Forecasts system...

SCADA, with its relatively expensive RTUs installed at distribution substations, can provide status and measurements for distribution feeders at the substation. Distribution automation equipment is now available to measure and control at locations dispersed along distribution circuits. This equipment can monitor sectionalizing devices (switches, interrupters, fuses), operate switches for circuit reconfiguration, control voltage, read customers’ meters, implement time-dependent pricing (on-peak, off-peak rates), and switch customer equipment to manage load. This equipment requires significantly increased functionality at distribution control centers. Distribution control center functionality varies widely from company to company, and the following list is evolving rapidly. • Data acquisition: Acquires data and gives the operator control over specific devices in the field, Includes data processing, quality checking, and storage. • Feeder switch control: Provides remote control of feeder...

Automatic generation control (AGC) consists of two major and several minor functions that operate online in real-time to adjust the generation against load at minimum cost. The major functions are load frequency control and economic dispatch, each of which is described below. The minor functions are reserve monitoring, which assures enough reserve on the system; interchange scheduling, which initiates and completes scheduled interchanges; and other similar monitoring and recording functions. LOAD FREQUENCY CONTROL Load frequency control (LFC) has to achieve three primary objectives, which are stated below in priority order: 1. To maintain frequency at the scheduled value 2. To maintain net power interchanges with neighboring control areas at the scheduled values 3. To maintain power allocation among units at economically desired values The first and second objectives are met by monitoring an error signal, called area control error (ACE), which is a combination of net interchange err...

Although the activities of engineers are quite varied, there are some personality traits and work habits that typify most of today’s successful engineers. • Engineers are problem solvers. • Good engineers have a firm grasp of the fundamental principles of engineering, which they can use to solve many different problems. • Good engineers are analytical, detailed oriented, and creative. • Good engineers have a desire to be lifelong learners. For example, they take continuing education classes, seminars, and workshops to stay abreast of innovations and new technologies. This is particularly important in today’s world because the rapid changes in technology will require you as an engineer to keep pace with new technologies. Moreover, you will risk being laid off or denied promotion if you are not continually improving your engineering education. • Good engineers, regardless of their area of specialization, have a core knowledge that can be applied to many areas. Therefore, well-trained en...

Engineers apply physical and chemical laws and principles and mathematics to design millions of products and services that we use in our everyday lives. These products include cars, computers, aircraft, clothing, toys, home appliances, surgical equipment, heating and cooling equipment, health care devices, tools and machines that makes various products, and so on. Engineers consider important factors such as cost, efficiency, reliability, and safety when designing these products. Engineers perform tests to make certain that the products they design withstand various loads and conditions. They are continuously searching for ways to improve already existing products as well. They also design and supervise the construction of buildings, dams, highways, and mass transit systems and the construction of power plants that supply power to manufacturing companies, homes, and offices. Engineers play a significant role in the design and maintenance of a nation’s infrastructure, including communi...

MODELING The basic concepts of transient stability are based on highly simplified models. Practical power systems consist of large numbers of generators, transmission circuits, and loads. For stability assessment, the power system is normally represented using a positive sequence model. The network is represented by a traditional positive sequence power flow model that defines the transmission topology, line reactances, connected loads and generation, and pre-disturbance voltage profile. Generators can be represented with various levels of detail, selected based on such factors as length of simulation, severity of disturbance, and accuracy required. The most basic model for synchronous generators consists of a constant internal voltage behind a constant transient reactance, and the rotating inertia constant (H). This is the so-called classical representation that neglects a number of characteristics: the action of voltage regulators, variation of field flux linkage, the impact of th...

Many factors affect the transient stability of a generator in a practical power system from which few are discussed here. THE POST-DISTURBANCE SYSTEM REACTANCE: The weaker the post-disturbance system, the lower P max  will be. THE DURATION OF THE FAULT CLEARING TIME: The longer the fault is applied, the longer the rotor will be accelerated and the more kinetic energy will be gained. The more energy that is gained during acceleration, the more difficult it is to dissipate it during deceleration. THE INERTIA OF THE GENERATOR: The higher the inertia, the slower the rate of change of angle and the less the kinetic energy gained during the fault. THE GENERATOR INTERNAL VOLTAGE AND INFINITE BUS VOLTAGE: The lower these voltages, the lower P max  will be. THE GENERATOR LOADING PRIOR TO THE DISTURBANCE: The higher the loading, the closer the unit will be to P max ,  which means that during acceleration, it is more likely to become unstable. THE GE...

Transient stability is an important consideration that must be dealt with during the design of power systems. In the design process, time-domain simulations are conducted to assess the stability of the system under various conditions and when subjected to various disturbances. Since it is not practical to design a system to be stable under all possible disturbances, design criteria specify the disturbances for which the system must be designed to be stable. The criteria disturbances generally consist of the more statistically probable events which could cause the loss of any system element and typically include three-phase faults cleared in normal time and line-to-ground faults with delayed clearing due to breaker failure. In most cases, stability is assessed for the loss of one element (such as a transformer or transmission circuit) with possibly one element out-of-service pre-disturbance. Therefore, in system design, a wide number of disturbances are assessed and if the system is fo...

While it is true that power systems are designed to be transiently stable, and many of the methods described above may be used to achieve this goal, in actual practice, systems may be prone to instability. This is largely due to uncertainties related to assumptions made during the design process. These uncertainties result from a number of sources, including: • LOAD AND GENERATION FORECAST : The design process must use forecast information about the amount, distribution, and characteristics of the connected loads, as well as the location and amount of connected generation. These all have a great deal of uncertainty. If the actual system load is higher than planned, the generation output will be higher, the system will be more stressed, and the transient stability limit may be significantly lower. • SYSTEM TOPOLOGY : Design studies generally assume all elements in service, or perhaps up to two elements out of service. In actual systems, there are usually many elements out of service ...

For reliable service, a power system must remain intact and be capable of withstanding a wide variety of disturbances. Owing to economic and technical limitations, no power system can be stable for all possible disturbances or contingencies. In practice, power systems are designed and operated so as to be stable for a selected list of contingencies, normally referred to as “design contingencies”. Experience dictates their selection. The contingencies are selected on the basis that they have a significant probability of occurrence and a sufficiently high degree of severity, given the large number of elements comprising the power system. The overall goal is to strike a balance between costs and benefits of achieving a selected level of system security. While security is primarily a function of the physical system and its current attributes, secure operation is facilitated by: Proper selection and deployment of preventive and emergency controls. Assessing stability limits and operating...

NEED FOR CLASSIFICATION Power system stability is a single problem; however, it is impractical to deal with it as such. Instability of the power system can take different forms and is influenced by a wide range of factors. Analysis of stability problems, including identifying essential factors that contribute to instability and devising methods of improving stable operation is greatly facilitated by classification of stability into appropriate categories. These are based on the following considerations: • The physical nature of the resulting instability related to the main system parameter in which instability can be observed. • The size of the disturbance considered indicates the most appropriate method of calculation and prediction of stability. • The devices, processes, and the time span that must be taken into consideration in order to determine stability. Figure 11.1 shows a possible classification of power system stability into various categories and subcategories. FIGURE...

Power system stability is the ability of the system, for a given initial operating condition, to regain a normal state of equilibrium after being subjected to a disturbance. Stability is a condition of equilibrium between opposing forces; instability results when a disturbance leads to a sustained imbalance between the opposing forces. The power system is a highly nonlinear system that operates in a constantly changing environment; loads, generator outputs, topology, and key operating parameters change continually. When subjected to a transient disturbance, the stability of the system depends on the nature of the disturbance as well as the initial operating condition. The disturbance may be small or large. Small disturbances in the form of load changes occur continually, and the system adjusts to the changing conditions. The system must be able to operate satisfactorily under these conditions and successfully meet the load demand. It must also be able to survive numerous disturbances ...

• Oscillations are due to natural modes of the system and therefore cannot be eliminated. However, their damping and frequency can be modified. • As power systems evolve, the frequency and damping of existing modes change and new ones may emerge. • The source of “negative” damping is power system controls, primarily excitation system automatic voltage regulators. • Interarea oscillations are associated with weak transmission links and heavy power transfers. • Interarea oscillations often involve more than one utility and may require the cooperation of all to arrive at the most effective and economical solution. • Power system stabilizers are the most commonly used means of enhancing the damping of interarea modes. • Continual study of the system is necessary to minimize the probability of poorly damped oscillations. Such “beforehand” studies may have avoided many of the problems experienced in power systems.. It must be clear that avoidance of oscillations is only one of many aspects...

The following methods can be used to mitigate voltage stability problems. MUST-RUN GENERATION: Operate uneconomic generators to change power flows or provide voltage support during emergencies or when new lines or transformers are delayed. SERIES CAPACITORS: Use series capacitors to effectively shorten long lines, thus decreasing the net reactive loss. In addition, the line can deliver more reactive power from a strong system at one end to one experiencing a reactive shortage at the other end. SHUNT CAPACITORS: Though the heavy use of shunt capacitors can be part of the voltage stability problem, sometimes additional capacitors can also solve the problem by freeing “spinning reactive reserve” in generators. In general, most of the required reactive power should be supplied locally, with generators supplying primarily active power. STATIC VAR COMPENSATORS (SVC): SVCs, the modern counterpart to the synchronous condenser, are effective in controlling voltage and preventing ...

There are three basic reasons for grounding a power system which are personal safety, protective device operation, and noise control. All three of these reasons will be addressed. PERSONAL SAFETY: The most important reason for grounding a device on a power system is personal safety. The safety ground, as it is sometimes called, is provided to reduce or eliminate the chance of a high touch potential if a fault occurs in a piece of electrical equipment. Touch potential is defined as the voltage potential between any two conducting materials that can be touched simultaneously by an individual or animal. Figure 15.2 illustrates a dangerous touch potential situation. The “hot” conductor in the piece of equipment has come in contact with the case of the equipment. Under normal conditions, with the safety ground intact, the protective device would operate when this condition occurred. However, in Fig. 15.2, the safety ground is missing. This allows the case of the equipment to fl...

In general, shunt compensators are classified depending on the technology used in their implementation. Rotating and static equipments were commonly used to compensate reactive power and to stabilize power systems. In the last decades, a large number of different static FACTS controllers, using power electronic technologies and digital control schemes have been proposed and developed. There are two approaches to the realization of power electronics-based compensators: the one that employs thyristor-switched capacitors and reactors with tap-changing transformers, and the other group that uses self-commutated static converters. A brief description of the most commonly used shunt compensators is presented below. FIXED OR MECHANICALLY SWITCHED CAPACITORS Shunt capacitors were first employed for power factor correction in the year 1914. The leading current drawn by the shunt capacitors compensates the lagging current drawn by the load. The selection of shunt capacitors depends on many...

Figure 25.1 shows the principles and theoretical effects of shunt reactive power compensation in a basic ac system, which comprises a source V 1 , a transmission line, and a typical inductive load. Figure 25.1 a shows the system without compensation, and its associated phasor diagram. In the phasor diagram, the phase angle of the current has been related to the load side, which means that the active current (I P ) is in phase with the load voltage V 2 . Since the load is assumed inductive, it requires reactive power for proper operation, which must be supplied by the source, increasing the current flow from the generator and through the lines. If reactive power is supplied near the load, the line current is minimized, reducing power losses and improving voltage regulation at the load terminals. This can be done with a capacitor, with a voltage source, or with a current source. In Fig. 25.1 b, a current-source device is being used to compensate the reactive component of the l...

Shunt compensation is used basically to control the amount of reactive power that flows through the power system. In a linear circuit, the reactive power is defined as the ac component of the instantaneous power, with a frequency equal to 100/120 Hz in a 50 or 60 Hz system. The reactive power generated by the ac power source is stored in a capacitor or a reactor during a quarter of a cycle, and in the next quarter cycle is sent back to the power source. The reactive power oscillates between the ac source and the capacitor or reactor, and also between them, at a frequency equal to two times the rated value (50 or 60 Hz). For this reason it can be compensated using static equipments or VAR generators, avoiding its circulation between the load (inductive or capacitive) and the source, and therefore improving voltage regulation and stability of the power system. Reactive power compensation can be implemented with VAR generators connected in parallel or in series.

Figures 24.6, 24.7, and 24.8 show three types of hybrid active/passive filters, the main purpose of which is to reduce initial costs and to improve efficiency. The shunt passive filter consists of one or more tuned LC filters and/ or a high-pass filter. Table 24.2 shows comparisons among the three hybrid filters in which the active filters are different in function from the passive filters. Note that the hybrid filters are applicable to any current harmonic source, although a harmonic-producing load is represented by a thyristor rectifier with a DC link inductor in Figs. 24.6, 24.7, and 24.8. Such a combination of a shunt active filter and a shunt passive filter as shown in Fig. 24.6 has already been applied to harmonic compensation of naturally-commutated twelve-pulse cycloconverters for steel mill drives. The passive filters absorbs 11th and 13th harmonic currents while the active filter compensates for 5th and 7th harmonic currents and achieves damping of harmonic reson...

Nonlinear loads drawing non-sinusoidal currents from utilities are classified into identified and unidentified loads. High-power diode/thyristor rectifiers, cycloconverters, and arc furnaces are typically characterized as identified harmonic-producing loads because utilities identify the individual nonlinear loads installed by high-power consumers on power distribution systems in many cases. The utilities determine the point of common coupling with high-power consumers who install their own harmonic producing loads on power distribution systems, and also can determine the amount of harmonic current injected from an individual consumer. A ‘‘single’’ low-power diode rectifier produces a negligible amount of harmonic current. However, multiple low-power diode rectifiers can inject a large amount of harmonics into power distribution systems. A low-power diode rectifier used as a utility interface in an electric appliance is typically considered as an unidentified harmonic-producing load. ...