ENGINEERING ARTICLES

A typical modern power system is a high-order multivariable process whose dynamic response is influenced by a wide array of devices with different characteristics and response rates. Stability is a condition of equilibrium between opposing forces. Depending on the network topology, system operating condition and the form of disturbance, different sets of opposing forces may experience sustained imbalance leading to different forms of instability. A systematic basis for classification of power system stability is given below. Need for Classification Power system stability is essentially a single problem; however, the various forms of instabilities that a power system may undergo cannot be properly understood and effectively dealt with by treating it as such. Because of high dimensionality and complexity of stability problems, it helps to make simplifying assumptions to analyze specific types of problems using an appropriate degree of detail of system representation and appropriate ...

“Power system stability is the ability of an electric power system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that practically the entire system remains intact”. The definition applies to an interconnected power system as a whole. Often, however, the stability of a particular generator or group of generators is also of interest. A remote generator may lose stability (synchronism) without cascading instability of the main system. Similarly, stability of particular loads or load areas may be of interest; motors may lose stability (run down and stall) without cascading instability of the main system. The power system is a highly nonlinear system that operates in a constantly changing environment; loads, generator outputs and key operating parameters change continually. When subjected to a disturbance, the stability of the system depends on the initial o...

The term alternative energy is referred to the energy produced in an environmentally friendly way (different from conventional means, i.e., through fossil-fuel power plants, nuclear power plants and hydropower plants). Alternative energy considered in this dissertation is either renewable or with high energy conversion efficiency. There is a broad range of energy sources that can be classified as alternative energy such as solar, wind, hydrogen (fuel cell), biomass, and geothermal energy. Nevertheless, as mentioned in the previous section, at present the majority of the world electricity is still generated by fossil fuels, nuclear power and hydropower. However, due to the following problems/concerns for conventional energy 5 technologies, the renewable/alternative energy sources will play important roles in electricity generation. And, sooner or later, today’s alternatives will become tomorrow’s main sources for electricity. Conventional generation technologies are not environme...

The five major factors that contribute to the renewed interest in distributed generation (DG) system: Electricity market liberalization  Developments in DG technologies  Increased customer demand for highly reliable electricity. Environmental concerns. Constraints on the construction of new transmission lines. ADVANTAGES OF DISTRIBUTED GENERATION SYSTEMS: UTILITY PERSPECTIVE: On-site power supply avoids transmission and distribution losses. Increasing the efficiency compared with central power generation. Diversification of power sources. A possible solution to constraints on new transmission lines. Provides cleaner power by using renewable sources such as wind and sun. Better quality of power. Hedge against uncertain load growth and high market. CUSTOMER PERSPECTIVE: Improving energy efficiency and reducing greenhouse- gas emission through combined heat and power (CHP) plants and renewable sources. Improved reliability by having back-up ge...

Distributed Generation (DG) also called as site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from the many small energy sources. In recent years, micro electric power systems such as photovoltaic generation systems, wind generators and micro gas turbines, etc., have increased with the deregulation and liberalization of the power market. Under such circumstances the environment surrounding the electric power industry has become ever more complicated and provides high-quality power in a stable manner which becomes an important topic. Here DG is assumed to include Wind power Generation (WG) and Fuel Cells (FC), etc. Wind energy is the world‘s fastest-growing energy technology. It is a clean energy source that is reliable, efficient and reduces the cost of energy for homeowners, farmers and businesses. Wind turbines can be used to produce electricity for a single home or building, o...

Main features of power MOSFETs are: a) MOS type FETs are basically majority carrier devices; consequently, they differ greatly from bipolar transistors which are minority carrier devices. b) MOS type FETs are not current controlled devices as are bipolar transistors. They are voltage controlled devices and are controlled by the voltage applied between the gate and source. c) Since they are majority carrier devices, high-frequency switching operation is possible because there is no storage time-lag due to the carrier storage effect. d) With bipolar transistors, current concentration occurs in the high-voltage area and junction failure occurs due to secondary breakdown. Therefore, bipolar transistors require considerable derating. Power MOSFETs, on the other hand, have a negative temperature coefficient which makes it difficult for secondary breakdown to occur, making these devices highly resistant to failure and thus enabling use right up to the maximum rating. e)...

The circuit, shown below, uses a UJT to trigger a SCR. The UJT is used to more accurately trigger the SCR. When the source voltage exceeds 20V, the zener diode (DZ) will begin to conduct, applying a DC voltage across the base connections of the UJT. At the same time, diode D1 will be forward biased, and the capacitor will quickly charge through R1 and R2. This represents the left-hand pedestal portion of the of the emitter voltage. Once the capacitor charges to the voltage across R3, D1 will become reverse biased and the capacitor will continue to slowly charge through R4. This represents the ramp portion of the emitter voltage. The capacitor continues to charge until the UJT fires. At this point the capacitor will quickly discharge through R6, and this represents the right-hand pedestal of the emitter voltage. The capacitor discharge is sufficient to trigger the SCR. Figure: RAMP AND PEDESTAL UJT-SCR CONTROL CIRCUIT The point at which the UJT fires can be adjusted by vary...

GTO being a monolithic p-n-p-n structure just like a thryistor its basic operating principle can be explained in a manner similar to that of a thyristor. In particular, the p-n-p-n structure of a GTO can be though of consisting of one p-n-p and one n-p-n transistor connected in the regenerative configuration as shown in Figure. Figure: Current distribution in a GTO (a) During turn on; (b) During turn off.  With applied forward voltage VAK less than the forward break over voltage both ICBO1 and ICBO2 are small. Further if IG is zero IA is only slightly higher than (ICBO1 + ICBO2). Under this condition both ∝n and ∝p are small and (∝p + ∝n) <<1. The device is said to be in the forward blocking mode. TURNING ON PROCESS OF GTO To turn the device on either the anode voltage can be raised until ICBO1 and ICBO2 increases by avalanche multiplication process or by injecting a gate current. The current gain ∝ of silicon transistors rises rapidly as the emitter curr...

In many engineering situations it is useful to scale, or normalize, dimensioned quantities. This is commonly done in power system analysis. The standard method used is referred to as the per-unit system. Historically, this was done to simplify numerical calculations that were made by hand. Although this advantage is eliminated by the calculator, other advantages remain. • Device parameters tend to fall into a relatively narrow range, making erroneous values conspicuous. • Using this method all quantities are expressed as ratios of some base value or values. • The per-unit equivalent impedance of any transformer is the same when referred to either the primary or the secondary side. • The per-unit impedance of a transformer in a three-phase system is the same regardless of the type of winding connections (wye-delta, delta-wye, wye-wye, or delta-delta). • The per-unit method is independent of voltage changes and phase shifts through transformers where the base voltages in the wi...

The heart of any power electronic circuit is its semiconductor-switching network. The question arises here as to whether we have to use switches to perform electrical power conversion from the source to the load. The answer, of course, is no, as there are many circuits that can perform energy conversion without switches, including linear regulators and power amplifiers. However, the need to use semiconductor devices to perform conversion functions is very much related to converter efficiency. In power electronic circuits, the semiconductor devices are generally operated as switches, that is, either in the on-state or the off-state. This is unlike the case for power amplifiers and linear regulators where semiconductor devices operate in the linear mode. As a result, a very large amount of energy is lost within the power circuit before the processed energy reaches the output. Semiconductor switching devices are used in power electronic circuits because of their ability to control and man...