ENGINEERING ARTICLES

It depends on what is limiting the power flow and how much of an increase is needed to solve the problem. In most circumstances, power flow limits are the result of concerns over electrical phase shift, voltage drop or thermal effects in lines, cables or substation equipment. SURGE IMPEDANCE LOADING LIMITS As power flows along a transmission line, there is an electrical phase shift, which increases with distance and with power flow. As this phase shift increases, the system in which the line is embedded can become increasingly unstable during electrical disturbances. Typically, for very long lines, the power flow must be limited to what is commonly called the Surge Impedance Loading (SIL) of the line. Surge Impedance Loading is equal to the product of the end bus voltages divided by the characteristic impedance of the line. Since the characteristic impedance of various HV and EHV lines is not dissimilar, the SIL depends approximately on the square of system voltage. T...

In order to prevent over voltages at light loads, it is necessary to have devices for absorbing reactive power (like shunt reactors) not only at either end of a long line but even at intermediate points. Generators connected at the ends of the line have limited reactive power absorption capability as defined by their capability curves. If transmission redundancy exists (i.e., parallel transmission paths exist), then a very lightly loaded long line may be tripped to avoid overvoltage. However this may be detrimental to system security if some additional line trippings take place due to faults. If shunt reactors are permanently connected, they result in large sags in the voltage under heavy loading conditions. Moreover, reactive power demanded by long transmission lines under these situations may be excessive and may lead to system-wide low voltage conditions. Compensation of a line involves changing the effective line parameters by connecting (lumped) capacitors in series and shun...

As the network becomes complex, application of Ohm’s law for solving the networks becomes tedious and hence time consuming. For solving such complex networks, we make use of Kirchhoff’s laws. Gustav Kirchhoff (1824-1887), an eminent German physicist, did a considerable amount of work on the principles governing the behaviour of electric circuits. He gave his findings in a set of two laws: (i) current law and (ii) voltage law, which together are known as Kirchhoff’s laws. KIRCHHOFF'S CURRENT LAW The first law is Kirchhoff’s current law (KCL), which states that the algebraic sum of currents entering any node is zero. Let us consider the node shown in Figure 1. The sum of the currents entering the node is -i a +i b -i c +i d =0 Or i a -i b +i c -i d =0 Which simply states that the algebraic sum of currents leaving a node is zero. Alternately, we can write the equation as i b +i d =i a +i c Which states that the sum of currents entering a node is equal to the s...

An active two-terminal element that supplies energy to a circuit is a source of energy. An ideal voltage source is a circuit element that maintains a prescribed voltage across the terminals regardless of the current flowing in those terminals. Similarly, an ideal current source is a circuit element that maintains a prescribed current through its terminals regardless of the voltage across those terminals. These circuit elements do not exist as practical devices, they are only idealized models of actual voltage and current sources. Ideal voltage and current sources can be further described as either independent sources or dependent sources. An independent source establishes a voltage or current in a circuit without relying on voltages or currents elsewhere in the circuit. The value of the voltage or current supplied is specified by the value of the independent source alone. In contrast, a dependent source establishes a voltage or current whose value depends on the value of the ...

A SCADA System typically provides the following functions: • Comprehensive monitoring of primary and secondary plant • Secure control of primary plant • Supervision of secondary plant • Operator controlled display of non-SCADA data • Alarm management • Event logging • Sequence of events recording • Trend recording All functions must be provided with a high level of security and reliability. The control system itself must be highly self-monitoring and problems brought immediately to the operator’s attention. Operator access must also be protected by a security system. In addition, certain performance standards are required, for both data acquisition and the user interface. For example, time recording of events to one millisecond resolution is now possible. Whilst user interface performance is less critical, operators expect that their actions will result in display delays measured in only a few seconds: for example, from the execution of a circuit breaker control...

The slack or swing bus is usually a PV-bus with the largest capacity generator of the given system connected to it. The generator at the swing bus supplies the power difference between the “specified power into the system at the other buses” and the “total system output plus losses”. Thus swing bus is needed to supply the additional real and reactive power to meet the losses. Both the magnitude and phase angle of voltage are specified at the swing bus, or otherwise, they are assumed to be equal to 1.0 pu and 0 0 , as per flat-start procedure of iterative solutions. The real and reactive powers at the swing bus are found by the computer routine as part of the load flow solution process. It is to be noted that the source at the swing bus is a perfect one, called the swing machine, or slack machine. It is voltage regulated, i.e., the magnitude of voltage fixed. The phase angle is the system reference phase and hence is fixed. The generator at the swing bus has a torque angle and excitati...

Each bus in the system has four variables: voltage magnitude, voltage angle, real power and reactive power. During the operation of the power system, each bus has two known variables and two unknowns. Generally, the bus must be classified as one of the following bus types: 1. SLACK OR SWING BUS This bus is considered as the reference bus. It must be connected to a generator of high rating relative to the other generators. During the operation, the voltage of this bus is always specified and remains constant in magnitude and angle. In addition to the generation assigned to it according to economic operation, this bus is responsible for supplying the losses of the system. 2. GENERATOR OR VOLTAGE CONTROLLED BUS During the operation the voltage magnitude at this the bus is kept constant. Also, the active power supplied is kept constant at the value that satisfies the economic operation of the system. Most probably, this bus is connected to a generator where the voltage i...

GS method is very useful for very small systems. It is easily adoptable, it can be generalized and it is very efficient for systems having less number of buses. However, GS LFA fails to converge in systems with one or more of the features as under: • Systems having large number of radial lines • Systems with short and long lines terminating on the same bus • Systems having negative values of transfer admittances • Systems with heavily loaded lines, etc. GS method successfully converges in the absence of the above problems. However, convergence also depends on various other set of factors such as: selection of slack bus, initial solution, acceleration factor, tolerance limit, level of accuracy of results needed, type and quality of computer/ software used, etc.

Load flow studies are important in planning and designing future expansion of power systems. The study gives steady state solutions of the voltages at all the buses, for a particular load condition. Different steady state solutions can be obtained, for different operating conditions, to help in planning, design and operation of the power system. Generally, load flow studies are limited to the transmission system, which involves bulk power transmission. The load at the buses is assumed to be known. Load flow studies throw light on some of the important aspects of the system operation, such as: violation of voltage magnitudes at the buses, overloading of lines, overloading of generators, stability margin reduction, indicated by power angle differences between buses linked by a line, effect of contingencies like line voltages, emergency shutdown of generators, etc. Load flow studies are required for deciding the economic operation of the power system. They are also required in transi...

Electric circuit theorems are always beneficial to help find voltage and currents in multi loop circuits. These theorems use fundamental rules or formulas and basic equations of mathematics to analyze basic components of electrical or electronics parameters such as voltages, currents, resistance, and so on. These fundamental theorems include the basic theorems like Superposition theorem, Norton’s theorem, Maximum power transfer theorem and Thevenin’s theorems. Other group of network theorems which are mostly used in the circuit analysis process includes Reciprocity theorem and Millman’s theorem. 1) SUPERPOSITION THEOREM: As applicable to AC networks, it states as follows:  In any network made up of linear impedances and containing more than one source of emf, the current flowing in any branch is the phasor sum of the currents that would flow in that branch if each source were considered separately, all other emf sources being replaced for the time being, by their respectiv...

This theorem applies to networks containing linear bilateral elements and a single voltage source or a single current source. This theorem may be stated as follows: If a voltage source in branch A of a network causes a current of 1 branch B, then shifting the voltage source (but not its impedance) of branch B will cause the same current I in branch A. It may be noted that currents in other branches will generally not remain the same. A simple way of stating the above theorem is that if an ideal voltage source and an ideal ammeter are inter-changed, the ammeter reading would remain the same. The ratio of the input voltage in branch A to the output current in branch B is called the transfer impedance. Similarly, if a current source between nodes 1 and 2 causes a potential difference of V between nodes 3 and 4, shifting the current source (but not its admittance) to nodes 3 and 4 causes the same voltage V between nodes 1 and 2. In other words, the interchange of an ideal...

The statements of Kirchhoff’s laws are shown in Art. For DC networks except that instead of algebraic sum of currents and voltages, we take phasors or vector sums for AC networks. 1. KIRCHHOFF’S CURRENT LAW: According to this law, in any electrical network, the phasors sum of the currents meeting at a junction is zero.  In other words, ∑ I = 0 ----- at a junction. Put in another way, it simply means that in any electrical circuit the phasors sum of the currents flowing towards a junction is equal to the phasors sum of the currents going away from that junction.  2. KIRCHHOFF’S VOLTAGE LAW: According to this law, the phasors sum of the voltage drops across each of the conductors in any closed path (or mesh) in a network plus the phasors sum of the emfs connected in that path is zero.  In other words, ∑ IR + ∑ emf = 0 ---- round a mesh.

Following are few characteristics of Moving Coil Meter Movement. (1) Full-scale deflection current (Im), (2) Internal resistance of the coil (Rm), (3) Sensitivity (S). 1. FULL-SCALE DEFLECTION CURRENT (IM) It is the current needed to deflect the pointer all the way to the right to the last mark on the calibrated scale. Typical values of Im for D’ Arsonval movement vary from 2 μA to 30 mA. It should be noted that for smaller currents, the number of turns in the moving coil has to be more so that the magnetic field produced by the coil is strong enough to react with the field of the permanent magnet for producing reasonable deflection of the pointer. Fine wire has to be used for reducing the weight of the moving coil but it increases its resistance. Heavy currents need thick wire but lesser number of turns so that resistance of the moving coil is comparatively less. The schematic symbol is shown in Figure. 2. INTERNAL RESISTANCE (RM) It is the dc ohmic r...

All instruments, whether electrical or electronic, are calibrated at the time of manufacture against a measurement standard. 1. INTERNATIONAL STANDARDS These are defined by international agreement and are maintained at the international Bureau of Weights and Measurements in Paris. 2. PRIMARY STANDARDS These are maintained at national standards laboratories in each country. They are not available for use outside these laboratories. Their principal function is to calibrate and verify the secondary standards used in industry. 3. SECONDARY STANDARDS These are the basic reference standards used by industrial laboratories and are maintained by the particular industry to which they belong. They are periodically sent to national laboratory for calibration and verification against primary standards. 4. WORKING STANDARDS These are the main tools of a measurement laboratory and are used to check and calibrate the instrument used in the labora...

Electrical isolation by means of transformers is needed in switch-mode dc power supplies for three reasons: SAFETY : It is necessary for the low-voltage dc output to be isolated from the utility supply to avoid the shock hazard. DIFFERENT REFERENCE POTENTIALS : The dc supply may have to operate at a different potential, for example, the dc supply to the gate drive for the upper MOSFET in the power-pole is referenced to its Source. VOLTAGE MATCHING : If the dc-dc conversion is large, then to avoid requiring large voltage and current ratings of semiconductor devices, it may be economical and operationally more suitable to use an electrical transformer for conversion of voltage levels.

There are several deleterious effects of high distortion in the current waveform and the poor power factor that results due to it. These are as follows: Power loss in utility equipment such as distribution and transmission lines, transformers, and generators increases, possibly to the point of overloading them. Harmonic currents can overload the shunt capacitors used by utilities for voltage support and may cause resonance conditions between the capacitive reactance of these capacitors and the inductive reactance of the distribution and transmission lines. The utility voltage waveform will also become distorted, adversely affecting other linear loads, if a significant portion of the load supplied by the utility draws power by means of distorted currents.

Power electronic converters are switch-mode circuits that process power between two electrical systems using power semiconductor switches. The electrical systems can be either DC or AC. Therefore, there are four possible types of converters; namely DC/DC, DC/AC, AC/DC, and AC/AC. The four converter types are described below: DC/DC CONVERTER : is also known as ‘‘Switching Regulator’’. The circuit will change the level voltage available from a DC source such as a battery, solar cell, or a fuel cell to another DC level, either to supply a DC load or to be used as an intermediate voltage for an adjacent power electronic conversion such as a DC/AC converter. DC/DC converters coupled together with AC/DC converters enable the use of high voltage DC (HVDC) transmission which has been adopted in transmission lines throughout the world. DC/AC CONVERTER : Also described as ‘‘Inverter’’ is a circuit that converts a DC source into a sinusoidal AC voltage to supply AC loads, c...

The concept of the ideal switch is important when evaluating circuit topologies. Assumptions of zero-voltage drop, zero-leakage current, and instantaneous transitions make it easier to simulate and model the behavior of various electrical designs. Using the characteristics of an ideal switch, there are three classes of power switches: UNCONTROLLED SWITCH : The switch has no control terminal. The state of the switch is determined by the external voltage or current conditions of the circuit in which the switch is connected. A diode is an example of such switch. SEMI-CONTROLLED SWITCH : In this case the circuit designer has limited control over the switch. For example, the switch can be turned-on from the control terminal. However, once ON, it cannot be turned-off from the control signal. The switch can be switched off by the operation of the circuit or by an auxiliary circuit that is added to force the switch to turn-off. A thyristor or a SCR is an example of this switch t...

The aim of power electronics is to optimize the power efficiency, minimal size, minimal weight and meeting the requirements for user loads by modifying the voltages and currents. Fig.1 shows a block diagram of a power electronic system. Power processors, depending on the application, the output of the load may have the following forms DC: Regulated or adjustable magnitude. AC: Constant frequency and adjustable magnitude or adjustable frequency and adjustable magnitude. Figure 1: Block Diagram of a Power Electronic System Power conversions (converters) consist of four different conversion functions as shown in Figure 2 and described in below.  AC-DC (rectification)  Possibly control DC voltage and AC current  Examples: Diode rectifiers and thyristor rectifiers.  DC-DC (conversion)   Modify and control voltage magnitude.  Examples: Buck and Boost Converters.  DC-AC (inversion)  Single and three-phase converters and ...

Stability is a notion that describes whether the system will be able to follow the input command, that is, be useful in general. In a non-rigorous manner, a system is said to be unstable if its output is out of control. To investigate the effect of feedback on stability, from the below above. If GH = - 1, the output of the system is infinite for any finite input, and the system is said to be unstable. Therefore, we may state that feedback can cause a system that is originally stable to become unstable. Certainly, feedback is a double-edged sword; when it is improperly used, it can be harmful. It should be pointed out, however, that we are only dealing with the static case here and in general, GH = — 1 is not the only condition for instability. It can be demonstrated that one of the advantages of incorporating feedback is that it can stabilize an unstable system If we introduce another feedback loop through a negative Feedback gain of F, as shown in Fig. given below, the input-output r...

Feedback is used to reduce the error between the reference input and the system output. Feedback also has effects on such system performance characteristics as stability, bandwidth, overall gain, impedance, and sensitivity. Feedback affects the gain G of a non-feedback system by a factor of 1 + GH. The system of Fig. give below is said to have negative feedback, because a minus sign is assigned to the feedback signal. The quantity GH may itself include a minus sign, so the general effect of feedback is that it may increase or decrease the gain G. In a practical control system, G and H are functions of frequency, so the magnitude of 1 - GH may be greater than 1 in one frequency range but less than 1 in another. Therefore, feedback could increase the gain of system in one frequency range but decrease it in another. Feedback may increase the gain of a system in one frequency range but decrease it in another. Figure: Effect of Feedback on Overall Gain

Closed-Loop Control Systems is also known as Feedback Control Systems. Disadvantages of open loop control system are corrected through the close loop control system. The input transducer converts the form of the input to the form used by the controller. An output transducer, or sensor, measures the output response and converts it into the form used by the controller. For example, if the controller uses electrical signals to operate the valves of a temperature control system, the input position and the output temperature are converted to electrical signals. The input position can be converted to a voltage by a potentiometer, a variable resistor, and the output temperature can be converted to a voltage by a thermistor. A device whose electrical resistance changes with temperature. The first summing junction algebraically adds the signal from the input to the signal from the output, which arrives via the feedback path, the return path from the output to the summing junction. The...

Open loop system is also known as non-feedback system. An open-loop control system is shown in Fig. It starts with a subsystem called an input transducer, which converts the form of the input to that used by the controller. The controller drives a process or a plant. The input is sometimes called the reference, while the output can be called the controlled variable. Other signals, such as disturbances, are shown added to the controller and process outputs via summing junctions, which yield the algebraic sum of their input signals using associated signs. For example, the plant can be a furnace or air conditioning system, where the output variable is temperature. The controller in a heating system consists of fuel valves and the electrical system that operates the valves. Open-loop systems, then, do not correct for disturbances and are simply commanded by the input. For example, toasters are open-loop systems, as anyone with burnt toast can attest. The controlled variable (output) o...