ENGINEERING ARTICLES

The growing number of power electronics base equipment has produced an important impact on the quality of electric power supply. Both high power industrial loads and domestic loads cause harmonics in the network voltages. At the same time, much of the equipment causing the disturbances is quite sensitive to deviations from the ideal sinusoidal line voltage. Therefore, power quality problems may originate in the system or may be caused by the consumer itself. For an increasing number of applications, conventional equipment is proving insufficient for mitigation of power quality problems. Harmonic distortion has traditionally been dealt with by the use of passive LC filters. However, the application of passive filters for harmonic reduction may result in parallel resonances with the network impedance, over compensation of reactive power at fundamental frequency, and poor flexibility for dynamic compensation of different frequency harmonic components. The increased severity of powe...

The electrical powers generated are either transferred onto a bus to be distributed (small scale), or into a power grid for transmission purposes (larger scale). This is done either directly or through power transformers, depending on the generated voltage and the required voltage of the bus or power grid. The next step is power transmission, whereby the generated electrical potential energy is transmitted via transmission lines, usually over long distances, to high-voltage (HV) substations. High-voltage substations will usually tap directly into the power grid, with two or more incoming supplies to improve reliability of supply to that substation’s distribution network. A typical electrical power network is illustrated in Figure 1. Electrical transmission is normally done via high to extra high voltages, in the range of 132–800 kV. Mega volt systems are now being developed and implemented in the USA. The longer the distance, the more economical higher voltages become. Figur...

VAR compensation is defined as the management of reactive power to improve the performance of ac power systems; maximizing stability by increasing flow of active power. Few Problems forced while reactive power compensation which are: 1. Load compensation 2. Voltage support LOAD COMPENSATION OBJECTIVES : are to increase the value of the system power factor to balance the real power drawn from the ac supply, compensate voltage regulation and to eliminate current harmonic components produced by large and fluctuating non-linear industries loads. VOLTAGE SUPPORT OBJECTIVES : It’s generally required to reduce voltage fluctuations at a given terminal of a transmission line. VAR compensation helps to maintain a substantially flat voltage profile at all levels of power transmission improves HVDC conversion terminal performance increases transmission efficiency ,controls steady state and temporary over-voltage and can avoid disastrous blackout. Series and shunt VAR compensation a...

SVC : Uses thyristor valves to rapidly add or remove shunt connected reactors and or capacitors often in coordination with mechanically controlled reactors and/or capacitors. NGH-SSR DAMPER : a resonance damper, a thyristor ac-switch connected in series with a small inductor and resistor across the series capacitor. STATCOM (static condenser) : A 3 phase inverter that is driven from voltage across a dc storage capacitor and whose there output voltages are in phase with the ac system voltage. When the output voltages are higher or lower than the ac system voltage the current flow is caused to lead or lag and difference in voltage amplitudes determine how much current flows. Reactive power and its polarity can be controlled by controlling voltage. PHASE ANGLE REGULATOR : The phase shift is accomplished by adding or subtracting a variable voltage concept that is perpendicular to the phase voltage of the line. UNIFIED POWER CONTROL : In this concept an ac voltage vector gen...

Flexible transmission system is akin to high voltage dc and related thyristors developed designed to overcome the limitations of the present mechanically controlled ac power transmission system.  Use of high speed power electronics controllers, gives 5 opportunities for increased efficiency. Greater control of power so that it flows in the prescribed transmission routes.  Secure loading (but not overloading) of transmission lines to levels nearer their required limits.  Greater ability to transfer power between controlled areas, so that the generator reserve margin- typically 18 % may be reduced to 15 % or less.  Prevention of cascading outages by limiting the effects of faults and equipment failure.  Damping of power system oscillations, which could damage equipment and or limit usable transmission capacity.  Flexible system requires tighter transmission control and efficient management of inter-related parameters that constrains today’s system ...

A bus is a node at which one or many lines, one or many loads and generators are connected. In a power system each node or bus is associated with 4 quantities, such as magnitude of voltage, phage angle of voltage, active or true power and reactive power in load flow problem two out of these 4 quantities are specified and remaining 2 are required to be determined through the solution of equation. Depending on the quantities that have been specified, the buses are classified into 3 categories. Buses are classified according to which two out of the four variables are specified LOAD BUS No generator is connected to the bus. At this bus the real and reactive power are specified.it is desired to find out the voltage magnitude and phase angle through load flow solutions. It is required to specify only P d  and Q d  at such bus as at a load bus voltage can be allowed to vary within the permissible values. GENERATOR BUS OR VOLTAGE CONTROLLED BUS Here the voltage magnitu...

1) Power flow analysis is very important in planning stages of new networks or addition to existing ones like adding new generator sites, meeting increase load demand and locating new transmission sites. 2) The load flow solution gives the nodal voltages and phase angles and hence the power injection at all the buses and power flows through interconnecting power channels. 3) It is helpful in determining the best location as well as optimal capacity of proposed generating station, substation and new lines. 4) It determines the voltage of the buses. The voltage level at the certain buses must be kept within the closed tolerances. 5) System transmission loss minimizes. 6) Economic system operation with respect to fuel cost to generate all the power needed 7) The line flows can be known. The line should not be overloaded, it means, we should not operate the close to their stability or thermal limits.

Transient stability is very much affected by the type of the fault. A three phase dead short circuit is the most severe fault; the fault severity decreasing with two phase fault and single line-to ground fault in that order. If the fault is farther from the generator the severity will be less than in the case of a fault occurring at the terminals of the generator. Power transferred during fault also plays a major role. When, part of the power generated is transferred to the load, the accelerating power is reduced to that extent. Theoretically an increase in the value of inertia constant M reduces the angle through which the rotor swings farther during a fault. However, this is not a practical proposition since, increasing M means, increasing the dimensions of the machine, which is uneconomical. The dimensions of the machine are determined by the output desired from the machine and stability cannot be the criterion. Also, increasing M may interfere with speed governing system. Thus look...

The controlled rectifier circuit is divided into three main circuits, (1) POWER CIRCUIT This is the circuit contains voltage source, load and switches as diodes, thyristors or IGBTs. (2) CONTROL CIRCUIT This circuit is the circuit, which contains the logic of the firing of switches that may, contains amplifiers, logic gates and sensors. (3) TRIGGERING CIRCUIT This circuit lies between the control circuit and power thyristors. Sometimes this circuit called switch drivers circuit. This circuit contains buffers, opt coupler or pulse transformers. The main purpose of this circuit is to separate between the power circuit and control circuit. The thyristor is normally switched on by applying a pulse to its gate.  The forward drop voltage is so small with respect to the power circuit voltage, which can be neglected. When the anode voltage is greater than the cathode voltage and there is positive pulse applied to its gate, the thyristor will turn on. The thyristor can be nat...

DIAC (Diode for Alternating Current) is like a TRIAC without a gate terminal. DIAC conducts current in both directions depending on the voltage connected to its terminals. When the voltage between the two terminals greater than the breakdown voltage, the DIAC conducts and the current goes in the direction from the higher voltage point to the lower voltage one. The following figure shows the layers construction, electric circuit symbol and the operating characteristics of the DIAC. Figure 1 shows the DIAC construction and electric symbol. Figure 2 shows a DIAC V-I characteristics. Figure1: DIAC construction and schematic symbol The DIAC used in firing circuits of thyristors since its breakdown voltage used to determine the firing angle of the thyristor. Figure2: DIAC V-I characteristics

In the center tap full wave rectifier, current flows through the load in the same direction for both half cycles of input AC voltage. The circuit shown in Figure has two diodes D1 and D2 and a center tapped transformer. The diode D1 is forward bias “ON” and diode D2 is reverse bias “OFF” in the positive half cycle of input voltage and current flows from point a to point b. Whereas in the negative half cycle the diode D1 is reverse bias “OFF” and diode D2 is forward bias “ON” and again current flows from point a to point b. Hence DC output is obtained across the load. Figure: Center-tap diode rectifier ADVANTAGES OF CENTER TAP DIODE RECTIFIER • The need for center-tapped transformer is eliminated, • The output is twice that of the center tapped circuit for the same secondary voltage, and, • The peak inverse voltage is one half of the center-tap circuit. DISADVANTAGES OF CENTER TAP DIODE RECTIFIER • It requires four diodes instead of two, in full wave circuit, and, • Ther...

TRIAC are used for the control of power in AC circuits. A TRIAC is equivalent of two reverse parallel-connected SCRs with one common gate. Conduction can be achieved in either direction with an appropriate gate current. A TRIAC is thus a bi-directional gate controlled thyristor with three terminals. Figure 1 shows the schematic symbol of a TRIAC. The terms anode and cathode are not applicable to TRIAC. Figure 2 shows the V-I characteristics of the TRIAC. Figure1: Schematic symbol of a TRIAC Figure2: Operating characteristics of TRIAC

A GTO thyristor can be turned on by a single pulse of positive gate current like conventional thyristor, but in addition it can be turned off by a pulse of negative gate current. The gate current therefore controls both ON state and OFF state operation of the device. GTO V-I characteristics is shown in Figure 2. The GTO has many advantages and disadvantages with respect to conventional thyristor among which few are discussed here. Figure1: Schematic Symbol of GTO Figure2: V-I characteristics of GTO ADVANTAGE OF GTO OVER THYRISTOR 1- Elimination of commutating components in forced commutation resulting in reduction in cost, weight and volume, 2- Reduction in acoustic and electromagnetic noise due to the elimination of commutation chokes, 3- Faster turn OFF permitting high switching frequency, 4- Improved converters efficiency, and, 5- It has more di/dt rating at turn ON. ADVANTAGE OF THYRISTOR OVER GTO 1- ON state voltage drop and associated losses are higher in...

The thyristor is the most important type of the power semiconductor devices. They are used in very large scale in power electronic circuits. The thyristor are known also as Silicon Controlled Rectifier (SCR). The thyristor has been invented in 1957 by general electric company in USA. The thyristor consists of four layers of semiconductor materials (p-n-p-n) all brought together to form only one unit. Figure 1 shows the schematic diagram of this device and its symbolic representation. The thyristor has three terminals, anode A, cathode K and gate G as shown in Figure 1.The anode and cathode are connected to main power circuit. The gate terminal is connected to control circuit to carry low current in the direction from gate to cathode. Figure1: Schematic diagram of SCR and its circuit symbol. The operational characteristics of a thyristor are shown in Figure 2 In case of zero gate current and forward voltage is applied across the device i.e. anode is positive with respect to ca...

The V-I characteristics of the silicon diode and germanium diode. The icon used to represent the diode is drawn in the upper left corner of the figure, together with the polarity markings used in describing the characteristics. The icon 'arrow' itself suggests an intrinsic polarity reflecting the inherent non-linearity of the diode characteristic. As shown in the figure the diode characteristics have been divided into three ranges of operation for purposes of description. Diodes operate in the forward- and reverse-bias ranges. Forward bias is a range of 'easy' conduction, i.e., after a small threshold voltage level (>> 0.7 volts for silicon) is reached a small voltage change produces a large current change. In this case the diode is forward bias or in "ON" state. The 'breakdown' range on the left side of the figure happened when the reverse applied voltage exceeds the maximum limit that the diode can withstand. At this range the diode destro...

There are many bad effects of harmonics on the power system components. These bad effects can derated the power system component or it may destroy some devices in severe cases. The following is the harmonic effects on power system components. IN TRANSFORMERS AND REACTORS • The eddy current losses increase in proportion to the square of the load current and square harmonics frequency, • The hysterics losses will increase, • The loading capability is derated by harmonic currents, • Possible resonance may occur between transformer inductance and line capacitor. IN CAPACITORS • The life expectancy decreases due to increased dielectric losses that cause additional heating, reactive power increases due to harmonic voltages, and, • Over voltage can occur and resonance may occur resulting in harmonic magnification. IN CABLES • Additional heating occurs in cables due to harmonic currents because of skin and proximity effects which are function of frequency, and, • The  I...

There are several striking features of power electronics, the foremost among them being the extensive use of inductors and capacitors. In many applications of power electronics, an inductor may carry a high current at a high frequency. The implications of operating an inductor in this manner are quite a few, such as necessitating the use of litz wire in place of single-stranded or multi-stranded copper wire at frequencies above 50 kHz, using a proper core to limit the losses in the core, and shielding the inductor properly so that the fringing that occurs at the air-gaps in the magnetic path does not lead to electromagnetic interference. Usually the capacitors used in a power electronic application are also stressed. It is typical for a capacitor to be operated at a high frequency with current surges passing through it periodically. This means that the current rating of the capacitor at the operating frequency should be checked before its use. In addition, it may be preferable if the c...

Rectifiers can be classified as uncontrolled and controlled rectifiers, and the controlled rectifiers can be further divided into semi-controlled and fully controlled rectifiers. Uncontrolled rectifier circuits are built with diodes, and fully controlled rectifier circuits are built with SCRs. Both diodes and SCRs are used in semi-controlled rectifier circuits. There are several rectifier configurations. The most famous rectifier configurations are listed below. • Single-phase semi-controlled bridge rectifier, • Single-phase fully-controlled bridge rectifier, • Three-phase three-pulse, star-connected rectifier, • Double three-phase, three-pulse star-connected rectifiers with inter-phase transformer (IPT), • Three-phase semi-controlled bridge rectifier, • Three-phase fully-controlled bridge rectifier, • Double three-phase fully controlled bridge rectifiers with IPT. Apart from the configurations listed above, there are series-connected and 12-pulse rectifiers for deliverin...

Whenever faults occur in power system large currents flow. Especially, if the fault is a dead short circuit at the terminals or bus bars enormous currents flow damaging the equipment and its components. To limit the flow of large currents under these circumstances current limiting reactors are used. These reactors are large coils covered for high self-inductance. They are also so located that the effect of the fault does not affect other parts of the system and is thus localized. From time to time new generating units are added to an existing system to augment the capacity. When this happens, the fault current level increases and it may become necessary to change the switch gear. With proper use of reactors addition of generating units does not necessitate changes in existing switch gear. CONSTRUCTION OF REACTORS These reactors are built with non-magnetic core so that saturation of core with consequent reduction in inductance and increased short circuit currents is avoided. Al...

Knowledge of short circuit current values is necessary for the following reasons. 1. Fault currents which are several times larger than the normal operating currents produce large electro-magnetic forces and torques which may adversely affect the stator end windings. The forces on the end windings depend on both the dc and ac components of stator currents. 2. The electro dynamic forces on the stator end windings may result in displacement of the coils against one another. This may result in loosening of the support or damage to the insulation of the windings. 3. Following a short circuit, it is always recommended that the mechanical bracing of the end windings to checked for any possible loosening. 4. The electrical and mechanical forces that develop due to a sudden three phase short circuit are generally severe when the machine is operating under loaded condition. 5. As the fault is cleared within 3 cycles generally the heating efforts are not considerable. Short circuits may occur in...

In the analysis of symmetrical three-phase short circuits the following assumptions are generally made. 1. Transformers are represented by their leakage reactances. The magnetizing current, and core fusses are neglected. Resistances, shunt admittances are not considered. Star-delta phase shifts are also neglected. 2. Transmission lines are represented by series reactances. Resistances and shunt admittances are neglected. 3. Synchronous machines are represented by constant voltage sources behind sub-transient reactances. Armature resistances, saliency and saturation are neglected. 4. All non-rotating impedance loads are neglected. 5. Induction motors are represented just as synchronous machines with constant voltage source behind a reactance. Smaller motor loads are generally neglected.

PER UNIT SYSTEM The per-unit system expressed the voltages, currents, powers, impedances, and other electrical quantities basis by the equation: Quantity per unit (pu) = Actual value/ Base value of quantity ADVANTAGES OF PER UNIT SYSTEM While performing calculations, referring quantities from one side of the transformer to the other side serious errors may be committed. This can be avoided by using per unit system. Voltages, currents and impedances expressed in per unit do not change when they are referred from one side of transformer to the other side. This is a great advantage. Per unit impedances of electrical equipment of similar type usually lie within a narrow range, when the equipment ratings are used as base values. Transformer connections do not affect the per unit values. Manufacturers usually specify the impedances of machines and transformers in per unit or percent of name plate ratings. Transformers can be replaced by their equivalent series impedances. ...

LOAD BUS A bus where there is only load connected and no generation exists is called a load bus. At this bus real and reactive load demand P d  and Q d  are drawn from the supply. The demand is generally estimated or predicted as in load forecast or metered and measured from instruments. Quite often, the reactive power is calculated from real power demand with an assumed power factor. A load bus is also called a P, Q bus. Since the load demands P d  and Q d  are known values at this bus. The other two unknown quantities at a load bus are voltage magnitude and its phase angle at the bus. In a power balance equation P d  and Q d  are treated as negative quantities since generated powers P g  and Q g  are assumed positive. VOLTAGE CONTROLLED BUS OR GENERATOR BUS A voltage controlled bus is any bus in the system where the voltage magnitude can be controlled. The real power developed by a synchronous generator can be varied b: changing the pri...

Power flow studies are performed to determine voltages, active and reactive power etc. at various points in the network for different operating conditions subject to the constraints on generator capacities and specified net interchange between operating systems and several other restraints. Power flow or load flow solution is essential for continuous evaluation of the performance of the power systems so that suitable control measures can be taken in case of necessity. In practice it will be required to carry out numerous power flow solutions under a variety of conditions. NECESSITY FOR POWER FLOW STUDIES Power flow studies are undertaken for various reasons, some of which are the following: I. The line flows 2. The bus voltages and system voltage profile 3. The effect of change in configuration and incorporating new circuits on system loading 4. The effect of temporary loss of transmission capacity and (or) generation on system loading and accompanied effects. 5. The effec...

The magnetic rotor field is generated by a field winding F on the rotor which is fed with an adjustable direct current. In addition, the rotor has a short circuited damper winding D at the surface. This winding serves to dampen electrical and mechanical oscillations and to shield the field winding from inverse rotating fields in case of asymmetries or harmonics in the stator currents. (In rotors without an explicitly realized damper winding, eddy currents in the rotor iron can have a similar effect.) Depending on the application of the generator, two different types of rotors are used that are shown in Figure. Figure:  Cross-sections through different rotor types. ROUND ROTOR Round rotors are used with high-speed turbines such as steam or gas turbines. For this reason, generators with round rotors are also called turbo generators. They can have ratings as high as 1800 MVA per unit. Due to the large centrifugal forces, the rotor consists of a long, narrow, solid steel c...