ENGINEERING ARTICLES

Saturday, September 26, 2015

FACTORS LIMITING POWER FLOW THROUGH TRANSMISSION LINE

By Engr. Aneel Kumar September 26, 2015 No comments:

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.

Typically, stability limits may determine the maximum allowable power flow on lines that are more than 150 miles in length. For very long lines, the power flow limitation may be less than the SIL as shown in Table. Stability limits on power flow can be as low as 20% of the line’s thermal limit.

Typical stability limits as a function of system voltage are:

Table: Power Flow Limits on Lines and Cables.

VOLTAGE DROP LIMITS

In addition to electrical phase shift, voltage magnitude decreases with distance. Generally, for transmission lines, the maximum allowable drop in voltage is limited to between 5% and 10% of the sending end bus voltage. The power flow (in MVA or MW) that corresponds to the maximum allowable decrease in voltage magnitude is called the line’s voltage drop limit. As with phase shift, a transmission line’s “voltage drop limit” decreases with transmission distance and is generally higher than the line’s thermal limit for short lines but less than the line’s stability limit for very long lines.

“Voltage drop” normally limits power flow on HV or EHV lines that are between 50 and 150 miles in length. Voltage drop limits on power flow can be as low as 40% of the line’s thermal limit.

Voltage drop limits may be increased by the addition of shunt capacitors at the end of the line. Such solutions are typically much cheaper than rebuilding the line.

THERMAL LIMITS

Thermal power flow limits on overhead lines are intended to limit the temperature attained by the energized conductors and the resulting sag and loss of tensile strength. In most cases, the maximum conductor temperature applied to modern transmission lines reflect ground clearance concerns rather than annealing of aluminum.

Thermal limits, as typically calculated, are not a function of line length. Thus for a given line design, a line 1 km long and one 500 km long typically have the same thermal limit. Thermal limits usually determine the maximum power flow for lines less than 50 miles in length.

There are a number of possible methods by which the MVA thermal capacity of an existing line may be increased. Some of these methods are technically straightforward, such as reinforcing the structures and restringing the line with a larger conductor. These methods come at a price, however. In addition to the dollar cost involved, there is construction out on the line, and either outage time or special construction methods to allow service while the work is in progress.

Other methods of thermal uprating, such as the use of weather dependent dynamic thermal ratings or voltage uprating by reduction from normal phase spacing, may require little or no line outage time and less capital investment than reconductoring and reinforcing the structures. The price here lies in the greater degree of technical sophistication required to ensure safe and reliable operation at higher loadings.

LINE LOADABILITY IN AC LINES

By Engr. Aneel Kumar September 26, 2015 No comments:

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 shunt. These compensating elements effectively reduce the line reactance and increase the shunt susceptance, thereby decreasing the surge impedance. Thus the effective SIL of a capacitor compensated line is higher than an uncompensated line. This increases the loadability of a long line.

Since total conductor cross-sectional area for EHV lines is mainly decided by electric field considerations (corona), these lines have large thermal capabilities, much in excess of the SIL. For long EHV lines, one cannot deviate much from SIL due to voltage constraints. Therefore, the thermal limit of a long EHV line is not the key limiting factor. However, thermal limit is the main limiting factor for short lines (< 100 km) wherein voltage constraints are not violated even for large deviations from SIL.

KIRCHHOFFS LAWS

By Engr. Aneel Kumar September 20, 2015 No comments:

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

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 sum of currents leaving the node. If the sum of the currents entering a node were not equal to zero, then the charge would be accumulating at a node. However, a node is a perfect conductor and cannot accumulate or store charge. Thus, the sum of currents entering a node is equal to zero.

Figure1: Kirchhoffs current law

KIRCHHOFF’S VOLTAGE LAW

Kirchhoff’s voltage law (KVL) states that the algebraic sum of voltages around any closed path in a circuit is zero.

In general, the mathematical representation of Kirchhoff’s voltage law is

∑Nj=1Vj(t)=0

Where Vj(t) is the voltage across the jth branch (with proper reference direction) in a loop containing N voltages.

In Kirchhoff’s voltage law, the algebraic sign is used to keep track of the voltage polarity.

In other words, as we traverse the circuit, it is necessary to sum the increases and decreases in voltages to zero. Therefore, it is important to keep track of whether the voltage is increasing or decreasing as we go through each element. We will adopt a policy of considering the increase in voltage as negative and a decrease in voltage as positive.

Consider the circuit shown in Figure 2, where the voltage for each element is identified with its sign. The ideal wire used for connecting the components has zero resistance, and thus the voltage across it is equal to zero. The sum of voltages around the loop incorporating V6, V3, V4, V5 is

−V6−V3+V4+V5=0

The sum of voltages around a loop is equal to zero. A circuit loop is a conservative system, meaning that the work required to move a unit charge around any loop is zero.

However, it is important to note that not all electrical systems are conservative. Example of a non-conservative system is a radio wave broadcasting system.

Figure2: Kirchhoff’s voltage law

ACTIVE CIRCUIT ELEMENTS (ENERGY SOURCES)

By Engr. Aneel Kumar September 20, 2015 No comments:

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 voltage or current elsewhere in the circuit. We cannot specify the value of a dependent source, unless you know the value of the voltage or current on which it depends.

The circuit symbols for ideal independent sources are shown in Figure (a) and (b).

Note that a circle is used to represent an independent source. The circuit symbols for dependent sources are shown in Figure (c), (d), (e) and (f). A diamond symbol is used to represent a dependent source.

Figure: (a) An ideal independent voltage source (b) An ideal independent current source (c) Voltage controlled voltage source (d) Current controlled voltage source (e) Voltage controlled current source (f) Current controlled current source

FUNCTIONS OF SCADA SYSTEMS

By Engr. Aneel Kumar September 19, 2015 No comments:

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, to the change of indications on the display.

IMPORTANCE OF SWING BUS

By Engr. Aneel Kumar September 19, 2015 No comments:

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 0⁰, 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 excitation which vary or swing as the demand changes. This variation is such as to produce fixed voltage.

CLASSIFICATION OF POWER SYSTEM BUSES

By Engr. Aneel Kumar September 19, 2015 No comments:

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 is controlled using the excitation and the power is controlled using the prime mover control (as you have studied in the last experiment). Sometimes, this bus is connected to a VAR device where the voltage can be controlled by varying the value of the injected VAR to the bus.

3. LOAD BUS

This bus is not connected to a generator so that neither its voltage nor its real power can be controlled. On the other hand, the load connected to this bus will change the active and reactive power at the bus in a random manner. To solve the load flow problem we have to assume the complex power value (real and reactive) at this bus.

Table: Classification of power system buses

LIMITATIONS OF GAUSS SEIDEL METHOD FOR LOAD FLOW ANALYSIS

By Engr. Aneel Kumar September 19, 2015 No comments:

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.

WHAT IS LOAD FLOW STUDIES

By Engr. Aneel Kumar September 19, 2015 No comments:

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 transient stability studies. Hence, load flow studies play a vital role in power system studies.

Thus the load flow problem consists of finding the power flows (real and reactive) and voltages of a network for given bus conditions. At each bus, there are four quantities of interest to be known for further analysis: the real and reactive power, the voltage magnitude and its phase angle. Because of the nonlinearity of the algebraic equations, describing the given power system, their solutions are obviously, based on the iterative methods only. The constraints placed on the load flow solutions could be:

The Kirchhoff’s relations holding well,
Capability limits of reactive power sources,
Tap-setting range of tap-changing transformers,
Specified power interchange between interconnected systems,
Selection of initial values, acceleration factor, convergence limit, etc.

ELECTRICAL NETWORK THEOREMS

By Engr. Aneel Kumar September 19, 2015 No comments:

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 respective internal impedances (if any).

Note. It may be noted that independent sources can be ‘killed’ i.e. removed leaving behind their internal impedances (if any) but dependent sources should not be killed.

2) THEVENIN’S THEOREM:

As applicable to AC networks, this theorem may be stated as follows:

The current through a load impedance ZL connected across any two terminals A and B of a linear network is given by Vth/(Zth + ZL) where Vth is the open-circuit voltage across A and B and Zth is the internal impedance of the network as viewed from the open-circuited terminals A and B with all voltage sources replaced by their internal impedances (if any) and current sources by infinite impedance.

3) RECIPROCITY THEOREM

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 current source and an ideal voltmeter in any linear bilateral network does not change the voltmeter reading.

However, the voltages between other nodes would generally not remain the same. The ratio of the input current between one set of nodes to output voltage between another set of nodes is called the transfer admittance.

4) NORTON’S THEOREM:

As applied to AC networks, this theorem can be stated as under:

Any two terminal active linear network containing voltage sources and impedances when viewed from its output terminals is equivalent to a constant current source and a parallel impedance. The constant current is equal to the current which would flow in a short-circuit placed across the terminals and the parallel impedance is the impedance of the network when viewed from open-circuited terminals after voltage sources have been replaced by their internal impedances (if any) and current sources by infinite impedance.

5) MAXIMUM POWER TRANSFER:

For any power source, the maximum power transferred from the power source to the load is when the resistance of the load R_L is equal to the equivalent or input resistance of the power source (R_in = R_Th or R_N). The process used to make R_L = R_in is called impedance matching.

This theorem is particularly useful for analyzing communication networks where the goals is transfer of maximum power between two circuits and not highest efficiency.

Cases:

1. When load is purely resistive and adjustable, MPT is achieved when R_L = | Z_g | = √ ( R2_g + X²_g).

2. When both load and source impedances are purely resistive (i.e. XL= X_g= 0), MPT is achieved when RL = Rg.

3. When RL and XL are both independently adjustable, MPT is achieved when XL= -X_gand R_L = Rg.

4. When XL is fixed and RL is adjustable, MPT is achieved when RL = √ [R2_g + (X_g+ XL)2]

6) MILLMAN’S THEOREM

It permits any number of parallel branches consisting of voltage sources and impedances to be reduced to a single equivalent voltage source and equivalent impedance. Such multi-branch circuits are frequently encountered in both electronics and power applications.

RECIPROCITY THEOREM

By Engr. Aneel Kumar September 19, 2015 No comments:

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.

In other words, the interchange of an ideal current source and an ideal voltmeter in any linear bilateral network does not change the voltmeter reading.

KIRCHHOFFS LAWS

By Engr. Aneel Kumar September 19, 2015 No comments:

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.

CHARACTERISTICS OF MOVING COIL METER MOVEMENT

By Engr. Aneel Kumar September 18, 2015 No comments:

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 resistance of the wire of the moving coil. A movement with smaller Im has higher Rm and vice versa. Typical values of Rm range from 1.2 Ω for a 30 mA movement to 2 kΩ for a 50 μA movement.

3. SENSITIVITY (S)
It is also known as current sensitivity or sensitivity factor. It is given by the reciprocal of full scale deflection current Im.

S=1/ Im ohms/volt

The sensitivity of a meter movement depends on the strength of the permanent magnet and number of turns in the coil. Larger the number of turns, smaller the amount of current required to produce full-scale deflection and, hence, higher the sensitivity. A high current sensitivity means a high quality meter movement. It also determines the lowest range that can be covered when the meter movement is modified as an ammeter or voltmeter.

MEASUREMENT STANDARDS

By Engr. Aneel Kumar September 18, 2015 No comments:

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 laboratory.

NEED FOR ELECTRICAL ISOLATION IN SWITCH-MODE DC POWER SUPPLIES

By Engr. Aneel Kumar September 13, 2015 No comments:

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.

DELETERIOUS EFFECTS OF HARMONIC DISTORTION AND A POOR POWER FACTOR

By Engr. Aneel Kumar September 13, 2015 No comments:

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 CONVERTER TOPOLOGIES

By Engr. Aneel Kumar September 13, 2015 No comments:

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, control AC motors, or even connect DC devices that are connected to the grid. Similar to a DC/DC converter, the input to an inverter can be a stiff source such as battery, solar cell, or fuel cell or can be from an intermediate DC link that can be supplied from an AC source.

AC/DC CONVERTER:

This type of converter is also known as ‘‘Rectifier’’. Usually the AC input to the circuit is a sinusoidal voltage source that operates at 120 V, 60 Hz or a 230 V, 50 Hz, which are used for power distribution applications. The AC voltage is rectified into a unidirectional DC voltage, which can be used directly to supply power to a DC resistive load or control a DC motor. In some applications the DC voltage is subjected to further conversion using a DC/DC or DC/AC converter. A rectifier is typically used as a front-end circuit in many power system applications. If not applied correctly, rectifiers can cause harmonics and low power factor when they are connected to the power grid.

AC/AC CONVERTER:

This circuit is more complicated than the previous converters because AC conversion requires change of voltage, frequency, and bipolar voltage blocking capabilities, which usually requires complex device topologies. Converters that have the same fundamental input and output frequencies are called ‘‘AC controllers’’. The conversion is from a fixed voltage fixed frequency (FVFF) to a variable voltage fixed frequency (VVFF). Applications include: light dimmers and control of single-phase AC motors that are typically used in home appliances. When both voltage and frequency are changed, the circuits are called ‘‘Cycloconverters’’, which convert a FVFF to variable voltage variable frequency (VVVF) and when fully controlled switches are used, this class of circuit is called ‘‘Matrix Converter’’. Another way of achieving AC/AC conversion is by using AC/DC and DC/AC through an intermediate DC link. This type of combined converter approach can be complex as the correct control approach must be implemented including simultaneous regulation of the DC link, injection of power with a prescribed power factor and bidirectional control of energy flow.

CLASSIFICATIONS OF POWER SWITCHES

By Engr. Aneel Kumar September 12, 2015 No comments:

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 type.

FULLY CONTROLLED SWITCH:

The switch can be turned-on and off via the control terminal. Examples of this switch are the BJT, the MOSFET, the IGBT, the GTO thyristor, and the MOS-controlled thyristor (MCT).

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Friday, September 11, 2015

Engr. Aneel Kumar

WHAT IS POWER ELECTRONICS

By Engr. Aneel Kumar September 11, 2015 No comments:

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 different modulations schemes.

AC-AC (conversion)

Cycloconverter, Matrix Converter, AC choppers

Figure 2: a Power Electronic System with Four Possible Conversions

Power electronics can be used in different categories such as:

1. Switch-mode (dc) power supplies and uninterruptible power supplies

2. Energy conservation

3. Process control and factory automation

4. Transportation

Power electronics are widely used in modern days in applications where power processing is required such as:

Residential: Refrigeration and freezers, Air conditioning, Cooking, Lighting and Electronics.

Commercial: Uninterruptible power supplies (UPS), Heating, ventilating, and air conditioning, Lighting, and Computers and office equipment.

Industrial: Pump, Compressors, Machine tools, Welding and Induction heating.

Transportation: Battery chargers for electric vehicles, Cars, Buses, Subways and automotive electronics including engine controls.

Utility systems: High-voltage dc transmission (HVDC), Static VAR compensation (SVC), Supplemental energy sources (wind, photovoltaic), fuel cells, Energy storage systems and Induced-draft fans and boiler feed water pumps.

Aerospace: Space shuttle power supply systems, Satellite power systems and Aircraft power systems.

Telecommunications: Battery chargers and Power supplies (dc and UPS).

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Thursday, September 10, 2015

Engr. Aneel Kumar

EFFECT OF FEEDBACK ON STABILITY

By Engr. Aneel Kumar September 10, 2015 No comments:

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 relation of the overall system is

It is apparent that although the properties of G and H are such that the inner-loop feedback system is unstable, because GH = -1, the overall system can be stable by properly selecting the outer-loop feedback gain F. In practice, GH is a function of frequency, and the stability condition of the closed-loop system depends on the magnitude and phase of GH. The bottom line is that feedback can improve stability or be harmful to stability if it is not properly applied. Sensitivity considerations often are important in the design of control systems. Because all physical elements have properties that change with environment and age, we cannot always consider the parameters of a control system to be completely stationary over the entire operating life of the system. For instance, the winding resistance of an electric motor changes as the temperature of the motor rises during operation. Control systems with electric components may not operate normally when first turned on because of the still changing system parameters during warm-up. This phenomenon is sometimes called "morning sickness." Most duplicating machines have a warm-up period during which time operation is blocked out when first turned on. In general, a good control system should be very insensitive to parameter variations but sensitive to the input commands. We shall investigate what effect feedback has on sensitivity to parameter variations. We consider G to be a gain parameter that may vary. The sensitivity of the gain of the overall system M to the variation in G is defined as

Where dM denotes the incremental change in M due to the incremental change in G, or dG. The sensitivity function is written

This relation shows that if GH is a positive constant, the magnitude of the sensitivity function can be made arbitrarily small by increasing GH, provided that the system remains stable. It is apparent that, in an open-loop system, the gain of the system will respond in a one-to-one fashion to the variation in G. Again, in practice, GH is a function of frequency; the magnitude of 1+GH may be less than unity over some frequency ranges, so feedback could be harmful to the sensitivity to parameter variations in certain cases. In general, the sensitivity of the system gain of a feedback system to parameter variations depends on where the parameter is located. The reader can derive the sensitivity of the system in Fig due to the variation of H.

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Engr. Aneel Kumar

EFFECT OF FEEDBACK ON OVERALL GAIN

By Engr. Aneel Kumar September 10, 2015 No comments:

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

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Wednesday, September 09, 2015

Engr. Aneel Kumar

CLOSED LOOP CONTROL SYSTEMS

By Engr. Aneel Kumar September 09, 2015 No comments:

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 output signal is subtracted from the input signal. The result is generally called the actuating signal.

Figure: Closed-Loop Control Systems (Feedback Control Systems)

The closed-loop system compensates for disturbances by measuring the output response, feeding that measurement back through a feedback path, and comparing that response to the input at the summing junction. If there is any difference between the two responses, the system drives the plant, via the actuating signal, to make a correction. If there is no difference, the system does not drive the plant, since the plant's response is already the desired response.

Closed-loop systems, then, have the obvious advantage of greater accuracy than open-loop systems. They are less sensitive to noise, disturbances, and changes in the environment. Closed-loop systems are more complex and expensive than open-loop systems.

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Engr. Aneel Kumar

OPEN LOOP CONTROL SYSTEMS

By Engr. Aneel Kumar September 09, 2015 No comments:

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) of a toaster is the color of the toast. The device is designed with the assumption that the toast will be darker the longer it is subjected to heat. The toaster does not measure the color of the toast; it does not correct for the fact that the toast is rye, white, or sourdough, nor does it correct for the fact that toast comes in different thicknesses.

The distinguishing characteristic of an open-loop system is that it cannot compensate for any disturbances that add to the controller's driving signal (Disturbance 1 in Fig.). For example, if the controller is an electronic amplifier and Disturbance 1 is noise, then any additive amplifier noise at the first summing junction will also drive the process, corrupting the output with the effect of the noise.

Figure: Open loop control system (Non-feedback System)

The output of an open-loop system is corrupted not only by signals that add to the controller's commands but also by disturbances at the output (Disturbance 2 in fig). The system cannot correct for these disturbances. Other examples of open-loop systems are mechanical systems consisting of a mass, spring, and damper with a constant force positioning the mass.

The greater the force, the greater the displacement. Again, the system position will change with a disturbance, such as an additional force, and the system will not detect or correct for the disturbance.

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