ENGINEERING ARTICLES

Friday, February 20, 2015

ACTIVE FILTERS OR ACTIVE POWER LINE CONDITIONERS

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

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 power quality in power networks has attracted the attention of power engineers to develop dynamic and adjustable solutions to the power quality problems. Such equipment, generally known as active filters, are also called active power line conditioners, and are able to compensate current and voltage harmonics, reactive power, regulate terminal voltage, suppress flicker, and to improve voltage balance in three phase systems.

Advantage of active power filter:

The advantage of active filtering is that it automatically adapts to changes in the network and load fluctuations. They can compensate for several harmonic orders, and are not affected by major changes in network characteristics, eliminating the risk of resonance between the filter and network impedance. Another plus is that they take up very little space compared with traditional passive compensators.

WHAT IS ELECTRICAL POWER DISTRIBUTION

By Engr. Aneel Kumar February 15, 2015 No comments:

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.

Figure1: Typical electrical power network

Normally, the transmission voltage will be transformed at the HV substation to a lower voltage for distribution purposes. This is due to the fact that distribution is normally done over shorter distances via underground cables. The insulation properties of three-phase cables limit the voltage that can be utilized, and lower voltages, in the medium-voltage range, are more economical for shorter distances. Figure 2 is a schematically illustration of a typical power grid.

Critical medium-voltage (MV) distribution substations will generally also have two or more incoming supplies from different HV substations. Main distribution substations usually supply power to a clearly defined distribution network, for example, a specific plant or factory, or for town/city reticulation purposes.

Figure2: Typical power grid

Power distribution is normally done on the medium-voltage level, in the range of 6.6–33 kV. Three-phase power is transferred, mostly via overhead lines or 3-core MV power cables buried in trenches. Single-core-insulated cables are also used, although less often. Low-voltage distribution is also done over short distances in some localized areas.

A power distribution network will therefore typically include the following:

• HV/MV power transformer(s) (secondary side)
• MV substation and switch-gear
• MV power cables (including terminations)
• MV/LV power transformer(s) (primary side).

The distribution voltage is then transformed to low voltage (LV), either for lighting and small power applications, or for electrical motors, which is usually fed from a dedicated motor control center (MCC). This is illustrated in Figure 3.

Note: Voltage levels are defined internationally, as follows:

• Low voltage: up to 1000 V
• Medium voltage: above 1000 V up to 36 kV
• High voltage: above 36 kV

Supply standards variation between continents by two general standards have emerged as the dominant ones:

• In Europe

IEC governs supply standards
The frequency is 50 Hz and LV voltage is 230/400 V

• In North America

IEEE/ANSI governs supply standards
The frequency is 60 Hz and the LV voltage is 110/190 V.

Overhead lines are far cheaper than underground cables for long distances, mainly due to the fact that air is used as the insulation medium between phase conductors, and that no excavation work is required. The support masts of overhead lines are quite a significant portion of the costs, that is the reason why aluminum lines are often used instead of copper, as aluminum lines weigh less than copper, and are less expensive. However, copper has a higher current conducting capacity than aluminum per square mm, so once again the most economical line design will depend on many factors.

Figure3: Typical power distribution network

Overhead lines are by nature prone to lightning strikes, causing a temporary surge on the line, usually causing flash-over between phases or phase to ground. The line insulators are normally designed to relay the surge to ground, causing the least disruption and/or damage. This is of short duration, and as soon as it is cleared, normal operation may be resumed. This is why sophisticated auto-reclosers are employed on an increasing number of overhead lines. Overhead lines have the following properties:

Advantages of Overhead lines

• Less expensive for longer distances
• Easy to locate fault.

Disadvantages of Overhead lines

• More expensive for shorter distances
• Susceptible to lightning
• Not environment-friendly
• Maintenance intensive
• High level of expertise and specialized equipment needed for installation.

Underground (buried) cable installations are mostly used for power distribution in industrial applications. They have the following properties:

Advantages of Underground cable

• Less expensive for shorter distances
• Not susceptible to lightning
• Environment-friendly
• Not maintenance intensive.

Disadvantages of Underground cable

• Expensive for long distances
• Can be difficult to locate fault.

The focus of this manual will be on MV power distribution, specifically practical considerations regarding MV switch-gear, power cables, power factor correction and computer simulation studies.

VAR COMPENSATION

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

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 are used to modify the natural electrical characteristic of ac power system. Series compensation modifies the transmission or distribution system parameters while shunt compensation changes the equivalent impedance of the load.

Earlier, rotating synchronous condensers and fixed or mechanically switched capacitors or inductors have been used for reactive power compensation. Now a days static VAR compensators employing thyristor switched capacitors and thyristor controlled reactor to provide or absorb the required reactive power have been developed. Use of self-commutated PWM convertors with appropriate control scheme permits the implementation of static compensators capable of generating or absorbing reactive current components with faster time response.

FACTS SYSTEM CONTROLLER

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

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 generated by a thyristor based inverter is injected in series with phase voltage. The driving dc voltage for inverter is obtained by rectifying the ac to dc from the same transmission line. In such an arrangement the injected voltage may have any phase angle relationship to the phase voltage. It is possible to obtain a net phase and amplitude voltage change that confers control of both active and reactive power.

DYNAMIC BRAKE: A shunt connected resistive load, controlled by thyristor switches. such a load can be selectively applied in each pass, half cycle by half cycle to damp any specific power flow oscillation, so that generating unit run less risk of losing synchronism, as a result more can be transferred over systems subjected to stability constraints.

A thyristor controlled resistor in parallel with the transmission line can be used effectively to damp power swing oscillations in the transmission system. FACT technology ensures power flow through prescribed routes, maximization of capacity, securing loading capacity enhancement under various scenarios of uprating or upgrading the lines thermal current capacity. One of the important function of FACT is VAR compensation.

TYPES	ATTRIBUTES
NGH- SSR Damper	Damping of oscillation, series impedance control, transient stability
SVC-static var-compensator	Voltage control, var-compensation damping of oscillation
TCSC-Thyristor controlled series capacitor	Power control, voltage control, series impedance control, damping of oscillations, transient stability
Static-condensor	Voltage control, VAR-compensator damping of oscillations, transient stability.
Thyristor controlled phase angle regulator	Power control, voltage control, var-compensator, damping of oscillation, transient stability.
Thyristor controlled dynamic brake	Damping of oscillation, transient stability.

FLEXIBLE AC TRANSMISSION SYSTEM FACTS

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

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 including:

Series impedance- phase angle.
Shunt impedance- occurrence of oscillations at various frequencies below rated frequency.

This results in transmission line to operate near its thermal rating. E.g. a 1000kv line may have loading limit 3000-4000Mw .but the thermal limit may be 5000Mw.

CLASSIFICATION OF ELECTRICAL POWER SYSTEM BUSES

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

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 magnitude corresponding to the generator voltage and real power P_g corresponds to its rating are specified. It is required to find out the reactive power generation Q_g and phase angle of the bus voltage.

SLACK (SWING) BUS

For the Slack Bus, it is assumed that the voltage magnitude |V| and voltage phase Θ are known, whereas real and reactive powers P_g and Q_g are obtained through the load flow solution.

OBJECTIVES OF LOAD FLOW STUDY

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

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.

FACTORS AFFECTING TRANSIENT STABILITY

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

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 looking at the swing equations

The possible methods that may improve the transient stability are:

(i) Increase of system voltages, and use of automatic voltage regulators.
(ii) Use of quick response excitation systems
(iii) Compensation for transfer reactance X_I2 so that P_e increases and P_m - P_e = P_a reduces.
(iv) Use of high speed circuit breakers which reduce the fault duration time and hence the accelerating power.

When faults occur, the system voltage drops. Support to the system voltages by automatic voltage controllers and fast acting excitation systems will improve the power transfer during the fault and reduce the rotor swing.

Reduction in transfer reactance is possible only when parallel lines are used in place of single line or by use of bundle conductors. Other theoretical methods such as reducing the spacing between the conductors-and increasing the size of the conductors are not practicable and are uneconomical.

Quick opening of circuit breakers and single pole reclosing is helpful. Since majority of the faults are line to ground faults selective single pole opening and reclosing will ensure transfer of power during the fault and improve stability.

THYRISTOR AS A CONTROLLED CONVERTERS

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

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 naturally turned off if the voltage of its anode becomes less than its cathode voltage or it can be turned off by using commutation circuit. If the voltage of its anode is become positive again with respect to its cathode voltage the thyristor will not turn on again until gets a triggering pulse to its gate.

The method of switching off the thyristor is known as Thyristor commutation. The thyristor can be turned off by reducing its forward current below its holding current or by applying a reverse voltage across it. The commutation of thyristor is classified into two types,

1- NATURAL COMMUTATION

If the input voltage is AC, the thyristor current passes through a natural zero, and a reverse voltage appear across the thyristor, which in turn automatically turned off the device due to the natural behavior of AC voltage source. This is known as natural commutation or line commutation. This type of commutation is applied in AC voltage controller rectifiers and cycloconverters. In case of DC circuits, this technique does not work as the DC current is unidirectional and does not change its direction. Thus the reverse polarity voltage does not appear across the thyristor. The following technique work with DC circuits:

2- FORCED COMMUTATION

In DC thyristor circuits, if the input voltage is DC, the forward current of the thyristor is forced to zero by an additional circuit called commutation circuit to turn off the thyristor. This technique is called forced commutation. Normally this method for turning off the thyristor is applied in choppers.

There are many thyristor circuits we cannot present all of them. In the following items we are going to present and analyze the most famous thyristor circuits. By studying the following circuits you will be able to analyze any other circuit.

DIAC

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

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

CENTER TAP DIODE RECTIFIER

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

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,
• There are always two diodes in series are conducting. Therefore, total voltage drop in the internal resistance of the diodes and losses are increased.

BIDIRECTIONAL TRIODE THYRISTOR TRIAC

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

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

GATE TURN OFF THYRISTOR GTO

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

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 GTO than thyristor,
2- Triggering gate current required for GTOs is more than those of thyristor,
3- Latching and holding current is more in GTO than those of thyristor,
4- Gate drive circuit loss is more than those of thyristor,
5- Its reverse voltage block capability is less than its forward blocking capability.

SEMICONDUCTORS SWITCH THYRISTOR

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

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 cathode, junction J1 and J3 are forward bias while J2 remains reverse biased, and therefore the anode current is so small leakage current. If the forward voltage reaches a critical limit, called forward break over voltage, the thyristor switches into high conduction, thus forward biasing junction J2 to turn thyristor ON in this case the thyristor will break down. The forward voltage drop then falls to very low value (1 to 2 Volts). The thyristor can be switched to on state by injecting a current into the central p type layer via the gate terminal. The injection of the gate current provides additional holes in the central p layer, reducing the forward break over voltage. If the anode current falls below a critical limit, called the holding current IH the thyristor turns to its forward state.

Figure2: Thyristor V-I characteristics.

If the reverse voltage is applied across the thyristor i.e. the anode is negative with respect to cathode, the outer junction J1 and J3 are reverse biased and the central junction J2 is forward biased. Therefore only a small leakage current flows. If the reverse voltage is increased, then at the critical breakdown level known as reverse breakdown voltage, an avalanche will occur at J1 and J3 and the current will increase sharply. If this current is not limited to safe value, it will destroy the thyristor.

The gate current is applied at the instant turn on is desired. The thyristor turn on provided at higher anode voltage than cathode. After turn on with IA reaches a value known as latching current, the thyristor continuous to conduct even after gate signal has been removed. Hence only pulse of gate current is required to turn the Thyristor ON.

SEMICONDUCTORS SWITCH DIODE

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

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 destroyed. On the other hand if the polarity of the voltage is reversed the current flows in the reverse direction and the diode operates in 'reverse' bias or in "OFF" state. The theoretical reverse bias current is very small.

Figure The diode V-I characteristics

In practice, while the diode conducts, a small voltage drop appears across its terminals. However, the voltage drop is about 0.7 V for silicon diodes and 0.3 V for germanium diodes, so it can be neglected in most electronic circuits because this voltage drop is small with respect to other circuit voltages. So, a perfect diode behaves like normally closed switch when it is forward bias (as soon as its anode voltage is slightly positive than cathode voltage) and open switch when it is in reverse biased (as soon as its cathode voltage is slightly positive than anode voltage). There are two important characteristics have to be taken into account in choosing diode. These two characteristics are:

• PEAK INVERSE VOLTAGE (PIV): Is the maximum voltage that a diode can withstand only so much voltage before it breaks down. So if the PIV is exceeded than the PIV rated for the diode, then the diode will conduct in both forward and reverse bias and the diode will be immediately destroyed.

• MAXIMUM AVERAGE CURRENT: Is the average current that the diode can carry.

It is convenient for simplicity in discussion and quite useful in making estimates of circuit behavior (rather good estimates if done with care and understanding) to linearize the diode characteristics. Instead of a very small reverse-bias current the idealized model approximates this current as zero. (The practical measure of the appropriateness of this approximation is whether the small reverse bias current causes negligible voltage drops in the circuit in which the diode is embedded. If so the value of the reverse-bias current really does not enter into calculations significantly and can be ignored.) Furthermore the zero current approximation is extended into forward-bias right up to the knee of the curve. Exactly what voltage to cite as the knee voltage is somewhat arguable, although usually the particular value used is not very important.

HARMONICS EFFECTS ON POWER SYSTEM COMPONENTS

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

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 I²R losses increase.

IN SWITCHGEAR

• Changing the rate of rise of transient recovery voltage, and,
• Affects the operation of the blowout.

IN RELAYS

• Affects the time delay characteristics,
• False tripping may occurs.

IN MOTORS

• Stator and rotor I²R losses increase due to the flow of harmonic currents,
• In the case of induction motors with skewed rotors the flux changes in both the stator and rotor and high frequency can produce substantial iron losses, and,
• Positive sequence harmonics develop shaft torque that aid shaft rotation; negative sequence harmonics have opposite effect.

IN GENERATORS

• Rotor and stator heating,
• Production of pulsating or oscillating torques, and,
• Acoustic noise.

IN ELECTRONIC EQUIPMENT

• Unstable operation of firing circuits based on zero voltage crossing,
• Erroneous operation in measuring equipment,
• Malfunction of computers allied equipment due to the presence of ac supply harmonics.

AWARENESS OF POWER ELECTRONICS

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

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 capacitor has self-healing property. Hence an inductor or a capacitor has to be selected or designed with care, taking into account the operating conditions, before its use in a power electronic circuit.
In many power electronic circuits, diodes play a crucial role. A normal power diode is usually designed to be operated at 400 Hz or less. Many of the inverter and switch-mode power supply circuits operate at a much higher frequency and these circuits need diodes that turn ON and OFF fast. In addition, it is also desired that the turning-off process of a diode should not create undesirable electrical transients in the circuit. Since there are several types of diodes available, selection of a proper diode is very important for reliable operation of a circuit.

Analysis of power electronic circuits tends to be quite complicated, because these circuits rarely operate in steady state. Traditionally steady state response refers to the state of a circuit characterized by either a DC response or a sinusoidal response. Most of the power electronic circuits have a periodic response, but this response is not usually sinusoidal.

Typically, the repetitive or the periodic response contains both a steady state part due to the forcing function and a transient part due to the poles of the network. Since the responses are non-sinusoidal, harmonic analysis is often necessary. In order to obtain the time response, it may be necessary to resort to the use of a computer program.

Power electronics is a subject of interdisciplinary nature. To design and build control circuitry of a power electronic application, one needs knowledge of several areas, which are listed below.

• Design of analogue and digital electronic circuits, to build the control circuitry.

• Microcontrollers and digital signal processors for use in sophisticated applications.

• Many power electronic circuits have an electrical machine as their load. In AC variable speed drive, it may be a reluctance motor, an induction motor or a synchronous motor. In a DC variable speed drive, it is usually a DC shunt motor.

• In a circuit such as an inverter, a transformer may be connected at its output and the transformer may have to operate with a non-sinusoidal waveform at its input.

• A pulse transformer with a ferrite core is used commonly to transfer the gate signal to the power semiconductor device. A ferrite-cored transformer with a relatively higher power output is also used in an application such as a high frequency inverter.

• Many power electronic systems are operated with negative feedback. A linear controller such as a PI controller is used in relatively simple applications, whereas a controller based on digital or state-variable feedback techniques is used in more sophisticated applications.

• Computer simulation is often necessary to optimize the design of a power electronic system. In order to simulate, knowledge of software package such as MATLAB, Pspice, Orcad,…..etc. and the know-how to model nonlinear systems may be necessary.

The study of power electronics is an exciting and a challenging experience. The scope for applying power electronics is growing at a fast pace. New devices keep coming into the market, sustaining development work in power electronics.

RECTIFICATION

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

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 delivering high quality high power output. Power rating of a single-phase rectifier tends to be lower than 10 kW.

Three-phase bridge rectifiers are used for delivering higher power output, up to 500 kW at 500 V DC or even more. For low voltage, high current applications, and a pair of three-phase, three-pulse rectifiers interconnected by an inter-phase transformer (IPT) is used. For a high current output, rectifiers with IPT are preferred to connecting devices directly in parallel.

There are many applications for rectifiers. Some of them are:

• Variable speed DC drives,
• Battery chargers,
• DC power supplies and Power supply for a specific application like electroplating

DC to AC CONVERSION

The converter that changes a DC voltage to an alternating voltage, AC is called an inverter. Earlier inverters were built with SCRs. Since the circuitry required turning the SCR off tends to be complex, other power semiconductor devices such as bipolar junction transistors, power MOSFETs, insulated gate bipolar transistors (IGBT) and MOS-controlled thyristors (MCTs) are used nowadays. Currently only the inverters with a high power rating, such as 500 kW or higher, are likely to be built with either SCRs or gate turn-off thyristors (GTOs). There are many inverter circuits and the techniques for controlling an inverter vary in complexity.

Some of the applications of an inverter are listed below:

• Emergency lighting systems,
• AC variable speed drives,
• Uninterrupted power supplies, and,
• Frequency converters.

DC to DC CONVERSION

When the SCR came into use, a DC-to-DC converter circuit was called a chopper. Nowadays, an SCR is rarely used in a DC-to-DC converter. Either a power BJT or a power MOSFET is normally used in such a converter and this converter is called a switch-mode power supply. A switch-mode power supply can be one of the types listed below:

• Step-down switch-mode power supply,
• Step-up chopper,
• Fly-back converter,
• Resonant converter.

The typical applications for a switch-mode power supply or a chopper are:

• DC drive,
• Battery charger, and,
• DC power supply.

AC to AC Conversion

A cycloconverter or a Matrix converter converts an AC voltage, such as the mains supply, to another AC voltage. The amplitude and the frequency of input voltage to a cycloconverter tend to be fixed values, whereas both the amplitude and the frequency of output voltage of a cycloconverter tend to be variable especially in Adjustable Speed Drives (ASD). A typical application of a cycloconverter is to use it for controlling the speed of an AC traction motor and most of these cycloconverters have a high power output, of the order a few megawatts and SCRs are used in these circuits. In contrast, low cost, low power cycloconverters for low power AC motors are also in use and many of these circuit tend to use TRIAC in place of SCRs. Unlike an SCR which conducts in only one direction, a TRIAC is capable of conducting in either direction and like an SCR, it is also a three terminal device. It may be noted that the use of a cycloconverter is not as common as that of an inverter and a cycloinverter is rarely used because of its complexity and its high cost.

REACTORS

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

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. Alternatively, it is possible to use iron core with air-gaps included in the magnetic core so that saturation is avoided.

CLASSIFICATION OF REACTORS

(i) Generator reactors (ii) Feeder reactors (iii) Bus-bar reactors

The above classification is based on the location of the reactors. Reactors may be connected in series with the generator in series with each feeder or to the bus bars.

(I) GENERATOR REACTORS

The reactors are located in series with each of the generators as shown in Figure 1 so that current flowing into a fault F from the generator is limited.

Figure: 1

Disadvantages:

(a) In the event of a fault occurring on a feeder, the voltage at the remaining healthy feeders also may loose synchronism requiring resynchronization later.

(b) There is a constant voltage drop in the reactors and also power loss, even during normal operation. Since modern generators are designed to with stand dead short circuit at their terminals, generator reactors are now-a-days not used except for old units in operation.

(II) FEEDER REACTORS

In this method of protection each feeder is equipped with a series reactor as shown in Figure 2. In the event of a fault on any feeder the fault current drawn is restricted by the reactor.

Figure: 2

Disadvantages:

(a) Voltage drop and power loss still occurs in the reactor for a feeder fault. However, the voltage drop occurs only in that particular feeder reactor.

(b) Feeder reactors do not offer any protection for bus bar faults. Never the less, bus-bar faults occur very rarely.

As series reactors inhererbly create voltage drop, system voltage regulation will be impaired. Hence they are to be used only in special case such as for short feeders of large cross-section.

(III) BUS BAR REACTORS

In both the above methods, the reactors carry full load current under normal operation. The consequent disadvantage of constant voltage drops and power loss can be avoided by dividing the bus bars into sections and inter connect the sections through protective reactors. There are two ways of doing this.

(a) RING SYSTEM:

In this method each feeder is fed by one generator. Very little power flows across the reactors during normal operation. Hence, the voltage drop and power loss are negligible. If a fault occurs on any feeder, only the generator to which the feeder is connected will feed the fault and other generators are required to feed the fault through the reactor.

(b) TIE BAR SYSTEM:

This is an improvement over the ring system. This is shown in Figure 3. Current fed into a fault has to pass through two reactors in series between sections.

Another advantage is that additional generation may be connected to the system without requiring changes in the existing reactors.

The only disadvantage is that this systems requires an additional bus-bar system, the tie-bar.

Figure: 3

IMPORTANCE OF SHORT CIRCUIT CURRENTS

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

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 power systems due to system over voltages caused by lightning or switching surges or due to equipment insulation failure or even due to insulator contamination. Sometimes even mechanical causes may create short circuits. Other well-known reasons include line-to-line, line-to-ground, or line-to-line faults on overhead lines. The resultant short circuit has to the interrupted within few cycles by the circuit breaker.

It is absolutely necessary to select a circuit breaker that is capable of operating successfully when maximum fault current flows at the circuit voltage that prevails at that instant. An insight can be gained when we consider an R-L circuit connected to an alternating voltage source, the circuit being switched on through a switch.

ANALYSIS OF SYMMETRICAL THREE PHASE SHORT CIRCUITS

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

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.

ADVANTAGES OF PER UNIT SYSTEM

By Engr. Aneel Kumar February 10, 2015 1 comment:

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.
Equipment impedances can be easily estimated since their per unit impedances lie within a relatively narrow range.
Reduced calculations in three-phase systems.
By the choice of voltage bases, the solution of networks containing several transformers is easy.
More usefully for digital computation.
For apparatus of the same general type the p.u. and volt drops or losses are in the same order, regardless of size.
For transformers the p.u. of impedances are same for primary and secondary sides.

PER UNIT CONVERSION PROCEDURE OF SINGLE PHASE

Pick a VA base for the entire system, S_base
Pick a voltage base for each different voltage level, V_base.
Voltage bases are related by transformer turns ratios.
Voltages are line to neutral.
Calculate the impedance base, Z_base = (V_base)²/S_base
Calculate the current base, I_base =V_base/Z_base
Convert actual values to per unit
Convert to per unit (p.u.) (many problems are already in per unit)
Solve
Convert back to actual as necessary

CLASSIFICATION OF POWER SYSTEM BUSES

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

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 prime mover input. This in turn changes the machine rotor axis position with respect to a synchronously rotating or reference axis or a reference bus. In other words, the phase angle of the rotor δ is directly related to the real power generated by the machine. The voltage magnitude on the other hand, is mainly, influenced by the excitation current in the field winding. Thus at a generator bus the real power generation P_g and the voltage magnitude [V_g] can be specified. It is also possible to produce vars by using capacitor or reactor banks too. They compensate the lagging or leading vars consumed and then contribute to voltage control. At a generator bus or voltage controlled bus, also called a PV-bus the reactive power Q_g and δ_g are the values that are not known and are to be computed.

SLACK BUS

In a network as power flow from the generators to loads through transmission lines power loss occurs due to the losses in the line conductors. These losses when included, we get the power balance relations

Pg-P_d–P_L=O ---- (1)

Qg-Q_d-Q_L=0 ---- (2)

Where Pg and Qg are the total real and reactive generations, P_d and Q_d are the total real and reactive power demands and PL and QL. are the power losses in the transmission network. The values of Pg, Qg and P_d and Q_d are either known or estimated. Since the flow of currents in the various lines in the transmission lines are not known in advance, P_L and Q_L remains unknown before the analysis of the network.

But these losses have to be supplied by the generators in the system. For this purpose, one of the generators or generating bus is specified as 'slack bus' or 'swing bus'. At this bus the generation Pg and Qg are not specified. The voltage magnitude is specified at this bus. Further, the voltage phase angle δ is also fixed at this bus. Generally it is specified as 0° so that all voltage phase angles are measured with respect to voltage at this bus. For this reason slack bus is also known as reference bus. All the system losses are supplied by the generation at this bus. Further the system voltage profile is also influenced by the voltage specified at this bus.

POWER FLOW STUDIES AND ITS IMPORTANCE

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

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 effect of in-phase and quadrative boost voltages on system loading
6. Economic system operation
7. System loss minimization
8. Transformer tap setting for economic operation
9. Possible improvements to an existing system by change of conductor sizes and system voltages.

For the purpose of power flow studies a single phase representation of the power network is used, since the system is generally balanced. When systems had not grown to the present size, networks were simulated on network analyzers for load flow solutions. These analyzers are of analogue type, scaled down miniature models of power systems with resistances, reactances, capacitances, autotransformers, transformers, loads and generators. The generators are just supply sources operating at a much higher frequency than 50 Hz to limit the size of the components. The loads are represented by constant impedances. Meters are provided on the panel board for measuring voltages, currents and powers. The power flow solution in obtained directly from measurements for any system simulated on the analyzer.

With the advent of the modern digital computers possessing large storage and high speed the mode of power flow studies have changed from analog to digital simulation. A large number of algorithms are developed for digital power flow solutions. The methods basically distinguish between themselves in the rate of convergence, storage requirement and time of computation. The loads are generally represented by constant power.

Network equations can be solved in a variety of ways in a systematic manner. The most popular method is node voltage method. When nodal or bus admittances are used complex linear algebraic simultaneous equations will be obtained in terms of nodal or bus currents.

However, as in a power system since the nodal currents are not known, but powers are known at almost all the buses, the resulting mathematical equations become non-linear and are required to be solved by interactive methods. Load flow studies are required for power system planning, operation and control as well as for contingency analysis. The bus admittance matrix is invariably utilized in power flow solutions.

CONDITIONS FOR SUCCESSFUL OPERATION OF A POWER SYSTEM

There are the following:

1. There should the adequate real power generation to supply the power demand at various load buses and also the losses
2. The bus voltage magnitudes are maintained at values very close to the rated values.
3. Generators, transformers and transmission lines are not over loaded at any point of time or the load curve.

SYNCHRONOUS MACHINE ROTOR TYPES

By Engr. Aneel Kumar February 07, 2015 No comments:

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 cylinder.

The field windings are mounted in slots that are mill-cut into about 2/3 of the perimeter. Because of the discrete distribution of the windings on the rotor surface, the magnetic flux density in the air gap always has a stair-step form. Through proper distribution of the windings these stair-steps can be made approximately sinusoidal!

SALIENT POLE ROTOR

Salient pole rotors are used with low-speed hydro turbines with rated powers of up to 800 MVA per unit. In order to obtain the appropriate electrical power frequency in spite of the low rotor speed, salient pole rotors typically have multiple pole pairs. For run-of-river power stations the number of poles can be as high as p = 200! Such rotors have very large diameters (several meters) and short lengths.

The field windings are mounted on the individual poles. By properly designing the geometric form of the poles, the magnetic flux density in the air gap at the stator surface can also be made approximately sinusoidal!

Friday, February 20, 2015

Sunday, February 15, 2015

Friday, February 13, 2015

LOAD BUS

GENERATOR BUS OR VOLTAGE CONTROLLED BUS

SLACK (SWING) BUS

Thursday, February 12, 2015

(1) POWER CIRCUIT

(2) CONTROL CIRCUIT

(3) TRIGGERING CIRCUIT

1- NATURAL COMMUTATION

2- FORCED COMMUTATION

ADVANTAGES OF CENTER TAP DIODE RECTIFIER

DISADVANTAGES OF CENTER TAP DIODE RECTIFIER

ADVANTAGE OF GTO OVER THYRISTOR

ADVANTAGE OF THYRISTOR OVER GTO

IN TRANSFORMERS AND REACTORS

IN CAPACITORS

IN CABLES

IN SWITCHGEAR

IN RELAYS

IN MOTORS

IN GENERATORS

IN ELECTRONIC EQUIPMENT

Wednesday, February 11, 2015

DC to AC CONVERSION

DC to DC CONVERSION

AC to AC Conversion

CONSTRUCTION OF REACTORS

CLASSIFICATION OF REACTORS

(I) GENERATOR REACTORS

(II) FEEDER REACTORS

(III) BUS BAR REACTORS

(a) RING SYSTEM:

(b) TIE BAR SYSTEM:

Tuesday, February 10, 2015

PER UNIT SYSTEM

ADVANTAGES OF PER UNIT SYSTEM

PER UNIT CONVERSION PROCEDURE OF SINGLE PHASE

LOAD BUS

VOLTAGE CONTROLLED BUS OR GENERATOR BUS

SLACK BUS

NECESSITY FOR POWER FLOW STUDIES

CONDITIONS FOR SUCCESSFUL OPERATION OF A POWER SYSTEM

Saturday, February 07, 2015

ROUND ROTOR

SALIENT POLE ROTOR

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