ENGINEERING ARTICLES

Sunday, September 29, 2013

PLANNING OF HYDROELECTRIC FACILITIES

By Engr. Aneel Kumar September 29, 2013 No comments:

1) Siting

Hydroelectric plants are located in geographic areas where they will make economic use of hydraulic energy sources. Hydraulic energy is available wherever there is a flow of liquid and accumulated head.

Head represents potential energy and is the vertical distance through which the fluid falls in the energy conversion process. The majority of sites utilize the head developed by freshwater; however, other liquids such as saltwater and treated sewage have been utilized. The siting of a prospective hydroelectric plant requires careful evaluation of technical, economic, environmental, and social factors. A significant portion of the project cost may be required for mitigation of environmental effects on fish and wildlife and relocation of infrastructure and population from flooded areas.

2) Hydroelectric Plant Schemes

There are three main types of hydroelectric plant arrangements, classified according to the method of controlling the hydraulic flow at the site:

1. Run-of-the-river plants, having small amounts of water storage and thus little control of the flow through the plant

2. Storage plants, having the ability to store water and thus control the flow through the plant on a daily or seasonal basis

3. Pumped storage plants, in which the direction of rotation of the turbines is reversed during off-peak hours, pumping water from a lower reservoir to an upper reservoir, thus “storing energy” for later production of electricity during peak hours

3) Selection of Plant Capacity, Energy, and Other Design Features

The generating capacity of a hydroelectric plant is a function of the head and flow rate of water discharged through the hydraulic turbines, as shown in the following equation:

P = 9.8 η QH

Where

P is the power (kW)

η is the plant efficiency

Q is the discharge flow rate (m3/s)

H is the head (m)

Flow rate and head are influenced by reservoir inflow, storage characteristics, plant and equipment design features, and flow restrictions imposed by irrigation, minimum downstream releases, or flood control requirements. Historical daily, seasonal, maximum (flood), and minimum (drought) flow conditions are carefully studied in the planning stages of a new development. Plant capacity, energy, and physical features such as the dam and spillway structures are optimized through complex economic studies that consider the hydrological data, planned reservoir operation, performance characteristics of plant equipment, construction costs, the value of capacity and energy, and financial discount rates. The costs of substation, transmission, telecommunications, and off-site control facilities are also important considerations in the economic analysis. If the plant has storage capability, then societal benefits from flood control may be included in the economic analysis.

Another important planning consideration is the selection of the number and size of generating units installed to achieve the desired plant capacity and energy, taking into account installed unit costs, unit availability, and efficiencies at various unit power.

HYDROELECTRIC POWER GENERATION

By Engr. Aneel Kumar September 29, 2013 No comments:

Hydroelectric power generation involves the storage of a hydraulic fluid, water, conversion of the hydraulic (potential) energy of the fluid into mechanical (kinetic) energy in a hydraulic turbine, and conversion of the mechanical energy to electrical energy in an electric generator.

The first hydroelectric power plants came into service in the 1880s and now comprise approximately 20% (875 GW) of the worlds installed generation capacity (World Energy Council, 2010). Hydroelectricity is an important source of renewable energy and provides significant flexibility in base loading, peaking, and energy storage applications. While initial capital costs are high, the inherent simplicity of hydroelectric plants, coupled with their low operating and maintenance costs, long service life, and high reliability, makes them a very cost-effective and flexible source of electricity generation.

Especially valuable is their operating characteristic of fast response for start-up, loading, unloading, and following of system load variations. Other useful features include their ability to start without the availability of power system voltage (black start capability), ability to transfer rapidly from generation mode to synchronous-condenser mode, and pumped storage application.

Hydroelectric units have been installed in capacities ranging from a few kilowatts to nearly 1 GW. Multiunit plant sizes range from a few kilowatts to a maximum of 22.5 GW.

ELECTRICAL ENGINEERING PROGRAM EDUCATIONAL OBJECTIVES

By Engr. Aneel Kumar September 17, 2013 No comments:

Successfully practice electrical engineering to serve state and regional industries, government agencies, or national and international industries.

Work professionally in one or more of the following areas: analog electronics, digital electronics, communication systems, signal processing, control systems, and computer-based systems.

Achieve personal and professional success with awareness and commitment to their ethical and social responsibilities, both as individuals and in team environments.

Maintain and improve their technical competence through lifelong learning, including entering and succeeding in an advanced degree program in a field such as engineering, science, or business.
Electrical Engineering Student Outcomes

Student outcomes are statements that describe what students are expected to know and are able to do by the time of graduation, the achievement of which indicates that the student is equipped to achieve the program objectives.

The generalized outcomes for the electrical engineering program are given below.

To Understand - to understand the mathematical and physical foundations of electrical engineering and how these are used in electronic devices and systems. An understanding that engineering knowledge should be applied in an ethically responsible manner for the good of society.

To Question - to critically evaluate alternate assumptions, approaches, procedures, tradeoffs, and results related to engineering problems.

To Design - to design a variety of electronic and/or computer-based components and systems for applications including signal processing, communications, computer networks, and control systems.

To Lead - to lead a small team of student engineers performing a laboratory exercise or design project; to participate in the various roles in a team and understand how they contribute to accomplishing the task at hand.

To Communicate - to use written and oral communications to document work and present project results.

SELF INDUCTANCE

By Engr. Aneel Kumar September 13, 2013 No comments:

A current-carrying coil produces a magnetic field that links its own turns. If the current in the coil changes the amount of magnetic flux linking the coil changes and, by Faraday’s law, an emf is produced in the coil. This emf is called a self-induced emf.

Let the coil have N turns. Assume that the same amount of magnetic flux F links each turn of the coil. The net flux linking the coil is then NF. This net flux is proportional to the magnetic field, which, in turn, is proportional to the current I in the coil. Thus we can write NF µ I. This proportionality can be turned into an equation by introducing a constant. Call this constant L, the self-inductance (or simply inductance) of the coil:

As with mutual inductance, the unit of self-inductance is the henry.

The self-induced emf can now be calculated using Faraday’s law:

The above formula is the emf due to self-induction.

Example

Find the formula for the self-inductance of a solenoid of N turns, length l, and cross-sectional area A.

Assume that the solenoid carries a current I. Then the magnetic flux in the solenoid is

MUTUAL INDUCTANCE

By Engr. Aneel Kumar September 13, 2013 No comments:

Suppose we hook up an AC generator to a solenoid so that the wire in the solenoid carries AC. Call this solenoid the primary coil. Next place a second solenoid connected to an AC voltmeter near the primary coil so that it is coaxial with the primary coil. Call this second solenoid the secondary coil. As shown in figure.

The alternating current in the primary coil produces an alternating magnetic field whose lines of flux link the secondary coil (like thread passing through the eye of a needle). Hence the secondary coil encloses a changing magnetic field. By Faraday’s law of induction this changing magnetic flux induces an emf in the secondary coil. This effect in which changing current in one circuit induces an emf in another circuit is called mutual induction.

Let the primary coil have N1 turns and the secondary coil have N₂ turns. Assume that the same amount of magnetic flux F₂from the primary coil links each turn of the secondary coil. The net flux linking the secondary coil is then N₂F₂. This net flux is proportional to the magnetic field, which, in turn, is proportional to the current I1 in the primary coil. Thus we can write N₂F₂µI₁. This proportionality can be turned into an equation by introducing a constant. Call this constant M, the mutual inductance of the two coils:

The unit of inductance is WB/A=Henry (H) named after Joseph Henry.

The emf induced in the secondary coil may now be calculated using Faraday’s law:

The above formula is the emf due to mutual induction.

Example

The apparatus used in Experiment EM-11B consists of two coaxial solenoids. A solenoid is essentially just a coil of wire. For a long, tightly-wound solenoid of n turns per unit length carrying current I the magnetic field over its cross-section is nearly constant and given by

. Assume that the two solenoids have the same cross-sectional area A. Find a formula for the mutual inductance of the solenoids.

EFFICIENCY OF A TRANSFORMER

By Engr. Aneel Kumar September 12, 2013 No comments:

Since the equivalent circuit contains two winding resistances and a core-loss resistance then power is lost as heating energy inside the transformer. Hence the conversion of power through the transformer cannot be 100%, a small loss of efficiency occurs. This is usually less than about 2% for power transformers. Assume all resistances and reactances are referred to the secondary winding. The efficiency can be expressed as,

Where cosØ is the power factor of the load

Pc is the core-loss
Is is the secondary current
Vs is the secondary voltage
Es is the secondary emf.

This formula applies to single-phase transformers, or to one phase of a three-phase transformer.

OPERATING PRINCIPLES OF T RANSFORMERS

By Engr. Aneel Kumar September 12, 2013 No comments:

A single-phase power system transformer consists basically of two windings wound onto an iron core. The iron core concentrates the flux and restricts it to a defined path. It also creates the maximum possible amount of flux for a given excitation. In order to maximize the mutual coupling the two windings are wound concentrically on to the same part of the iron core. Figure 6.1 shows the typical winding arrangement of a single-phase transformer. This is called shell-type construction.

Not all the flux created by one winding couples with the other winding. Furthermore the flux which does not couple both windings does not flow completely round the iron core, some of it flows in the air close to the windings. The common flux in the iron circuit is called the mutual or magnetizing flux. The flux that escapes into the air and does not couple the windings is called the leakage flux. One winding is referred to as the primary winding and is connected to the source of supply voltage. The second winding is the secondary winding and is connected to the load. The primary may be either the low or the high voltage winding.

The magnetizing flux is determined by the applied voltage to the primary winding. In power transformers the current drawn from the supply to magnetize the core is only a fraction of one percent of the rated primary winding current. The core design and type of iron is specially chosen to minimize the magnetizing current.

When current is drawn from the secondary winding the effect on the magnetizing flux is to reduce it. However, the magnetizing flux density must be maintained and this is achieved by the primary winding drawing more current from the supply.

METHODS OF STARTING INDUCTION MOTORS

By Engr. Aneel Kumar September 07, 2013 No comments:

When the maximum kW rating of an induction motor is reached for direct-on-line starting, it becomes necessary to introduce an alternative method of starting the motor. There are several methods used in the oil industry. The object is to reduce the starting current drawn from the supply during all or part of the run-up period. There are two basic approaches that can be used:-

• Select special-purpose designs for the motor in which the winding arrangements are modified by external switching devices that are matched to the motor, e.g. star-delta motor and starter.

• Select conventional motors but use special external starting devices, e.g. Korndorfer starter, autotransformer starter, ‘soft-starter’ using a controlled rectifier-inverter system.

In all cases of reduced voltage starting, care must be taken to check that the motor will create sufficient torque at the reduced voltage to accelerate the load to the desired speed in as short a time as possible. Excessive run-up times must be avoided as. When the run-up time is expected to be high the manufacturer of the motor should be consulted regarding the possibility of damage and infringement of its guarantees. The following methods are the most commonly used, typically in the order shown below.

• Star-delta method.
• Korndorfer auto-transformer method.
• Soft-start power electronics method.
• Series reactor method.
• Part winding method.

Star-Delta Method Of Starting Induction Motors

A specially designed motor is used. The stator windings are arranged so that the start and finish of each phase winding in the stator is brought out to the main terminal box so that six terminals are available for connection to cables. Usually two three-core or four-core cables are used unless their conductor size becomes too large, in which case single-core cables would be used. The windings are connected externally in star for starting and delta for running. The external connections are made by using a special starter in the motor control centre which also provides control relays and current transformers that determine when the transfer from star to delta should take place.

Disadvantages of Star-Delta Method

• The windings are open-circuited during the transfer and this is not considered good practice, a delay should be incorporated to allow the flux in the motor to decay during the transfer.

• The starting current and torque are reduced to 33% of their value during the run-up period. This reduction may be too much for some applications.

• The running condition requires a delta winding connection and this has the disadvantage that harmonic currents can circulate in the windings.

Figure shows the basic circuit for a star-delta starter.

Korndorfer Auto-Transformer Method Of Starting Induction Motors

A standard design of motor is used. An external auto-transformer is connected between the main circuit breaker, or contactor, and the motor during the starting and run-up period. Figure 5.8 shows the connections that are commonly used in a balanced three-phase arrangement. The voltage ratio of the auto-transformer needs to be carefully selected. If it is too high then the full benefit is not achieved. If too low then insufficient torque will be created. The most effective ratio is usually found between 65% and 80%.

Disadvantage of Korndorfer Auto-Transformer Method

Two extra three-phase circuit breakers or contactors are necessary, thus making three in total for the motor circuit, which require space to be allocated. Retro-fitting a Korndorfer starter may therefore be difficult if space is scarce.

Figure shows the basic circuit for Korndorfer Auto-Transformer Method.

Soft-Start Power Electronics Method Of Starting Induction Motors

A standard design of motor is used. An external rectifier-inverter is connected between the main circuit breaker, or contactor, and the motor during the starting and run-up period. The starter varies the frequency and voltage magnitude of the applied three-phase supply to the motor. Upon starting the frequency and voltage are set to their lowest values, and thereafter they are slowly raised as the shaft speed increases. The intent is to operate the motor in its near-synchronous speed state for each frequency that the motor receives.

The rectifier-inverter equipment is expensive when compared with other switching and transformer methods, but it has several advantages:-

• The starting current can be limited to a value that is equal to or a little higher than the full-load current of the motor.

• The torque created in the motor during the whole run-up period can be in the order of the full-load value, and so the shape of the inherent torque-speed curve of the motor is not a critical issue for most standard designs of motors.

Series Reactor Method Of Starting Induction Motors

A standard design of motor is used. This is a simple method that requires the insertion of a series reactor during starting. The reactor is bypassed once the motor reaches its normal working speed.

Only one extra circuit breaker or contactor is required. The amount of reactance is calculated on the basis of the desired reduction of line current during starting, but the limiting factor is the reduced starting torque. The torque is reduced for two reasons, firstly because the total circuit impedance is increased and secondly the reactance-to-resistance ratio is increased.

Part Winding Method Of Starting Induction Motors

A special design of motor is used. The stator has two three-phase windings that are arranged in parallel and wound in the same slots. If the two windings are the same then on starting and during run-up one winding would provide half of the total torque at any speed. Hence one winding is used for starting and both for running. The method is not suited to small or high speed motors. With two equal windings the starting current and torque are half of their totals. This method is seldom used in the oil industry because of the preference for standard motors, and the availability of satisfactory alternative methods.

CRITICAL TIMES FOR INDUCTION MOTORS

By Engr. Aneel Kumar September 07, 2013 No comments:

There are two important time periods that are critical in the application of induction motors. One is the allowable run-up or starting time and the other is the maximum stalling time.

The run-up time is determined by the static torque versus speed characteristic, and the moment of inertia of the load. High inertia loads can cause very long run-up times. However, a long runup time in itself is not usually a problem for the driven machine. Most induction motors in the oil industry are started direct-on-line and the starting and run-up currents drawn by the motor can be in the range between about 4 and 7 times the rated current. When these currents exist for, say, 20 seconds, the amount of heat created in the stator windings and the rotor bar conductors is considerable.

The surface temperature of these conductors can reach values high enough to cause damage to the winding insulation and slot wedges. With hazardous area applications this temperature rise can be very significant for some types of enclosures.

When considering the run-up time it is also necessary to know how many times the motor needs to be started in, say, one hour because successive starting would not permit the conductors or the insulation time to cool down before the next start takes place. (In that event the insulation temperature would creep up and the material would eventually fail. This process could also cause the windings to become loose in their slots and such damage would be followed by vibrational wear of the insulation.)

The stalling time that can be tolerated needs to be known. This will enable the relay protection for stalling to be correctly set. A motor can withstand a stall condition for a limited period of time, during which the starting (or stalling) current will be much higher than the normal current. The same kind of damage that can occur during prolonged run-up times will be caused by a stalling condition, but the time taken will be less because the rotor remains stationary and so no air can be circulated to remove the heat. Therefore the rate of rise of surface temperature is bound to be faster in a stalling situation. Stalling can be caused by the drive shaft being seized, for example due to a loss of lubricating oil, corrosion of bearing surfaces, fluid in the driven machine becoming very thick or even solidifying. It can also be caused by an open circuit of one of the supply phases. Modern protective relays are available for detecting a stalling condition and a loss of one phase of the supply.

PRINCIPLE OF OPERATION OF THREE PHASE INDUCTION MOTOR

By Engr. Aneel Kumar September 07, 2013 No comments:

Induction motors have two main components, the stator and the rotor. The stator carries a three-phase winding that receives power from the supply. The rotor carries a winding that is in the form of a set of single-bar conductors placed in slots just below the surface of the rotor. The slots have a narrow opening at the surface of the rotor, which serves to lock the conductor bars in position. Each end of each bar conductor is connected to a short-circuiting ring, one at each end of the rotor. The stator winding is a conventional type as found in three-phase generators and synchronous motors.

The three-phase stator winding produces a rotating field of constant magnitude, which rotates at the speed corresponding to the frequency of the supply and the number of poles in the motor. The higher the number of poles the lower the speed of the rotation. Assume that the rotor is stationary and the motor has just been energized. The magnetic flux produced by the stator passes through the rotor and in so doing cuts the rotor conductors as it rotates. Since the flux has a sinusoidal distribution in space its rotation causes a sinusoidal emf to be induced into the rotor conductors. Hence currents are caused to flow in the rotor conductors due to the emfs that are induced. The emfs are induced in the rotor by transformer action, which is why the machine is called an ‘induction’ motor. Since currents now flow in both the stator and the rotor, the rotor conductors will set up local fluxes which interact with the excitation flux from the stator. This interaction causes a torque to be developed on the rotor. If this torque exceeds the torque required by the mechanical load the shaft will begin to rotate and accelerate until these two torques are equal. The rotation will be in the direction of the stator flux since the rotor conductors are being driven by the stator flux.

Initially the speed is much less than that of the stator field, although it is increasing. Consequently the rate at which the stator flux cuts the rotor conductors reduces as the shaft speed increases.

The frequency and magnitude of the induced rotor emfs therefore decrease as the shaft accelerates.

The local flux produced by the rotor conductors therefore rotates at a slower speed relative to the rotor surface. However, since the rotor body is rotating at a slow speed, the combined effect of the body speed plus the rotational speed of the local rotor flux causes the resulting rotor flux to rotate at the same speed as the stator field.

The rotor currents are limited by the short-circuit impedance of the rotor circuit. This circuit contains resistance and reactance. The inductive reactance is directly proportional to the frequency of the induced emfs in the rotor. As the rotor accelerates two effects take place:-

a) The rotor impedance increases.

b) The rotor emf reduces.

These effects result in the supply current is being nearly constant during most of the run-up period.

The rotor speed cannot reach the same speed as that of the stator field, otherwise there would be no induced emfs and currents in the rotor, and no torque would be developed. Consequently when the rotor speed is near to the synchronous speed the torque begins to decrease rapidly until it matches that of the load and rotational friction and windage losses. When this balance is achieved the speed will remain constant.

MAIN AND PILOT EXCITER

By Engr. Aneel Kumar September 07, 2013 No comments:

Main Exciter

The exciter (sometimes called the main exciter) is a synchronous generator that has its stator and rotor windings inverted. Its field winding is fixed in the stator, and the rotor carries the armature or AC . In addition the rotor carries the semiconductor bridge rectifier that converts the armature voltages to a two-wire DC voltage system. The AC voltages and currents in the armature are often alternating at a higher frequency than those in the main generator, e.g. 400 Hz. The higher frequency improves the speed of response of the exciter. The DC power circuit is coupled to the field of the main generator by the use of insulated conductors that pass coaxially inside the rotor of the exciter and the rotor of the main generator. This eliminates the use of slip rings, which were traditionally used before shaft mounted rectifiers were developed. A slight disadvantage of this technique is that the derivative feedback cannot be taken from the output of the exciter. However, with modern electronic devices used throughout the AVR, this can be regarded as an insignificant disadvantage.
The time constant Te of the exciter is mainly related to its field winding. The saturation block in Figure 4.1 accounts for the magnetic saturation of the iron core of the exciter, and it is important to represent this because the expected range of the performance of the exciter is wide. Its terminal voltage may have a value of typically 3.0 per unit when the generator is fully loaded. This may increase to about 6.5 per unit when the generator needs to maintain a full short circuit at or near to its terminals. The maximum excitation voltage is called the ‘ceiling voltage’ of the exciter.

Pilot exciter

The AVR system requires a source of power for its amplifier, its reference voltage and other electronic circuits that may be involved e.g. alarms. There are several methods of obtaining this necessary power,

• An external power supply.
• Self-excitation.
• Pilot exciter.

An external supply could be an uninterrupted power supply (UPS) that is dedicated to the generator. Although this is feasible it is not a method that is used, the main reason being that it departs from the requirement of self-containment. The equipment involved would require external cables and switchgear, both of which add a factor of unreliability to the scheme.

The self-excitation method relies upon the residual magnetism in the iron core of the main generator that remains in the core after the generator is shut down. When the generator is started again and run up to speed a small emf is generated by the residual magnetism. A special circuit detects the residual emf at the main terminals and amplifies it to a predetermined level. This amplified voltage is rendered insensitive to a wide range of emf values and has sufficient power to feed all the auxiliary requirements of the AVR. The advantage of this method is its low cost compared with using a pilot exciter. Its main disadvantage is an inferior performance when a short circuit occurs at or near the main generator. The detected emf, or terminal voltage, when the generator is connected to the busbars, falls to near zero when the short circuit exists. The AVR may lose its supply during this period or perform in an unpredictable manner. The excitation of the generator may collapse, which is not desirable.

The pilot exciter method is highly reliable and has a fully predictable performance. A small alternator is mounted on the same shaft, and often within the same frame, as the main exciter. It receives its excitation from a shaft mounted permanent magnet rotor system. Hence its level of excitation is constant and dependable. The AC output from the pilot exciter is rectified and smoothed by components within the AVR cubicle. It can be seen that this method is completely independent of the conditions existing in the main generator. This is the method usually specified in the oil industry.

PHSOR DIAGRAM OF A TWO AXIS SALIENT POLE GENERATOR

By Engr. Aneel Kumar September 07, 2013 No comments:

Following phasor is phsor diagram of a two-axis salient pole generator.

The following points apply to the drawing of phasor diagrams of generators and motors:-

• The terminal voltage V is the reference phasor and is drawn horizontally.
• The emf E lies along the pole axis of the rotor.
• The current in the stator can be resolved into two components, its direct component along the ‘direct or d-axis’ and its quadrature component along the ‘quadrature or q-axis’.

The emf E leads the voltage V in an anti-clockwise direction when the machine is a generator.

Each reactance and resistance in the machine has a volt drop associated with it due to the stator current flowing through it. Consider a generator. The following currents and voltages can be shown in a phasor diagram for both the steady and the dynamic states.

E the emf produced by the field current If .

V the terminal voltage.

V_d the component of V along the d-axis.

V_q the component of V along the q-axis.

I the stator current.

I_d the component of I along the d-axis.

I_q the component of I along the q-axis.

IR_a the volt drop due to the armature or stator current.

I_dR_a the component of IRa along the d-axis.

I_qR_a the component of IRa along the q-axis.

I_dX_d the volt drop due to the d-axis synchronous reactance.

I_dX^’d the volt drop due to the d-axis transient reactance.

I_dX^’’d the volt drop due to the d-axis sub-transient reactance.

I_qX_q the volt drop due to the q-axis synchronous reactance.

I_qX^’q the volt drop due to the q-axis transient reactance (normally taken as IqXq ).

I_qX^’’q the volt drop due to the q-axis sub-transient reactance.

E^’ the emf behind the transient impedance.

E^’’ the emf behind the sub-transient impedance.

OPERATION OF GENERATOR

By Engr. Aneel Kumar September 07, 2013 No comments:

The stator, also called the armature, carries the three-phase AC winding. The rotor, also called the field, carries the DC excitation or field winding. The field winding therefore rotates at the shaft speed and sets up the main magnetic flux in the machine.

The fundamental magnetic action between the stator and rotor is one of tangential pulling. In a generator, the rotor pole pulls the corresponding stator pole flux around with it. In a motor, the stator pole pulls the rotor pole flux around with it. The action is analogous to stretching a spring, the greater the power developed, the greater the pull and greater the corresponding distance that is created between the rotor and stator flux axes.

When a machine is not connected to the three-phase supply but is running at rated speed and with rated terminal voltage at the stator, there exists rated flux in the iron circuit and across the air gap. This flux cuts the stator winding and induces rated emf in winding and hence rated voltage at the main terminals. Consider what happens in a generator. Let the generator be connected to a load, or the live switchboard bus bars. Stator current is caused to flow. The current in the stator winding causes a stator flux to be created which tends to counteract the air-gap flux that is produced by the excitation. This reduction of air-gap flux causes the terminal voltage to fall. The terminal voltage can be restored by increasing the rotor excitation current and hence the flux. So the demagnetizing effect of the stator current can be compensated by increasing the field excitation current. This demagnetizing effect of the stator current is called ‘armature reaction’ and gives rise to what is known as the synchronous reactance, which is also called a ‘derived’ reactance.

Steady State Armature Reaction

The rotating field in the air gap of a synchronous machine is generally considered to be free of space harmonics, when the basic operation of the machine is being considered. In an actual machine there are space harmonics present in the air gap, more in salient pole machines than a cylindrical rotor machine. It is acceptable to ignore the effects of space harmonics when considering armature reaction and the associated reactance. Therefore the flux wave produced by the rotating field winding can be assumed to be distributed sinusoidally in space around the poles of the rotor and across the air gap.

If the stator winding, which consists of many coils that are basically connected as a series circuit, is not connected to a load then the resulting emf from all the coils is the open circuit emf of the phase winding. Closing the circuit on to a load causes a steady state current to flow in the stator coils. Each coil creates a flux and their total flux opposes the field flux from the rotor. The resulting flux in the air gap is reduced. The emf corresponding to the air-gap flux drives the stator current through the leakage reactance and conductor resistance of the stator coils. The voltage dropped across this winding impedance is small in relation to the air-gap voltage. Deducting this voltage drop from the air-gap voltage gives the terminal voltage of the loaded generator. In the circumstance described thus far the reduction in air-gap flux is called armature reaction and the resulting flux is much smaller than its value when the stator is open circuit. Restoring air gap and terminal voltage requires the field current to be increased, which is the necessary function of the automatic voltage regulator and the exciter.

When the rotor pole axis coincides with the axis of the stator coils the magnetic circuit seen by the stator has minimum reluctance. The reactance corresponding to the armature reaction in this rotor position is called the direct axis synchronous reactance X_sd . If the stator winding leakage reactance, X_a, is deducted from X_sd the resulting reactance is called the direct axis reactance X_d .

A similar situation occurs when the rotor pole axis is at right angles to the axis of the stator coils. Here the magnetic reluctance is at its maximum value due to the widest part of the air gap facing the stator coils. The complete reactance in this position is called the quadrature axis synchronous reactance X_sq . Deducting X_a results in the quadrature axis reactance X_q .

Transient State Armature Reaction

Assume the generator is loaded and operating in a steady state. If the peak-to-peak or rms value of the stator current changes in magnitude then its corresponding change in magneto-motive force (mmf) will try to change the air-gap flux by armature reaction. Relatively slow changes will allow the change in flux to penetrate into the rotor. When this occurs an emf is induced in the field winding. This emf drives a transient current around a circuit consisting of the field winding itself and the exciter that is supplying the winding. The induction of current is by transformer action. An increase in stator current will be matched by an increase in field current during the transient state. A voltage drop will occur in the machine due to the armature reaction and the reduction in air-gap flux. Reactance is associated with this type of armature reaction.

When the rotor poles are coincident with the stator coils axis the armature reaction is a maximum and the reactance is called the direct axis transient reactance X^’_d .

The situation is different when the rotor poles are at right angles to the stator coils. There is no induction in the field circuit and the reluctance is high, being almost the same as for the steady state condition. In this situation the corresponding quadrature axis transient reactance X^’_q approximately equals the reactance X_q . Cylindrical rotors of two-pole high speed generators have a nearly uniform rotor diameter and almost constant air gap all around the periphery. Hence the reactance X^’_q is almost equal to X^’_d .

Sub-Transient State Armature Reaction

Again assume that the generator is loaded and operating in a steady state. In this situation the magnitude of the stator current is allowed to change rapidly, as in the case of a short circuit in the stator circuit. The additional flux produced by the stator winding will try to penetrate the surface of the rotor poles. Most oil industry generators are provided with damper bars to reduce the excursions in rotor speed during major disturbances. The bars are made of copper or copper alloy and placed longitudinally in the face of the rotor poles. They function in a manner similar to a squirrel cage induction motor when there is a transient change in rotor speed relative to the synchronous speed. As soon as the additional flux passes through the pole faces it will induce currents in the damper bars and the solid pole tips, by the process of transformer induction. These induced currents will set up flux in opposition in order to maintain constant flux linkages with the stator.

During this transient condition, or more appropriately called a sub-transient condition, the additional flux is forced to occupy a region consisting of air and the surface of the rotor poles. This is a high reluctance condition which gives rise to reactances of low values.

Some generators have the damper bars connected to a ring at either end of the pole structure, which provides some damping action from the quadrature axis. This provides a set of short-circuited coils in the quadrature axis, which are air cored and able to repel the flux that is attempting to enter their region.

By the same reasoning as for the ‘transient’ reactances so the sub-transient reactances are derived, and are called the direct axis sub-transient reactance X^’’_d and the quadrature axis subtransient reactance X^’’_q.

SUB TRANSIENT STATE ARMATURE REACTION

By Engr. Aneel Kumar September 07, 2013 No comments:

TRANSIENT STATE ARMATURE REACTION

By Engr. Aneel Kumar September 07, 2013 No comments:

STEADY STATE ARMATURE REACTION

By Engr. Aneel Kumar September 07, 2013 No comments:

GENERATOR OPERATION

By Engr. Aneel Kumar September 06, 2013 No comments:

COMMON ASPECTS BETWEEN GENERATORS AND MOTORS

By Engr. Aneel Kumar September 06, 2013 No comments:

The theoretical operation of synchronous generators and synchronous motors is almost the same. The main differences are the direction of stator current and the flow of power through these machines.

The construction of generators and motors, of the same kW ratings, used in the oil and gas industry is very similar. Variations that are noticeable from the external appearance exist mainly due to the location of the machine and its surrounding environment. It is uncommon for generators to be placed in hazardous areas, whereas it is occasionally necessary to use a synchronous motor in a hazardous area, e.g. driving a large gas compressor. Large induction motors are often used for driving oil pumps and gas compressors that need to operate in hazardous areas.

The rotor of generators may be either ‘cylindrical’ or ‘salient’ in construction. Synchronous motors nearly always have salient pole rotors. Machines with four or more poles are always of the salient pole rotor type. Cylindrical pole rotors are used for two-pole generators, and these generators are usually driven by steam or gas turbines at 3600 rpm for 60 Hz or 3000 rpm for 50 Hz operation and have power output ratings above 30 megawatts.

The methods of cooling and the types of bearings are generally the same.

GOVERNING SYSTEMS FOR GAS TURBINES

By Engr. Aneel Kumar September 06, 2013 No comments:

In all power systems the requirement is that the steady state speed deviation, and hence frequency, is kept small for incremental changes in power demand, even if these power increments are quite large – 20%, for example.

There are two main methods used for speed governing gas turbines,

a) Droop governing.

b) Isochronous governing.

Droop governing requires a steady state error in speed to create the necessary feedback control of the fuel value. ‘Droop’ means that a fall in shaft speed (and hence generator electrical frequency) will occur as load is increased. It is customary that a droop of about 4% should occur when 100% load is applied. Droop governing provides the simplest method of sharing load between groups of generators connected to the same power system

In control theory terminology this action is called ‘proportional control’. This method of governing is the one most commonly used in power systems because it provides a reasonably accurate load sharing capability between groups of generators.

Isochronous governing causes the steady state speed error to become zero, thereby producing a constant speed at the shaft and a constant frequency for the power system. Isochronous governing is also a form of ‘integral control’. This method is best suited to a power system that is supplied by one generator. This type of power system has very limited application. However, there are situations where one isochronously governed generator can operate in parallel with one or more droop-governed generators. The droop-governed generators will each have a fixed amount of power assigned to them for the particular system frequency. This is achieved by adjusting their set points. As the demand on the whole system changes, positively or negatively, the isochronously governed generator will take up or reject these changes, and the steady state frequency will remain constant. This hybrid type of load sharing is seldom used in the oil industry.

Accurate power sharing and constant speed control can be obtained by using a specially designed controller. This controller incorporates load measurement of each generator, measurement of common system frequency and a sub-system to reduce the power mismatches of each generator to zero. The controller regularly or even continuously trims the speed set points of each gas turbine to maintain zero mismatches. A slowly operating integrator can be superimposed onto these set points to adjust them simultaneously so that the frequency is kept constant. This is a form of ‘proportional integral’ control.

The basic control system of most gas turbine generator systems is shown in Figure.

Where

ω = shaft speed

ω_ref = reference speed

P_e = electrical power at the generator shaft

P_m = mechanical output power of the gas turbine

P_a = accelerating power

P_f = friction and windage power

STARTING METHODS FOR GAS TURBINES

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Gas turbines are usually started by a DC motor or an air motor. Either system is available for most turbines up to about 20 MW. Occasionally AC motors are used. Beyond 20 MW, when heavy industrial machines tend to be used, it becomes more practical to use air motors or even diesel engine starters. DC motors require a powerful battery system. The DC motor and battery systems tend to be more reliable and less space consuming, which is important for offshore systems. Air motors require air receivers and compressors. The compressors require AC motors or diesel engines. Air start and diesel start systems are more popular for onshore plants especially remote plants, e.g. in the desert. This is partly due to the fact that batteries tend to suffer from poor maintenance in hot, dry locations. Air systems require regular maintenance and must be kept fully charged in readiness for a quick start. Air system receivers can become very large if more than three successive starting attempts are required. More starts can probably be obtained by a battery system that occupies the same physical space.

Occasionally process gas can be used instead of air to drive the air/gas starter motor. This eliminates the need for receivers and compressors. However, there should always be a reliable source of gas available. The exhaust gas from the starter motor should be safely discharged e.g. into a ventilating pipeline.

FACTORS TO BE CONSIDERED AT THE DESIGN STAGE OF A POWER PLANT

By Engr. Aneel Kumar September 06, 2013 No comments:

The electrical engineer should take full account of the site location and environmental conditions that a gas turbine generator will need to endure. These conditions can seriously affect the electrical power output that will be achievable from the machine. The starting point when considering the possible output is the ISO rating.

This is the declared rating of the machine for the following conditions:-

• Sea level elevation.
• 15⁰C (59⁰F) ambient temperature.
• Basic engine, no losses for inlet or exhaust systems, no losses for gearbox and mechanical transmission.
• Clean engine, as delivered from the factory.

The gas turbine manufacturer provides a standardized mechanical output power versus ambient temperature characteristic. (Some manufacturers also give the electrical output power versus ambient temperature characteristic. Therefore care must be exercised to be sure exactly which data are to be given and used.)

The following derating factors should be used in the estimation of the continuous site rating for the complete machine:

• ISO to a higher site ambient temperature, typically 0.5 to 0.8% per ◦C.
• Altitude, usually not necessary for most oil industry plants since they are near sea level.
• Dirty engine losses and the ageing of the gas turbine, assume 5%.
• Fuel composition and heating value losses, discuss with the manufacturer.
• Silencer, filter and ducting losses, assume 2 to 5%.
• Gearbox loss, typically 1 to 2%.
• Generator electromechanical inefficiency, typically 2 to 4%.
• Auxiliary loads connected to the generator, typically 1 to 5%.

Dirty engine losses

Consideration should be given to the fact that engines become contaminated with the combustion deposits, the lubrication oil becomes less efficient, blades erode and lose their thermodynamic efficiency and air filters become less efficient due to the presence of filtered particles. These effects combine to reduce the output of the machine. A rule-of-thumb figure for derating a gas turbine for dirty engine operation is 5%. This depends upon the type of fuel, the type of engine, the environment and how long the engine operates between clean-up maintenance periods. Individual manufacturers can advise suitable data for their engines operating in particular conditions. Dirty engine conditions should be considered, otherwise embarrassment will follow later once the machine is in regular service.

Fuel composition and heating value losses

The chemical composition and quality of the fuel will to some extent influence the power output. However, it is usually the case that more or less fuel has to be supplied by the fuel control valve for a given throughput of combustion air. Hence it is usually possible to obtain the declared normal rating from the machine, but attention has to be given to the supply of the fuel. In extreme cases the profile of the fuel control valve may require modification so that adequate feedback control is maintained over the full range of power output. The appropriate derating factor is usually 100%, i.e. no derating.

Silencer, filter and ducting losses

The amount of silencing and filtering of the inlet combustion air depends upon the site environment and the operational considerations.

Site environmental conditions may be particularly bad, e.g. deserts where sand storms are frequent; offshore where rain storms are frequent and long lasting. The more filtering that is required, the more will be the pressure lost across the filters, both during clean and dirty operation. This pressure drop causes a loss of power output from the machine.

The amount of inlet and exhaust noise silencing will depend upon, the location of machine with respect to people in say offices or control rooms, how many machines will be in one group since this affects the maintenance staff and total noise level permitted by international or national standards. The effects of a silencer are similar to a filter since the silencing elements cause a pressure drop.

With offshore platforms it is not always practical to locate the main generators in a good place regarding the position and routing of the inlet and exhaust ducting. Long runs of ducting are sometimes unavoid- able. It is then necessary to allow a derating factor for the pressure drop that will occur. The manufacturer should be consulted for advice on this aspect. For a typical offshore or onshore situation with a reason- able degree of silencing a rule-of-thumb derating factor would be 98%. In a poor location assume 95%.

FUEL FOR GAS TURBINES

By Engr. Aneel Kumar September 06, 2013 No comments:

The fuels usually consumed in gas turbines are either in liquid or dry gas forms and, in most cases, are hydrocarbons. In special cases non-hydrocarbon fuels may be used, but the machines may then need to be specially modified to handle the combustion temperatures and the chemical composition of the fuel and its combustion products.

Gas turbine internal components such as blades, vanes, combustors, seals and fuel gas valves are sensitive to corrosive components present in the fuel or its combustion products such as carbon dioxide, sulphur, sodium or alkali contaminants.

The fuel can generally be divided into several classifications:-

• Low heating value gas.
• Natural gas.
• High heating value gas.
• Distillate oils.
• Crude oil.
• Residual oil.

SINGLE AND TWO SHAFT GAS TURBINES

By Engr. Aneel Kumar September 06, 2013 No comments:

There are basically two gas turbine driving methods, known as ‘single-shaft’ and ‘two (or twin) shaft’ drives. In a single-shaft gas turbine, all the rotating elements share a common shaft. The common elements are the air compressor, the compressor turbine and the power turbine. The power turbine drives the generator.

In some gas turbines, the compressor turbine and the power turbine are an integral component. This tends to be the case with heavy-duty machines.

The basic arrangement is shown in Figure 2.3.

In a two-shaft gas turbine the compressor is driven by a high pressure turbine called the compressor turbine, and the generator is driven separately by a low pressure turbine called the power turbine

The basic arrangement is shown in Figure 2.4.

Two-shaft systems are generally those which use aero-derivative engines as ‘gas generators’, i.e. they produce hot, high velocity, high pressure gas which is directed into the power turbine. Some light industrial gas turbines have been designed for either type of drive. This is achieved by fitting a removable coupling shaft between the two turbines. Some points to consider with regard to the two types of driver are:-

a) High speed of rotation tends to improve the compressor and turbine efficiency. Hence, with two separate shafts, the best thermodynamic performance from both turbines and the compressor is obtainable.

b) Using aero-derivative machines means that a simple ‘add on’ power turbine can be fitted in the exhaust streams of the aero engine. This enables many manufacturers to design a simple power turbine and to use a particular aero engine.

c) Two-shaft machines are often criticized as electrical generators because of their slower response to power demands in comparison with the single-shaft machines. This can be a problem when a two-shaft machine may have to operate in synchronism with other single-shaft machines or steam turbine generators. Sometimes the slower response may affect the power system performance during the starting period of large motors. A power system computerized stability study should be carried out to investigate these types of problem.

Some of the recent aero engines could be called ‘three-shaft’ arrangements because within the gas generator there are two compressor turbines and two compressors.

ENGINEERING ARTICLES

Sunday, September 29, 2013

PLANNING OF HYDROELECTRIC FACILITIES

HYDROELECTRIC POWER GENERATION

Tuesday, September 17, 2013

ELECTRICAL ENGINEERING PROGRAM EDUCATIONAL OBJECTIVES

Friday, September 13, 2013

SELF INDUCTANCE

MUTUAL INDUCTANCE

Thursday, September 12, 2013

EFFICIENCY OF A TRANSFORMER

OPERATING PRINCIPLES OF T RANSFORMERS

Saturday, September 07, 2013

METHODS OF STARTING INDUCTION MOTORS

CRITICAL TIMES FOR INDUCTION MOTORS

PRINCIPLE OF OPERATION OF THREE PHASE INDUCTION MOTOR

MAIN AND PILOT EXCITER

PHSOR DIAGRAM OF A TWO AXIS SALIENT POLE GENERATOR

OPERATION OF GENERATOR

SUB TRANSIENT STATE ARMATURE REACTION

TRANSIENT STATE ARMATURE REACTION

STEADY STATE ARMATURE REACTION

Friday, September 06, 2013

GENERATOR OPERATION

COMMON ASPECTS BETWEEN GENERATORS AND MOTORS

GOVERNING SYSTEMS FOR GAS TURBINES

STARTING METHODS FOR GAS TURBINES

FACTORS TO BE CONSIDERED AT THE DESIGN STAGE OF A POWER PLANT

FUEL FOR GAS TURBINES

SINGLE AND TWO SHAFT GAS TURBINES

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