Independent Generation of Electric Power

David Stephen

• Publisher: Butterworth-Heinemann
• Year: 2014
• Print ISBN:
  9780750616911
• eBook ISBN:
  9781483102368
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Independent Generation of Electrical Power explains the different operations involved in the generation of power in power plants and the concepts and principles behind them. The book covers topics such as the parameters and requirements of generator performance; configurations of generators; and the operation and modes of control of generators; system control logic; and different energy management systems. The book also includes three appendices. Appendix 1 contrasts induction generation and synchronous generation; Appendix 2 covers different protection equipment, and Appendix 3 discusses the analyses involved in electrical systems. The monograph is recommended for engineers who would like to know more about the design and operation of plants and how it generates power.

1

Generator performance

Publisher Summary

This chapter discusses generator performance. The purpose of an electrical generator is to produce electrical energy to some load, but it must do so within specified operating limits of frequency and voltage. The load behavior affects the generator requirements but the environmental conditions and the characteristics of the prime mover and the generator control systems are all relevant to the actual generator performance during steady duty or rated condition and also during transient, sub-transient, pre-transient, and all dynamic conditions encountered. With electrical generators, it is easy to determine when the load changes, and this can be done before the speed has changed. It can enable the governor to preempt this signal and give a faster and more accurate response to the actual load change being imposed. Electrical generators themselves are not ideal devices and produce system conditions that may have undesirable consequences in particular situations. When combined with the wide range of characteristics that can result from practical electrical loads, these problems can be very significant. An independent generator must have characteristics suitable to all its possible operating functions, and must be connected into the system and controlled in such a manner as to provide the overall requirements of the connected loads.

The purpose of an electrical generator is to produce electrical energy to some load but it must do so within specified operating limits of frequency and voltage. When it operates in island mode it is responsible for determining these parameters but when operating in parallel mode with other generators it only has limited control, in general, and the required mode of operation can be as significant as the electrical load being supplied.

The load behaviour affects the generator requirements but the environmental conditions, the characteristics of the prime mover and the generator control systems are all relevant to the actual generator performance, during steady duty or rated condition and also during transient, subtransient, pretransient and all dynamic conditions encountered. The actual design of any generator, therefore, has to make allowance for all these factors; however, before considering generator performance it is necessary to consider in detail the relevant aspects of the following factors:

1. Significance of prime mover.

2. Electrical loads and systems.

3. Environmental effects.

4. Generator control systems.

Significance of the prime mover

The purpose of a prime mover is to convert energy in one form into rotating mechanical power to drive the electrical generator, and the important characteristic is the torque–speed performance as a function of load absorbed by the generator. The inherent (or design) performance of an engine is affected by several secondary factors such as variation in fuel or energy medium, or environmental effects such as ambient air. The resultant performance will seldom result in a satisfactory generator performance to meet the requirements of the electrical load and a control system is added to ensure reasonable compatibility. This may consist of a simple governor or be a more complicated energy-regulating system and it differs significantly between types and sizes of prime mover.

The prime mover and generator form an integral package with mechanical connections between them and the wide range of possible features associated with this interaction requires the generator to have mechanical features which can affect its electrical performance.

Prime mover

The type of prime mover chosen for any generator must meet the following basic requirements:

1. Have a rated continuous running speed which ensures that the generator will produce its desired electrical frequency, if necessary by using a gearbox or other speed-changing device between prime mover and generator, and possess some means of adjusting the speed.

2. Use a suitable, available, economic fuel, and have an effective fuel supply system including any necessary filtering, pressurization etc., with a standby alternative and a suitable changeover procedure, if desired.

3. Have acceptable initial and operating costs, including running costs under actual operating conditions of load, fuel and environment and allowing for effects of running efficiency, heat/energy recovery, and also expense involved in supervision, erection and maintenance.

4. Be capable of operating satisfactorily and safely in its specific environment, such as available cooling media, weather, hazardous area, marine or other contaminated location, and adverse effects of noise, vibration, exhaust etc.

5. Have a competitive first cost in relation to number and size of units selected, including any auxiliaries required, together with structural or installation requirements.

In addition to these basic requirements, however, the following technical aspects must also be satisfied:

6. The complete interconnected shaft system, including all rotors, gears, couplings, etc., must be designed to meet all operating torque and speed conditions for steady-state load conditions and also any cyclic, torsional, transient and pretransient electrical short circuit duties, as well as starting and accelerating conditions. It must also be compatible with all vertical and horizontal displacements, deflections and line-ups, including axial effects, and their resultant effects on the components of the system and its supporting structure.

7. It must also be suitable for any abnormal torque conditions or irregularities due to maloperation, cyclic and torsional effects and resonances, whether mechanical, electrical or a combination.

It is also necessary to incorporate a suitable speed-governing and compatible fuel supply control system, which must meet all the following conditions:

8. Provide a suitable steady-state and transient speed change control allowing for the specific fuel system available and auxiliary supply arrangements, including, if required, a suitable on-load fuel changeover facility.

9. Provide a suitable steady load–speed characteristic, droop or isochronous as required, with acceptable levels of error, fluctuation and repeatability, with adequate range of control facilities and maintenance requirements.

10. Prevent undesirable interaction of multiple sets in parallel, whether similar or dissimilar, through any energy medium whether electrical, mechanical, fuel or any other, and avoid any interactive electromechanical resonances.

General

While the above summary is useful as an aide-memoire, in practice it is usually found that there are other factors which may have an overriding influence on the optimum choice of plant. Thus prospective purchasers may have an aversion to a particular type of machine, or a supplier, or there may be national or political reasons for supporting home manufacturers’ products in preference to those from other countries.

Some units have been specifically designed for particular applications and match the requirements more closely than others; some, by reason of long proven service, have an obvious advantage over newer designs which may appear technically superior on paper, but do not have a proven service record.

It is necessary, therefore, for a system designer or project engineer to take into consideration such factors, as it is not usual for the choice of every feature of a unit design to be unrestricted.

Usually, any investigatory study for a suitable generation plant will result in a final choice between significantly different concepts: for example, a low-speed diesel-driven generator versus a high-speed gas turbine driving a generator through gears. Nearly all the items listed in the above summary have significantly different effects on such options, and the final assessment as to which unit is more suitable for a particular installation may have to involve completely different concepts which are not normally of much technical significance. The psychological reaction of operating or maintenance staff can also play a big part in such choices, and the necessity for providing special training or educational courses may have to be included in the final cost factors.

The primary function of a generator prime mover is to convert energy from fuel or other source into mechanical movement which can be used by the generator to provide electrical energy to the load. The most common source of energy is fossil fuel or water power; nuclear, wind, wave power etc. are also used, but to a lesser extent. Fuels such as oil or gas can be used directly in engines, but they can also be used in the same way as coal or wood for combustion in a boiler to raise steam which can drive other forms of prime mover.

There are distinct types of prime mover, those that have a pure rotational movement, such as a steam or gas turbine, and those such as a diesel or steam reciprocating engine; the method of producing torque to drive the generator results in very significant advantages and disadvantages of these alternative types.

Development over the years has resulted in each type being employed for specific purposes and for the range of power and speeds where it offers significant advantages. Thus, in general, diesel engines and hydraulic turbines are directly connected to electrical generators, whereas gas and steam turbines, which operate efficiently at high speeds, are usually connected to generators through speed-reducing gearboxes.

Special applications may justify more unusual arrangements to match particular requirements, but the same basic conditions require to be met if satisfactory generator performance is to be obtained: in some instances mixed types of prime mover can be incorporated with advantage, but it is always necessary to evaluate not only first cost of special arrangements but hidden costs related to expensive or difficult maintenance and possibly operator training to handle complicated plant.

Characteristics

The main difference between reciprocating and rotary engines is that the former generates a fluctuating torque while the latter is inherently a uniform torque. A variety of features have been used to minimize this difference, such as the use of multiple cylinders which are utilized in a time sequence to produce a greater number of torque pulsations but of a smaller magnitude. The addition of a flywheel can also reduce the effective output torque by acting as an energy damper and converting torque pulsation to speed pulsations of small magnitude.

In many instances this torque–speed pulsation has a negligible effect on generator performance. However, it can be very significant if the frequency of the pulsation, which naturally results in a pulsation of the electrical energy output, coincides with a critical response frequency of some electrical load, such as the phenomenon of lamp-flicker, or the more common condition where the frequency of pulsation is such as to excite an electromechanical resonance or even a purely mechanical resonance in some part of the associated structure.

The electromechanical resonance effects, although originating in the prime mover, can be propagated through the electrical system and can affect other units on the system. This is of particular importance when running generators in parallel where their prime movers are of the reciprocating type.

The factors that determine the power output of different types of prime mover are quite different: operation at low power can cause serious deterioration in a diesel engine, whereas it can often provide considerable overpower for a short time; rotary machines do not exhibit this to the same extent. Since gas turbines utilize large volumes of air for combustion, their power output capability increases considerably at lower ambient temperatures.

The quality of fuel used can also affect the capability of any prime mover, quite apart from the possible damage that low grade or contaminated fuel can do.

Different types of prime mover are designed for specific types of duty and the basis for their rating takes this into account. For example, industrial-type gas turbines are heavier and more robust than the aero-derivative types and are intended for continuous rated operation over considerable periods, whereas the lighter, more compact aero-derivatives have advantages in short-term emergenc...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Introduction
• Chapter 1: Generator performance
• Chapter 2: LIQUID AMMONIA
• Chapter 3: Generator application and control
• Chapter 4: System control logic
• Chapter 5: Energy management systems
• Appendix 1
• Appendix 2
• Appendix 3
• INDEX