reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

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at least one of the Type A2 keys to satisfy the Safety Rule earthing requirements.

(h)Reinstatement of supplies will be established in the reverse order.

Note: It has been assumed that the Chlorination Plant Electrolysers are 'dead end feeders' in the above procedure.

8.6 Other safety interlocking

During normal operation it is imperative that all

equipment which contains live metal must be designed such that access is denied at all times whilst the equipment is energised. Typical examples of this are switchgear cubicles where internal access is not possible when the isolator is 'on', which is done by ensuring that in this position the door cannot be opened. Although these requirements are written in the equipment specification, there are several ways to achieve the objective. The choice must be one of a balanced design, preventing defeat from the unintentional act but at the same time not building in complications which makes normal operation more difficult.

1 Principles of electrical system analysis

1.1Introduction

1.2System design assessments

1.3Analysis program developments

1.4Analysis techniques

1.4,1 Reliability evaluation

1.4.2 Power system performance analysis 1.5 Quality assurance of analysis programs

2 Reliability evaluation of power systems

2.1Introduction

2.2Quantitative reliability evaluation

2.2.1Choice of numerical indices

2.2.2Scope of reliability evaluation assessments

2.3Computer programs for reliability evaluation

2.3.1Batch program — RELAPSE

2.3.2Interactive program — GRASP (Version 11

2.3.3Interactive program — GRASP (Version 21 2.4 Data requirements

2.4.1Active failure rate

2.4.2Passive failure rate

2.4.3Total failure rate

2.4.4Average repair time

2.4.5Switching time

2.4.6Maintenance rate

2.4.7Maintenance time

2.4.8Stuck probability

2.4.9Time limit of a limited energy source

2.4.10Common mode failure rate

2.5 Techniques employed

2.5.1 Graphical representation of the station electrical System

2.5.2Component and branch numbering

2.5.3Branch definition

2.5.4Criteria of failure

2.5.5Analysis control procedures

2.5.6Deduction of minimal paths

2.5.7Deduction of minimal cut sets 2.5,8 Types of failure/restoration event

2.5.9Switching effects of component active failure

2.5.10Markov state-space models

2.5.11Evaluation techniques (busbar indices)

2.5.12Evaluation techniques (system indices)

2.5.13Presentation of results

2.6Quality assurance

2.7Typical applications

2.7.1Example of the calculation and use of busbar indices

2.7.2Example of the calculation and use of system indices

3 Power system performance analysis

3.1Load flow analysis

3.1.1Introduction

3.1.2Program construction [4,5]

3.1.3Use of programs

3.2Fault level analysis

3.2.1Introduction

3.2.2Program construction

3.2.3Use of programs

3.3Stability analysis

3.3.1Introduction

3.3.2Analytical and programming considerations

3.3.3Use of programs

3.4Future developments of electrical analysis programs

4 References

1 Principles of electrical system analysis

1.1 Introduction

The assessment and validation of electrical power systems in terms of their predicted performance and comparative levels of reliability and availability are essential tasks in the early design phases for all major projects. This is particularly important in respect of those aspects of the design and performance necessary for ensuring the safe and satisfactory integration of various types of power station into the CEGB grid supply system.

Broadly, the primary functional design objectives of electrical system analysis are to:

• Ensure that the designs for plant and system net-

works comply with specified operating performance and safety criteria.

•Evaluate the comparative performance and cost effectiveness of alternative plant designs and system configurations.

•Determine and optimise plant and system parameters for technical performance specification purposes.

•Provide the electrical characteristics necessary for the design of electrical protection systems.

•Define system and plant constraints for operational and maintenance procedural purposes.

System and plant parameters are Currently fully represented by mathematical models in the design process

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of electrical auxiliary systems and associated plant. They involved network reduction techniques, with many design performance approximations and assumptions; sensitivity and iterative studies, invoking permissible system and plant design tolerances, were necessarily of an extremely constrained nature.

With present day computer programs and computing facilities, the analysis techniques are vastly more comprehensive, efficient and precise than older methods. Batch mode and interactive analysis programs enable the predictive performance of station electrical system designs to be optimised on an iterative basis, with much more detail and a higher degree of accuracy than previously, in an extremely short time.

Batch mode computing systems are capable of analysing the performance of more than one power system at a time and are primarily suitable for processing the extensive amounts of information and results associated with the dynamic transient analysis of large systems. Such studies may, for example, involve a computational time of about 10 minutes for a single

short term transient condition of 5 seconds, but they can of course be submitted for analysis during computer off-peak periods at night times and weekends.

Interactive on-line computing, by comparison, allows the design engineer to interface with, and directly control, the analysis process from a computer terminal in respect of the network representation, type of study required, system parameters, corrective actions and output result displays. While interactive computing is highly user orientated and consequently more time demanding at the terminal for the design engineer, it is much faster than batch mode analysis systems in the overall turn round time of producing results, especially for steady state power system studies.

The availability of separate suites of batch mode and interactive computer programs, when coupled with their respective databases, enables a series of specific power system performance or reliability evaluation analysis studies to be undertaken in a uniform manner, by varying the system parameters and the type of study. As a consequence of the ability to control the type

of study and sequence of studies from within a suite of programs, the efficiency and speed of response in producing assessments of large complex systems has increased significantly.

For example, having entered all the system and plant data, the design engineer is able to produce the results of a power system steady state analysis or reliability evaluation study within a matter of seconds for a complete power station electrical system, including the grid interface connections. It is also a relatively simple exercise to undertake sensitivity studies to identify the system components and parameters having a critical impact on the overall system performance.

One should not, however, overlook the considerable design effort and time that is involved in setting up the system networks and the collation, checking and entry of the system plant and connection data for modelling and simulating complete station electrical systems fully. This may amount to something in excess of eight man weeks for a large power system.

1.4.1 Reliability evaluation

A reliability evaluation study forms the normal starting point for comparing the effectiveness of alternative

interface connection arrangements between the grid system, the turbine-generator units and the associated station electrical supply systems. The extent of the system evaluation undertaken, using interactive computing programs, incorporates the following analysis features:

•Failure rates.

•Outage durations.

•Outage times.

•Common mode failures.

•Limited energy sources.

•Time dependent reliability.

The basic techniques required to analyse and evaluate the reliability of the systems include the simulation of realistic failure events, restoration procedures, switching actions, maintenance events and operational constraints.

For multiple turbine-generator power stations with identical unit and station systems, it is more efficient to divide the systems into subsystems. This technique, as explained in detail later, is not only efficient and

precise but reduces the design engineering effort involved in the preparation and input of system component data.

1.4.2 Power system performance analysis

Batch mode and interactive computing programs are used to assess the performance of integrated and isolated power systems in respect of:

•Load flows, voltage regulation and transformer tapchanger optimisation.

•Short-circuit (symmetrical and asymmetrical) fault levels.

•Dynamic motor starting performance.

•Transient fault stability performance of systems and motors.

•Fast transient switching of systems and motors.

•Transient recovery voltage levels.

•Controller modelling of AVRs, governors and prime movers.

•Power system protection co-ordination of relays and fuses.

Power system performance analysis is largely based on load flow, short-circuit and transient stability studies. The analysis, using nominal plant parameters, commences with the calculation of the steady state power

flows, voltage distributions and short-circuit fault levels throughout the system during no load, minimum load, start-UP and CMR sequence loading (block loading) conditions of the turbine-generator unit operation. During this stage of the analysis, transformer tap changer positions are adjusted, as appropriate, to achieve the optimum system voltage profiles for these conditions of operation. Dynamic motor starting and system transient fault .tability performance evaluation studies are also intro& during this process for the optimisation

of system and plant parameters.

Further stcady state, dynamic response and transient fault stability sensitivity analyses are then performed, selectively invoking permissible plant manufacturing tolerances, defined system operating ranges of voltage and frequency, and credible system and plant outage operating conditions. Calculation of system dynamic responses uses a step-by-step approach, starting from a specified balanced steady state operating condition produced by a load flow study.

Analysis programs contain a number of models for the representation of synchronous machines and induction motors; they automatically select the most suitable model, according to the amount of data provided by the user in terms of the machine model parameters. The most detailed models of a synchronous machine include the transient and sub-transient and saturation function parameters, together with the excitation and speed governor controller model block diagram transfer function elements. Other loads are represented as fixed impedances, while the transformers, transmission and cable connections are represented by their equivalent circuit parameters.

Inclusion of the effects of instantaneous voltage and frequency deviations into the calculations, produces a more realistic assessment of system behaviour under transient conditions. The widely used diesel or gasturbine generator supported essential supply systems in nuclear power stations are examples of such operating situations and a special purpose transient analysis program has been developed for assessing their dynamic performance. This program recalculates the plant and s,stern parameters throughout the transient variations of instantaneous voltage and frequency, thereby enabling the systems and plant to be designed and operated closer to their limits, coupled with the ability to define the settings of the associated protective devices more accurately.

Detailed descriptions of power system reliability ev aluation and performance analysis are given in the following Sections 2 and 3 respectively. These descriptions are comprehensively illustrated by authentic computer printouts generated by the software programs under discussion, i.e., they have not been subjected to any form of artistic licence.

1.5 Quality assurance of analysis programs

Particular emphasis is placed on achieving the highest

possible standard of quality by instituting rigorous verification and validation testing disciplines from the inception of a program up to the stage when it is formally released in a production status version. Test systems, representative of the wide spectrum of the electrical systems encountered within power stations, are used for the quality assurance of all program versions and enhancements.

Each development and production version of a program is identified with a unique identification reference, which is displayed on the VDU during analysis and is subsequently printed automatically on the system input, network diagrams, graphical and output data sheets.

At an early period in the development process, an appropriate level of verification and validation testing is initiated and formally recorded with respect to program source codes, models, calculations and control routines. Verification activities are progressively increased during the development, involving extensive testing and comparison of the source codes with the mathematical models by the specialist software programmer.

The verification and validation tasks are the responsibility of the design engineer, who undertakes a comprehensive comparison of the mathematical models and program results with manual calculations, field measurements of plant and system performance tests and, whenever applicable, with earlier versions of the program and alternative validated analysis programs. The widespread utilisation of a program by many design engineers for the analysis of various types of integrated and isolated power systems is an additional i mportant contribution to the verification and validation process.

Notwithstanding the procedures and disciplines outlined it is recognised that the verification, validation and modification of computer programs may continue for many years following their formal introduction at a production status level. Rigid quality control disciplines must continue to be exercised in this regard in accordance with the procedures outlined.

2 Reliability evaluation of power systems

2.1 Introduction

The reliability of larger turbine-generator units (500 MW and above) in modern power stations is assuming increasing significance in terms of system security and the economics of supply, since they are used in the main to meet the base load requirements of the grid system.

It follows that all the supporting auxiliary plant and systems within each station must have a similar high degree of reliability, in order to minimise the impact of their failure on the overall reliability of the main units.

The Station Electrical System (SES) is an example of one such internal system: it can be considered to have three constituent parts:

•The MAIN system, the principal function of which is to distribute a sufficient supply of electrical power of the required quality to all the drives and auxiliary plant associated with the boilers/reactors and turbine-generators. Failure of all or even part of this system could have a direct impact on the availability of a main generator, causing either the total loss or reduction of output from the generator to the grid system.

•The GUARANTEED system which supplies power of guaranteed quality to computers, instruments, etc., necessary for the operational control and monitoring of the power station. Sometimes this is referred to as the No-break system and is required to continue to function during large system disturbances, unit start, shutdown, etc.

•The ESSENTIAL system which comprises standby generators and associated electrical connections and drives (some of which are common with the MAIN system). This is sometimes called the Short-break system and is generally used (with or without the standby generators) to supply power to all those auxiliaries necessary for the safe operation and posttrip cooling requirements of nuclear stations and for black starting of fossil-fired stations.

It is important, therefore, that a reliability assessment of all three parts of the SES is performed during, and as an integral part of, the design process and should cover all the system operating configurations and station running modes.

Reliability assessment, as such, is not a new requirement. Engineers have always striven to design systems that will continue to operate in a safe and reliable manner. In the past, this reliability has generally been achieved by drawing on the experience of design and operational staff and using purely engineering and qualitative criteria. Quantitative assessment, due mainly to the sheer tedium of the calculations involved, tended to be limited to the evaluation of relatively small systems (comprised of few components).

2.2 Quantitative reliability evaluation

It is clear that a quantitative evaluation of the SES reliability is of considerable help in decision making during the design phases of any power station project. It consists of the calculation of a set of numerical indices which provides a measure of the 'goodness' of the system and can be used during the design phase to compare one system with another.

2.2.1 Choice of numerical indices

There are many reliability indices that could be cal-

culated and used as a measure of the 'goodness' of a SES. The following are typical examples of suitable indices:

OT or OD The total Outage Time or total Outage Downtime in a specified period

The interactive computer programs developed specifically for the quantitative reliability evaluation of power station electrical systems (described in Section 2.3 of this chapter) calculate, list and/or display the reliability indices. The following indices have been selected as the most useful and appropriate in the iterative process of system design:

(a)For a particular load point or node of a SES:

F R Expected average number of failures of supply to the load point in a year (failures/year)

AOD Average outage duration or downtime — average time for which no supply is available at the load point (hours)

AOT Total annual outage time — total time in a year when no supply is available at the load point — (hours/year).

(b)For the SES as a whole:

AOT Total time of operation in each of the derated states in a year (hours/year)

LOG Expected loss of generation in a year due to the unreliability of the SES (MWh/year).

2.2.2 Scope of reliability evaluation assessments

The availability of very powerful modern mainframe computers led to the development of efficient computer