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6 Power system planning and switchgear engineering
6.1 Planning of switchgear installations 6.1.1 Concept, boundary conditions, pc calculation aid
The process of planning switchgear installations for all voltage levels consists of establishing the boundary conditions, defining the plant concept and deciding the planning principles to be applied.
The planning phase is a time of close cooperation between the customer, the consult- ing engineer and the contractor.
The boundary conditions are governed by environmental circumstances (plant location, local climatic factors, influence of environment), the overall power system (voltage level, short-circuit rating and arrangement of neutral point), the frequency of operation, the required availability, safety requirements and also specific operating conditions.
Table 6-1 gives an indication of the boundary conditions which influence the design concept and the measures to be considered for the different parts of a switchgear installation.
In view of the equipment and plant costs, the necessity of each measure must also be examined from an economic standpoint.
Taking the busbar concept as an example (Table 6-3), the alternatives are evaluated technically and economically. The example is valid for h.v. installations, and to some extent m.v. installations as well.
PC calculation aid
Numerous computer programs are available for use in planning switchgear installations, particularly for design calculation. Sections 6.1.6 and 6.1.7 deal with computer-aided methods for:
– short-circuit current – cable cross sections.
Table 6-2 summarizes the computer programs used in planning switchgear installations, together with their fields of application and contents.
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274 Table 6-1
Choice of plant concept and measures taken in relation to given boundary conditions Boundary conditions Concept and measures
Environment, climate, Outdoor/indoor
location: Conventional/GlS/hybrid
Equipment utilization Construction
Protection class of enclosures Creepage, arcing distances Corrosion protection Earthquake immunity Network data,network form: Short-circuit loadings Protection concept Lightning protection Neutral point arrangement Insulation coordination Availability and redundancy Busbar concept of power supply: Multiple infeed
Branch configuration Standby facilities Uninterruptible supplies Fixed/drawout apparatus Choice of equipment Network layout
Power balance: Scope for expansion
Equipment utilization Instrument transformer design Ease of operation: Automatic/conventional control
Remote/local control Construction/configuration Safety requirements: Network layout
Arcing fault immunity Lightning protection Earthing
Fire protection Touch protection Explosion protection
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Table 6-2
PC programs for project planning and calculations for switchgear installations Program Applications Testing, calculation and dimensioning name
EMTP/ATP Calculation of transient – Internal and external overvoltages processes in any meshed – Interference voltage affecting multiphase electrical telecommunications cables
systems – Transient voltage boost in
earthing systems on lightning strike – Operational response of battery
power systems
ERSO2005 Calculation of earthing systems – Determination of the propagation resistance
– Determination of step and touch voltages
PRESSURE Calculation of the pressure – Verification of the pressure with characteristic in switchgear stand capability of medium voltage
rooms on arcing switchgear
– Dimensioning of pressure relief equipment
NEPLAN® Program system for network calculation with the following modules:
Phase fault current calculation – Switchgear installations (busbars, Calculation of symmetrical connections)
and non-symmetrical fault – Equipment (switches, transformers) currents to
– IEC 60909 (VDE 0102) – Superposition method
Load flow calculation – Protection systems – Switchgear installations – Equipment and power – Minimum loss system operation
methods
– Critical system states
– Useful switchovers on equipment failure
Load curve simulation – Voltage drop on motor start-up – Load behaviour against time Optimized load flow – Load prediction
– Distribution network – Minimization of losses – Transmission network – Voltage stability Transient stability – Power plant use
– Study of network behaviour in dynamic processes – Generator and turbine control – Faults and switching processes (continued)
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Table 6-2 (continued)
Computer programs for project planning and calculations for switchgear installations Program Application area Testing, determination, dimensioning Name
NEPLAN® Motor start-up Simulation in time range Harmonic analysis – Harmonic currents and voltages
in networks with converters – Filtering and compensation
equipment
– Propagation of audiofrequency ripple control signals
Selectivity analysis – Protection coordination in MV (overcurrent-time protection) and LV networks – demonstration
of selectivity
– Checking of switch-off conditions Distance protection – Compilation of selective tripping
schedules
Dimensioning of medium – Optimum cable cross-section and low voltage cables – Protection from overload
– Protection on short-circuit – Voltage drop
Reliability analysis – Determination of reliability characteristics in networks Maintenance – Determination of an optimum
maintenance and replacement strategy for systems and equipment
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6.1.2 Planning of high-voltage installations
The following criteria must be considered when planning high-voltage switchgear in- stallations:
Voltage levels
High-voltage installations are primarily for power transmission, but they are also used for distribution and for coupling power supplies in three-phase and HVDC systems.
Factors determining their use include network configuration, voltage, power, distance, environmental considerations and type of consumer:
Distribution and urban networks > 52 – 245 kV
Industrial centres > 52 – 245 kV
Power plants and transformer stations > 52 – 800 kV
Transmission and grid networks 245 – 800 kV
HVDC transmission and system ties > 300 kV
Railway substations 123 – 245 kV
Plant concept, configuration
The circuitry of an installation is specified in the single-line overview diagram as the basis for all further planning stages. Table 6-3 shows the advantages and disadvantages of some major station concepts. For more details and circuit configurations, see Section 11.1.2.
The availability of a switching station is determined mainly by:
– circuit configuration, i. e. the number of possibilities of linking the network nodes via circuit-breakers and disconnectors, in other words the amount of current path redundancy,
– reliability/failure rate of the principal components such as circuit-breakers, disconnectors and busbars,
– maintenance intervals and repair times for the principal components.
278 Table 6-3
Comparison of important busbar concepts for high-voltage installations
Concept Advantages Disadvantages
configuration
Single – least cost – BB fault causes complete
busbar station outage
– maintenance difficult – no station extensions without disconnecting the installation – for use only where loads can be disconnected or supplied from elsewhere
Single – low cost – extra breaker for bypass tie
busbar with – each breaker accessible for – BB fault or any breaker fault bypass maintenance without causes complete station outage
disconnecting
Double busbar – high changeover flexibility with – extra breaker for coupling with one two busbars of equal merit – BB protection disconnects circuit-breaker – each busbar can be isolated for all feeders connected with the
per feeder maintenance faulty bus
– each feeder can be connected – fault at branch breaker to each bus with tie breaker and disconnects all feeders on the BB disconnector without affected busbar
interruption
– fault at tie breaker causes complete station outage
2-breaker system – each branch has two circuit- – most expensive method
breakers – breaker defect causes half
– connection possible to either the feeders to drop out if
busbar they are not connected to both
– each breaker can be serviced bus bars
without disconnecting the feeder – feeder circuits to be – high availability considered in protection
system; applies also to other multiple-breaker concepts
(continued)
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Table 6-3 (continued)
Comparison of important busbar concepts for high-voltage installations
Concept Advantages Disadvantages
configuration Ring bus
1¹⁄₂-breaker system
– breaker maintenance and any faults interrupt the ring – potential draw-off necessary in all feeders
– little scope for changeover switching
– three circuit-breakers required for two feeders – greater outlay for protection and auto-reclosure, as the middle breaker must respond independently in the direction of both feeders
Dimensioning
On the basis of the selected voltage level and station concept, the distribution of power and current is checked and the currents occurring in the various parts of the station under normal and short-circuit conditions are determined. The basis for dimensioning the station and its components is defined in respect of
– insulation coordination – clearances, safety measures – protection scheme
– thermal and mechanical stresses For these, see Sections 3, 4, and 5.
– low cost
– each breaker can be maintained without disconnecting load – only one breaker needed per feeder
– no main busbar required – each feeder connected to network by two breakers – all changeover switching done with circuit-breakers
– great operational flexibility – high availability
– breaker fault on the busbar side disconnects only one feeder – each bus can be isolated at any time
– all switching operations executed with circuit-breakers
– changeover switching is easy, without using disconnectors – BB fault does not lead to feeder disconnections
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Basic designs and constructions
The basic designs available for switching stations and equipment together with different forms of construction offer a wide range of possibilities, see Table 6-4. The choice depends on environmental conditions and also constructional, operational and economic considerations.
For further details, see Sections 10 and 11.
Table 6-4
The principal types of design for high-voltage switchgear installations and their location
Basic design Insulating Used mainly for
medium voltage level (kV) Location Outdoor Indoor
Conventional Air >52 – 123
Conventional Air 123 – 800
GIS SF6 >52 – 800 1)
Hybrid2) Air/SF6 245 – 500
1)GIS used outdoors in special cases
2)Hybrid principle offers economical solutions for station conversion, expansion or upgrading, see Section 11.4.2.2.
There are various layouts for optimizing the operation and space use of conventional outdoor switchgear installations (switchyards), with different arrangement schemes of busbars and disconnectors, see Section 11.3.3
6.1.3 Planning of medium voltage systems
Medium voltage networks are networks with rated voltages of over 1 kV and up to 36 kV. Medium voltage switchgear is used in transformer substations and switching substations, and in secondary unit substations and customer substations (in public utility networks only). This section is exclusively concerned with medium voltage switchgear for transformer substations and switching substations.
On account of the widely differing structures which result from the demands placed upon them, a distinction is made between industrial and power station service networks on the one hand, and public utility networks on the other hand.
The most frequent network voltages in Germany are
Public utility networks: 10 kV and 20 kV
Industrial and power station service networks: 6 kV, 10 kV and 30 kV.
While the load density in public utility networks ranges from 10 kW/km2or less in rural areas to 20 MW/km2or more in major cities, it often reaches even much higher levels in industrial and power station service networks.
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Apart from the high load density, industrial and power station service networks are frequently characterized by:
– high demands for reliability of supply, – high demands for voltage quality, – a high proportion of motor loads, and – high short-circuit powers.
As a consequence, the structure of industrial and power station service networks is strongly influenced by the requirements of the relevant production process. The requirements for configuration and the rated data of the medium voltage switchgear installations are to be deduced individually from the network structure concerned. It is normally essential to perform a network calculation (e.g. with NEPLAN®, cf. Table 6-2) in order, for example, to take account of the contribution of motor loads to the short- circuit current or to be able to formulate requirements for network protection.
The function of public utility networks is primarily to provide blanket supply of electrical energy while fulfilling the following conditions:
– Appropriate reliability of supply – Adequate voltage quality – Cost-effectiveness
As there are no restrictive planning criteria with regard to the appropriate reliability of supply, a number of standard network concepts which provide a level of reliability generally accepted as appropriate have become established in the course of time.
The demands placed on the medium voltage switchgear differ with the different network concepts. The choice of suitable system configuration is therefore always associated with the network planning, which matches the network concepts to the supply functions of the relevant network operator. The fundamental functions of network planning and the most important standard network concepts are therefore presented in the following section.
Planning and optimization of medium voltage networks for public power supply The function of network planning is nowadays less that of planning the network configuration than rather questioning and optimizing the existing, in many cases historically determined, network structures when replacement investments are impending. Frequently, the background conditions dictated by the network operator's supply function no longer correspond to the background conditions against which the medium voltage networks were established and expanded. Typical changes which can have feedback effects on the network structure and provide occasion for fundamental replanning or target network planning comprise the following:
– changed supply areas, for instance as a result of corporate mergers, – New or shifted load centres, with total load nearly stagnating or only rising
slightly,
– inaccurate load forecasts,
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– integration of decentralized generation facilities, and also
– increased pressure on costs, – increased equipment reliability, – improved network management.
Fundamental planning comprises the following aspects and focal points:
– analysis of the actual situation,
– definition of planning principles and criteria, – load forecast,
– development of target network variants,
– technical and commercial comparison of the network variants, and – definition of the target network.
The target network defines not only the network structure, but also the locations and sizes of transformer substations and switching substations and the requirements for the medium voltage switchgear.
Network calculations (e.g. with NEPLAN®, cf. Table 6-2) are performed in the course of analysis of the actual situation and to assess the target network variants. In that process, load flow and phase fault current calculations are accompanied above all by probability calculations to estimate reliability.
Probability calculations on reliability facilitate quantification of the reliability of supply in distribution networks. The influence of different network and system configurations on service reliability can be quantified in this way on the basis of reliability characteristics.
Examples of reliability characteristics include interruption frequency (stated as 1/a), i.e.
the expected value for the frequency with which interruptions are to be expected by consumers in the network, and non-availability (stated in min/a), i.e. the probability of finding a consumer without supply.
The reliability calculations are based on models of characteristic disturbance sequences, which are applied to the equipment in a specified network.
All significant contributions to failure events in the network are examined on the basis of the reliability characteristics of the equipment and the failure models, and the effects of these failures on the service to consumers are determined. The reliability characteristics then describe the cumulative effects of all failures in the network.
Furthermore, if the costs of interruptions are known, details of the annual costs caused by interruptions to supply can be presented.
The results of the reliability calculations for each consumer are as follows:
– HU: Expected interruption frequency (in 1/a) – PrU: Expected non-availability (in min/a)
– TU: Expected average interruption duration (in min or h) – WU: Expected energy not supplied on time (in MWh/a) Those for the system as a whole are as follows:
– SAIFI (System Average Interruption Frequency Index): Expected average interruption frequency (in 1/a)
– SAIDI (System Average Interruption Duration Index): Expected average non-availability (in min/a)
– WU,total: Expected energy not supplied on time (in MWh/a)
In addition, the contributions of the individual pieces of equipment to the "unreliability”
of the network can be quantified. This allows the importance of the equipment for the
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function of the network to be determined and, for example, priorities to be deduced for maintenance and replacement work.
a) Ring network b) Network with opposite station
c) Network with load- centre substation
Fig. 6-1:
Networks in which the individual transformer substations are not interconnected on the medium voltage side
Fig. 6-2:
Networks in which the individual transformer substations are interconnected on the medium voltage side
a) Corresponding transformer substations
b) Corresponding transformer substations with opposite station
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It is impossible to say in general which network concept provides the most efficient ratio of network costs to service reliability for which supply job, as the forms of the higher level high voltage networks and the subordinate low voltage networks and, last but not least, the rated system voltage which is usually specified, also have to be taken into account.
For transformer substations in explosion-proof design, there are no medium voltage connections between the substations. In these cases, the simple ring network concept as shown in fig. 6-1 a) is usually preferred. In extensive catchment areas with low load density, it is often more economical to implement a concept with opposite stations as shown in fig. 6-1 b). The opposite stations can be designed as switch-disconnector systems. When load centres far from the transformer substations are to be supplied, it is useful to provide a load centre substation as shown in fig. 6-1 c). This is connected to the transformer substation by feed cables and supplies power close to the load, for example via a ring network. Load centre substations are to be designed as circuit- breaker systems.
Networks in which the transformer substations are connected on the medium voltage side are used when those substations are not in intrinsically safe design. Back-up supply is then effected through the medium voltage network. The transformer substations are connected together either directly by distribution cables (fig. 6-2 a)) or by feed cables as in the opposite station concept shown in fig. 6-2 b). These network concepts can provide an economical alternative when the construction of an additional transformer substation is to be avoided, or initially implemented with one transformer only. The cost-effectiveness is however to be checked for the individual case concerned. Operational management of these network concepts is more complex, as coupling of the transformer substations through the medium voltage network has to be avoided when switching operations are performed in the network.
Otherwise, for example, high circulating currents could flow or the short-circuit withstand capability of equipment could be exceeded.
In practice, the load density within the supply territory of a distribution network operator frequently varies over quite a large range, and the higher and lower level networks are not uniform. This means that the network concepts presented are mostly not encountered in their pure form, but as hybrids.
A common feature of all the network concepts presented is the radial operating mode of the distribution cables, which not only facilitates simple operational management (apart from the exception mentioned above) but also a simple protection strategy and troubleshooting, particularly with single phase faults (earth faults / short-circuits to earth).
Planning of medium voltage switchgear
The standard structure of medium voltage switchgear today is the factory-assembled type-tested switchgear installation conforming to IEC 60298 (VDE 0670 Part 6). The most common structural types are described in Section 8.2.
The most important distinguishing characteristics of the currently available structural types and the associated decision-making criteria are:
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Distinguishing characteristics Technical decision-making criteria
Low costs Higher costs –
Single Double Network concept
busbars busbars
Air-insulated Gas-insulated Dimensions of the installation Environmental conditions (contamination, moisture, service requirements, cleaning)
Cubicle Metal-partitioned Personnel safety during wiring work Restriction of damage in the event of internal arcing (if compartmentalization is designed for this)
Switch disconnector Circuit-breaker Rating data
system system – Short-circuit currents
– Operating currents – Switching frequency Protection concept
6.1.4 Planning of low-voltage installations
Low-voltage installations are usually near the consumer and generally accessible, so they can be particularly dangerous if not installed properly.
The choice of network configuration and related safety measures is of crucial importance. The availability of electricity is equally dependent on these considerations.
Table 6-5 compares the advantages and disadvantages of commonly used network configurations, see also Section 5.1.
Another important step in the planning of low-voltage switchgear installations consists of drawing up a power balance for each distribution point. Here, one needs to consider the following:
– nominal power requirement of consumers, – short-time power requirement (e.g. motor startup), – load variations.
The IEC recommendations and DIN VDE standards give no guidance on these factors and point out the individual aspects of each installation.
For power plants and industrial installations, the circumstances must be investigated separately in each case.
The following Tables 6-5 and 6-6 are intended as a planner’s guide. The planners can use the information in Table 6-6 for reference.The total power is derived from the sum of the installed individual power consumers multiplied by the requirement factor with the formula:
Pmax = ΣPi · g Pmax= power requirement
Pi = installed individual power producer g = requirement factor
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Table 6-5 Summary of network configurations and protection measures for low-voltage installations System1)AdvantagesDisadvantagesMain application TN systemFast disconnection of fault orHigh cost of wiring and cablePower plants, public power short circuit. Least danger fordue to protective conductors.supply and networks. people and property.Any fault interrupts operations. TT systemLess wiring and cable required.Complex operational earthingLivestock farming. Zones with different touch(≤2 Ω). Equipotential bonding voltages permitted. Can benecessary for each building. combined with TN networks. IT systemLess expensive in respect ofEquipment must be insulatedHospitals wiring and cables.throughout for the voltage bet-Industry. Higher availability:ween the outside conductors. 1st fault is only signalled,Equipotential bonding necessary. 2nd fault is disconnected. Total insulationMaximum safety.Equipment doubly insulated,Residential, small-scale Can be combined with othereconomical only for small consumers.switchboards and equipment networks.With heat-generating loads, insulation constitutes fire hazard. Safety/extra-low voltageNo dangerous touch voltages.Limited power with cost-effectiveSmall apparatus. Functional extra-low voltageequipment use. Special requirements for circuitry. 1)For definitions and block diagram of the systems, see Section 5.1.2
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Table 6-6
Demand factor g for main infeed of different electrical installations
Type of installation Demand factor g for Remarks
or building main infeed
Residential buildings
Houses 0.4 Apply g to average use per
dwelling.
Blocks of flats Total demand = heating + a.c.
– general demand (excl. elec. heating) 0.6 typical + general.
– electric heating and air-conditioning 0.8 to 1.0 Public buildings
Hotels, etc 0.6 to 0.8 Power demand strongly
Small offices 0.5 to 0.7 influenced by climate, e.g.
Large offices (banks, insurance – in tropics high demand for
companies, public administration) 0.7 to 0.8 air-conditioning
Shops 0.5 to 0.7 – in arctic high heating de-
Department stores 0.7 to 0.9 mand
Schools, etc. 0.6 to 0.7
Hospitals 0.5 to 0.75
Places of assembly (stadiums,
theatres, restaurants, churches) 0.6 to 0.8
Railway stations, airports, etc. no general figure Power demand strongly influenced by facilities Mechanical engineering
Metalworking 0.25 Elec. drives often generously
Car manufacture 0.25 sized.
Pulp and paper mills 0.5 to 0.7 g depends very much on standby drives.
Textile industry
Spinning mills 0.75
Weaving mills, finishing 0.6 to 0.7 Miscellaneous Industries
Timber industry 0.6 to 0.7
Rubber industry 0.6 to 0.7
Leather industry 0.6 to 0.7
Chemical Industry
0.5 to 0.7 Infeed must be generously
Petroleum Industry sized owing to sensitivity of
chemical production processes to power failures.
Cement works 0.8 to 0.9 Output about 3500 t/day with
500 motors. (Large mills with h.v. motor drives.)
Food Industry 0.7 to 0.9
Silos 0.8 to 0.9
Mining Hard coal
Underground working 1
Processing 0.8 to 1
Brown coal
General 0.7
Underground working 0.8
(continued)
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Table 6-6 (continued)
Demand factor g for main infeed of different electrical installations
Type of installation Demand factor g for Remarks
or building main infeed
Iron and steel industry (blast furnaces, convertors)
Blowers 0.8 to 0.9
Auxiliary drives 0.5
Rolling mills
General 0.5 to 0.81) 1)g depends on number of
Water supply
0.8 to 0.91) standby drives.
Ventilation Aux. drives for
– mill train with cooling table 0.5 to 0.71) – mill train with looper 0.6 to 0.81) – mill train with cooling table
0.3 to 0.51) and looper
Finishing mills 0.2 to 0.61)
Floating docks
Pumps during lifting 0.9 Pumping and repair work do
Repair work without pumps 0.5 not occur simultaneously.
Lighting for road tunnels 1
Traffic systems 1 Escalators, tunnel ventilation,
traffic lights Power generation
Power plants in general
– low-voltage station services no general figure
– emergency supplies 1
Nuclear power plants
– special needs, e.g. pipe heating,
sodium circuit 1
Cranes 0.7 per crane Cranes operate on short-time:
power requirements depend on operation mode (ports, rolling mills, ship-yards) .
Lifts 0.5 varying widely Design voltage drop for
with time of day simultaneous startup of several lifts
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The type of construction depends on the station’s importance and use (required availability), local environmental conditions and electromechanical stresses.
Construction Main application
Type-tested draw-out Main switching stations
switchgear Emergency power distribution
Motor control centres
Type-tested fixed-mounted Substations
switchgear a.c./d.c. services for h.v.
stations Load centres
Cubicles or racks Light/power switchboards
Load centres
Box design Local distribution,
Miniature switchboards
The short-circuit currents must be calculated in terms of project planning activity, the equipment selected in accordance with thermal stresses and the power cable ratings defined. See also Sections 3.2, 7.1 and 13.2. Particularly important is the selectivity of the overload and short-circuit protection.
Selective protection means that a fault due to overloading or a short circuit is inter- rupted by the nearest located switchgear apparatus. Only then can the intact part of the system continue to operate. This is done by suitably grading the current/time characteristics of the protection devices, see also Sections 7.1.4,14.3 and 15.4. The choice of relays can be difficult if account has to be taken of operating conditions with powerful mains infeeds and comparatively weak standby power sources. In some cases changeover secondary protective devices have to be provided.
6.1.5 Station services switchgear in power plants
In a power plant, the electrical station services (abbreviated to SS in the following) consists of all the d.c. facilities from 24 to 220 V and a.c. facilities up to about 20 kV for controlling and supplying power to the equipment needed to keep the plant running.
Hence, these auxiliary services clearly play a vital role in assuring the plant`s reliable operation. Close attention must therefore be paid to requirements affecting the particular plant and the safety considerations, such as the provision of backup systems.
Alternating current (a.c.) station services
The a.c. services for a generating plant unit consist essentially of the SS power transformer, in most cases a medium-voltage distribution net, and low-voltage distribution facilities. They may include a power transformer and the necessary distribution gear fed from separate m.v. network for supplying general loads, i.e. not directly related to the generating unit, and possibly for starting the units and shutting them down. Standby power supplies are dealt with in Chapter 15.
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Bus-type system configurations are employed in plants with smaller generators and hence reduced SS power requirements, and have the advantage of lower capital cost.
The station services of all power plants are based on these two principal arrangements.
Redundancy in the form of double busbars, cross-links between generating units, etc are possible, giving rise to a grat many alternative layouts.
The basic SS arrangement in a so-called unit-type system for a power plant composed of separate units is shown in Fig. 6-3, and corresponding layout for a bus-type system in Fig.6-4. The advantages of the unit-type configuration are that the ratings for the SS distribution facilities are lower, making it easier to cope with short-circuit currents, and that the units are self-contained, so enhanced availability.
Fig. 6-3:
Basic diagram of SS power supply for a unit-type generating plant Generator
transformer
Unit service transformer
Main generator
Unit services switchgear
Unit services load Unit 1
Grid network
External network
Unit 2 Unit services load
General services load
Auxiliary generator General services switchgear General services transformer
General systems
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Special requirements of auxiliary systems for different types of power plants are described on the following pages.
Hydro power plants
In hydro power plants the station services require 1-3% of the generators rated power, depending on type and size. Most of the individual motor ratings are below 50 kW.
Often the only exceptions to this are ratings of the power house crane, governor oil pumps and dewatering pumps. For this reason, medium-voltage distribution can be dispensed with. Only in the case of long distances to outside installations such as water intake stations, or very extensive power house, is medium- voltage switchgear required.
The unit-type layout is preferred with unit ranges >50MW. An uninterruptible SS power supply is not an absolute necessity. A brief interruption of up to a few minutes does not cause the turbines to trip out because the governor oil pumps are either fed from the station battery, or the governor system has an oil accumulator tank. Nevertheless, standby power facilities (usually a diesel set) or an external supply from a separate medium-voltage network cannot be omitted, since the energy sources mentioned above are available for only a short time and power must be secured for the gates and valves.
Fig 6-4:
Basic diagram of bus-type SS power supply Main busbar
Generator transformer
Services transformer
Main generator
Auxiliary generator Services busbar
Station services (generator set and other)
Grid network
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Diesel power plants
The SS power requirement in relation to generator output lies between 1% in the case of small sets and 3% for heavy-oil plants. Few of the auxiliaries have ratings above 50 kW. There is therefore no need for medium-voltage equipment. Much of the services power is determined by the number of auxiliaries, which with large heavy-oil engines (> 10 MW) can amount to 100 separate drives per set.
The failure of auxiliary systems such as fuel supply or lube oil causes the generating set to trip within a few seconds. Redundancy with automatic selection is hence necessary both for the auxiliaries and their power feeds. In the case of diesel plants burning heavy fuel oil, facilities should be provided for switching from heavy to light diesel oil if the main services supply fails, to prevent the heavy fuel oil from thickening and so clogging the pipes.
Diesel power plants are chiefly used in "island” networks. Consequently, there is often no secure external supply available and one has to resort to standby power units. With relatively small sets, auxiliaries fed from the station battery (possibly through inverters) can be used in the event of emergency shutdowns. This also applies to starting if the a.c. supply falls, which is more frequently the case with plants running isolated.
Gas turbine / combined-cycle power plants
In pure gas turbine plants, i.e. where the heat in the exhaust gases is not utilized, the SS power of the set when running is about 1% of the unit range. The auxiliaries are usually fairly small, so a medium-voltage level can be dispensed with. This is not so with the starting gear, where gas turbines of any substantial size will need a medium- voltage supply. Widely used are starting systems with static frequency changers, which provide the variable frequency necessary for starting.
Unit-type system is the preferred arrangement. On economy grounds, one starting facility is commonly provided for several units.
In the case of combined-cycle plants, where the exhaust from a gas turbine passes to a heat recovery boiler to raise steam for a turbine set, the proportion of supply power rises to some 3% because of the additional loads in the stream cycle , the cooling- water loop and for the steam turbine-generator.
Conventional steam power plants
The power needed fort he services in coal-fired steam plants amounts to about 7-10%
of the unit rated output, and some 5-6% in oil-fired stations. In conventional steam plants the station services are predominantly arranged on the unit-type principle. A medium-voltage level is required chiefly because of the large feedwater drives. A general power supply system is usually provided for starting and shutting down the generator units, but also as a provisional source if a unit`s own supply falls, and to feed loads that are separate from the units. In view of the extended size of such plants there may be several distribution centres for this general supply.The latter obtains its power either from the utility network or, if this cannot be relied on, from the station`s own diesel or gas turbine generating set.
Combined heat and power stations
While the station services for the different types of power plants are to a large extent standardized, the remarkable feature of the SS facilities for combined heat and power
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(CHP) stations is their wide variety. The reason is that CHP stations produce such diverse commodities as electricity and heat.
The design of these power plants, and hence of their SS systems, is governed by which of their products has priority.
If the main emphasis is on producing heat, the station services are more likely to be fed from a separate network than by the turboset itself. On the steam side, CHP stations are usually operated in a busbar configuration, in which case this arrangement is also applied on the electrical side. If, in addition to the boilers providing steam for the turbines, peak heating loads call for the use of back-up boilers, these will require power from the station power supply, as will the pumps for circulating hot water if the chosen heating medium is water.
In the event of a boiler or turbine outage, it is rarely possible to draw heat from another network to make up the deficit, a situation that is usually no problem with electricity.
The aim is therefore to operate smaller units in parallel, with redundancy.
Direct current (d.c.) station services
Direct-current systems are used for control and monitoring purposes, but also for supplying power to d.c. drives and, as part of an emergency (UPS) system, via inverters to alternating-current drives. The required energy is stored in batteries, with conversion by means of rectifiers and inverters. In normal operations the d.c. loads are fed via the rectifiers. The batteries serve only as a buffer in the event of shortlived high load currents or to brief interruptions in the a.c. supply, particularly to ensure that the generating units are shut down safely.
The d.c. systems employed in generating stations for providing power and for control purposes have voltages of 110 or 220 V, while the increasing use of electronics has also led to self-contained rectifier-battery systems of 24 or 48 V.
As with a.c. station services systems, both unit-type and busbar arrangements are possible, or a mixture of the two (e.g. 220 V as a unit system and 24 V as a busbar system for higher-level control facilities. Redundancy needs to be provide in order to achieve the required reliability.
Automatic station service power transfer
Reliability is greatly enhanced by duplicating parts of an station service system, the power supply for instance. Automatic transfer facilities are therefore needed to ensure trouble-free operation or to guarantee secure startup (see also section 15). Their purpose is to switch the station service loads to another power feed quickly and as far as possible without upsetting the running of the power plant.
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6.1.6 Computer-aided calculation of short-circuit currents
A knowledge of the expected short-circuit currents in an installation is essential to the correct selection of the switchgear and the line-side connected networks. The methods of calculation are described in chapter 3.
The upper limit value of these fault currents determines – the power ratings of the switching devices, – the mechanical design of the installation, – the thermal design of the equipment,
– the electrical design and configuration of earthing systems,
– the maximum permissible interference in telecommunications systems.
The lower limit value of these fault currents determines – overcurrent protection relays and their settings.
The calculation of short-circuit currents therefore helps to solve the following problems:
– Dimensioning of equipment on the basis of (dynamic) stresses on closing and opening and also thermal stresses.
– Design of the network protection system.
– Questions of compensation and earthing.
– Interference problems (e.g. in relation to telecommunications lines).
The NEPLAN®computer program enables simple but comprehensive calculation of short-circuit currents. It takes account of
– different switching conditions of the installations, – emergency operation,
– cold and hot states of the cable network, – contribution of motors to short-circuit currents.
The program output provides the short-circuit currents at the fault location and in the branches
a) for the transient phase after occurrence of the fault:
– Initial symmetrical short-circuit current I"k
– Peak short-circuit current ip
– Symmetrical short-circuit breaking current Ia b) for the steady-state phase after occurrence of the fault:
– Sustained short-circuit current Ikk – Short-circuit powers S"k – Voltages at the nodes
The results can be printed out both as phase values (L1, L2, L3) and as component values (positive-sequence, negative-sequence, zero-sequence system).
The comprehensive graphics functions provided by NEPLAN® allow not only the network topology but also phase fault results to be displayed on screen and plotted, see fig. 6-5. The user creates and edits the graphical network display interactively with the mouse. The calculations performed by the program strictly follow the method set out in IEC 60909 (VDE 0102) and described in section 3.3.
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Figure 6-5:
Example of graphic output (plot) of a computer-aided short-circuit current calculation (partial section) by the NEPLAN®PC program.
6.1.7 Computer-aided calculation of cable cross-sections
Before the cross-sections of the cables between the equipment in the switchgear system and the connected loads are finalized, they are to be calculated in relation to the operating conditions and cable lengths.
Factors determining the cross-section in this calculation are as follows:
– Permissible load carrying capacity in normal operation, taking account of the ambient temperature and method of laying.
– Response of protective devices on overload and at the smallest possible short- circuit current to interrupt hazardous touch voltages.
– Permissible voltage drop along the cable line in normal operation and, where applicable, during the start-up phase of motors.
– Thermal short-circuit strength.
The NEPLAN® module developed at ABB makes it possible to carry out this comprehensive calculation for every circuit. By entering the circuit data, such as operating current, max. and min. short-circuit current, tripping currents/times of the protective devices and maximum permitted voltage drops, the program selects the appropriate minimum cross-section to be laid for the relevant cable length. The method of calculation is in accordance with DIN VDE 0100, DIN VDE 0276 and the respective cable manufacturer’s data.
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6.2 Planning of substations 6.2.1 Modular planning of substations
To deal with ever tighter project schedules, it is essential to continue to increase the degree of prefabrication of switchgear components, to support project management with computerized aids as much as possible, to reduce engineering during the project and to save as much time as possible in assembling and commissioning the equipment.
Efforts similar to the previously achieved progress in modularization and standardization in
– LV switchgear design using type-tested switchgear assemblies (TTA, PTTA) as modular LV switchgear system (ABB MNS system),
– MV switchgear design using type-tested switchbays with standard programs, – high-current technology with modular structure of generator busducts and
circuitbreakers,
– HV switchgear design with gas-insulated switchbay series in modular technology as preassembled, type-tested and pretested bays have been made with optimized primary and secondary technical design in the area of HV outdoor switchgear installations.
6.2.2 Definition of modules
More highly integrated modules and function groups as modules are required to reduce the project periods for switchgear installations.
A module in this sense is a unit or a function group, – that can execute a self-contained function,
– that has a minimum of interfaces, which are as standardized as possible, – whose complex function can be described with few parameters, – that can be prefabricated and pretested to a great extent and
– that can be altered within narrow limits by the smallest possible degree of adaptation engineering for customer demands and requirements while adhering to standards as much as possible.
It is essential that any changes to modules do not detract from the rationalization and quality achieved by type testing, degree of prefabrication and pre-testing.
6.2.3 From the customer requirement to the modular system solution
The progressive deregulation in energy markets and the accompanying downward pressure on costs is resulting in new requirements on the project planning of transformer substations. In addition to the engineering of classical customized installations, the modular switchgear installation concept offers the chance of developing largely standardized and therefore more economical solutions. This is done by implementing a systematic pattern of thinking to yield products with high functionality and combined installation modules. This means that the interfaces are unified and also reduced in number by grouping products into modules.
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For project planning and engineering, this means that system solutions are generated from a modular system of components in which the individual modules are precisely described as derived from the technical and economical requirements of a new substation in the network. The available CAD systems are ideally suited for quick and easy combination of complete station components from a catalogue of individual components. The current integrated enterprise resource planning (ERP) software also offer suitable databases and structures that enable quick access to descriptions, parts lists and prices.
The substation planner will have the greatest optimization effect when the customer provides requirements that describe functions only instead of detailed requirements in the form of comprehensive specifications. This gives the engineer the greatest possible freedom to bring the system requirements into conformity with the available modular solutions. In the modular concept, detailed installation requirements that go far beyond the description of functions result in expensive adaptation work, making the overall installation more expensive. Adaptation work in the modular concept is possible, but it always results in extra work in preparing the tender, project planning, engineering, processing and documentation of the installation.
Fig. 6-6
From the functional requirements of the network to the modular system solution Modular box of
building blocks
Technical and economic requirements
Optimized modular system
solution Concept
and planning
Conventional components
➱ ➡
➱ ➱
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6.2.4 IT tools for computer-aided project management 6.2.4.1 Terms and standards
Schedule of the most important IT terms PDM Systems
(Product Data Management)
Supports product-related processes in product development, CAD integration, revision management, document management, etc.
SCM Systems
(Supply Chain Management)
The integrated, process-orientated planning and control of flows of goods, information and money throughout the chain of added value from the customer to the raw materials supplier.
E-procurement
Support of the procurement processes by information technology (Internet).
ERP Systems
(Enterprise Resource Planning)
Planning and control of the entire chain of added value in an enterprise. (Purchasing, materials management, production planning and control, quality assurance, warehouse management, personnel and financial management.)
DMS Systems
(Document Management System)
A document management system manages documents created electronically and non-electronically throughout their life cycle.
CAD Systems
(Computer Aided Design/Drafting)
Engineering development and design; compilation of drawings and calculation.
MCAD Mechanical design
ECAD Electrical design (circuit diagrams etc.) CASE Systems
(Computer Aided Software Engineering)
Systems to support and verify software development.
Collaborative Engineering (CE)
Also known as "Simultaneous Engineering”. Individual steps in product development do not take place in chronological sequence, but simultaneously, using Internet technologies.
MRO Materials
Standard commercial consumable materials and products not relevant to production, which are as a rule procured in large quantities.
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Tabelle 6-7
Schedule of the most important standards
Standard Title
IEC 61346 Industrial systems, installations and equipment and industrial products - Structuring principles and reference designations
IEC 61355 Classification and designation of documents for plants, systems and equipment
IEC 61082 Preparation of documents used in electrotechnology
IEC 61360 Standard data element types with associated
classification scheme for electric components IEC 81714 Design of graphical symbols for use in the technical
documentation of products IEC 60617 Graphical Symbols (IEC database)
IEC 82045 Document management
IEC 61286 Information technology - Coded graphic character set for use in the preparation of documents used in electrotechnology and for information interchange IEC 62023 Structuring of technical information and documen-
tation
IEC 62027 Preparation of parts lists
IEC 61175 Industrial systems, installations and equipment and industrial products - Designation of signals ISO 10303 Product data representation and exchange
6.2.4.2 Schedule of the most important file formats and interfaces BMP (Bitmap Picture)
Raster image in bitmap format
JPEG (Joint Photographic Experts Group) Image file
GIF (Graphics Image Format) Image file
DGN
File format of the MicroStation CAD program from Bentley DWG
Standard file format for saving vector graphics in AutoCAD DXF (Drawing Exchange Format)
Drawing exchange format for vector data, originally for AutoCAD EDIF (Electronic Design Interchange Format)
Mostly for digital and analog components HPGL (Hewlett Packard Graphics Language) Standard file format for vector graphics
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IGES (Initial Graphics Exchange Specification)
Interface developed in the USA under the auspices of the National Bureau of Standards, orientated towards the transfer of geometry data between different CAD/CAM systems, mainly in the field of mechanical design.
PDF (Portable Document Format) Document format from Adobe
Texts and image data can be presented together in a uniform document. As PDF documents can be exchanged without problems between various software platforms, this format has achieved a certain popularity.
TIFF (Tagged image file format) Raster data, pixel graphics
VNS (Verfahrensneutrale Schnittstelle für Schaltplandaten – Process-neutral interface for circuit diagram data)
Unofficial but widespread German interface for exchange of documentation for electrotechnical systems.
Interfaces for high quality data exchange are increasingly gaining in importance for CAD/CAE applications.
The familiar interfaces IGES and DXF are only suitable for the exchange of simple graphical information. Higher quality interfaces, such as VNS (Verfahrens-Neutrale Schnittstelle für Schaltplandaten to DIN V 40950 2ndedition) provide opportunities on a significantly higher level to transfer graphical and logical information between electrical engineering systems and CAD systems. This interface is however only customary in Germany. A fully comprehensive exchange of information is, finally, achieved by STEP (STandard for the Exchange of Product model data to ISO/IEC 10303). Data transfer via STEP presupposes the availability of STEP-compliant tools with object-orientated databases. The interface properties defined as application models for the various applications have been published as standards for mechanical engineering (AP 214) and for electrical engineering (AP 212).
VRML (Virtual Reality Markup Language)
Language describing 3D scenes and their geometry, lighting, animation and interaction features. Most 3D modelling tools facilitate import and export of VRML files, which has allowed this file format to become established as an exchange format for 3D models.
JT
JT is a file format from the UGS PLM Solutions company which facilitates product visualization, information distribution and common data use by various PLM programs. It is also used by CAD systems for open data exchange with PDM systems.
6.2.5 Computer-aided project management
As a result of the different viewpoints of those concerned with planning with regard to the trades involved in a switchgear installation and the sometimes lengthy planning stages between compilation of a bid and the construction of the system, project planning places extremely high demands on the computer-aided procedures and data management systems.
These procedures are predominantly aimed at facilitating and supporting the business models qualitatively (by reducing error and defect costs) and quantitatively (reducing costs by reuse). The basis of this is the modularization described in 6.2.1.
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Current interest focuses especially on the data management and communication processes (Product Data Management and Collaborative Engineering).
As the persons involved in the planning process are often working together and communicating from different parts of the world, the need for processes to support information exchange and graphically display various views of the switchgear system has been continuously growing in recent years.
IT technologies to support a distributed work platform and communication in global projects with customers, their consultants and the subcontractors are of especial importance. These processes represent the backbone of rapid and effective project planning.
The widespread availability of the Internet has brought about major changes in the selection of and communication with suppliers in recent years (eCommerce).
In addition, the customers and future system operators now have increased demand for integration of the system supplied in their networks and information systems.
Internet technologies now permit the online use of product configuration tools in early phases of bid compilation.
This presupposes a clearly structured product portfolio and a modular product structure.
In method support, the trend is one towards modular product platforms which can be adapted to suit various markets and applications.
A virtual switchgear model assists in distributed planning and communication.
In recent years, the communication processes, technologies and protocols of the Internet have enriched our opportunities for access to product information and navigation. Improved configuration tools can also be developed on the basis of a clearly structured model. The trend towards standardization and modularization of the fundamental components in a system will therefore continue. Only in this way can complex systems be configured and the overall delivery periods and costs reduced.
The virtual, digitally available product model has an influence on the structuring of documentation in various phases of the design, procurement, production and operation of the switchgear system (Product Data Management). That is why the infrastructure and logistics of passing on information play such an important role in this application.
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Figure 6-7 shows the planning phases which are most important here.
The individual phases/process steps are considered separately below.
SCM (Supply Chain Management)
Assures the quality and quantity of the information in the module kit, which forms the basis of reuse and the associated increases in productivity and efficiency addressed above.
Module kit
Represents the product portfolio, and is to be managed with a high quality standard as it is the basic requirement for the following processes.
Customer enquiry
The most important part of the entire process chain, as the requirements have to be defined and verified here both functionally and commercially, to be used as input for the following process steps.
Bid compilation
Compilation of the functional and commercial specification on the basis of the customer enquiry.
Importance must be attached here to high quality and short throughput times. This is ensured by a module kit. Special customer requirements are then defined in a further design step. It is important here to establish the product/project structure in such a way as not to incur additional revision work in subsequent steps.
Module kit
SCM
Customer equiry
Bid compilation
Substation project
Order design
Order delivery
Customer inspection
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Order design
On receipt of the order, the bid structure can be adopted and the detailed engineering of the system can begin. The planning is firmed up in this process step and all documents prepared and compiled for delivery.
In addition, the purchasing department has to order all the parts.
Order delivery
This process step is performed locally at site. In terms of information management this means that all the documents and information required must be available and, when amendments are made at site, these amendments are included accordingly in the project documentation, as this is the basis for customer inspection.
Customer inspection
Functional and commercial inspection and acceptance of the project/product delivered.
Transformer substation project
Protected software environment in which the project/product is virtually planned, specified and managed. Secure revision management is highly important for the performance of the project, and it is ensured that the persons responsible and involved are informed of the version and status of all revisions.
6.3 Reference designations and preparation of documents
Two important series of standards have guided the rules for the reference designation of equipment and the preparation of circuit documents for many years. The symbols for individual devices were specified in the DIN 40900 series, and the DIN 40719 series regulated reference designation and representation.
The two series of standards have been superseded in the context of international standardization in the IEC. DIN 40900 has been replaced by the IEC 60617 series. The changes are minor, because DIN 40900 was already based on an earlier version of the international standard IEC 60617. The most important parts of DIN 40719 were superseded by IEC 61082 as early as 1996/97. The structure of reference designation systems has been fundamentally revised on an international level. IEC 61346-1 describes the general rules. With the publication of IEC 61346-2 containing the tables of code letters, the last part of DIN 40719, Part 2, was withdrawn.
The change from "item designations for electrical equipment” in accordance with DIN 40719-2 to the new standard IEC 61346, "Structuring principles and reference designations” took place only very hesitantly. The following section presents the structuring principles and reference designation system of IEC 61346. The most important designation tables from the old standard DIN 40719 are included once again in section 6.3.4.
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6.3.1 Structuring principles and reference designations to IEC 61346
If, in the past, designations in electrical systems were determined by means of designation blocks and firmly assigned tables for specified data positions within those designation blocks, now, however, the focus is on a hierarchical structure with reference designations derived from that. Such structures are based on „component relationships”. The elements in a lower order level of a hierarchical structure are always complete components of the next higher level. The structure established in this way can be represented as a tree structure with nodes and branches.
Station: kein Kennzeichen
Kennbuchstabe nach Tabelle 2, evtl. mit Unterklasse (nur in Gliederungsstufe 1)
Kennbuchstabe nach Tabelle 1, evtl. mit Unterklasse (für alle folgenden Gliederungsstufen)
Gliederungs- stufe:
1
2
3 0
Figure 6-8 Assignment of classes to divisional levels
On divisional level 1 to figure 6-6, selection of code letters is performed on the basis of Table 2 of IEC 61346-2, „Classes of infrastructure objects”. Table 6-8 below shows this table 2 from the standard in a specific version for the field of application of switchgear.
Substation: no code letter
Divisional level
Code letter to table 2, with sub-class if appropriate
(in divisional level 1 only)
Code letter to table 1, with sub-class if appropriate (for all following divisional levels)
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Code Object class definition Examples
A Objects related to two or more classes of Supervisory control system infrastructure objects of classes B to Z. ripple control equipment
PLC-equipment
B Installations for > 420 kV C Installations for. ≤ 420 kV D Installations for 220 kV ... <380 kV E Installations for 110 kV ... <220 kV F Installations for 60 kV ... <110 kV G Installations for 45 kV ...<60 kV H Installations for 30 kV ...<45 kV J Installations for 20 kV ...<30 kV K Installations for 10 kV ...<20 kV L Installations for 6 kV ...<10 kV M Installations for 1 kV ...<6 kV N Installations for <1 kV
P Objects for equipotential bonding Earthing protection Lightning protection Q, R, S Free
T Transformer plants
U Free
V Objects for the storage of material or goods Finished good stores Raw material stores Water tank plant Oil tank plant W Objects for administrative or social Canteen
purposes or tasks Office
Recreation area Garage
X Objects for fulfilling auxiliary purposes or tasks Air-conditioning system outwith the main process (for example on a site, Alarm system, Clock system in a plant or building) Lighting installation,
Electric power distribution Fire protection system Security system Water supply Crane
Y Objects for communication and information tasks Computer network Telephone system Video surveillance system Antenna system Z Objects for housing or enclosing technical Building
systems or installations like areas and buildings Constructional facilities Factory site Road Wall, fence Objects for common tasksObjects for main process facilitiesObjects not related to the main process
Table 6-8 Classes of infrastructure objects
