POWER SYSTEM OVERVIEW

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					    ELECTRICITY SUPPLY INDUSTRY PLANNING COUNCIL

                                POWER SYSTEM OVERVIEW

     Information paper prepared for the South Australian Government
         Taskforce Inquiry into the National Electricity Market, 2001



1    Power System Overview .................................................................................................... 2

2    Network Elements.............................................................................................................. 3

3    Typical Structure................................................................................................................ 3

4    Limiting Factors to Power System Capability...................................................................... 4
     4.1 Thermal ................................................................................................................... 4
     4.2 Volt Drop.................................................................................................................. 4
     4.3 Dynamics ................................................................................................................. 5
     4.4 Interrelation of Factors.............................................................................................. 5

5    History of SA Network Development .................................................................................. 5

6    Current SA Transmission Network ..................................................................................... 5
     6.1 Simplified Overview.................................................................................................. 5
     6.2 Network Transfer Capability ..................................................................................... 6

7    Future Possible Network Developments............................................................................. 7
     7.1 Distribution ............................................................................................................... 7
     7.2 Transmission............................................................................................................ 7
           7.2.1 Potential Impact of Possible Future Generation Sources .............................. 7
           7.2.2 Interconnectors ............................................................................................ 8
           7.2.3 National Electricity Market Impacts............................................................... 8
     7.3 Projections for Network Development ....................................................................... 8




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                          Task Force Paper on Overview of Power System                    • April 2001




         1      POWER SYSTEM OVERVIEW
         Electricity has evolved as one of the forms of energy that is easy to produce, transport,
         distribute and transform to meet the diverse needs of a modern technological society. A
         power system is the infrastructure that enables the production of electrical energy and its
         transfer to the locations where it can be utilised.

         The modern electric power system converts energy from a raw form (typically mechanical
         rotational energy at power stations) into electricity, transmits it from the power station
                                                   s
         sources, and delivers it to a consumer’ site (where the electrical energy is converted into
         whatever form of useful work the consumer requires). Unlike many other forms of energy
         systems, the modern electric power system is not designed to store energy. Its operation
         requires the rate of conversion of energy into electricity at the sources to be equal, within a
         very tight tolerance, to the rate of consumption of electricity at the consumer load sites, at all
         times.

         A practical power system is a network that interconnects a variety of power system elements,
         including:

         \   power stations, where fuel (coal, gas, oil, wind, water flow) is converted into rotational
             energy by means of an appropriate turbine. The turbine drives the rotor of an electrical
             generator and produces electrical energy,

         \   switching stations which enable the electrical circuits of the network to be switched and
             the configuration rearranged as appropriate,

         \   substations where network voltage levels are increased or decreased to allow efficient
             transmission and distribution of the electrical energy, and

         \   capacitors, reactors and other special power conversion or controlling devices necessary
             for the efficient management and operation of the power system.

         Networks provide the corridors that transport the electrical energy between the power stations
         and the consumers.

         All modern power systems are based upon alternating current theory and principles. (Some
         power systems also incorporate direct current elements within the networks). There are two
         main parameters that describe the performance and quality of a power system:

         \   frequency, and

         \   voltage.

         At any instant in time, frequency is the measure of the supply/demand energy balance. It is
         directly related to the rotational energy at the power stations and under all circumstances
         must be kept within a close band surrounding a value of 50 hertz.

                           s
         A power system’ voltage is equivalent to its “                      .
                                                          pressure indicator” Good operating practice
         requires that the voltage be maintained within a reasonable operating range that is generally
         defined in terms of a nominal value for the network under consideration. Typical practice is for
                                                               s
         voltage to be maintained within +/- 5% of the network’ nominal voltage.



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Task Force Paper on Overview of Power System                     • April 2001




2      NETWORK ELEMENTS
A power system typically contains a hierarchy of networks which are based upon voltage
(275kV, 132kV, 66kV, 33kV, 11kV, 0.415kV) and function (transmission, high voltage (HV)
distribution, low voltage (LV) distribution, reticulation).

The nominal voltage of a network is a design parameter that is a compromise between:

\   power transfer capability over the transport corridor,

\   losses incurred in transferring power over the element,

\   cost of element (initial capital, annual operating costs).

Typically the term:

\   transmission is used to describe bulk transportation of large amounts of power at high
    voltages between power stations and major geographic locations. Transmission generally
    implies transport corridors that are reliable and continuously available under most
    foreseeable operating conditions. Consequently, transmission network design implies use
    of concepts involving planned element redundancy such as multiple parallel circuits,
    alternate paths between points and diversification of element paths. The quantities and
    direction of power flows over transmission elements depends upon network configuration
    and power exchanges directed by the network system dispatcher. Transmission
    networks, their protection, and backup and support services are technically complex,
    frequently utilise leading edge technology and are expensive commensurate with their
    function of being the central core of a power system network. Failures of transmission
    corridors generally have major repercussions in the overall functioning of a power system.

\   distribution is applied for transporting smaller amounts of power over widely dispersed
    areas, generally at lower voltages, and at lower levels of security and availability.
    Distribution systems often are characterised by designs utilising circuits that are
    predominantly radial in nature and with limited ability to be switched into alternative
    configurations. Distribution systems frequently span wide geographic areas. Power flow is
    typically unidirectional. The failure of a distribution circuit has a relatively minor impact
    upon the overall functioning of a large power system.

3      TYPICAL STRUCTURE
The power system networks may be viewed as arranged in a pyramid (or tree) structure
                                                                               s
where typically generators feed a common transmission network at the pyramid’ apex, and
consumers withdraw power through individual customer connections (reticulation) from the
                                                s
low voltage distribution networks at the pyramid’ base. (In practice, some small generators
connect lower than at the apex, while some large consumers connect higher than at the
base).

Figure 1 (refer to page 10) illustrates the typical power system structure.




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                           Task Force Paper on Overview of Power System                    • April 2001




         4      LIMITING FACTORS TO POWER SYSTEM
                CAPABILITY
         The individual power system components are each subject to a number of limitations that
         define the upper and lower bounds of their operation. Acceptable operation of a power system
         means that the power system is operated in a manner such that the most restrictive of all the
         constraints is not violated. The particular constraint that dictates an operational binding limit
         may change depending upon the power system configuration and operating point.

         Due to the interaction between all power system elements in determining power flows in the
         network branches, it can be very misleading to extrapolate performance from one operating
         configuration to another or to consider an element in isolation from the rest of the network. It
         is generally necessary for technical studies to be based upon specific configurations of the
         total networks. The evaluation techniques involve simulation of the power system
         performance in an appropriate reference time frame; the techniques are very mathematical
         and interpretation of the simulation results require specialised expertise.

         Some principal factors which must be considered when determining power system capability
         are the following.

         4.1   Thermal

         The passage of an electric current through a circuit (exposed conductors, insulated cable)
         generates heat. The continued ability of the circuit to pass electricity depends upon its
         conductors’ abilities to dissipate the heat (ie losses) through surrounding medium (air,
         insulation, soil) without significant physical deterioration (annealing, losing physical strength)
         of the conductor. The maximum current carrying capability defines the thermal rating of the
         circuit beyond which it is not wise to transmit additional power.

         4.2   Volt Drop

         The passage of electric current through a circuit also causes the voltage at the end of the
         circuit to be lower than that at the sending end under most circumstances.

         To ensure reliable operation and quality of supply, voltage conditions at all points on the
         network must be operated within a small band surrounding the nominal design voltage of the
         network (say typically within +/- 5% of nominal).

         A longer line results in a larger voltage drop at the end when transmitting a given amount of
         power, if all other line parameters remain the same. For a particular line segment (fixed length
         and construction configuration), the voltage drop increases as the power transferred across
         the circuit increases. Consequently, the voltage drop occurring across a line segment varies
                                   s
         depending upon the segment’ physical parameters (length, construction details) and the
         power transfer flowing.

         Thus voltage drop considerations may be the limiting factor in the ability of an electrical circuit
         to transfer power.




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Task Force Paper on Overview of Power System                    • April 2001




4.3     Dynamics

To be useful, a practical power system must be able to continue to operate and perform in a
predictable manner at times when some component elements are under stress (including
transmission or distribution circuits short circuiting, a generating unit failure, a generating unit
ancillary function not responding as desired). Restricting power system dynamic excursions
within acceptable operating bounds is an operational practice necessary to meet a desired
quality of supply. Such limitations (transient stability, small signal stability) may impose
limitations upon power transfer capability of some network elements.

4.4     Interrelation of Factors

In a tightly meshed network (such as transmission within a metropolitan area incorporating
multiple circuits, short length lines and multiple switching stations), it may be a reasonable
engineering judgment to base transmission capability and capacity upon thermal transmission
line ratings.

However for a network where transmission lines are long and spread over wide geographic
areas (as generally encountered external to metropolitan areas), it frequently occurs that
voltage drop considerations dictate transmission line capability (at a level considerably below
the thermal limit capacity).

Use of power system components (such as capacitors and special purpose control systems)
may permit upgrading of specific circuit limitations. Specialised technical assessment is
required to determine applicable transmission circuit limits in these cases.

5       HISTORY OF SA NETWORK DEVELOPMENT
                  s
The origins of SA’ present transmission network began when the former Adelaide Electricity
Supply Company established connections between its East Terrace and Osborne power
stations and surrounding Adelaide Hills supply regions at 66kV. This has been expanded over
the years by overlaying a:

\                                                      s
      132kV network, commencing in the early 1950’ when additional transmission circuits
      were established between Adelaide and a major remote generation source (Playford A
      power station burning Leigh Creek coal) at Port Augusta,

\                                                 s
      275kV network, commencing in the late 1950’ when additional transmission circuits were
      established between Adelaide and the north of the State following the next major
      increment in Port Augusta generating capacity at Playford B power station.

Both the 132kV and 275kV networks have been broadened and expanded as necessary to
supply the present transmission and distribution needs of the state.

6       CURRENT SA TRANSMISSION NETWORK
6.1     Simplified Overview

The current SA transmission network is a meshed 275 and 132kV network covering the more
heavily populated areas of the state. This feeds an extensive distribution network operating at



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                           Task Force Paper on Overview of Power System                 • April 2001




         a variety of voltages pertinent to the customer demands and areas served. The backbone of
         the SA grid may be visualised as a broad transmission corridor running in a north/south
         direction as per Figure 2 (refer to page 11).

         For clarity and simplicity Figure 2 highlights that the major customer demand area is the
         Adelaide metropolitan area, with smaller demands in the north and south of the State. Power
         stations exist in each of the three demand areas. However as the supply/demand capacities
         of each of the areas are not balanced, there may be considerable power exchanges flowing
         over the north, south or both corridors depending upon total network customer demand and
         the particular dispatch from each power station.

         There are 6 major switching stations that primarily describe the SA transmission network:

         \     northern area:                     Davenport

         \     southern area:                     South East

         \     metropolitan Adelaide area:        Para, Cherry Gardens, TIPS, Magill

                                                    s
         Figure 3 (refer to page 12) illustrates SA’ transmission backbone with these major transport
         corridor switching stations. Other intermediate switching stations and substations that have a
         secondary impact or a distribution role are omitted from Figure 3 for clarity of presentation.

         6.2     Network Transfer Capability

         The northern transfer corridor is served by four 275kV transmission circuits. The two original
                                                                         s
         circuits (so called east and west circuits) which founded SA’ 275kV network are now
         50 years in age having been designed to service the power transfer requirements of
                                                                              s
         Playford B power station, which is now only a small component of SA’ generating capability.
         The capacity of these circuits are low and are not able to significantly contribute to power
         transfer over the northern corridor. These circuits are now more realistically regarded as
                                             s
         elements supporting the network’ distribution role rather than as primary transmission
         elements for the northern corridor.

         The northern transfer corridor now relies upon two circuits of more modern vintage (1985)
         which can be described as having a medium level transfer capability. These two circuits are
         built on common towers meaning that there exists a moderate risk that the northern transfer
         corridor could be severed should some event cause a failure of the line towers.

         The southern transfer corridor comprises two circuits generally of 1985 vintage with a medium
         transfer capability. Between the metropolitan area and a location at Tungkillo, the two circuits
         are built using a single set of line towers (ie double circuit construction); between Tungkillo
         and Tailem Bend, each circuit is constructed as a separate line with some physical
         separation; however south of Tailem Bend, a single route using double circuit towers is again
         utilised. Thus the southern transfer corridor is also at moderate risk of severance should an
         event initiate double circuit failures.

         The metropolitan region is served by a mixture of medium and high capacity 275kV circuits,
         some serving predominantly a transmission role while other circuits act more in the vein as a
         distributor. The number of circuits and their connectivity means there is a low impact upon the




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Task Force Paper on Overview of Power System                   • April 2001




overall 275kV network capability should significant circuit failures occur, even where double
circuit construction is employed on some circuit segments.

The SA network also contains an extensive underlying 132kV network. In some areas it still
provides some power transfer functionality but with limited capability. In most areas,
(particularly the more recent segments, and with future additions) the 132kV network
functionality is becoming more akin to that of a distribution role.

7       FUTURE POSSIBLE NETWORK
        DEVELOPMENTS
7.1     Distribution

Regional customer demand typically increases by small annual increments. The rate may
                                  s
vary depending upon the region’ economic growth and existing industry characteristics. Such
growth is typically accommodated within the networks by utilising existing infrastructure
capacities supplemented by augmentations as necessary. Block customer demands may lead
to specific circuit augmentations. Distribution network development is therefore expected to
evolve in a manner consistent with past practice, ie based upon an efficient and optimum use
of relevant infrastructure assets (existing distribution assets coupled with a timely migration of
aged and/or capacity limited transmission assets to a distribution role).

7.2     Transmission

A power station dictates the requirement for the transmission network to have the capacity
and capability to transport large blocks of power. Consequently, establishing major new
power stations typically requires specific transmission network augmentations. Some smaller
new generation units can frequently be accommodated within the capacities of existing
network infrastructure where they will never fully supply and exceed local demand.

At all times the extremes between local generation/demand imbalance become the
parameters for determining transport corridor capacities.

7.2.1    POTENTIAL IMPACT OF POSSIBLE FUTURE GENERATION SOURCES

The geography of the State dictates that transmission network development on the west coast
can be considered basically as a radial network emanating from Port Augusta. As such, any
impact that substantial west coast generation (wind farms, Port Lincoln, Whyalla) might have
to the main north/south transmission corridor, can be visualised as a component for transport
over the northern transmission corridor. The existing west coast network infrastructure is of a
                                         s
distribution nature reflecting the region’ low aggregate and dispersed local demand. Should
substantial generation capacity materialise on the west coast, reasonably substantial
transmission works are likely to be required on the west coast and possibly on the northern
corridor.

Development of additional generators within or near to the metropolitan region are most likely
to impact the detailed transmission network within the metropolitan area. They are expected
to have little impact upon the northern and southern transport corridors.




                                                                                                     Page 7
                          Task Force Paper on Overview of Power System                 • April 2001




         Additional south east generation (power stations, windfarms, industrial cogeneration plants)
         are likely to change the south area supply/demand balance from the present situation where
         there exists a net customer demand, to one where south area supply/demand are at times
         roughly balanced. The levels of south east generation that materialise will dictate possible
         south corridor transmission augmentations.

         7.2.2   INTERCONNECTORS

         The proposed Murraylink and SNI interconnectors will join the existing transmission network
         approximately mid way along the northern transport corridor. Although the projects do not
                                                         s
         provide any additional augmentation of SA’ existing transmission networks, they have the
         potential to significantly alter established power exchanges over the northern corridor.

         The existing Heywood interconnector with Victoria is effectively a large generating source
         within the southern area. It dictated the existing south corridor transmission design capacity.
         Any proposals to increase Victorian import over the Heywood interconnector or other
         interconnectors to the south east are likely to require reinforcement works to the south
         transmission corridor.

         7.2.3   NATIONAL ELECTRICITY MARKET IMPACTS

         Building a generating plant within predominantly a distribution area to contribute local supply
                          s
         against the area’ customer demand, is a technique that has traditionally been used in some
         instances rather than the alternative of building additional transmission network links. There
                                         s
         are several instances within SA’ power system development where this has occurred.

         With the introduction of the National Electricity Market (NEM), the former regime of vertically
         integrated electric utility companies which operated generating plants as necessary (even if
                                                               s
         out of economic merit order) to balance the utility’ requirements of quality of supply and
         overall economic operation of the total power system, has been removed. Under the NEM,
         dispatch occurs on the basis of generator bids in the market with local load requirements
         being accounted for by applying relevant regional loss factors. Participant bidding behaviour
         and regional marginal loss factors may however interact to highlight local network problems.

         7.3     Projections for Network Development

         At all times the extremes between local generation/demand imbalances remain the
         parameters for determining transport corridor capacities. Limiting power exchange over a
         corridor due to binding constraints on transmission elements may have an impact of
         increasing pool prices.

                                            s
         The major enhancements to SA’ existing transmission network are likely to arise from the
         cumulative impact of opportunities to develop additional power stations and interconnector
         import/export capacity. NEM dispatch recognises the available capacity of the transmission
         networks and interconnectors (between regions and/or states) and loads them such that an
         optimal price outcome is achieved within their capabilities and with negligible impact upon
         distribution functionality (and end customers). Thus it is likely that there will evolve a more
         marked distinction and segregation between transmission and distribution network
         functionality (both in operations and network design) than has been previously apparent in the



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Task Force Paper on Overview of Power System                 • April 2001




SA electricity industry. Transmission augmentations are therefore likely to be designed with
additional headroom (larger available “free floating” capacity) than in the past.

        s
For SA’ northern transport corridor, it is likely that the lower portion of the corridor (say
between Robertstown and the metropolitan area) will require further development over time.
Similar drivers will also dictate development of the southern transport corridor; however it is
anticipated such development will span the complete southern corridor.

The existing main transmission corridor routes around the metropolitan fringe have set the
scene for establishing a major terminal to the east of the metropolitan area at Tungkillo.
Continuing customer growth in the south-eastern hills and adjacent country regions and the
need to rationalise roles of existing older 132kV transmission networks in the surrounding
region are likely to favour establishing a new major hub and network development at this
location.

Notwithstanding the above, the practical implementation of the anticipated trends will be slow
and gradual as the transitions between transmission and distribution are defined, and
development will be driven by growth and flexible operational requirements whilst extracting
efficient utilisation and life from the existing infrastructure assets.

Figure 4 (refer to page 13) illustrates how the power system transport corridors may possibly
develop in the future.




                                                                                                  Page 9
                           Task Force Paper on Overview




             ∼        generators
                                     ∼
                  transmission network




          high voltage distribution networks




          low voltage distribution networks




                     customers




                    Figure 1




Page 10
Task Force Paper on Overview of Power System       • April 2001




                                          ∼
                                                   Northern
                                                   Playford
                                                   Mintaro
                                                   Pt Lincoln


                                      north area




       ∼
   Torrens Island
                                 metropolitan area



   Pelican Point
   Dry Creek
   Osborne




                                      south area




                                          ∼
                                                   Ladbroke Grove
                                                   Snuggery
                                                   Heywood Interconnector


                               Figure 2




                                                                            Page 11
                 Task Force Paper on Overview of Power System   • April 2001




                                Davenport




                                       Para




          TIPS                                                  Magill


                                       Cherry Gardens




                                  South East




                                   Figure 3




Page 12
      Task Force Paper on Overview of Power System             • April 2001




                                                                           ∼      Northern
                                                                                  Playford




  ∼
Pt Lincoln
Whyalla
                     west coast area                                     north area



new generation
                                                                                             Murraylink
                                                                                             Interconnector




                                                        ∼                                          ∼
                                                                                             SNI Interconnector


                                             Mintaro                    mid north area




  ∼
 TIPS
 Pelican Point
                        metropolitan area
                                                                            hub
                                                                                                new∼
                                                                                                interconnectors
 Dry Creek
 Osborne




                 ∼
            new generation
                                                       ∼
                                            new generation




                                                                         south area




                                                        Snuggery
                                                                  ∼
                                                        Ladbroke Grove
                                                                                         ∼
                                                                                      new generation
                                                                                      new interconnectors
                                                        Heywood Interconnector


                                             Figure 4




                                                                                                   Page 13

				
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Description: POWER SYSTEM OVERVIEW