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Limits to renewables - how electricity grid issues may constrain the growth of distributed generation

 

By Professor Michael Laughton, B.A.Sc., PhD, DSc(Eng.), FREng., CEng., FIEE,

Paul Spare MSc, CEng, FInstE, MIMechE

 

Summary

To meet Kyoto commitments and to preserve a degree of diversity of electricity supply, the UK Government has a target of 10% of electrical energy to be generated by renewable sources in 2010. This target is ambitious; nevertheless Grid system control will therefore have to cope with an increasing proportion of small, embedded power stations, most of which will be connected to the distribution networks. These developments will generate significant challenges for the control of the grid system and need to be understood more widely.

 

 1. INTRODUCTION

The UK Government has a target of 10% of electricity to be generated by renewable sources in 2010 (note that this is a target for energy not power). More ambitious targets have been recommended by a parliamentary committee of 20% by 2010 and 50% by 2030. In addition the Government has simultaneous ambitions of increasing the electrical contribution of Combined Heat and Power (CHP) plant to at least 10, 000 MW by 2010, more than twice the 2000 capacity of 4,300 MW. The proportion of new forms of plant, both renewable and CHP, within the UK’s overall generating capacity is estimated to rise from 8.6% in 2001 to 20% in 2010 and likely to increase further in subsequent years. Over the same period to 2010, some twenty Magnox reactors are scheduled for closure, followed in the decade 2010-2020 by most of the AGR stations and many large coal-fired plants.

 

 

2. ENERGY SOURCES IN ELECTRICITY SUPPLY

2.1 Differences between energy resources

There are important differences between our current major power sources and renewables. Conventional large stations, e.g. coal, gas and nuclear plant, each with an output of at least 100 MW, generate the bulk of electricity in the UK. In England and Wales in mid-2001 there is 67,000 MW of such plant, 94% of which is connected directly to the high voltage transmission network and supplying 92% of approximately 375 TWh of electricity produced in the UK. This integrated system, which operates under co-coordinated central control, allows responsive generation capability and standby reserves to be shared across the system, whilst allowing demand at each moment to be met by the most economic generation, largely irrespective of where it is located.

 

Renewable sources of electricity, on the other hand, take many forms, e.g. the marine sources - tidal barrages, tidal streams and waves – wind both on-shore and off-shore, biomass - combustible waste, landfill gas and energy crops – hydro - both large and small-scale - and solar - both active (PV) and passive. With most outputs being much less than 100 MW, it is uneconomic for them to connect directly to the transmission system and so are generally connected directly to their local distribution network. Individually most of such small stations will not be subject to central control as exercised by the system operators, partly because the high cost of conventional telemetry means that system control and data acquisition (SCADA) has only low penetration at voltages below 132kV. At these levels, therefore, distribution networks operate largely with incomplete and uncertain information.

 

2.2 The Government’s renewable target

Using the electricity growth rates forecast by the DTI as a guide, the Government’s 10% target for renewable generation is 39.4 – 40.8 TWh, four times the present level. The respective renewable resources contributions in 2000 are as shown in Table 1. The biggest growth in contributions before 2010 will be from wind and biofuels, in the latter case especially energy from waste. The EC Landfill Directive will enforce major changes in landfill practices requiring reductions in the biodegradable domestic waste sent to landfill by 2016 to be no more than 35% of its level in 1995. The development of more energy from waste schemes (EfW) is a virtual certainty. With the limitations imposed on the landfill option the major route for EfW is by mass burn incinerators.

 

The likely of achievement of the 10% target is a matter of speculation. Major uncertainties surround the necessary planning permissions to be obtained for various schemes because of growing environmental opposition. Last year only 60 MW of onshore wind powered capacity was installed and this year the figure is likely to be about 100 MW; however the DTI intends that offshore wind alone will supply 1.8% of total UK electricity supply by 2010, i.e. from approximately 3000 MW of installed offshore capacity. Simple arithmetic shows that the rates of build necessary for wind turbine and small biomass fuelled plants, plus the associated electrical substation and network connection changes, impose severe practical limits on the capacity that might be in place by 2010. Perhaps 6-7% might be possible in total, but no matter, the10% target serves as a useful spur to the development to a young industry.

 

 

Table 1 Likely Annual Generation of Electrical Energy by Renewables.

 

 

Technology

TWh in 2000

Wind - onshore

0.946

Wind - offshore

-

Solar

0.001

Hydro - small

0.239

Hydro - large

4.989

Biofuels

-

-Landfill gas

2.188

-Sewage

0.366

-MSW combustion

1.368

-Other

0.499

-Energy crops

-

TOTAL

10.476

 

 

3. IMPLICATIONS OF RENEWABLES

3.1 Security and diversity of supply

It seems in many publications that the benefit of increased diversity and hence security of electricity supply is linked to policies geared to the development of renewable energy sources. This does not coincide with the view advanced by the NGC, viz.

 

"It may seem at first that security of supply is potentially at its greatest value when the source of power is close to the demand it supplies. However transmission circuits tend to be far more reliable than individual generating units. Accordingly enhanced security is delivered by providing sufficient transmission capacity between customers and the national stock of generation. The transmission system is able to exploit the diversity between individual generating sources and demand"

This is an engineering view based on actual statistics and reliability analysis. To argue to the contrary is just wishful thinking unless supported by different facts.

    1. Power supply quality

The following three points with regard to electrical power are paramount.

 

 

Power is required at the rated frequency and voltage, free from harmonics, voltage surges and interruptions. A modern industrialised society depends heavily on high quality power supplies for computerised control processes and information technology. Even a millisecond interruption is sufficient to cause many millions of pounds of damage to continuous processes

The estimated cost to European businesses of periods of poor power quality amounts to some 13 – 20 billion euros per year. Even higher amounts are quoted for the USA.

 

A large number of small embedded generators make control more difficult. Small, dispersed renewable generation plant with, in some cases, randomly intermittent output cannot provide the whole capacity needed to meet the national demand for electrical energy for technical reasons. There is room for renewables in the UK power supply, but only to a limit determined by the ability of the rest of the conventional generating plant to guarantee the integrity of operation and power supply quality.

 

3.3 Variable and Unpredictable Output/Quality

3.3.1 Voltage

Consumer loads can be sensitive to both frequency and voltage. The significance of voltage dips was seen in Singapore where an unprecedented five voltage dips occurred in the last few months of 1999, with large outcries from the high technology consumers. The cost to one chip manufacturer was claimed to be $1million per event. The cost of voltage dips to US industry is estimated to be $10 billion / year.

This issue is further illustrated by the problems created for the network voltage by the pulsation or random fluctuation of the power generated by large wind farms. Voltage control and quality problems arise when generators embedded within the distribution network start/stop generating. This can cause other network users to suffer voltage fluctuation, dips and steps outside of the statutory limits and inject unwanted harmonics into the voltage waveform. The problem would be exacerbated if the wind farm were to be connected to the grid, despite certain technical fixes.

 

3.3.2 Frequency

Frequency is controlled to a certain degree by generators responding automatically to changes in power output; however many new forms of generation, renewable and CCGT plant included, do not provide the necessary response to low frequency drift.

The frequency of the electricity supplied has to be controlled to within 1% of the rated 50 Hz specified. Outside of these limits automatic load shedding occurs and in practice frequencies are maintained much closer than these limits to avoid other, less serious system problems. This requirement needs continuous balancing of load demand with generation on a minute-by-minute basis.

 

Added to this technical picture are the economic constraints imposed by privatisation. Now the frequency depends on the price offered to ensure sufficient generation is made available to match demand, i.e. market price is made by market equilibrium. The system frequency effectively represents the market equilibrium. Frequency above 50 Hz would mean surplus generation, under which condition all costlier generation should be backed down; frequency below 50 Hz would mean a generation deficit, calling for even costlier generation to be brought in. The new electricity trading arrangements (NETA) place greater responsibility on companies to balance actual production with last-minute customer demands, which may cause short-term price volatility.

 

Today there are many non-utility players in the electric power industry and the potential levels of their power generation depend on associated heat output or renewable resource availability, which is mostly uncertain. They are not in the business of maintaining good engineering practices on the generation and transmission systems. Since there is generally a profit built into each kWh sold (however small), there is no incentive for independent generators to reduce generation during periods of high frequency (and possibly to increase generation during periods of low frequency); thus increasing levels of renewable and CHP plant could amplify frequency and also voltage instability.

 

3.3.3 System stability

System stability problems will arise when intermittent generators, especially at light load, supply a significant proportion of system demand. Informed Electricity Supply Industry opinion asserts that when the power presented to the grid by randomly intermittent sources rises above 15-20% of total system demand, then grid stability becomes an increasing risk.

This operational constraint relates to power and not to energy, which is not widely appreciated. The annual load factors of wind turbines are between 25 and 30%. There would be times, however, when most turbines would be operating at full load and delivering 100 % of power. It is this maximum power output, not the energy output, which causes stability problems. Furthermore the system endures a daily load demand cycle as shown in Fig.1 so it follows that the system can only accommodate lower levels of wind turbine-generator output power at night or in the summer. With a 20% limit the embedded wind generation would have to stay between 10 GW maximum at a peak winter load 50 GW to about 3.8 GW at the minimum summer load of 19 GW.

Image12.gif (8116 bytes)

 

Figure 1 Daily Load Demand on the England and Wales Power Supply System

Curve A Maximum winter demand, 16/01/2001.

Curve B Typical winter demand, 04/12/2000. Curve C Typical summer demand, 04/07/2000.

Curve D Minimum summer demand, 30/07/2000.

In Denmark the wind power production comprised 13-14% of the total electricity consumption in 2000 and growing. The energy sector as well as individuals has already complained about the problem of stability in the electrical network for quite some time, and the Danish Energy Minister has eventually admitted that there is a problem and that it should be solved through discussion and openness.

 

3.3.4 Uncertainties in plant scheduling

Many factors set operating limits on the flexibility of power system operation and there must be operating reserves of various kinds to ensure that the system can meet all likely immediate dynamic and longer term steady-state conditions. Normally within a time-scale of:

In all of these situations Operator decisions are an integral part of the control process and negotiation of such reserve requirements is an ongoing dynamic process.

The problems caused by large amounts of randomly intermittent generation are well illustrated by the situation in West Denmark faced by the transmission company Eltra.
The Eltra Magazine had a front-page story on the subject, which read  "We have between 0 and 2000 MWe of wind power tomorrow".  The windpower is 0 MW if the wind is below force 3, 800 MW if the wind force is 4/5 or 2000 MW if the wind force is 5/6. In the same edition, an article entitled "Eltra's Chairman – Wind Power has created an acute need for new thinking", explores the problems in some detail.


Figure 2 illustrates the aggregated output from the windfarms in the region feeding the Eltra system selected arbitrarily from four days in April 2001. There are big errors in almost a third of wind forecasts from the Danish Meteorological Institute. It is thus impossible to plan production, and, therefore, both an over-production of power or, in certain wind-deficient cases, a shortage of power can occur. In the latter case Eltra is forced to buy power at maximum prices on the open market.

 

 

Figure 2 Aggregated wind generation in Jutland and Furen, Western Denmark

Eltra’s problems arise because in the Danish system it is completely impossible to control the production of environmentally friendly electricity. Eltra is forced to accept all the power not only from wind turbines but also from decentralised heating and power plants, irrespective of the need for electricity.

Denmark has a small electric system with an annual demand less than 10% of the UK demand. Having to balance supply and demand with input from a very substantial installed wind power capacity (1600 MWe in West Denmark, alone) requires that conventional thermal power stations continually follow customer demand less wind input, a task for which they were never designed. In addition interconnectors with Norway, Sweden and Germany provide the added flexibility needed in operation, allowing power importing if need be or surplus power to be exported. This situation results in the highly efficient thermal power stations running at less than optimal efficiency and requiring a certain amount of the thermal capacity to be kept in "spinning reserve" where they consume energy but deliver no useful power.  Further plant must be "kept hot" ready to start up on short notice, again a highly inefficient process. This cost of operating stand by plant is never attached to the costs quoted for plant with intermittent generation, but, in the case of Denmark, might explain why the Danes have the highest domestic electricity costs in Europe at 12.21 p / kWh (c.f. UK cost of 7.97 p/kWh).

In the case of the UK until now the level of generation contributed by randomly intermittent sources has been relatively small and caused little difficulty. It is sufficient to note that at some level of supply the cost of operating stand by plant would become prohibitive. Such plant would be needed especially in the four or five times in the winter when the country is covered by a large, cold high-pressure system with little or no wind. This conclusion runs counter to that of a three-year study by Forum for the Future, an environmental charity, where researchers investigated the possibility of sourcing 30 per cent of electricity from renewable sources by 2020 and of increasing combined heat and power schemes. If half of the contribution came from wind, about 24GW of turbine capacity would be needed plus a correspondingly large capacity of conventional plant for stand by purposes. Clearly there is a practical and economic limit to such enthusiasms.

 

3.4 Network considerations

The UK power system is characterised by a 400kV Grid Transmission system (Fig.3) that delivers the large flows of power through bulk supply points to the low voltage distributors. The arrival of renewable and other embedded generation may not alter the need for bulk transfers of power along the transmission network and could even lead to their increase.

The various forms of embedded generation are connected to distribution networks that are designed and operated in radial configurations and are not designed to accommodate active sources of energy. This historic practice is based on the aim of minimising infrastructure costs. Embedded generation adds to local fault levels and hence, sooner or later leads to the need for larger switchgear as well as the restructuring of the protective systems.

 

In addition and as a consequence of this design practice, the distribution networks are often tapered in power flow capacity from the bulk supply point down to the customer in much the same way as a road or water network. Much renewable generation, e.g. wind, is sited away from the bulk supply points and nearer to the ends of the network, hence the difficulties in finding suitable connection points on the joint grounds of limited power flow and switchgear capacity.

The wind and wave resources in NW Scotland are very large, but transmission capabilities southwards to the Scottish markets for electrical energy are inadequate or non-existent and those from Scotland to the larger English markets are already being used to capacity. The Scotland/England interconnector has only a capacity of 1,600 MW, although this is being enhanced to 2,200 MW. To remove these limitations will require major investment in reinforcing and expanding the transmission grid. Gaining rights of way to build such new circuits will face determined and prolonged opposition from environmental groups.

 

4. CONCLUSIONS

The present high quality power supply in the UK is the result of immense capital and intellectual investment over many years. The importance of maintaining high quality power supplies is perhaps not appreciated by those who debate the future of the electricity supply industry in political, economic and environmental circles, yet without high power supply quality the operation of a modern industrialised society is not possible.

 

Several issues need to be addressed with regard to the future mix of generating capacity if a significant proportion of renewable generating plant is to be included.

 

 

 

 

 

 

 

Figure 2 Aggregated wind generation in Jutland and Furen, Western Denmark

 

 


[This map can be found at: www.parliament.the-stationary-office.co.uk/pa/cm200001/cmselect/cmstech/29101.gif]

 

Figure 3: Electricity supply system as at 1 April 1999

 

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