Voltage control Reactive energy

March 1997 Ref : 03005Ren9712 ...................................................................................................... Voltage control...
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March 1997 Ref : 03005Ren9712

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Voltage control – Reactive energy ......................................................................................................

Hydro Power and other Renewable Energies Study Committee ......................................................................................................

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Voltage control – Reactive energy ............................................................................................ Hydro Power and other Renewable Energies Study Committee ............................................................................................

Paper prepared by:

Hubert WEIS (LU); Maurice DUPUY (FR); Myriam BARIL (CA); Mike BARLOW (GB); Mario CAMBI (IT); Maurice GENIER (CH); Jean Pierre GERMEAU (BE); Carlos MADUREIRA (PT); G.SCHILLER (AT); Antonio TAHULL (ES )

Copyright © Union of the Electricity Industry - EURELECTRIC, 2000 All rights reserved Printed at EURELECTRIC, Brussels (Belgium)

VOLTAGE CONTROL - REACTIVE ENERGY

SUMMARY

EXECUTIVE SUMMARY ......................................................................................................1 1.

THE ISSUE OF VOLTAGE CONTROL .........................................................................2

2.

THE CONTRIBUTION OF HYDRO POWER.................................................................4

3.

REACTIVE PERFORMANCES OF THE HYDRO GENERATION MIX .......................5

4.

VALUATION OF REACTIVE ENERGY........................................................................6

5.

COST OF INDUCTOR-CAPACITOR UNITS .................................................................8

6.

COST OF SUPPLY WITH SYNCHRONOUS COMPENSATOR ...................................9

7.

INTEREST OF HYDROPOWER FOR VOLTAGE CONTROL ......................................9

VOLTAGE CONTROL - REACTIVE ENERGY

EXECUTIVE SUMMARY

Voltage control, like frequency control, is an essential part of the operation of power systems. Indeed, energy exchanges must be carried out with voltage differences within well-specified limits and the voltage at the consumers’terminals must remain within equally stringent limits. Also essential are: the

primary secondary and tertiary controls

required to be provided to a degree dependent on the response and operational requirements of the system. This may be performed either by thermal or hydro generation and static equipment made up of inductor-capacitor units. Hydroelectricity takes part in these actions according to its voltage connection level on the network and geographical location, but the latter is not always favourable to hydro power .The capacities are similar to the UCPTE European sample examined (12 countries wholly or partially ) of 50,000 MVAr. To use this energy, it turned out that the only substitution which could be used was the static equipment installed on all the networks .An average cost of 3.9 XEU 95/kVAr, including investment depreciation and operating expenses, has been estimated. On this basis, the capacity of the hydro power plants taken into account, for all the UCPTE countries considered, represents an annual static equipment saving of 195 MXEU 95 and an immediate investment of 1500 MXEU 95. However, it should be noted that:

estimated

⇒ The operation as synchronous compensator of hydro generating sets has an cost of 0.4 cXEU 95/kVArh which reduces the initial benefit.

⇒ As reactive energy is not optimally transmitted more than a few dozen kilometres, the use of hydro power plants to supply or absorb reactive energy is usually limited by their remote location from demand centres. Hydro power, except in some regions where it can be well placed in the network (South-East of France, North of Scotland), does not have a specific benefit for voltage control.

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1.

THE ISSUE OF VOLTAGE CONTROL

(see UCPTE document: Reactive power and voltage control in the UCPTE network of 31/07/91). The operator of power systems generates, transmits and distributes energy at EHV (400 and 225 kV in Europe and at HV (63-90 and 110 kV)) and then distributes it to end customers at MV and LV. These energy exchanges must be managed at voltages maintained at limits due to constraints of various origins: • first of all, the contractual level fixed by the customer to satisfy his uses and industrial or domestic processes, • • the sizing of the equipment: dielectric strength of transmission facilities, operating diagram of the generators, • • the operating ranges of equipment: adjustment of protective devices, tapping ranges of transformers, etc., • • operating conditions of power systems and of their interconnections: static stability under stationary operating conditions, transient stability during disturbances, • • finally, the economic concern for least cost generation management by limiting transmission losses. Frequency is an element common to the overall network, but voltage is a local variable greatly influenced by reactive power flows accompanying any active energy exchange. As is the case for active energy, the control equipment may be classified into: ◊ r Primary voltage control It is provided by the controllers of the generating sets, which, through their action on excitation voltage of the set's generator, tends to reduce the difference between the actual terminal voltage and the actual scheduled (set point) value. This control is provided within the constructive limits of the generators: diagram P - Q and stability margin. Compensation between the voltage of the generating sets and that of the outlet of the connection transformers may also be performed, as well as a distribution of reactive energy between the sets connected to the same busbar according to a signal taken from the latter, may improve the control capacities.

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◊ Secondary voltage control The automatic primary controller maintains the terminals of each generator at a constant voltage equal to the local set value through action of the generator excitation voltage. During a disturbance: tripping of a generating set, of a line, change in demand - if one lets the generators operate at the operating points defined by the action of their primary controller, a non optimum solution for the overall network is soon reached: generators producing reactive energy absorbed by others, poorly distributed total reserve, degrading network voltage plan due to major reactive power flows, increase of line losses, generator overloads. To return to a more optimum situation, one has to act on the set point levels of the primary controllers. Manual action on a case-by-case basis soon proves to be inefficient for a larger network. The purpose of secondary voltage control is to carry out this modification in real time and in a coordinated manner. The network is divided into control areas that are semi-independent or at least have no effect on the adjoining areas,and the voltage plan of each of them is fixed and controlled by acting in a coordinated way on control equipment available and on the set point voltage of the generators in operation. This action is performed on a "pilot" point of the area which should be representative of the change in voltages throughout the area. The area must also be characterized by the presence of reactive power generating facilities of sufficient number to be able to meet the reactive requirements of the area in due time, so as to avoid the transmission of reactive power and substantial interactions between neighbouring areas. The response time of the secondary control is a matter of minutes (200 to 300 s), whereas primary control is instantaneous (a few seconds). A more effective secondary control taking into account interactions between areas is being implemented on the French grid. ◊ r Tertiary voltage control The constant changes of the operating conditions of generation, transmission and demand systems: if the secondary control set point adjustment levels were not readapted accordingly, the demand variability, generating set startups/shutdowns, scheduled or unscheduled tripping of lines would lead to situations remote from an economic optimum (reactive energy flows inducing significant losses) and to degraded network operation safety (generating sets at reactive stop and therefore not operational for voltage control, overloaded lines, etc.). The role of tertiary voltage control is to adapt and harmonize the various set point voltages to the pilot points in all the networks so as to economically optimize the operation of the system and ensure its optimum safety.

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This control is still manual and performed by the national or regional load dispatching centres. Studies for its automation are under way so as to further improve optimality. ◊ r Static control equipment The setting up on MV and HV networks (or even EHV networks in some countries, such as Canada) of static equipment, such as: • • Capacitors which provide reactive power, • • Reactors which absorb this reactive power contribute with the on-load tap changers of transformers in maintaining an acceptable voltage level in the network areas. The synchronous compensators which supply or absorb reactive energy are also effective control equipment. De-energizing lines or line sections in off-peak periods may also be a means of control without compromising system security. 2.

THE CONTRIBUTION OF HYDRO POWER

Most hydro generating sets are equipped with voltage controllers and take part in primary voltage control. In many power plants, the coupling transformers are fitted with on-load tap changers. The largest of these sets connected to the EHV networks take part in the secondary control under the influence of the pilot points situated in the transmission substations to which they are connected. Due to their construction, hydro generators are capable within their operational area of the P/Q diagram of a major range of reactive energy supply and demand. A number of them, following turbine runner dewatering and watering, may operate as a synchronous compensator and participate directly in voltage control. The geographical location of hydro power plants, generally quite remote from demand centres, is both: • • favourable: they are the only means of control in areas that are under-equipped (e.g. in France: LA DURANCE, in Scotland: FOYERS). In addition, hydro generating sets can start up, then operate without calling upon external sources and are therefore able to resupply a collapsed network, or restore the voltage to the auxiliaries of power plants and major users. • • unfavourable: reactive energy is not transmitted and the control effect of the hydro generating sets is insufficient to ensure stable control in urban areas of high demand.

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It should be noted that conventional thermal generating sets, as well as nuclear sets, play a significant part in voltage control (e.g. 600 MW standardized series, coal: + 310 MVAr, - 255 MVAr ; P4 1300 MW standardized series, nuclear: + 500 MVAr , - 500 MVAr). Furthermore, the operation of hydro generating sets as synchronous compensator has an effect on the costs: ⇒ ð Investments, often additional, for: * dewatering, degassing and watering devices, * mechanical disconnection devices between generators and hydraulic turbines, * on-load tap changers of generating set transformers. ⇒ ð Operating expenses: * for additional maintenance due to operation as synchronous compensator, * for the demand of auxiliary services (sprinkling water, oil and water systems, etc.) * for the active power extracted from the network to run the system, ranging from 1 to 10% of the rated capacity of the generating set. 3.

REACTIVE PERFORMANCES OF THE HYDRO GENERATION MIX

An evaluation of the potential of the UCPTE generation mix was performed on the basis of an analysis of the generating facilities of a number of utilities and countries represented in the group of experts. It can be observed that : ⇒ For the overall generation mix studied, hydro power shows major reactive energy generation or absorption capacities, but which often remain poorly known (and this is often the case for the other means of generation ) For the UCPTE countries : 50,000 MVAr supply 35,000 MVAr absorption ⇒ The other thermal generating facilities generally have lower and, of course, extremely variable capacities, according to the composition of each generation mixes. ⇒ Static equipment is at least as large as the equipment spread out in the power plants and their generators. A great variability from country to country is to be seen, due to the extent of the territory, the location of the power plants and the territorial breakdown of demand and generation ⇒ Unlike the performances of frequency control and of various active generations, the problem concerning voltage control, its performances and the means to be used does not appear to be fully known: more local uses and regional network configurations emerge .

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4.

VALUATION OF REACTIVE ENERGY

There still do not exist economic optimization elements for reactive energy as there are for the distribution and regulation of active energy (marginal costs). Comparative or simulation methods consequently have to be used. Of course, it is not possible at the service rendered value level to compare one with the other the various energy sources capable of generating reactive energy: indeed, regardless of the fuel used, the MVAR produced or consumed will have the same effect on voltage control. The differences which may arise will be at the generation cost level. For all of the results set out below, the currencies of the various countries have been converted to Ecus: XEU (European Union Unit of Account) for 1995 by using the trend rates of the respective currencies and the foreign exchange rates. The EURO designation is equivalent. Simulations have been performed in several countries: • • In France, through the use of a model for network optimization and particularly line losses: optimum economic difference both in investments to be made (static equipment) and cost of losses between two situations: one real, the other by assuming the unavailability of hydro generating sets of an area. The result leads to a valuation from 770 to 1,550 XEU 95/MVAr (0.8 to 1.6 XEU 95/kVAr). • • In Germany, a comparative study between reactive energy generation from thermal generating sets and inductor-capacitor sets placed on the high voltage network (380 kV and 110 kV, respectively) concludes in an average annual cost of about 3.3 XEU 95/kVARr for a 3,000 h utilization of the installation. Under the same utilization conditions, the reactive supply of a generator operating in synchronous compensator mode results in a similar value (3 XEU 95/kVAr. When line losses and additional generator outages are taken into account, the economic advantages of inductor-capacitor units are affirmed from the low utilization time values. ♦ Mention may also be made of the United Kingdom where, as part of the electricity market, voltage control is remunerated by the National Grid and evaluated on the bases of: 3.1 XEU 95 / kVAr) for an annual supply from 1.2 to 2.40 XEU 95/ kVAr of annual availability

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♦ In Italy, legislation concerning the connection of customers to the network imposes penalties corresponding to: 0.8 CXEU 95 / kVArh if 0.8 ≤cos ϕ≤0.9 and 1.02 CXEU 95 / kVArh if cos ϕ< 0.8 i.e. for 3,000 h/year: 24 to 31 XEU 95/ kVAr It therefore does not seem possible to perform a valuation of this energy on the basis of global economic simulations. Those carried out on limited cases may serve to validate the orders of magnitude of the value chosen. However, a comparison with voltage control static equipment: inductor-capacitor units or adjustable FACTS ( Flexible Alternative Currency Transmission System ), may be performed. It is obvious that there are limits to this comparison: • • static units are generally placed on MV or HV networks, whereas generating sets are connected at HV or EHV (225-400 kV). • • they are by definition adjustable at the very most in stages, whereas the control is continuous for the generating sets. • • the unit power values range from 20 to 50 MVAr for the static equipment; they may reach 200 MVAR for a hydro generating set and 500 MVAr for a nuclear set. Nevertheless, it appears that this comparison element is quite universal and does reflect reality: static units are actually used where there is no longer possibility of control by generating sets.

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5.

COST OF INDUCTOR-CAPACITOR UNITS

So as to be able to value the overall hydro generation mix considered, the cost to be taken into account for static equipment will have to be defined. A survey of the various countries has given the following results: In XEU 95 COUNTRY AUSTRIA BELGIUM

Cost of kVAr 19.2 26

CANADA (Hydro-Québec) FRANCE GERMANY ITALY

83.6

IRELAND NORWAY PORTUGAL SPAIN UNITED KINGDOM

21.5 42.5 15.2 12.3 32

AVERAGE

15.4 25 32.2

27

REFERENCES 50 MVAr units Unit from 6 to 80 MVAr (MV to HV) Static Unit 300 MVAr at EHV (36 M $ Can) Unit 20-30 MVAr Inductor-capacitor Unit 100 MVAr (70 M lires / MVAr) SVC Capacitor 63 kV 54 MVAr units for 75 MVAr (2 M £) For 63 to 150 networks

REMARKS -from 18.3 to 32.6 (SVC: 61 XEU/kVAr)

With bay: cost 2x at 110 kV

at 20 kV SVC unit (10 M £/75 MVAr) kV

The valuation will be performed on the basis of: 30 XEU 95 / kVAr ( to take into account margins of error and ancillary connection of these devices on the networks ) to which will have to be added operating and maintenance expenses (that are assumed to be equal to 1/10th of the investment cost) amounting to: 3 XEU 95 / kVAr that is spread out evenly over a 15-year lifetime, or a total annual cost at the 10% discount rate of: 3.9 XEU 95 / kVAr

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6.

COST OF SUPPLY WITH SYNCHRONOUS COMPENSATOR

If one can consider that the fact of operating at any control point means that the supplying or absorbing of reactive energy has no effect on the cost criteria and should therefore change the kVARh supply value. The same does not hold true for a generating set operating as synchronous compensator. To make such a means of operation possible, the following would be necessary: • • an extra investment: means of operation with dewatering of the hydraulic runners, • • additional maintenance expenses both for specific equipment and the turbines themselves due to the additional running time. • • water for the operation of the turbines and watering of the moving parts bringing about losses from the economic standpoint, • • additional generation costs: consumed energy, auxiliary services, etc. A study carried out on a sample of power plants from the French generation mix showed an additional cost of about: 0.075 to 0.4 cXEU 95 / kVArh that can be selected in the present study. This value has been confirmed by investigations carried out on a number of pumped storage power plants in Germany and Luxembourg: 0.3 to 0.45 cXEU 95 / kVArh. 7.

INTEREST OF HYDROPOWER FOR VOLTAGE CONTROL

On the basis of the hydro generation mix elements such as they are defined in section 3 and the values that may be used (section 5), one obtains an annual value of the service provided by voltage control due to its supply/reactive energy absorption capabilities of: 195 MXEU 95 If this capacity did not exist, it would have to be replaced by static control units (inductors-capacitors) or by autonomous synchronous compensators the annual investment payments of which would be this value. Of course, an optimization of the generation mix could be performed through enhanced operation of thermal power plants and network voltage plans. It follows that the above figure is a major factor which has to be weighted if it is to be used as such.

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Also to be weighted are additional costs brought about by operation as synchronous compensator (which represents a not inconsiderable part of the power used for the voltage control). For use of half of the power for 2,000 h per year, the overcost comes to about : 120 MXEU 95 If one considers the level of investments to be agreed to replace the voltage control capacities provided by the UCPTE hydroelectric generation mix, the amount comes to: 1,500 MXEU 95 It can therefore be concluded that: ⇒ the hydro power plants of the UCPTE generation mix represent a supply reactive power of about: 50,000 MVAr the use of which avoids a static equipment investment estimated at: 1,500 MXEU 95 and annual investment and operating expenses of: 195 MXEU 95 per annum ⇒ however, the use of these power plants is not optimal when it concerns performing a voltage control on large geographical areas. It is more appropriate in this case to use static compensation equipment at the network nodes.

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