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9REGIMES BALANCING IN THE LOCAL ELECTRIC SYSTEM WITH RENEWABLE
SOURCES OF ELECTRICITY
Jean-Pierre Ngoma
Ph.D.
University of Douala, Cameroon Kravchuk S. Ph.D
Vinnitsa National Technical University, Ukraine
ABSTRACT
Greater part of renewable sources of energy is represented by renewable energy sources (RES). Their operation mode depends on natural conditions of the region, where they are located. That is why the problem of RES stability estimation is important in the problem of covering the daily schedule of energy consumption. One of the aims of the research is to evaluate the probability of providing the consumption schedule by means of corresponding source on the base of statistic data.
In this paper, the algorithm determination of stability covering load demand their own generation of renewable energy, particularly solar power, was proposed. On the basis of this indicator the method of determining the optimum by criterion of minimum reduced cost, power, which should provide a source of reserve to maintain an appropriate level of reliability in carrying the local electrical system. This method, based on the evaluation of stability data for generating renewable energy sources, including solar power, the problem of covering a given load demand and its impact on the number electricity.
Keywords: power generation, solar power, load schedule, reserves power, probability characteristics, given the cost.
Introduction. Recently, electric stations (ES) using renewable energy sources (RES) are actively introduced in electric networks. Such electric networks, when the generation of ES with RES in them reach 2030% and more of the total load power, have the signs of local electrical systems (LES) [1]. They are characterized by tasks, as well as for electric power systems (EPS) with large thermal, nuclear and hydroelectric power stations. In LES it is necessary to maintain the reliability of electricity supply to consumers and the quality of electricity, as well as to ensure the economy of regimes. These tasks can be effectively addressed only when the balance of power is constantly maintained in the LES. In the LES with combined power supply from local and centralized sources of electricity has its own characteristics. ES with RES are related to sources with non-guaranteed generation. The generation of electricity by them depends on the characteristics (meteorological parameters) of the environment. To ensure a balance of power in the LES or, which is almost the same, compensation for the instability of generating ES with RES in the LES should be a reserve
Pcs(t)
capacity. Here, there are options: the use of both the reserve capacity of EPS and the installation of power storage in the LES, which will store electricity that consumers have not used for a certain period of time. Today, with the latter option, there are technical and economic problems that still require an effective solution in order to adequately compete with the first option [2, 3].
The purpose of this article is to show the possibilities under which the capacity of EPS can be used as a backup for LES.
Conditions for the balance of active power in the LES. In Fig. 1 shows how power balance is formed in the local electrical system (LES) with combined power supply. Electricity is supplied from power plants using renewable energy sources located directly in the LES, as well as sources of centralized power. Power stations based on RES can be wind (WES), solar (SES), small hydroelectric power (HPP). In addition, cogener-ation (CP) and biogas plants (BP), generated electricity, can be used in LES. The load of transformer substations consists of the load of consumers and the generation of electricity sources that are on their balance sheets.
PRES (t)
Storrage
PREs(t) = PsEs(t) + PwEs(t) + +PHpp(t) + PBp(t) + Pcp(t)
I
S(t)
Figure 1 - Power balance in the local electrical system
The balance of active capacity in LES at the point of its connection to the electric power system (EPS)
with large thermal and nuclear power plants is recorded:
PCS(t) ^^ ^TSSCO PiES(t) PvES(t) PiPP(t) PBG(t) PCG(t)
где PCS (t) - graph of consumption of LES electric power from EPS; PTSS(t) - total load graph of transformer substations LES;
PsEs(t),PwEs(t),PHPp(t),Р»(0,PCo(t) - generation graphs of SES, WES, small hydropower plants, BG and CG, respectively.
In LES with combined power supply, three different tasks can be solved [4]: achieving maximum profit from the sale of electricity in conditions of a multistage
\ 1
i1 (Pres (t)
(1)
tariff for the energy market with technical limitations on the part of individual ES with RES; Reduction of dependence of LES with total load ZPTSS(t) from centralized power supply; minimization of deviations from the schedule set by the system operator for the generation of renewable energy stations in LES. With the increase in the volume of generation of stations with RES in LES, the most urgent task is in the energy market conditions. It can be formulated as follows:
2
IP (t ))2 *
^ min
(2)
i=1
при условиях
IPTSs(t) Píes(t) PwEs(t) -PhppC) "Pbg(0 -pco(t) "pcs(t) = 0
где PRES - the given schedule of total generation
n
of ES with RES; ^ P (t) - current total generation
i=1
of ES with RES; n - is the number of controllable stations in the LES.
Power PCS(t) depending on the balance of
power of ES with RES in LES can be with plus or minus signs. In the first case, centralized food is a reserve for generation in LES, and in another - LES gives out excess electricity in EPS. A prerequisite for determining PCS (t) is to maintain, with a given accuracy, the generation of ES with RES in the LES (ideal case is
n
PRES(t)-1P (t) =0). This should take into account b
the forecast information about the meteorological parameters, which is formed by the corresponding subsystem of the automated control system (ACS). It allows to adequately determine the possibilities of generating ES with RES on the following day [5]. Unstable sources of electric power of the type WES and SES in the objective function and the limitations of the optimal control problem can be represented by mathematical expectations of the time dependencies of generation
MwES{P(t)}, MSES{P(t)}, t e [to; tk].
Determination of costs for reserve capacity.
Since it is impossible to ensure the required level of balance and reliability of generating ES with renewable energy due to their instability, it is necessary to determine the capacity of the reserve that EPS can provide.
The task of determining the optimal capacity of the LES reserve, regardless of the management principles, should meet the minimum of the given costs for maintaining the reserve's capacity and take into account the costs that will be compensated in the event of an unavailability of the power balance in the LES. Thus, the objective function can be written:
Bs = B(PR ) + B(PTSS) + B(AW) ^ min , (3)
where B(P) = é^1 P - generation capacity
reserve costs
P
R
to
LES
in
EPS;
B(Pss ) = CPss - costs for the supply of transmission capacity transmission lines (TLES); bunit ,
TSS - the unit costs for creating a generation reserve for LES in EPS and the cost of creating a stock by the TLES, respectively; B(AW) = bQM[AW] -costs for compensation to consumers for unrealised electricity; b0 - unit cost of undelivered electric energy; M[AW] = T iL/^b - the mathematical
expectation of the unrealised of electricity in the LES due to the presence of an unstable generation of ES with RES, where T - is the time for which the mathematical expectation of the unrealised electricity is estimated, Pload - is the load power, kunstab is the instability factor or the probability of the power shortage in LES [6].
In Fig. 2 shows the dependence B£, which is determined by (3).
0
1
Cost, $
Pr, kW
Figure 2 - Dependence of costs for providing a reserve ofpower LES 1 - the total cost of the reserve, 2 - the dependence of costs for the under unrealised electricity, 3 - the cost of
creating and maintaining reserve capacity
Optimal power reserve. Optimal reliability of LES is the minimum of the reduced costs for reservation B^ (3) (dependence (1) in Figure 2 without taking into account the cost component of the TLES B (^bandwidth ) )• Taking into account the linearity of the relationship between the costs of creating a capacity reserve B (PR ) and the proportionality of the mathematical expectation of costs B (AW) from unrealized electricity M [AW ] to the end user, you can find the minimum of the functional (3):
SBv
SR SR
l unit p bR Rr + b
SM [AW ] SR
= 0 . (4)
1R R R
Dependence (2) in Fig. 2 can be approximated by a quadratic polynomial. Then (4) takes the form:
SBY
SR SR
bR?PR +b 3« + cFr
-d )
SR
= 0 , (5)
1 R R R
where a, c and d - coefficients of a polynomial. With (5), the capacity of the reserve, which ensures a minimum of costs for it, is defined as:
burnt + b c
R _ bR + b0C
R 2b0a
(6)
As follows from (6), the value of standby power for LES from EPS is largely dependent on its specific economic indicators.
Conclusions. To ensure a balance of power in LES with electrical stations using RES, and in order to meet the requirements for reliability of electricity supply and quality of electricity, a power reserve is required. A reserve power source can be beaten by EPS with large thermal and nuclear power plants. Determination of the optimal power reserve value is a technical and economic task, the solution of which must take into account the instability of generation of power plants
with renewable energy sources, the cost of reserve capacity, and the mathematical expectation of under-sup-ply to consumers. A method is proposed for determining the optimal reserve power by the criterion of the minimum of the given costs.
References
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