**1. Introduction**

100 Renewable Energy – Trends and Applications

Zhang, L. & Luh, P. B. (2005). Neural Network-based Market Clearing Price Prediction and

*IEEE Transactions on Power Systems,* Vol. 20, No. 1, February, 2005, pp. 59-66.

Confidence Interval Estimation with an Improved Extended Kalman Filter Method,

The chapter presents issues of renewable energy sources connection into Slovak power system. It describes calculation techniques for estimating acceptable RES capacities, which can be connected into the system, without jeopardizing security and quality of supplied electricity. Calculations of additional expenses for regulation power purchase for various values of RES capacities connected are presented in the second part of the chapter.

The interest in construction of photovoltaic power plants in Slovak Republic soared in 2008. A new act concerning renewable energy sources and remarkably generous prices paid for electricity from these sources (associated with the act) stimulated investors' interest in building new plants. Based upon experience from other countries and following analysis of possibilities for procuring own sources of balancing electricity for auxiliary services, Slovak Electricity Transmission System, Plc. (SEPS) placed an order to Department of Power Electrical Systems to elaborate a study in which a maximum capacity of PV plants would be determined, which can be connected into the power system, taking into account availability of balancing power for secondary and tertiary regulation. Developed study determined 120 MWp as maximum power of PV plants, which can be connected in 2010.

Influence from EU side and from investors and construction of new thermal and nuclear power plants in Slovakia necessitated an elaboration of a new study, which would analyse the situation in transmission system with respect of balancing electricity for the case, when new PV plants with capacity up to 1 200 MWp are connected. This chapter presents solution of this matter and some results.

Determination of connectable capacity of PV plants into power system of Slovakia can be accomplished from different points of view. One of them can be based on the basic condition, which is used in all studies concerned PV plants connection into distribution network. This condition states, that voltage change in point of common coupling should be less than 2 %, compared with the situation before plant connection. If this condition is applied, resultant connectable capacity should be relatively high, as 22 kV lines can easily withstand considerably higher loads than those of current ones. Probably 80 to 90 % of demanded capacity could be accepted in this case. This consideration can be hardly accepted due to sources features, which show a vast variability of power, practically in whole power range. In addition, capacities of PV plants are usually lower than 4 MW, thus the responsibility for balance is brought to the distribution company. But the power balance can only be solved on the Slovak transmission system level.

The other possible approach is according to the act [1], where in § 1 sect. 2 the following is stated: "Operator of distribution system is obliged, after the price for connection to distribution system is reimbursed, to connect installation of electricity producer into distribution system with priority, if the installation fulfils technological conditions and sales conditions of connection into the system so that security, reliability and stability of system operation is unchanged". Operator of regional distribution system uses electricity obtained according to sect. 6 for losses redemption. In case, when instantaneous power of obtained electricity exceeds the value necessary for losses redemption, operator of regional distribution system has the right to sell this electricity for the market price. This kind of electricity selling is not considered a business activity in energy sector and does not require a license for electricity supply.

The third possible approach for connectable capacity into distribution system determination is based on the assumption of PV sources variability and the responsibility for balance being bear by a distribution company. Purchase of auxiliary services is needed for balance compensation and this purchase can only be accomplished by Slovak transmission system operator. So the approach to PV plants connectivity with regard to the balance responsibility has to be solved on the Slovak transmission system level, cumulative for all PV plants connected into distribution systems in Slovakia. This approach is described in the following sections of the chapter. In this analysis wind power plants are also taken in consideration together with PV plants. Their connectable capacity was determined to maximum 200 MW. Generally, the term "renewable energy sources (RES)" is used in the chapter.
