**2. Input data for PV plants influence analysis**

Slovak Electricity Transmission System, Plc. was established on 1 April 1994. That day a former National Power System Dispatch Centre in Prague finished its activities and Slovak Power System Dispatch Centre in Zilina took over. Slovak power system was step by step transformed into several economically independent units.

Electricity production is concentrated in Slovenske Elektrarne, Plc, a part of ENEL Group (the company is partly owned by Slovak government).

Production sources are in the following structure:


Slovak Electricity Transmission System, Plc. with its National Power System Dispatch Centre in Zilina are performing transmission and controlling activities on transmission system.

This company operates:


Total transformation capacity is 10 010 MVA.

Three distribution companies provide distribution services for end-consumers. These are:


owned by foreign companies.

102 Renewable Energy – Trends and Applications

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

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

Slovak Electricity Transmission System, Plc. was established on 1 April 1994. That day a former National Power System Dispatch Centre in Prague finished its activities and Slovak Power System Dispatch Centre in Zilina took over. Slovak power system was step by step

Electricity production is concentrated in Slovenske Elektrarne, Plc, a part of ENEL Group

Slovak Electricity Transmission System, Plc. with its National Power System Dispatch Centre in Zilina are performing transmission and controlling activities on transmission

a license for electricity supply.

(RES)" is used in the chapter.

system.

This company operates: 1776 km of 400 kV lines 902 km of 220 kV lines 17 substations of 400 kV 8 substations of 220 kV

**2. Input data for PV plants influence analysis** 

transformed into several economically independent units.

nuclear sources 1820 MW thermal sources 2584 MW hydro sources 2478 MW others 898 MW

(the company is partly owned by Slovak government). Production sources are in the following structure:

Total transformation capacity is 10 010 MVA.

The maximum load of the power system in 2010 measured on 17 December 2010, 17:00 h was 4 342 MW.

Minimum load in the same year measured on 8 August 2010, 5:00 h was 2 190 MW.

Today, there is about 470 MW of installed capacity in photovoltaic power plants in Slovak power system. Wind sources capacity is practically negligible. Hydro power capacity was already mentioned. Photovoltaic sources are characterized by rapid changes of powers, depending on weather conditions. Despite the fact that prediction of performance of RES is in progress these sources are causing unbalance between electricity production and consumption – mainly during periods of low loads in system. The main problem is the possibility to build PV plants with the capacity up to 1 MW. These plants (after connection into system) are not obliged to control deviation between agreed and actual electricity supply into network. These plants are connected to distribution 22 kV system (in 99 % of cases) and are not obliged to measure parameters and transfer data to dispatching centre. Mentioned deviations in electricity supply have to be handled by dispatching centre through purchasing auxiliary services. These additional purchases increase electricity price for customers. Also quality of supplied electricity can be affected. Different types of auxiliary services are described in the following sections together with analysis of their impact on the Slovak power system operation.

A basic property of power system operation is that equilibrium between production and consumption of electricity has to be maintained in every single moment. The consumption of electricity is given by consumers themselves by switching on and off a large number of different appliances and that is why can hardly be affected. Thus equilibrium has to be maintained on the electricity production side and the sources' power must be adapted to instantaneous consumption.

Power system load planning is based on, considering permanent time changes of electricity consumption, the behaviour of system load during 24 hours depicted in daily load diagram (DLD). Expected load during a year is determined from typical DLDs for power system of two days with the highest and lowest loads. DLD for Slovak power system is depicted in Fig. 1 for a summer day.

Calculations for auxiliary services were done only for summer season in the time of minimal value of weekly maximum - *LMAX* for the values of 2261, 2394, 2660, 2926, 3192, 3458 MW. Evaluated scenarios of system electricity sources employment participating in coverage of DLD in regulation area of Slovakia during summer, in time of absolute minimum were considered according to available data and records from SEPS. In the calculation of the Slovak power system (SPS) operation substantial changes in electricity production installations can are considered. These are listed in Table 1 for individual years. For the purpose of simplified depiction of sources employment, the scenarios were marked A, B, C.

Fig. 1. DLD of summer day


Table 1. Expected changes of installed capacity for years 2010 – 2013

For each of the sources employment scenarios A, B, C the following installed capacities of RES were used: 300, 400, 500, 600, 800, 1000 and 1200 MW. Each capacity of RES was then employed in the values of 10, 20, 40, 60, 70, 80 and 90 % of installed capacity.

Based on actual operational states of existing RES in SPS the installed capacity utilisation of RES in summer is 0 – 75 % *Pinst* . The variability depends mainly on actual time changes of global solar radiation and used photovoltaic panels.

The coverage of DLD depends on possibilities of each power plant, their failure rate and planned repairs cycles. Employment of individual sources for DLD coverage was performed according to the standards used in preparation of transmission system operation. Pumped storage plants were used to cover peaks of the DLD. Operation of industrial power plants in different regions was considered according to previous years' information. Cut-offs of production facilities and used electricity production technology have substantial impact on source employment in summer season. In case of cut-offs of production facilities, data from SEPS for the years 2010 and 2011 were used.

Sources employment scenario A B C

Source/Year 2010 2011 2013

Nuclear power plant EMO 3,4 1000 MW

RES wind+PV variable variable variable

For each of the sources employment scenarios A, B, C the following installed capacities of RES were used: 300, 400, 500, 600, 800, 1000 and 1200 MW. Each capacity of RES was then

Based on actual operational states of existing RES in SPS the installed capacity utilisation of RES in summer is 0 – 75 % *Pinst* . The variability depends mainly on actual time changes of

The coverage of DLD depends on possibilities of each power plant, their failure rate and planned repairs cycles. Employment of individual sources for DLD coverage was performed according to the standards used in preparation of transmission system operation. Pumped storage plants were used to cover peaks of the DLD. Operation of industrial power plants in different regions was considered according to previous years' information. Cut-offs of production facilities and used electricity production technology have substantial impact on source employment in summer season. In case of cut-offs of production facilities, data from

Table 1. Expected changes of installed capacity for years 2010 – 2013

global solar radiation and used photovoltaic panels.

SEPS for the years 2010 and 2011 were used.

employed in the values of 10, 20, 40, 60, 70, 80 and 90 % of installed capacity.

Malženice 430 MW 430 MW

New 850 MW

Fig. 1. DLD of summer day

Combined cycle plant
