**4. Provision of auxiliary services and balancing electricity purchasing**

Slovakia, as one of the members of interconnected European power system ENTSO-E, has to meet basic requirements for parallel operation of power systems. One of these basic requirements is also range and quality of auxiliary services, taking to account on one side global view of secure and reliable operation and on the other side, local nature of consumption of individual country. While the range of some service is strictly ordered from ENTSO-E (i.e. range of the primary regulation, which principally performs locally, reacts according to instantaneous frequency deviation, in tens of seconds, but impulse, i.e. origin of imbalance between electricity production and consumption can occur anywhere in interconnected system), ranges of other services can vary based on local behaviour of system and thus it has to meet only certain frame requirements.

Basically every power system must have power reserve secured for coverage of an outage of the largest source in order to be balanced in power in relation to other countries. This condition is secured by so-called stacking of various kinds of auxiliary services, or bi- or multi-lateral contracts with international partners, usually neighbouring countries.

In Slovak environment TSO SEPS acts as a partner for ENTSO-E. SEPS after consideration of before mentioned criteria elaborates i.a. a proposal of range of individual auxiliary services and in sense of valid legislation (as a regulated subject) submits to the national Regulatory Office for Network Industries, who in the form of decision defines range and price for a instantaneous availability of auxiliary services and balancing electricity

Fluctuation of electricity production from RES primarily imposes higher demands on regulation sources of SPS either on amount available reserves or on the quality of regulation. In the frame of interconnected system ENTSO-E the SPS is regulated for an agreed balance. SEPS has to keep the agreed value and quality of this regulation is monitored at the coordination centre level. Potential deterioration or violation of accepted standards in area of agreed balance would result in investigation of SPS by ENTSO-E.
