Source: https://elering.ee/en/system-operation
Timestamp: 2019-04-22 12:54:27+00:00

Document:
In the interest of maintaining the electricity system frequency, electricity production and consumption must be in balance at all times.
Every day, preparations are made for the maintenance of the next day’s balance. For this, the production plans, forecast consumption and planned trade for all market participants are collected together for the balance plan. On the basis of the resulting plan, the electricity system’s operations are monitored and managed. In the event that actual consumption and production should differ too much from the planned volumes or if a generator must be disconnected due to an emergency, contingency plans provide options to restore the balance of the system balance – for this, agreements are signed for the use of balancing capacity and emergency reserve capacity.
The system’s uniformity and performance must also be preserved in the event of faults in the network. In addition, system operations must be statically and dynamically stable. To this end, system operation and development is planned so that all possible emergency situations are analysed proactively, and on that basis decisions are made on network element repair acceptability and system import and export capacity.
Maximum import and export capacities depend, on the one hand, on the thermal capacity of lines, and on the other hand on the system’s stability limit determined during regime calculations. The smaller of the two figures is taken as a basis in determining the final limit.
Countertrading is used when there is an event in the electricity system that causes cross-border transmission lines to be overloaded or which physically interrupts the transmission capacity.
Cross-border energy trades effected before the overload of cross-border transmission lines or outages of physical transmission are fully guaranteed by the TSO, meaning previously confirmed supplies are not cancelled.
The TSO into whose network the supplies are or were flowing activates the additional amount of generating capacity required in the area for which it is responsible.
The TSO that is or was sending out the flow reduces the amount of generating capacity by as much as is required in the area for which it is responsible.
Elering is required by European Union regulation (EC) No. 714/2009, Annex 1 p 5.4 to make public the principles for calculating transmission capacity. In the official Estonian translation of the regulation, this principle is described as the “general scheme for the calculation of the total transfer capacity and the transfer reliability margin based upon the electrical and physical features of the network”.
The standards were approved on 13 August 2009 by the Estonian Competition Authority in its decision No. 7.1-1/09-0612-002.
Total Transfer Capacity (TTC) is calculated from the technical parameters of the network with consideration for the requirements for operational reliability in the Grid Code (see §3, §6, §10, §11, §12, §13 and others). The most important of these requirements are the N-1 and N-2 criteria. These criteria state that the transmission capacity must be calculated to cover the switching-off of one or two of the elements that have most impact on the electricity system. Subsequently, the maximum transmission capacity that does not exceed the thermal capacity of lines or risk the static or dynamic stability of the system can be calculated.
The Transmission Reliability Margin (TRM) is calculated to cover unplanned events such as unforeseen short circuits, measuring errors in the measuring system or emergency transfers of capacity between TSOs. Experience in planning is of great importance in calculating this margin, as is information from the TSOs of neighbouring countries. The exact amounts of transmission capacity are agreed with the TSOs of the neighbouring systems by considering all of this information.
The TRM is subtracted from the TTC to give Net Transmission Capacity (NTC). NTC is the capacity that is given for market participants to use in cross-border energy trading.
Elering uses the principles mentioned above to calculate transmission capacities and then coordinates these capacities with the TSOs of neighbouring countries.
The system limits set by the TSO, their reasons and effect on the electricity system can be found on the Nord Pool homepage. In addition, information on actual disruptions in the transmission systems can be found on the same homepage.

References: §3
 §6
 §10
 §11
 §12
 §13