According to Article 3(2)(36) of the Network Code on System Operation, System State is the operational state of the transmission system in relation to the Operational Security Limits.
Network Code on System Operation differentiates the following System States:
According to Article 19 of the Network Code on System Operation each Transmission System Operator (TSO) is required in real-time operation, determine the System State of its transmission system.
In order to determine the System State, each Transmission System Operator (TSO) must at least every 15 minutes perform Contingency Analysis in real-time, monitoring the parameters against a common set of criteria, while taking into account the effect of potential Remedial Actions and measures of the System Defence Plan.
What are the consequences of the System States for Grid Users?
The first aim of a common definition for System State is to give to TSOs a common understanding of the situation, and of the coordination they must have to undertake. The principles for System State classification aims at differentiating when the system is N- and (N-1) compliant (Normal State), N compliant but not (N-1) compliant (Alert State), or not N- compliant (Emergency State). That means that in both Normal and Alert State, there is no significant violation of the Operational Security Limits.
When the Transmission System is in Alert State, although all Operational Security Limits are respected, there is at least one Contingency which is reasonably probable and which is part of the Contingency List, for which, in case of occurrence, at least one of the operational parameters will not remain within the Operational Security Limits. It's therefore important to mitigate the risk of occurrence of such Contingency, and this is the reason why TSO can ask for stopping actions which can increase the probability of Contingency.
1. In Emergency State, at least one of the operational parameters is outside of the Operational Security Limits, so it affects the quality and security of the electricity supply towards Significant Grid Users. Nevertheless, a precise definition of the impact on Grid Users in case of Emergency State cannot be done in the OS NC, since it varies depending on the sequence of events that have occurred: impact on frequency in case of big imbalance without sufficient amount of active reserves, impact on voltage in case of insufficient of Reactive Power reserves, etc.;
2. In Alert State the system is secure in N- situation, so there is no direct consequence for Grid Users, except for those currently conducting a test;
A precise definition of the impact on Significant Grid Users in case of Emergency State cannot be done in the OS NC, since it varies depending of the sequence of events that have occurred.
ENTSO-E Supporting Document for the Network Code on Operational Security of 24 September 2013 2nd Edition Final, p. 155
Network Code on System Operation
Monitoring and determination of system states by TSOs
1. Each TSO shall, in real-time operation, determine the system state of its transmission system.
2. Each TSO shall monitor the following transmission system parameters in real-time in its control area, based on real-time telemetry measurements or on calculated values from its observability area, taking into account the structural and real-time data in accordance with Article 42:
(a) active and reactive power flows;
(b) busbar voltages;
(c) frequency and frequency restoration control error of its LFC area;
(d) active and reactive power reserves; and
(e) generation and load.
3. In order to specify the system state, each TSO shall perform contingency analysis at least once every 15 minutes, monitoring the transmission system's parameters defined in accordance with paragraph 2, against the operational security limits defined in accordance with Article 25 and the criteria for system states defined in accordance with Article 18. Each TSO shall also monitor the level of available reserves against the reserve capacity. When carrying out the contingency analysis, each TSO shall take into account the effect of remedial actions and the measures of the system defence plan.
4. If its transmission system is not in normal state and if that system state is qualified as a wide area state the TSO shall:
(a) inform all TSOs about the system state of its transmission system via an IT tool for the exchange of real-time data at pan-European level; and
(b) provide with additional information on its transmission system elements which are part of the observability area of other TSOs, to those TSOs.
Network Code on System Operation, Article 3(2)(36), Article 19