Energy management system

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The nature of transmission networks will change and grow in importance due to Smart Grid. The increased distance of bulk power generation and load centres will result in a tendency to interconnect systems that used to be independent. Furthermore, the exchange and trade of power over long distances will grow in the future.

Information exchange may be necessary across large geographical areas and across traditional systems operation boundaries.

Transmission networks are equipped for obtaining a large number of measurement values; they are able to determine the current load flow situation by means of estimation algorithms. In an estimate, the algorithm uses a numerical network model to try to find a load flow solution in which the difference between the load flow solution and measurement values is minimal. The estimation of the network state supplies the operator with a complete load flow solution for supervising the network, including those sections of the network for which no measurement values are transmitted to the control system.

The network state estimation is generally followed by a limit value monitoring process that compares the result of the estimation with the operating limits of the individual operational equipment, in order to inform the operator about overloads or other limit value infringements in a timely fashion.

The load flow solution of the network state estimation is then used for ongoing functions such as outage analysis, short-circuit analysis or optimizing load flow as a basic solution for further calculations.

The outage analysis carries out “What if?” studies in which the failure of one or more items of operational equipment is simulated. The results of these load flow calculations are then compared with the operational equipment limits in order to be able to detect secondary faults resulting from an operational equipment failure. If such violations of the so-called (n-1) security are detected, an attempt can be made by, for example, using a bottleneck management application to define measures with which (n-1) security can be re-established.

The short-circuit analysis simulates short-circuit situations for all kinds of different network nodes on the basis of numerical model calculations. It checks whether the ensuing short-circuit currents are within the operational equipment limits. The quantities to be checked are the breaking power and the trip time of the circuit breakers and the peak short-circuit current strength of the systems. Here again, the operator is informed about any limit violations so that suitable remedial action can be taken in a timely fashion.

The optimizing load flow attempts to determine an optimum network state by varying the controlled variables in the power supply system. The following target functions for “optimum” are possible:

The voltage/reactive power optimization attempts to minimize the reactive power flow in the network in order to reduce voltage drops and transmission losses. In particular, the reactive power generation of the generators or compensation equipment act as controlled variables.

The active power optimization system tries to minimize the transmission losses by re-dispatching the incoming supplies from the generator.

If system reliability has been selected as the target function of the optimization, the optimizing load flow tries to find a system state in which the capacity of all operational equipment is utilized as evenly as possible. The purpose of this is to avoid further secondary failures in the event of failure of heavily utilized resources.

The challenge posed by Smart Grid implementation and the increased use of bulk power transmission will be a change from the quasi-static state of the transmission grid to a more complex and dynamic behaviour. Therefore, the current available supervision, management and control functions will need to be adapted.

State estimation, for example, will have to include the transient behaviour of the network. In addition, the traditional power, voltage and current measurements need to be extended to phasor measurement provided by PMUs (Phasor Measurement Units).

An optimal representation and visualization as well as decision-supporting tools are expected to be developed in order to support the operator of such complex systems. This will require a large amount of data need to be transmitted, synchronized and represented in a way to safeguard the system integrity of the overall transmission network.

The standards listed below are available on  IEC webstore.

The EMS SCADA component architecture is given in the Figure above. Data and information of the actual status of the transmission system is online available through the RTUs of all substations in the network. The transmission network is operated and controlled from the dispatch centres by remote controlled circuit breakers in all relevant fields of the network. The operators are supported (coached and controlled) by the EMS SCADA system regarding energy flows in the network, during normal, maintenance and emergency operation of the network or parts of it.

Communication protocols from the substation automation domain can be used because the EMS SCADA system interacts with the protection, monitoring and control systems in the substations. Furthermore, the EMS SCADA will have direct interaction with power plants connected to the transmission system and Transmission System Operators (TSO) are responsible for balancing power generation and demand. Finally, TSOs have a responsibility in supporting the energy market interactions to bulk generation connected to the substations in their EHV and HV transmission networks.

The set of standards representing the related protocols regarding EMS SCADA can be positioned as shown in the Figure above. This diagram shows the communication layer of Smart Grid Architecture Model. The significant standards regarding communication are IEC 60870-5-101 and IEC 60870-5-104 to connect power plants to the grid.

The information layer of EMS SCADA, as shown in the Figure above, is based on standards and guidelines that cover the Information Models relevant for EMS SCADA Systems used for operating the EHV and HV networks of TSOs.