T1 : Power system design is a key parameter for the introduction of Distributed Energy Resources(DER)
The basic principles used for developing and operating Power Systems are valid whatever the scale of the considered system. The main goal is to get targeted reliability for customers at an acceptable price. Such systems can be characterised by a set of properties. Some of them could be put under pressure following the introduction of Distributed Energy Resources (DER).
What are the consequences of an increasing deployment of Distributed Energy Resources on the validity of system design principles and on system reliability?
The basic principles behind power systems development and operation are valid whatever the system considered
Preventive security margins are the cornerstone of the design of electrical power systems. Respecting these margins means that the system is robust in the face of a predefined list of contingencies (i.e. “secured” events such as the sudden disconnection of a line or a generating unit). In case of more severe incidents, system security is restored by deploying dedicated defence actions.
Power system control in quasi steady-state is implemented for preserving security margins. It is deployed using a 2 or 3 layer control sheme. Short-term control is purely local (i.e. “primary control”) while the medium & long-term controls are based on means of communication (“secondary” & “tertiary” controls).
The principles of electrical power system operation can be prolonged in the presence of DER
Distributed Energy Resources will impact on the system as a whole as well as transmission and distribution networks, calling for adjustments of both networks’ operation. Built initially for rotating generators, AC power system operating principles are also compatible with power electronic interfaces and demand response.
This means that the present structure of control can be prolonged. To maintain security, the TSO, however, needs more information than before. This mainly concerns status, type and operating point of Distributed Generation (DG) units. Aggregation of DER units is a provisional way to manage this information before the emergence of Smartgrid solutions.
This figure from the EGIDE simulator shows the incidence of non-homogeneous feeders in rural distribution: MV feeder 1 with no generation and MV feeder 2 with micro-CHP installed in Low Voltage. Seven pairs of LV feeders are represented (not visible on the picture), each pair is supplied by a distribution transformer with 90% of micro-CHP installed along one LV feeder and 10% along the other one
This figure shows a more detailed representation of the simulated network. Two LV feeders are represented for each distribution transformer. The equivalent micro-CHP and load are connected at feeders end. Voltage at feeder end correctly represents the voltage drop of continuous distributed load and génération along LV feeder
For a good understanding of networks technical issues the distinction between transmission and distribution networks must be based on technical criteria rather than on administrative limits
Technical issues can be de facto classified into two main categories: local and global. The local ones concern the distribution networks and include overload, voltage control, protection, Power Quality and risk of islanding. The global ones related to system security concern the transmission system and include power–frequency control, voltage–reactive power compensation, system protections, emergency control procedures, defense plans and system restoration.
When studying DER issues, this limit between transmission and distribution must be located where the structural layout of the system is radically changing. This is usually where radially operated medium and low voltage networks connect to the meshed structure of the high voltage transmission system.
A certain amount of DER can be connected to distribution networks following the “fit & forget” principle
Currently the basic design is referred to as “fit & forget” principle. The system is set up for being unconditionally adequate. This means that such network respects voltage, flow and fault current requirements for all possible expected scheduled demand without any intervention from the distribution network operator, except in the case of a fault.
The current design rules depend on load density and are characterised by the duality between overload and voltage control. At distribution level, in medium and low voltage, adequacy is given by sufficient capacity for urban networks or voltage drop margin for rural networks. This means that margins exist in urban and semi-urban networks in terms of voltage control, while few or no margins exist in rural networks.
The available margins thus allow for a connection of a certain amount of DER. Going further is possible but is conditional on load diversity – materialised by the pphysical aggregation through the network- and “coincidence” between load and generation functions (i.e. load curves, generation profiles).
- Jacques DEUSETractebel EngineeringTechnical Director - WP2 Leaderemail
- Olof SAMUELSSONIEA-LTHWP2email
- Christine SCHWAEGERLSIEMENS PTDWP2email
- “Power System and Market Integration of DER, the EU-DEEP1 Approach”, Dr J. Deuse et al. CIRED 2005 Conference, Turin, Italy.
- “EU-DEEP Integrated Project – Technical Implications of the ‘Hosting Capacity’ of the System for DER”, Dr. J. Deuse et al. International Journal of Distributed Energy Resources, Volume 4, Number 1, January 2008.