Power converters for distributed generation ACES Advanced Control - - PowerPoint PPT Presentation

Problem statement Modeling and control challenges References

Power converters for distributed generation

ACES — Advanced Control of Energy Systems

Institute of Industrial and Control Engineering Universitat Polit` ecnica de Catalunya

Keywords: distributed generation, hierarchical models, networked control

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007

Problem statement Modeling and control challenges References Background Modeling approach

Background

Object: distributed power generation (DG) in large or very large (> 100kW) power farms operated by distribution network operators (DNO). Problems posed by small to medium DG plants (< 100kW) are of different nature. Power farm ∼ hundreds of unit of the same kind and size, with maybe a varying connecting topology for optimization. Plants may be made of photovoltaic, wind turbine, biogas turbine, fuel cell, or combined heat-power units.

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007

Problem statement Modeling and control challenges References Background Modeling approach

Background (cont’d)

Each unit (modeled as a controlled source) has an optimal (maximum power) operation point, which may depend on ambient factors. Power must be supplied to the grid in a given form (3-phase signal of given frequency and amplitude, with small harmonic distortion, and maybe with reactive content under some circumstances) ⇒ power quality (PQ) problem. A complex array of converters (both dc-dc (elevators) and dc-ac (inverters)) must be put in between to make the units work at the optimum point and to supply the power in the required form. For large plants, the interaction between converters can give rise to large harmonic pollution.

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007

Problem statement Modeling and control challenges References Background Modeling approach

Background (cont’d)

Special control algorithms must be designed to cope with disconnection or with reactive power injection to help the power grid recover after disconnection of power facilities elsewhere. Complex modeling and control problem, with varying network topology for both the plant and the power grid.

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007

Problem statement Modeling and control challenges References Background Modeling approach

Modeling approach

Model everything as variable structure systems in the PHS framework. Easy to do for the plant, including the converters. The external power grid may be modeled at several levels of complexity, from very simple, small, lumped parameter networks, to very large and complex ones, including genuine distributed phenomena and stochastic perturbations. Complete model: a system with hundreds of state variables and switches, plus the power grid model.

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007

Problem statement Modeling and control challenges References Models for power grid, DG plant and converters DG plant architecture Power converter architecture Control objectives specification Theory issues Modeling software

Models

The plant units can be modeled as controlled sources with parameter dependent iv characteristic curves. PHS models for all kind of converters are available. The local power grid can be modeled in detail, and assume that the rest of the grid is an ideal bus. Connect everything in a switched network modeled as a variable Dirac structure.

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007

Problem statement Modeling and control challenges References Models for power grid, DG plant and converters DG plant architecture Power converter architecture Control objectives specification Theory issues Modeling software

DG plant architecture

Individual power units may be grouped together and connected to a single converter. Rationale: nearby units share the same ambiental factors (for instance a shadowed zone in the photovoltaic case). Reconfigurable plant architecture to cope with failing or less than optimum performance: hierarchical series/parallel network.

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007

Problem statement Modeling and control challenges References Models for power grid, DG plant and converters DG plant architecture Power converter architecture Control objectives specification Theory issues Modeling software

Power converter architecture

Distributed inverters vs central inverter. 3-phase inverters vs monophasic inverters.

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007

Problem statement Modeling and control challenges References Models for power grid, DG plant and converters DG plant architecture Power converter architecture Control objectives specification Theory issues Modeling software

Control objectives

Hierarchical set of dc-dc elevators to make individual units work at optimum point and establish an stable dc bus. Hierarchical set of inverters to deal with PQ issues:

3-phase signal of required waveform. Harmonic pollution reduction.

Overall global strategy to deal with specific situations:

disconnection from the grid. reactive power injection to help the grid recover.

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007

Problem statement Modeling and control challenges References Models for power grid, DG plant and converters DG plant architecture Power converter architecture Control objectives specification Theory issues Modeling software

Theory issues

Modeling of large VSS in the PHS framework, using graph theory tools. ( Valentin, C., et al.) Control of VSS in the PHS framework. Alternative: averaged state models. Formulation of PQ problems in the energy language. Reduced order models in the PHS framework for control design. Hierarchical control strategy.

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007

Problem statement Modeling and control challenges References Models for power grid, DG plant and converters DG plant architecture Power converter architecture Control objectives specification Theory issues Modeling software

Modeling software

Template editor in 20-sim for easy definition of arrays of submodels or discretized distributed systems. Numerical power-crunch for large, possibly stiff systems in 20-sim.

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007

Problem statement Modeling and control challenges References

References

1 Enslin, J.H.R., and Heskes, P.J.M., Harmonic interaction between a large number of distributed power inverters and the distribution network, IEEE Trans. on Power Electronics 19, pp. 1586-1593 (2004). 2 Valentin, C., et al., A port-Hamiltonian formulation of physical switching systems with varying constraints, Automatica (2007), doi: 10.1016/j.automatica.2006.12.022. 3 Graaff, R.A.A., and Enslin, J.H.R., Profitable, plug and play dispersed generation: the future?, Distributed

• generation. Briefing Paper. January 2007. www.leonardo-energy.org

4 IEEE Standards Coordinating Committee 21, IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems (IEEE Std 1547-2003), The Institute of Electrical and Electronics Engineers,

• Inc. (2003).

Proposal for GEOPLEX-II — Bologna meeting 8th June 2007