Publication Detail

DC Distributed Power Systems Analysis, Design and Control for a Renewable Energy System

Per Karlsson
200 pp.

Renewable energy systems are likely to become wide spread in the future due to environmental demands. As a consequence of the dispersed nature of renewable energy systems, this implies that there will be a distributed generation of electric power. Since most of the distributed electrical energy sources do not provide their electric power at line frequency and voltage, a DC
bus is a useful common connection for several such sources. Due to the differences in output voltage among the sources, depending on both the type of source and their actual operating point, the sources are connected to the DC power system via power electronic converters. The intention behind the presented work is not to replace the existing AC power system, but to include local DC power systems. The AC and DC power systems are connected at some points in the network. The renewable energy sources are weak compared
to the present hydro power and nuclear power plants, resulting in a need of power conditioning before the renewable energy is fed to the transmission lines. The benefit of such an approach is that power conditioning is applied on
a central level, i.e. at the interface between the AC and DC power systems.

The thesis starts with an overview of related work. Present DC transmission systems are discussed and investigated in simulations. Then, different methods for load sharing and voltage control are discussed. Especially, the voltage
droop control scheme is examined thoroughly. Since the droop control method does not require any high-speed communication between sources and loads, this is considered the most suitable for DC distributed power systems. The voltage feed back design of the controller also results in a specification of the DC bus capacitors (equivalents to DC link capacitors of single converters) needed for filtering. If the converters in the DC distribution system are equipped with capacitors selected from this design criterion and if the DC bus impedance is neglected, the source converters share the total load equally in
per unit.

The same DC distribution bus configuration is studied in a wind power application. Especially the dynamic properties of load-source interactions are
highlighted. They are interesting since the sources are considered weak for a distributed power system. This is illustrated with simulations where the power is fed from wind turbines only and constant power loads are controlled at the
same time as the DC bus voltage level. The wind power generators are modeled as permanent-magnet synchronous machines. The controller needed for the machines, including position estimation and field weakening, is discussed. To control the DC bus voltage, the available wind power must be higher than the power consumed by the loads and the excess power removed
by pitch angle control. Pitch angle control is a comparably slow process and, therefore, the DC bus voltage controller must handle the transient power distribution.

Personal safety and prevention of property damage are important factors of conventional AC power systems. For the investigated DC power system this is maybe even more important due to the fact that the star point of the sources and loads is left ungrounded or grounded through high impedance. The
difficulty of detecting ground faults arises from the fact that the AC sources
and loads are ungrounded or have high impedance to ground in order to
effectively block zero-sequence currents flowing between the AC systems. A grounding scheme for the DC distribution system together with algorithms for detection of ground faults, are presented. The proposed method detects ground faults on both the AC and DC sides and is extended to cover short circuit faults with a minor work effort.

Two schemes for high voltage interconnection of DC systems are studied. One of them provides galvanic isolation, which is an advantage since elevated voltage might appear in the DC systems otherwise, in the case of a ground fault in the high voltage interconnection.

Experimental verifications follow the theoretical investigations introduced above. First, dynamic properties are studied and the behaviour predicted from theoretical analysis and simulations is verified. Then, load sharing is investigated. Also in this investigation, the experimental results agree with the simulated.

type: Technical reports

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