John Godsk Nielsen

A detailed comparison of system topologies for dynamic voltage restorers

In this paper, four different system topologies for dynamic voltage restorers (DVRs) are analyzed and tested, with particular focus on the methods used to acquire the necessary energy during a voltage sag. Comparisons are made between two topologies that can be realized with a minimum amount of energy storage, with energy taken from the grid during the voltage sag, and two topologies that take energy from stored energy devices during the voltage sag. Experimental tests using a 10-kVA DVR show that the no-energy storage concept is feasible, but an improved performance can be achieved for certain voltage sags using stored energy topologies. The results of this comparison rank the no-storage topology with a passive shunt converter on the load side first, followed by the stored energy topology with a constant dc-link voltage.

Control strategies for dynamic voltage restorer compensating voltage sags with phase jump

Voltage sags are an important power quality problem and the dynamic voltage restorer is known as an effective device to mitigate voltage sags. In this paper, different control strategies for a dynamic voltage restorer are analyzed with emphasis put on the compensation of voltage sags with phase jump. Voltage sags accompanied by a phase jump are in some cases more likely to trip loads and a satisfactory voltage compensation are more difficult to achieve. Different control methods to compensate voltage sags with phase jump are proposed and compared. Two promising control methods are tested with simulations carried out in Saber and finally tested on a 10 kVA rated dynamic voltage restorer in the laboratory. Both methods can be used to reduce load voltage disturbances caused by voltage sags with phase jump. One method completely compensates the phase jump, which is the best solution for very sensitive loads. The second method does only partly compensate the phase jump, but it is expected to have a better performance in compensating a broader range of voltage sags.

Control and testing of a dynamic voltage restorer (DVR) at medium voltage level

The dynamic voltage restorer (DVR) has become popular as a cost effective solution for the protection of sensitive loads from voltage sags. Implementations of the DVR have been proposed at both a low voltage (LV) level, as well as a medium voltage (MV) level; and give an opportunity to protect high power sensitive loads from voltage sags. This paper reports practical test results obtained on a medium voltage (10 kV) level using a DVR at a distribution test facility in Kyndby, Denmark. The DVR was designed to protect a 400 kVA load from a 0.5 p.u. maximum voltage sag. The reported DVR verifies the use of a combined feed-forward and feed-back technique of the controller and it obtains both good transient and steady state responses. The effect of the DVR on the system is experimentally investigated under both faulted and nonfaulted system states, for a variety of linear and nonlinear loads. Variable duration voltage sags were created using a controllable LV breaker fed by a 630 kVA distribution transformer placed upstream of the sensitive load. The fault currents in excess of 12 kA were designed and created to obtain the required voltage sags.The dynamic voltage restorer (DVR) has become popular as a cost effective solution for the protection of sensitive loads from voltage sags. Implementations of the DVR have been proposed at both a low voltage (LV) level, as well as a medium voltage (MV) level; and give an opportunity to protect high power sensitive loads from voltage sags. This paper reports practical test results obtained on a medium voltage (10 kV) level using a DVR at a distribution test facility in Kyndby, Denmark. The DVR was designed to protect a 400-kVA load from a 0.5-p.u. maximum voltage sag. The reported DVR verifies the use of a combined feed-forward and feed-back technique of the controller and it obtains both good transient and steady-state responses. The effect of the DVR on the system is experimentally investigated under both faulted and nonfaulted system states, for a variety of linear and nonlinear loads. Variable duration voltage sags were created using a controllable LV breaker fed by a 630 kVA distribution transformer placed upstream of the sensitive load. The fault currents in excess of 12 kA were designed and created to obtain the required voltage sags. It is concluded the DVR works well in all operating conditions.

Comparison of system topologies for dynamic voltage restorers

In this paper, different system topologies for dynamic voltage restorers are analyzed with the emphasis put on methods to acquire the necessary energy during a voltage sag. Four different system topologies are analyzed and tested. Two, which can be realized with insignificant energy storage and the energy is taken from the grid and two topologies, which are based on stored energy are compared. Experimental tests on a 10 kVA rated dynamic voltage restorer shows that the no-energy storage concept is feasible, but an improved performance can for certain voltage sags, be achieved with stored energy. In the comparison, the no-storage topology with a passive shunt converter at the load side is ranked highest followed by an energy storage topology with constant DC-link voltage.

Control and test of dynamic voltage restorer on the medium voltage grid

Power quality problems like voltage dips and sags are presently important issues in the utilities. Major economic losses might appear at the customers due to those failures. This paper presents a power electronic device, which keeps voltage sags away from sensitive loads. It is a dynamic voltage restorer (DVR), which creates a series injection of voltage into the medium voltage grid. Energy storage is keeping the sensitive load running in a limited time. The design and a new control method of a DVR are explained in this paper. The DVR is also tested at medium voltage level (10 kV) and methods to initiate voltage dips are described and implemented. Different test results at 10 kV level show that the DVR keep the voltage to a sensitive load fixed in the case of voltage dips. Furthermore, it is also demonstrated that in the case of a transformer failure the system also works very fine.

Local compensation in distribution network grids

At Aalborg University a project concerning local compensation of the distribution network was started in 1998. The main focus was to see how to employ and control custom power systems (CUPS) in the distribution network, to build a CUPS-device to clean out some of the disturbances and to see how the disturbances were transmitted through the network grid. The project was build up as three separate projects: design and control of a dynamic voltage restorer, employment and control of CUPS in the distribution network and broadband modelling of the distribution network. The main results from the projects are reported in this paper.