In English

Balancing Asymmetrical Load Using a Static Var Compensator

Amar Alsulami
Göteborg : Chalmers tekniska högskola, 2014. 78 s.
[Examensarbete på avancerad nivå]

In practical power systems, it is normally impossible to maintain perfect balance or symmetry in phase voltages and currents. The diversity of the load, such as single phase, arc furnaces and railway, enhances the amount of unbalance or negative sequence components. Alongside asymmetrical load, further increase in negative sequence components is introduced due to the network inherent asymmetry i.e. untransposed transmission. Thus, they are usually excessive and exceeds standards at weak nodes in the network. Utilities and customers have to comply certain code agreements to limit the degree of negative sequence components in the network. This is because that negative sequence components cause deterioration to the network equipments. For instance, higher loss, torque oscillation, speed reduction and excessive rotor heat are undesirable obstacles to rotating machines. Asymmetrical load compensator can be based on passive elements, i.e. inductors and capacitors such as a Static Var Compensator (SVC), or a Voltage Source Converter (VSC) such as a Static Synchronous Compensator (STATCOM). The utilization of the negative sequence controller, an SVC or a STATCOM provides, gains a significant interest to most utilities around the globe. The compensators basically inject a different capacitive or inductive negative sequence current that has an opposite phase of the load negative sequence current. As a result, the network see symmetrical load and phase voltages and currents are balanced without exchanging active power between the network and the compensator. The thesis investigates the benefits of the SVC negative sequence controller to a network with a detailed description about the SVC characteristics and control components. The thesis also presents unbalance assessment methods implemented in practical networks during the planning stage. Moreover, drawbacks of negative sequence components to network apparatus i.e. rotating machines and transmission lines are summarized. The analysis is carried out using PSCAD for a simple network representation and IEEE 14 bus system. The result illustrates that the SVC allows utilities to balance asymmetrical loads to mitigate negative sequence components. The SVC response to balance asymmetrical load depends on load type, network strength and sources of unbalance. The SVC exhibits a very fast response to reduce the negative sequence components in extreme cases of unbalance such as asymmetrical short circuit. The SVC in general can mitigate negative sequence components caused by a sources connected in the same bus which means that the SVC provides local balancing only. Besides the negative sequence controller, the SVC enable a power factor correction by compensating for the reactive components of the load positive sequence current.

Nyckelord: SVC, negative sequence components, voltage unbalance, imbalance, asymmetrical loads



Publikationen registrerades 2014-01-23. Den ändrades senast 2015-04-16

CPL ID: 193013

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