Monday, 04 July 2016 12:04
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
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.
Index Terms: SVC, negative sequence components, voltage unbalance, imbalance, asymmetrical loads
and unbalance assessments.
1.0 Cahlmers Thesis- weblink