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
Abstract—This paper presents an analysis and a new control
design of a voltage-source converter (VSC) transmission system
operating under unbalanced network conditions. The system
is analyzed in the positive and negative synchronous reference
frames. The proposed control strategy contains a main controller
and an auxiliary controller. The main controller is implemented in
the positive d–q frame using decoupling control without involving
positive/negative-sequence decomposition. The auxiliary controller
is implemented in the negative-sequence d–q frame using
cross-coupling control of negative-sequence current. Simulation
results using the SIMULINK power system blockset show good
performance of the proposed control strategy for a 300-MW
300-kV dc VSC transmission system during both balanced conditions
and unbalanced conditions as may be caused by a solid
1.0 IEEE Transaction Paper - Weblink
This paper presents an overview of the state of the art in reactive
power compensation technologies. The principles of operation, design
characteristics and application examples of Var compensators
implemented with thyristors and self-commutated converters are
presented. Static Var generators are used to improve voltage regulation,
stability, and power factor in ac transmission and distribution
systems. Examples obtained from relevant applications describing
the use of reactive power compensators implemented with newstatic
Var technologies are also described.
Keywords—Reactive power, static Var compensators (SVCs).
1.0 Reactive Power Compensation Technologies: State-of-the-Art Review Weblink
A description of how accurate system modelling of a dc system
with an electromagnetic transient simulation program can be
used to study and correct interbipole oscillations between converters
connected into a parallel multiterminal system is given. The
paper shows how to decide on the detail of modelling that is
required, and demonstrates that it is often not enough to use
prepackaged , generic HVDC control models, supplied with these
programs. The detail of control models that are used in the electromagnetic
simulation programs are shown to have a significant
impact, in some cases, on the simulation results. In particular, a
case of six hertz oscillations, in the dc currents of two converters
on the Nelson River System, is accurately simulated, and it is
shown that control modifications suggested by the simulation
actually do eliminate the interbipole oscillations.
1.0 ADJUSTING CONVERTER CONTROLS FOR PARALLELED DC CONVERTERS
USING A DIGITAL TRANSIENT SIMULATION PROGRAM- Web Link