Balancing Asymmetrical Load Using a Static Var Compensator

- Negative Sequence Assessments and Controller

Abstract

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.

Index Terms: SVC, negative sequence components, voltage unbalance, imbalance, asymmetrical loads

and unbalance assessments.

 Referencess :

1.0  Cahlmers Thesis- weblink 

VSC Transmission Operating Under Unbalanced AC Conditions—Analysis and Control Design

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

single-phase-to-ground fault.

 Reference:

1.0 IEEE Transaction Paper - Weblink