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Online Voltage Stability Monitoring and Coordinated Secondary Voltage Control

Duong, Dinh Thuc
Doctoral thesis
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URI
http://hdl.handle.net/11250/2410911
Date
2016
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  • Institutt for elkraftteknikk [2665]
Abstract
The development towards increasing use of renewable energy sources and smart grid

applications represent a paradigm shift in power system operation. Due to the large share

of variable generation and more dynamic load patterns, operation conditions in the future

grid will change faster than today’s situation. This will render the grid less predictable and

consequently affect system operation, and voltage stability will undoubtedly remain one

of the major concerns. In this scenario, it is necessary to have new real-time monitoring

tools to timely identify operational limits in order to devise preventive and corrective

schemes to maintain security of operation.

Aiming at developing new online monitoring tools for power systems, the PhD project

has produced two methods to assess voltage stability in real time. Both are based on estimation

of the Thevenin impedance and consequently the maximum power transfer. The

first one estimates the Thevenin impedance by only local phasor measurements; meanwhile,

the second approach obtains the Thevenin impedance by combination of system

topology and measurements from phasor measurement units (PMUs). Although the second

method takes the system topology into its calculation, it requires only information

of the studied area, which is quite small since it is limited by boundary nodes. Therefore,

both proposed methods are suitable for online implementation. Through simulations

of the dynamic model of the Norwegian transmission system, the two algorithms result

in comparable estimations of the Thevenin impedance, and both successfully detect the

margin to voltage instability. In addition, with the topology-based method, it is able to

estimate the post-contingency Thevenin impedance in real time. Thanks to this capability,

it is possible to issue early warning of impact of critical disturbances on voltage

stability, which is vital for the secure and reliable operation of power systems.

Apart from the estimation of the Thevenin impedance, the PhD work also introduces a

new indicator called S-Z sensitivity indicator (S-ZI) for online voltage stability monitoring.

The algorithm for the new indicator is simple and requires only PMU measurements of

voltage and current of the considered load. Since the S-ZI is computed directly from the

local phasor measurements, the method works robustly; it does not face any problems

with divergence as often experienced in other approaches. The proposed indicator also

shows a good performance in detecting the margin to the voltage stability limit, with both

simulation and real PMU data. In addition, the S-ZI also functions as a calibrating tool to

verify accuracy of the online estimated Thevenin impedance. Based on this new indicator

and the proposed algorithm for estimation of the Thevenin impedance, a prototype of

online voltage stability monitoring has been built and tested with live stream of PMU

data obtained from the Norwegian transmission system. The results from the tests at two

locations in the 420kV and 130kV networks have shown that the proposed methods have

performed very well with real measurements in power systems.

Additionally, a scheme for coordinated secondary voltage control for systems with multiple VAr reserves has been introduced in the PhD project. The proposed scheme is

inspired by the concept of multi-agent system. Like an agent, the local controller takes on

the assigned tasks itself and contacts its neighbors for support when needed. The control

structure incorporates not only controllable VAr sources but also mechanically switched

capacitor banks and reactors, resulting in increased online reactive power reserves for

critical contingencies. The approach is suitable for areas with high penetration of FACTS

devices, distributed generation connected to power systems through voltage source converters

(VSCs), or VSC-HVDC systems. Moreover, the proposed scheme is also simple

and flexible in terms of coordination, implementation and expansion. Through simulations,

the control scheme has shown a good performance in coordinating reactive power

sources to obtain a flat voltage profile and sufficient reactive power reserves, not only in

normal operation but also under disturbance conditions.
Publisher
NTNU
Series
Doctoral thesis at NTNU;2016:242

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