Today's electric power systems are increasingly facing the consequences of a project, i.e.,
low-carbon electricity production (the increase of electricity production from wind and solar
power plants, the closing of coal and nuclear power plants), which has a significant impact on
the redistribution of power flows in the existing electric power networks. One of the essential components of smart grids (which is a prerequisite for the integration of more renewable energy in modern electric power systems) is the electronic devices that can properly and dynamically regulate the parameters of the electric power systems (power flows, voltage).
FACTS technology has been developed, but at the beginning, mainly because of the price, it
only provided more or less theoretical answers to the problems in an EPS. Today, when the
costs of blackouts in electric power systems can rise to billions of dollars and power-thyristor technology is more affordable, FACTS technology is often employed in electric power systems.
The FACTS devices are distinguished by their operating speed (operating frequency cycle or
even less), the possibility to change the network admittance and the voltage phasors, which in turn affects the power flow. Depending on the construction, operating principles and models, FACTS devices may be classified as:
• controllable series compensation – CSC,
• static synchronous series compensator – SSSC,
• static var compensator – SVC,
• static synchronous compensator – STATCOM,
• unified power flow controller – UPFC
• and the others which are not the subject of this thesis.
The power flow through transmission lines depends not only on the static network parameters (admittances), but also on the location of the load centres and on the location of the production. Despite sufficient electricity production, the transmission lines can transmit only a limited amount of active and reactive electric power, which means that not only adequate production, but also its proper location, is important for a reliable supply. The equilibrium for the production and consumption of reactive power is greatly influenced by the location of the production units in the electric power system, which should be as close as possible to the place of consumption. A lack of reactive power can lead to voltage instability, which, in the worst case, can cause a system blackout.
The consequences of significant blackouts due to voltage collapse are still in the memory and
they can occur again if they are not predicted during the planning and operation of a power
system. Voltage stability is considered in terms of the extension of the disturbance (small,
large disturbance) and the duration: long-term and short-term voltage instability. Long-term
voltage instability includes the operations of on load tap changers, thermostatically controlled loads, generator excitation limiters, and the duration is from a few minutes to a few hours.
When dealing with such a slow phenomenon the solving of algebraic equations for a
calculation of the power flow is sufficient, as is the use of methods that do not take into
account the time component. In terms of the steady-state analysis the speed of FACTS devices is not relevant, as the occurrence of a slow voltage instability can be avoided by appropriate (“hand”) measures, if they are available. Short-term voltage instability is based on time dependant (dynamic) load models and differential algebraic equations. Some researches show the possibility of implementation of the direct methods into short-term voltage analysis, which are well-known approach in the (angle) transient-stability studies.
The doctoral thesis treats the topic of voltage stability in terms of the possibility to apply the
direct methods for the voltage stability estimation and the impact of FACTS-devices on the
voltage profile in an electric transmission power system. Direct methods for the analysis of
the transient-stability of power systems with FACTS devices are well-studied, so we focused
on their suitability for analysis of voltage stability by selecting the appropriate time-dependant (dynamic) load characteristics which is explained in the present thesis.
FACTS devices can vary the voltage profile of the transmission network, which we proved
with the analytical derivation of equations based on power system model with FACTS
devices. Application of analytical equations allows fast and accurate calculation of PU curves
for any value of controllable parameters of FACTS-devices. In the thesis is shown that the
surface area in PU diagram covered between the boundaries of the permissible voltage can be used for the evaluation criteria of the effectiveness of FACTS-device for achieving a stable voltage profile.
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