25-09-2014, 10:19 AM
DESIGN AND SIMULATION OF VSC FOR HVDC
TRANSMISSIONS
DESIGN AND IMPLIMENTATION.ppt (Size: 677 KB / Downloads: 24)
ABSTRACT
Voltage source converter HVDC technology have huge potential for the
modern world.The increasing rating and improved performance of self
commutated semiconductor devices have made possible High voltage dc(HVDC)
transmission based on voltage sourced converter(VSC).HVDC technology is an
efficient and feasible method to transmit large amounts of electrical power over
long distances.
The principal characteristic of VSC-HVDC transmission is its ability to
independently control the reactive and real power flow at each of the Ac system to
which it is connected ,at the point of common coupling(PCC). The main function
of the VSC-HVDC is to transmit constant DC power from the rectifier to the
inverter.The main objective of this paper is to design and develop a simple
molecule through an advanced process known as Simulation .
This paper deals about the modelling of VSC based HVDC transmission
line.The VSC based HVDC transmission system consists of two converter stations
connected by a DC cable.VSC-HVDC is currently available for small to medium
scale power transmission applications.It is a new DC transmission system
technology.It has several advantages such as Mitigation of power quality
disturbances, No contribution to short circuit currents, Reduced risk of
commutation failures.
HISTORICAL BACKGROUND
It can be said that it was in 1882 when the era of transmission of electric
energy began and it was precisely based on dc transmission. The Pearl Street
Station provided New York's Financial district with electrical energy produced by
the dynamos that Thomas Alva Edison's laboratory had developed.In November
December 1887, Nikola Tesla introduced a system for alternating current
generators, transformers, motors, wires and lights.The advantages of transmitting
ac electrical energy over long distances compared to dc transmission were made
clear in this very early beginings. The dominant position of dc transmission was
brief and some decades later ac transmission overtook the lead.
During the decade of 1930s dc transmission began to be considered again as
an option of transmitting electrical energy, but this time at high voltage levels.
Important research efforts on High Voltage Direct Current (HVDC) converter
technology were undertaken in this decade. In the decade of 1950s HVDC
technology was boosted probably because the technology seemed to be
commercially feasible. In fact in 1950 the first commercial order for an HVDC
system was given to ASEA by Vattenfall for a 20 MW, 100 km
undersea cable between the Swedish mainland and the island of Gotland.The
project was commissioned in March 1954. The converters in this project
were based in mercury arc-valves.
CHANGES IN POWER SYSTEMS
Electricity markets and environmental rules are two main factors that have led
the implementation of major changes in the way transmission systems are
designed, constructed and operated. For instance, electricity markets have made
utilities to perform an optimization of the assets in order to remain competitive
and survive. On the other hand strong environmental rules have made it more
difficult to obtain licenses for the construction of new electrical projects, specially
overhead transmission lines.
The demand for electric energy is in permanent growth. Although the growing
trend slows down in periods of worldwide economic crisis, as the one experienced
OBJECTIVE AND SCOPE
The main aim of this project is to analyze the impact of a VSC-HVDC
transmission on a power system, when it is embedded in the transmission
network.More specific objectives defining the scope of this research are:
• To derive control strategies for damping electromechanical oscillations
using linear control, nonlinear control and model predictive control.
• To analyze VSC-HVDC transmissions' electromechanical oscillations
damping capabilities by controlling either active power or reactive
power or both simultaneously.
• To use the single Machine Equivalent (SIME) method within control
strategies and to find a possible relation between the information
obtained from SIME and modal analysis.
• To analyze the use of local and remote information within control
strategies.
• To analyze possible interactions between the control system of
VSC HVDC transmissions and other controllers of the power systems
under the different modes of operation.
1 POWER SYSTEM MODELING
The basic structure of an electrical power system can be divided into three
main parts: Generation, Transmission and Distribution. The different parts of the
power system operate at different voltage levels. Typically generation and
distribution are classified within the medium voltage range (between 1 kV and
100 kV). Transmission is classified in high voltage (between 100 kV and 300 kV)
or extra-high voltage (between 300 kV and above).
Generation is produced by converting mechanical energy appearing on the
shaft of turbines into electrical energy. This conversion is almost universally done
by the use of synchronous generators. The synchronous generators feed their
electrical power into the transmission system via a step-up transformer.
FUTURE WORK
The following is a list of ideas that can be considered for future work
• Power system stability improvement can be achieved when control strategies are
used for modulation of active power and reactive power in VSCs. Future work
could determine an adaptive controller that finds the optimum relation between
how much active power and how much reactive power should be modulated in the
event of a disturbance.This optimum relation would translate into an improvement
of stability margins.
28• An initial approach to find a relation between SIME method and modal analysis
was carried out. However this relation was found for a specific operating point
only (in the case of modal analysis) and for some specific faults (in the case of the
SIME method). A deeper analysis and a more general relation between could be
developed.
• An approach to coordinate wind farms and POD from the VSC-HVDC
transmission was presented. The proposed coordination basically considered the
wind power production and the gain of the control strategy for active power. A
larger scale coordination of the controllers can be pursued. Coordination wind
power production, POD control of wind power, POD control and voltage control
of the VSC-HVDC transmission can be included in a more general approach.
• MPC showed to be effective for damping of electromechanical oscillations.
Future work in this area can focus on how to avoid the small ripple that resulted
after having damped the first swings occurring after a disturbance.
• MPC assumes that the control has access to all state variables in the power
system. An interesting improvement in the MPC-based control strategy is to make
an analysis on how to reduce the number of states variables without sacrificing
damping effectiveness.
• It would be interesting to perform an analysis of the effectiveness of the derived
control strategies in a real time digital simulator (for instance OPAL-RT simulator
available at KTH). Moreover a further analysis of the issues found in the reactive
power control mode in weak areas could also be done in the simulator