448 research outputs found
Optimal Control Design for Multiterminal HVDC
This thesis proposes an optimal-control based design for distributed frequency control in multi-terminal high voltage direct current (MTDC) systems. The current power grid has become overstressed by rapid growth in the demand for electric power and penetration of renewable energy. To address these challenges, MTDC technology has been developed, which has the potential to increase the flexibility and reliability of power transmission in the grid. Several control strategies have been proposed to regulate the MTDC system and its interaction with connected AC systems. However, all the existing control strategies are based on proportional and integral (PI) control with predetermined controller structures. The objective of the thesis is to first determine if existing control structures are optimal, and if improved controller structures can be developed.The thesis proposes a general framework to determine the optimal structure for the control system in MTDC transmission through optimal feedback control. The proposed method is validated and demonstrated using an example of frequency control in a MTDC system connecting five AC areas
Control of multi-terminal HVDC networks towards wind power integration: A review
© 2015 Elsevier Ltd.
More interconnections among countries and synchronous areas are foreseen in order to fulfil the EU 2050 target on the renewable generation share. One proposal to accomplish this challenging objective is the development of the so-called European SuperGrid. Multi-terminal HVDC networks are emerging as the most promising technologies to develop such a concept. Moreover, multi-terminal HVDC grids are based on highly controllable devices, which may allow not only transmitting power, but also supporting the AC grids to ensure a secure and stable operation. This paper aims to present an overview of different control schemes for multi-terminal HVDC grids, including the control of the power converters and the controls for power sharing and the provision of ancillary services. This paper also analyses the proposed modifications of the existing control schemes to manage high participation shares of wind power generation in multi-terminal grids.Postprint (author's final draft
Distributed Secondary Frequency Control through MTDC Transmission Systems
In this paper, we present distributed controllers for sharing primary and
secondary frequency control reserves for asynchronous AC transmission systems,
which are connected through a multi-terminal HVDC grid. By using Lyapunov
arguments, the equilibria of the closed-loop system are shown to be globally
asymptotically stable. We quantify the static errors of the voltages and
frequencies, and give upper bounds for these errors. It is also shown that the
controllers have the property of power sharing, i.e., primary and secondary
frequency control reserves are shared fairly amongst the AC systems. The
proposed controllers are applied to a high-order dynamic model of of a power
system consisting of asynchronous AC grids connected through a six-terminal
HVDC grid.Comment: arXiv admin note: text overlap with arXiv:1409.801
Control of MTDC Transmission Systems under Local Information
High-voltage direct current (HVDC) is a commonly used technology for
long-distance electric power transmission, mainly due to its low resistive
losses. In this paper a distributed controller for multi-terminal high-voltage
direct current (MTDC) transmission systems is considered. Sufficient conditions
for when the proposed controller renders the closed-loop system asymptotically
stable are provided. Provided that the closed loop system is asymptotically
stable, it is shown that in steady-state a weighted average of the deviations
from the nominal voltages is zero. Furthermore, a quadratic cost of the current
injections is minimized asymptotically
Distributed Primary Frequency Control through Multi-Terminal HVDC Transmission Systems
This paper presents a decentralized controller for sharing primary AC
frequency control reserves through a multi-terminal HVDC grid. By using
Lyapunov arguments, the proposed controller is shown to stabilize the
equilibrium of the closed-loop system consisting of the interconnected AC and
HVDC grids, given any positive controller gains. The static control errors
resulting from the proportional controller are quantified and bounded by
analyzing the equilibrium of the closed-loop system. The proposed controller is
applied to a test grid consisting of three asynchronous AC areas interconnected
by an HVDC grid, and its effectiveness is validated through simulation
Dynamical Decentralized Voltage Control of Multi-Terminal HVDC Grids
High-voltage direct current (HVDC) is a commonly used technology for
long-distance electric power transmission, mainly due to its low resistive
losses. When connecting multiple HVDC lines into a multi-terminal HVDC (MTDC)
system, several challenges arise. To ensure safe and efficient operation of
MTDC systems, the voltage of all terminals need to be steered to within an
operational range. In this paper we study the commonly used decentralized
voltage droop controller, and show that it in general does not steer the
voltages to within the operational range. We propose a decentralized PI
controller with deadband, and show that it always steers the voltages to within
the operational range regardless of the loads. Additionally we show that the
proposed controller inherits the property of proportional power sharing from
the droop controller, provided that both the loads and the line resistances are
sufficiently low. The results are validated through simulation in MATLAB
Stabilization of MT-HVDC grids via passivity-based control and convex optimization
This paper presents a model for stabilizing multi-terminal high voltage direct-current (MT-HVDC) networks with constant power terminals (CPTs) interfaced with power electronic converters. A hierarchical structure of hierarchical control is developed, which guarantees a stable operation under load variations. This structure includes a port-Hamiltonian formulation representing the network dynamics and a passivity-based control (PBC) for the primary control. This control guarantees stability according to Lyapunov’s theory. Next, a convex optimal power flow formulation based on semidefinite programming (SDP) defines the control’s set point in the secondary/ tertiary control. The proposed stabilization scheme is general for both point-to-point HVDC systems and MTHVDC grids. Simulation results in MATLAB/Simulink demonstrate the stability of the primary control and the optimal performance of the secondary/tertiary control, considering three simulation scenarios on a reduced version of the CIGRE MT-HVDC test system: (i) variation of generation and load, (ii) short-circuit events with different fault resistances and (iii) grid topology variation. These simulations prove the applicability and efficiency of the proposed approach
Distributed Voltage and Current Control of Multi-Terminal High-Voltage Direct Current Transmission Systems
High-voltage direct current (HVDC) is a commonly used technology for
long-distance power transmission, due to its low resistive losses and low
costs. In this paper, a novel distributed controller for multi-terminal HVDC
(MTDC) systems is proposed. Under certain conditions on the controller gains,
it is shown to stabilize the MTDC system. The controller is shown to always
keep the voltages close to the nominal voltage, while assuring that the
injected power is shared fairly among the converters. The theoretical results
are validated by simulations, where the affect of communication time-delays is
also studied
A power consensus algorithm for DC microgrids
A novel power consensus algorithm for DC microgrids is proposed and analyzed.
DC microgrids are networks composed of DC sources, loads, and interconnecting
lines. They are represented by differential-algebraic equations connected over
an undirected weighted graph that models the electrical circuit. A second graph
represents the communication network over which the source nodes exchange
information about the instantaneous powers, which is used to adjust the
injected current accordingly. This give rise to a nonlinear consensus-like
system of differential-algebraic equations that is analyzed via Lyapunov
functions inspired by the physics of the system. We establish convergence to
the set of equilibria consisting of weighted consensus power vectors as well as
preservation of the weighted geometric mean of the source voltages. The results
apply to networks with constant impedance, constant current and constant power
loads.Comment: Abridged version submitted to the 20th IFAC World Congress, Toulouse,
Franc
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