23 research outputs found
Linear Approximations to AC Power Flow in Rectangular Coordinates
This paper explores solutions to linearized powerflow equations with
bus-voltage phasors represented in rectangular coordinates. The key idea is to
solve for complex-valued perturbations around a nominal voltage profile from a
set of linear equations that are obtained by neglecting quadratic terms in the
original nonlinear power-flow equations. We prove that for lossless networks,
the voltage profile where the real part of the perturbation is suppressed
satisfies active-power balance in the original nonlinear system of equations.
This result motivates the development of approximate solutions that improve
over conventional DC power-flow approximations, since the model includes ZIP
loads. For distribution networks that only contain ZIP loads in addition to a
slack bus, we recover a linear relationship between the approximate voltage
profile and the constant-current component of the loads and the nodal active
and reactive-power injections
QuickFlex: a Fast Algorithm for Flexible Region Construction for the TSO-DSO Coordination
Most of the new technological changes in power systems are expected to take
place in distribution grids. The enormous potential for distribution
flexibility could meet the transmission system's needs, changing the paradigm
of generator-centric energy and ancillary services provided to a demand-centric
one, by placing more importance on smaller resources, such as flexible demands
and electric vehicles. For unlocking such capabilities, it is essential to
understand the aggregated flexibility that can be harvested from the large
population of new technologies located in distribution grids. Distribution
grids, therefore, could provide aggregated flexibility at the transmission
level. To date, most computational methods for estimating the aggregated
flexibility at the interface between distribution grids and transmission grids
have the drawback of requiring significant computational time, which hinders
their applicability. This paper presents a new algorithm, coined as QuickFlex}
for constructing the flexibility domain of distribution grids. Contrary to
previous methods, a priory flexibility domain accuracy can be selected. Our
method requires few iterations for constructing the flexibility region. The
number of iterations needed is mainly independent of the distribution grid's
input size and flexible elements. Numerical experiments are performed in four
grids ranging from 5 nodes to 123 nodes. It is shown that QuickFlex outperforms
existing proposals in the literature in both speed and accuracy
Experimental Validation of Feedback Optimization in Power Distribution Grids
We consider the problem of controlling the voltage of a distribution feeder
using the reactive power capabilities of inverters. On a real distribution
grid, we compare the local Volt/VAr droop control recommended in recent grid
codes, a centralized dispatch based on optimal power flow (OPF) programming,
and a feedback optimization (FO) controller that we propose. The local droop
control yields suboptimal regulation, as predicted analytically. The OPF-based
dispatch strategy requires an accurate grid model and measurement of all loads
on the feeder in order to achieve proper voltage regulation. However, in the
experiment, the OPF-based strategy violates voltage constraints due to
inevitable model mismatch and uncertainties. Our proposed FO controller, on the
other hand, satisfies the constraints and does not require load measurements or
any grid state estimation. The only needed model knowledge is the sensitivity
of the voltages with respect to reactive power, which can be obtained from
data. As we show, an approximation of these sensitivities is also sufficient,
which makes the approach essentially model-free, easy to tune, compatible with
the current sensing and control infrastructure, and remarkably robust to
measurement noise. We expect these properties to be fundamental features of FO
for power systems and not specific to Volt/VAr regulation or to distribution
grids
Fully Distributed Peer-to-Peer Optimal Voltage Control with Minimal Model Requirements
This paper addresses the problem of voltage regulation in a power
distribution grid using the reactive power injections of grid-connected power
inverters. We first discuss how purely local voltage control schemes cannot
regulate the voltages within a desired range under all circumstances and may
even yield detrimental control decisions. Communication and, through that,
coordination are therefore needed. On the other hand, short-range peer-to-peer
communication and knowledge of electric distances between neighbouring
controllers are sufficient for this task. We implement such a peer-to-peer
controller and test it on a 400~V distribution feeder with asynchronous
communication channels, confirming its viability on real-life systems. Finally,
we analyze the scalability of this approach with respect to the number of
agents on the feeder that participate in the voltage regulation task