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Abstract—This paper introduces and evaluates an auxiliary control
strategy for downstream fault current interruption in a radial
distribution line by means of a dynamic voltage restorer (DVR).
The proposed controller supplements the voltage-sag compensation
control of the DVR. It does not require phase-locked loop and
independently controls the magnitude and phase angle of the injected
voltage for each phase. Fast least error squares digital filters
are used to estimate the magnitude and phase of the measured
voltages and effectively reduce the impacts of noise, harmonics,
and disturbances on the estimated phasor parameters, and this enables
effective fault current interrupting even under arcing fault
conditions. The results of the simulation studies performed in the
PSCAD/EMTDC software environment indicate that the proposed
control scheme: 1) can limit the fault current to less than the nominal
load current and restore the point of common coupling voltage
within 10 ms; 2) can interrupt the fault current in less than two cycles;
3) limits the dc-link voltage rise and, thus, has no restrictions
on the duration of fault current interruption; 4) performs satisfactorily
even under arcing fault conditions; and 5) can interrupt the
fault current under low dc-link voltage conditions.
INTRODUCTION
T HE DYNAMIC voltage restorer (DVR) is a custom power
device utilized to counteract voltage sags [1], [2]. It injects
controlled three-phase ac voltages in series with the supply
voltage, subsequent to a voltage sag, to enhance voltage quality
by adjusting the voltage magnitude, waveshape, and phase angle
[3]–[6]. Fig. 1 shows the main components of a DVR (i.e., a series
transformer , a voltage- source converter (VSC), a harmonic
filter, a dc-side capacitor , and an energy storage device
[7], [8]). The line-side harmonic filter [5] consists of the
leakage inductance of the series transformer and the filter
capacitor .
The DVR is conventionally bypassed during a downstream
fault to prevent potential adverse impacts on the fault and to
protect the DVR components against the fault current [9]–[11].
A technically elaborate approach to more efficient utilization of
the DVR is to equip it with additional controls and enable it also to limit or interrupt the downstream fault currents. A control
approach to enable a DVR to serve as a fault current limiter
is provided in [9]. The main drawback of this approach is
that the dc-link voltage of the DVR increases due to real power
absorption during fault current-limiting operation and necessitates
a switch to bypass the DVR when the protective relays,
depending on the fault conditions, do not rapidly clear the fault.
The dc-link voltage increase can be mitigated at the cost of a
slow-decaying dc fault current component using the methods
introduced in [7] and [12].
To overcome the aforementioned limitations, this paper proposes
an augmented control strategy for the DVR that provides:
1) voltage-sag compensation under balanced and unbalanced
conditions and 2) a fault current interruption (FCI) function. The
former function has been presented in [13] and the latter is described
in this paper.
It should be noted that limiting the fault current by the DVR
disables the main and the backup protection (e.g., the distance
and the overcurrent relays). This can result in prolonging the
fault duration. Thus, the DVR is preferred to reduce the fault
current to zero and interrupt it and send a trip signal to the upstream
relay or the circuit breaker (CB).
It should be noted that the FCI function requires 100%
voltage injection capability. Thus, the power ratings of the
series transformer and the VSC would be about three times
those of a conventional DVR with about 30%–40% voltage
injection capability. This leads to a more expensive DVR
system. Economic feasibility of such a DVR system depends
on the importance of the sensitive load protected by the DVR
and the cost of the DVR itself.
The performance of the proposed control scheme is evaluated
through various simulation studies in the PSCAD/EMTDC
platform. The study results indicate that the proposed control
strategy: 1) limits the fault current to less than the nominal load
current and restores the PCC voltage within less than 10 ms, and
interrupts the fault current within two cycles; 2) it can be used
in four- and three-wired distribution systems, and single-phase configurations; 3) does not require phase-locked loops; 4) is not
sensitive to noise, harmonics, and disturbances and provides effective
fault current interruption even under arcing fault conditions;
and 5) can interrupt the downstream fault current under
low dc-link voltage conditions.
II. PROPOSED FCI CONTROL STRATEGY
The adopted DVR converter is comprised of three independent
H-bridge VSCs that are connected to a common dc-link
capacitor. These VSCs are series connected to the supply grid,
each through a single-phase transformer. The proposed FCI
control system consists of three independent and identical
controllers one for each single-phase VSC of the DVR.
Assume the fundamental frequency components of the supply
voltage , load voltage , and the injected voltage , Fig. 1
are
(1)
(2)
(3)
Two identical least error squares (LES) filters [14] are used
to estimate the magnitudes and phase angles of the phasors
corresponding to and (i.e., and
, respectively in 5 ms [13]).
The FCI function requires a phasor parameter estimator (digital
filter) which attenuates the harmonic contents of the measured
signal. To attenuate all harmonics, the filter must have a
full-cycle data window length which leads to one cycle delay in
the DVR response. Thus, a compromise between the voltage injection
speed and disturbance attenuation is made. The designed
LES filters utilize a data window length of 50 samples at the
sampling rate of 10 kHz and, hence, estimate the voltage phasor
parameters in 5 ms. Fig. 3 depicts the frequency response of the
LES filters and indicates significant attenuation of voltage noise,
harmonics, and distortions at frequencies higher than 200 Hz
and lower than 50 Hz. Reference [13] demonstrates the effectiveness
of this filter in attenuating the noise, harmonics, and
distortions for the sag compensation mode of operation as well.
The next section shows that this filter also performs satisfactorily
in the FCI operation mode, even under arcing fault conditions
where the measured voltage and current signals are highly
distorted.
Fig. 2 shows a per-phase block diagram of the proposed DVR
control system corresponding to the FCI operation mode, where
is the nominal rms phase voltage.
CONCLUSION
This paper introduces an auxiliary control mechanism to enable
the DVR to interrupt downstream fault currents in a radial
distribution feeder. This control function is an addition to
the voltage-sag compensation control of the DVR. The performance
of the proposed controller, under different fault scenarios,
including arcing fault conditions, is investigated based
on time-domain simulation studies in the PSCAD/EMTDC environment.
The study results conclude that:
• the proposed multiloop control system provides a desirable
transient response and steady-state performance and effectively
damps the potential resonant oscillations caused by
the DVR LC harmonic filter;
• the proposed control system detects and effectively interrupts
the various downstream fault currents within two cycles
(of 50 Hz);
• the proposed fault current interruption strategy limits the
DVR dc-link voltage rise, caused by active power absorption,
to less than 15% and enables the DVR to restore
the PCC voltage without interruption; in addition, it interrupts
the downstream fault currents even under low dc-link
voltage conditions.
• the proposed control system also performs satisfactorily
under downstream arcing fault conditions.