14-05-2013, 02:12 PM
An Inrush Mitigation Technique of Load Transformers for the Series Voltage Sag Compensator
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Abstract
Survey results suggest that 92% of interruption
at industrial facilities is voltage sag related. The voltage
sag compensator, based on a transformer-coupled seriesconnected
voltage source inverter, is among the most costeffective
solution against voltage sags. Transformers are
often installed in front of critical loads for electrical
isolation purposes. When voltage sags happen, the
transformers are exposed to the disfigured voltages and a
DC offset will occur in its flux linkage. When the
compensator restores the load voltage, the flux linkage
will be driven to the level of magnetic saturation and
severe inrush current occurs. The compensator is likely to
be interrupted because of its own over-current protection,
and eventually the compensation fails, and the critical
loads are interrupted by the voltage sag. This paper
proposes an inrush current mitigation technique together
with a state feedback controller for the voltage sag
compensator. The operation principles of the proposed
method are specifically presented, and experiments are
provided validate the proposed approach.
INTRODUCTION
Power quality issues have received much attention
in recent years. In many countries, high-tech
manufacturers concentrate in industry parks. Therefore,
any power quality events in the utility grid can affect a
large number of manufacturers. Records show that
voltage sag, transients and momentary interruption
constitute 92% of the power quality problems [1]. The
voltage sag means that the root mean square value of
fundamental voltage temporarily reduces to 0.1~0.9 per
unit and maintains 0.5 to 30 cycle [2], [3]. Voltage sags
often interrupt critical loads and results in substantial
productivity losses. Voltage sag compensators have
been one of the most cost-effective voltage sag ridethrough
solutions. Several closed-loop control
techniques have been proposed for voltage source
inverter-based sag compensators [4-6]. In this paper,
the inrush issue of load transformers under the
operation of the sag compensator is presented. An
inrush mitigation technique is proposed and
implemented in a synchronous reference frame sag
compensator controller. The proposed technique can be
integrated with the conventional closed-loop control on
load voltages.
SYSTEM CONFIGURATION OF THE PROPORSED
COMPENSATOR
The series compensator is consisted by a three-phase
voltage source inverter as shown in Fig. 1. The leakage
inductor of coupling transformer Lf and capacitor Cf is
recognized as the low-pass filter to suppress PWM
ripples of the inverter output voltage vm. Figure 2
illustrates the equivalent circuit of series voltage sag
compensator and its dynamic equation can be
expressed as (1) and (2).
THE PROPOSED CONTROL METHOD
Figure 3 shows the block diagram of the proposed
control method. Note that the d-axis controller is not
shown for simplicity. The block diagram consists of
the full state feedback controller [11] and the proposed
inrush current mitigation tech. Detailed explanations
are given in the following sections.
Decoupling control
Since cross coupling terms derived from the
synchronous reference frame transformation and the
external disturbances exists in the physical model of
voltage sag compensator, the control block utilizes the
decoupling control to improve the accuracy and the
disturbance rejection ability. Figure 3 shows the
decoupling terms is produced by measuring the load
current, filter capacitor voltage and the filter inductor
current. The cross coupling terms in physical model can
be eliminated completely.
CONCLUSION
This paper proposes an inrush current mitigation technique
incorporating with the full sate feedback controller to prevent
the inrush current during the voltage compensation process.
The controller includes a voltage control, a current control
and a flux linkage control. The proposed control method is
based on the synchronous reference frame which enables
voltage sag compensator to achieve fast voltage injection and
prevent the inrush current. When voltage sag occurs, the
controller can track the transient flux linkage and calculate a
required compensation voltage in real-time for fast
compensation and elimination of flux linkage DC offset
caused by voltage sags. The effectiveness of the proposed the
flux linkage compensation mechanism is validated by
laboratory test results.