30-10-2012, 02:16 PM
Development of a Technique for On-Line Detectionof Shorts in Field Windings of Turbine-Generator Rotors: Circuit Design and Testing
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Abstract
A technique for on-line detection of incipient faults in
the windings of turbine-generator rotors has been developed based
on the twin signal sensing method. This paper describes the development
of power electronic circuits for generation of the twin signals
and detection of the reflected signals. The design and fabrication
of a lab model to test this technique is summarized along with
results of laboratory experiments. The issues involved in using the
developed technique for practical applications are addressed and
the limitations of the technique are summarized.
INTRODUCTION
EARLY detection of shorted turns in the rotors of large synchronous
turbine-generators is a long standing problem
with no satisfactory solution [1], [2]. Shorted turns cause vibrations,
which can lead to catastrophic consequences such as
mechanical damage to rotors. To avoid such a situation, periodic
maintenance of rotors is required, which increases the machine
downtime and results in loss of revenue.
Many techniques have been reported for detection of shorted
turns, most of which require some modification of the stator
windings or placement of flux coils in the air gap [3], [4]. The
twin signal sensing technique reported in [4] can be used for
on-line detection without any modification to the machine under
test. To detect developing or incipient shorts in any winding,
two identical pulses are applied to both ends of the winding.
The pulses travel through the winding and are reflected back
[5]. Any abnormality in the winding results in the applied signals
being reflected through nonsymmetrical circuits. The difference
between the reflected signals is amplified and used as a
signature signal for further processing using novelty detection
techniques. This paper is mainly concerned with the description
of the hardware circuits for the twin signal detection technique
TWIN SIGNAL DETECTION TECHNIQUE
A block diagram of the hardware setup is shown in Fig. 1.
The winding under test is shown as excited by a DC source. The
signal generator applies twin signal pulses to the device under
test, through two series capacitors, which serve to isolate the
high winding voltage from the detection circuits. The circuit is
thus designed for use while the winding under test is in normal
operation. The reflected signals obtained at the winding terminals
are fed to the signal detector through the series capacitors.
The signal detector filters these signals, finds the difference between
the two and amplifies this difference for further signal
processing.
The final output of the signal detector circuit is called the signature
signal and reflects the current state of the winding. This
signature is captured using either a digital storage oscilloscope
or a custom digital scope PC add-on card. It is then compared
to the signature obtained from the same winding in the past. If
there is a significant deviation in the signature signal, a novelty
is found, signifying the existence of a short somewhere in the
winding. The nature of difference between a previously stored
healthy signal and the newly recorded signal required for a novelty
to be detected depends on the novelty detection scheme used
and the associated threshold. Use of an elliptical novelty detector
is seen to give the best results. This method projects a
number of healthy signature signals along the principal components
of the signals.
CIRCUIT DESIGN SPECIFICATIONS
The signal generator circuit produces two low going voltage
pulses, which are applied to either end of the winding. To ensure
minimum interference with the main circuit and to increase detection
sensitivity with respect to pulse magnitude, the failing
edge of the pulses needs to be very sharp. These pulses should
be applied at or near the zero crossing of the winding voltage if
the winding is excited by an AC source. Thus, the design specifications
for the signal generator circuit are:
• generate a small duration periodic pulse with a sharp
falling (or rising) edge,
• duplicate the first edge of this pulse exactly for application
to both ends of the winding, and
• generate the pulses at or near the zero crossing of the AC
voltage for an AC excited winding. This calls for synchronization
of the applied periodic pulse with the system AC
voltage.
Problems and Modifications
The amplified difference between the reflected twin signals is
captured using either a digital storage oscilloscope or a custom
digital scope PC add-on card. The amplified signal should be
triggered at the instant at which the twin signals are applied to
the rotor. The applied signal was fed to an opto-coupler and the
isolated signal was used for triggering. However, the delay introduced
by the opto-coupler varies as a function of the environmental
factors by approximately 800 ns. Since, the signal
range of interest has duration of only a few microseconds, the
variation in the triggering delay could not be tolerated. To overcome
this problem, the signal at the MOSFET drain (Fig. 1) is
directly used as a falling edge triggering signal for the oscilloscope.
This implies that either the machine winding under test
should be isolated from the supply neutral or that the oscilloscope
ground should not be connected to the neutral. This is not
restrictive since the PC add-on scope card is isolated from the
supply neutral.
TESTING
This technique was tested on transformer windings with AC
excitation. The test winding was built with a facility for creating
temporary shorts at various locations. Although the transformer
winding can simulate the effect of excitation of a rotor winding,
the rotation of the rotor imparts other unique characteristics to
the signature signal. The excitation current is fed to the rotor
through brushes, which adds brush noise to the signature signal.
The rotation causes additional vibration and variation of the signature
signal depending on the position of the rotor. To obtain
more realistic signals with a facility for creating shorts in any
given winding, a test rotor was built.
CONCLUSIONS
The twin-signal sensing technique has been shown to provide
excellent results for on-line detection of faults in rotor windings
of turbine-generators. The technique is simple and needs
only healthy rotor data. A hardware circuit has been developed
to generate the necessary signals. The circuits have the additional
advantage of being able to float on any applicable DC or
AC voltage so that the testing can be performed on-line for excited
windings. Rigorous testing of the technique is performed
on a transformer coil and by building a test rotor with a facility
for creation of shorts in a variety of locations along the windings.
The test results show the effectiveness of the developed
methods.