24-12-2012, 05:03 PM
On-line Monitoring of Partial Discharges in a HVDC Station Environment
On-line Monitoring.pdf (Size: 2.32 MB / Downloads: 35)
ABSTRACT
This paper deals with the on-line measurement of partial discharges (PD) in a highvoltage
direct current (HVDC) converter station. The HVDC station is a particularly
challenging environment for measurement due to elevated interference levels caused
from the switching of thyristor-controlled converters. In this work, online PD
measurements were performed on two specimens; one is an HVDC converter
transformer and the other an HVDC converter wall bushing. The measurements were
performed in the high-frequency range from 400 kHz to 30 MHz with modern
wideband PD measurement instrumentation. Results demonstrate that on-line
measurement of PD in an HVDC station environment is possible and that a
combination of input filtering and modern signal processing methods for feature
extraction can be used to mitigate the converter interference. The feature extraction
method used plots partial discharge data on a time-frequency classification map. The
map enabled isolation of individual PD phenomena, and demonstrated that trending to
monitor insulation degradation is possible with online partial discharge measurement.
Furthermore, specific strategies for PD measurement and analysis are developed for
each of the transformer and bushing specimens.
INTRODUCTION
PARTIAL discharge (PD) in electrical insulation is a
phenomenon that results from localized electrical breakdown, or
discharge, which occurs at defects within an insulation system to
which electric stress is applied. The physical nature of a defect
that may cause PD can include small internal voids within an
insulating material, or tracking across the insulation surfaces [1,
2]. Over time, sustained PD activity will degrade the insulation
level of the system and can cause eventual failure. Therefore, the
occurrence of PD in electrical insulation is an indication that the
integrity of an insulating system is compromised or degraded.
Presently within industry, the application of on-line PD
measurement to installed high-voltage direct current (HVDC)
equipment is rare. Some reasons for the rarity of their application
are that HVDC stations are a particularly challenging
environment for the performance of on-line PD measurement due
to high levels of electromagnetic interference caused from
thyristor valve switching in the HVDC conversion process.
There is also significant interference from external air-corona due
to high electric stress at the surface of conductors and hardware
that exceeds the corona inception level.
SIGNAL PROCESSING METHODS FOR
FEATURE EXTRACTION
Recent developments in the area of digital signal processing
have produced some modern PD measurement instruments
with noise separation. These instruments use the so-called
feature extraction methods to distinguish and separate multiple
PD phenomena from noise, based on characteristics about the
shapes of the pulse waveforms captured through measurement
[7, 8]. However, these instruments are relatively new
technologies so there has been little chance to confirm their
effectiveness [9].
A popular feature extraction algorithm that has displayed
some success for online PD measurements is known as the
method of moments algorithm and is described in [6, 7]. In
this work, the acquisition of PD data on the HVDC apparatus
is processed by the method of moments algorithm. This
processing is handled by a PD measurement instrument which
is equipped with signal processing software to perform the
algorithm.
The algorithm captures information about the shape of
individual PD pulses to allow for isolation of phenomena in
online PD measurements. Because discharges occurring
within the insulation system are likely to have differences in
waveform shape, the algorithm provides an efficient method
for separating discharge phenomena in a measurement. This
improves the ability to analyze the measurement. For example,
switching noise from the HVDC converter could be separated
from PDs which occur in response to the transient voltage
stress.
CHARACTERIZATION OF THE HVDC
STATION NOISE ENVIRONMENT
The largest source of electromagnetic interference in an
HVDC station environment is due to the regular switching of
the twelve-pulse thyristor controlled HVDC converter. In
Figure 3, a phase-resolved plot of the repetitive switching
noise caused by the twelve-pulse converter that was measured
from an HVDC wall-bushing is shown. The twelve-pulse
converter will produce switching pulses twelve times per 60
Hz cycle. As a result, the measurement of the interference in
Fig. 3 shows these twelve distinct discharge pulses. Because
these pulses occur at the exact same phase locations
repetitiously, they are identified as noise. This measurement
was acquired from an online PD measurement performed on
the valve-windings of a converter power transformer.
The produced switching noise creates a dilemma for the online
measurement of PD. The measurement instruments will
have poor signal to noise ratio, and furthermore the repetitious
interference can blind the measurement of PDs in the
insulation system, which occur in response to the transient
switching stress.
Figure 4 shows the frequency spectrum of pulse activity
that was measured in the range between 0-30 MHz.
Measurements for the frequency spectrum were performed at
three different locations, the wall-bushing, the transformer
valve-winding, and the transformer line-winding (refer to
Figure 1). The method of coupling for these measurements
was by high-frequency current transformers located on the
ground connection of each bushing capacitance tap, typical of
that shown in Figure 5. The bandwidth of the high-frequency
current transformer is approximately 400 kHz-40 MHz.
TRANSFORMER PD TEST SETUP
As mentioned in the previous section, the method of
coupling for online PD measurements is capacitive, achieved
through capacitance taps on the transformer line and valvewinding
bushings (Figure 5). The bushings are a condensertype,
having oil-impregnated paper with aluminum foil layers
concentrically wrapped around the central bushing conductor.
The capacitance tap located at the grounded flange of the
transformer bushing, is electrically connected to the condenser
foil that is second from the outermost condenser foil. The
capacitance tap allows for off-line maintenance diagnostic
tests to be performed on the transformer bushing. In service,
this tap must be grounded, however, it may serve as a
capacitive coupler for PD measurement by grounding the tap
through a high frequency current transformer. The HFCT has
negligible impact on the grounding and provides a channel for
the high frequency content of PD pulses that propagate
through the winding.
The bushing capacitances for this transformer are
approximately C1=475 pF, C2=13000 pF on the valve-winding
bushing and C1=510 pF, C2=7100 pF for the line-winding
bushings; where C1 is the capacitance between the high
voltage terminal and the tap, while C2 is the capacitance
between the tap and ground. The input impedance of the test
instrument is approximately 50 Ω.
VARIABILITY OF PHENOMENA DURING MEASUREMENT
It is noteworthy that on two separate monitoring sessions,
performed 17 January 2008 and 10 October 2008 (not shown
in Figure 8) the phenomenon (b1) was absent from the
measurement. It is possible that operating conditions were
such that this PD phenomenon did not occur at the time of the
measurement. If these discharges are produced by surface
discharges on the transformer bushing, then the PD
phenomenon could be dependent upon conditions due to
surface contamination. However, if the defect is internal to the
transformer insulation caused by discharges at an insulation
boundary or interface, variability in the occurrence of the
phenomena can depend on conditions such as
loading/temperature, or the transformer tap position. Based on
the small number of acquisitions performed in the three-year
monitoring period, it is difficult to determine if these variables
play a significant factor for the occurrence of the phenomenon
(b1).
HVDC WALL BUSHING MEASUREMENTS
On-line PD measurements were performed on two HVDC
wall bushings located at the rectifier end of Manitoba Hydro’s
HVDC link. The bushings are on the negative polarity pole of
a six-pulse thyristor-controlled HVDC converter, operated up
to -463.5 kVdc.
The motivation behind performing online PD measurements
had arisen because chemical laboratory results found high
concentrations of SF6 decomposition byproducts in a gas
sample taken from one of the wall bushings. The most
significant of these found, was sulfur deca-fluoride S2F10.
Sulfur deca-fluoride is a known decomposition byproduct of
SF6 resulting from electrical discharge activity [16]. Under
normal conditions, no S2F10 is present in SF6. In the case of
this bushing, approximately 0.5% of the total volume of the
sample was S2F10, suggesting a serious electrical insulation
defect where high levels of discharge were present within the
bushing.
The defective bushing was located in the B-phase position
of its converter group. Additionally, the C-phase bushing of
this same group was monitored as a benchmark for
comparison with the suspected faulty B-phase. A successful
recording of PD events from online measurements would
provide valuable information for the identification of similar
insulation defects in these wall bushings in the future.
PHYSICAL EVIDENCE OF PARTIAL DISCHARGES
After the B-phase bushing was removed from service,
physical evidence of PD was found. A chalky white deposit
was discovered on the stress cone of the bushing condenser
core near the joint between the inner and outdoor sections as
shown in Figure 13. This deposit was caused from the
chemical byproducts of PD occurring in the SF6 gas. These
findings support the online PD measurements which
suggested PD activity was occurring in the B-phase wall
bushing.
CONCLUSION
This work has demonstrated that on-line PD measurements
in an HVDC station environment are feasible. The work has
validated modern signal processing methods for feature
extraction and their ability to aid in the removal of noise from
measurement, as well as isolate multiple PD phenomena
occurring simultaneously in high-voltage equipment. It has
been demonstrated that the isolation of PD phenomena
enables trending, which is essential for online PD
measurement to provide value as a condition assessment tool.
The characterization of the mixed voltage stresses imposed
on HVDC equipment has identified that PD can occur in
response to direct, alternating, and/or switching transient
components.
The HVDC station noise environment was characterized as
having significant switching noise in the bandwidth region
around 1.5 MHz. As a result it was discovered that the
negative impact of electromagnetic interference (noise) in PD
measurement could be mitigated by a combination of highpass
filtering and signal processing methods for feature
extraction.