KINETIC RESOLUTION OF ESTER-SUBSTITUTED OXAZOLIDINONES

There are two approaches to detecting the electrical charge in a partial discharge;

in the external circuit by a conventional PD measurement system, and internally by detecting the resonances set up in the GIS chambers.

(i) Conventional method The test circuit is that given in IEC Publication 270, and the charge flowing through a coupling capacitor fitted in parallel with the GIS is measured using a quadrupole and detector. The PD current pulse at the defect has a duration of less than 1 ns, and propagates as a travelling wave in each direction along the chambers. The pulses are attenuated and undergo multiple reflections, but do not appear immediately in the external circuit. After about a microsecond or so, the pulses die away and the GIS appears to the external circuit as a lumped capacitor with a depleted charge. From then, a replacement charge flows into the GIS, and is measured by the detector.

To obtain the maximum sensitivity of measurement, a completely shielded test arrangement is required [27], which is possible for a test assembly but may be inconvenient when testing a complete GIS. Also, the total capacitance of a GIS is high, and it must be divided into sections for test. In addition, there is no means of locating the discharge, and since a coupling capacitor is not normally provided in a GIS the technique cannot be used for in-service measurements.

(ii) UHF method The current at the PD site rises in less than a nanosecond, and can radiate EM waves with energy spectra extending to frequencies of 2000 MHz or more. This excites the GIS chambers into various modes of electrical reso-nance, which because of the low losses in the chambers can persist for up to a microsecond. The resonances are indicative of PD activity, and if they are picked up by couplers installed in the GIS may be displayed on a spectrum analyser.

The resonant technique was developed in the UK [22], where over the past 15 years or so much experience has been gained using UHF for PD detection at frequencies from 300 to 1500 MHz [28].

The UHF technique will later be described in detail, when it will be seen that its advantages are its high sensitivity; the ability to locate discharges accurately by time of flight measurements; and that it can readily be used in a continuous and remotely operated monitoring system.

2.7.3 Comparison of the techniques

It is difficult to compare the various diagnostic techniques from results gained under different experimental conditions, so a CIGRE Working Group (15-03) arranged for them to be used simultaneously to detect a range of artificial defects [29]. The tests were made in a 6 m long section of 420 kV GIS chambers into which was placed one of the following defects:

• a free metallic particle

• a particle attached to the surface of a barrier

10 15

10

5

0 5

1

50 100 150 200 250 U, kV

300 b

c

a

Figure 2.8 Signal to noise ratio of PD from a needle on the busbar (taken from [29]) a IEC 270 (pC/pC)

b UHF (dB)

c acoustic (mV/mV)

• a corona point at the HV conductor

• a similar point on the chamber wall.

The vessel was energised by a 0–510 kV metalclad test transformer, the voltage being increased slowly until breakdown occurred. During this time, diagnostic measurements were made using the following techniques:

a conventional electrical detection according to IEC Publication 270 with either a standard detector at 1 MHz, or the phase-resolved partial discharge (PRPD) evaluation system at 200 kHz [30]

b UHF, using an internal coupler at frequencies up to 1500 MHz c acoustic, using an external acoustic emission sensor at 34 kHz

d chemical, using detector tubes; this technique proved too insensitive to give a result over the limited test period.

To illustrate the results reported in Reference 29, those for a needle attached to the busbar are reproduced as Figure 2.8, in which the data from the various techniques has been expressed as signal/noise ratios, so that the results can be compared.

The general conclusions of this investigation were that:

• the acoustic, IEC 270, and UHF techniques all show good sensitivity

• acoustic measurements are non-intrusive and can be made on any GIS, but the attenuation of the signal across barriers and along the chambers is rather high

• conventional PD measurements need an external coupling capacitor, and cannot be used on GIS in service

• the UHF technique is suitable for in-service monitoring.

2.7.4 Overview of UHF technology

A GIS installation consists of a network of coaxial transmission lines which acts as a waveguiding structure for UHF signals, with an inherently low loss. In the absence of barriers and discontinuities, the attenuation at 1 GHz in a waveguide of this size (typically 0.5 m diameter) would be only 3–5 dB/km. In practice, reflections at discontinuities within the GIS chamber cause a reduction in signal strength which has been observed to be in the region of 2 dB/m [28]. These reflections can cause resonances to appear, such as those set up between dielectric barriers [31].

When coaxial lines are used for signal transmission, the usual mode of signal propagation is the transverse electromagnetic (TEM) mode, in which the electric and magnetic field components are transverse to the direction of propagation. The frequency of operation is kept below the cut-off frequencies at which higher-order transverse electric (TE) and transverse magnetic (TM) modes begin to be excited, thus ensuring non-dispersive propagation, a normal requirement for the maintenance of signal fidelity.

In the case of a GIS, the coaxial structure is a consequence of the need to contain the gaseous insulation, and its dimensions are accordingly defined by high voltage requirements. At UHF (300–3000 MHz), the GIS dimensions are such that the TE and TM modes of propagation cannot be neglected [32]. Excitation of a purely TEM mode signal would require symmetrical excitation of the waveguide, whereas the location of a PD current pulse is always asymmetrical with respect to the coaxial cross-section and therefore couples strongly with higher-order modes. These modes are closely related to those of the hollow cylindrical waveguide, and are therefore capable of propagating across gaps in the HV busbar, which would block TEM signals.

For these reasons, it is necessary to account for all modes of propagation within the measurement bandwidth to describe adequately the UHF signal resulting from a PD.

The design of internal couplers for detection of UHF signals in GIS involves a compromise between the conflicting requirements of minimising the field enhance-ment while maximising the UHF sensitivity. The coupler must not create an additional risk of breakdown, and is normally mounted in a region of relatively weak HV field, at an inspection hatch for example, where it is shielded in a recess in the outer conductor. A disadvantage is that the UHF fields also tend to be weaker in these regions, since they are subject to the same boundary conditions as the HV field.

UHF antennas of a form which would be desirable for good sensitivity, such as a radial monopole, are unacceptable as they would invite breakdown. However, other forms of broadband planar couplers, such as the spiral, have been investigated [33]

and shown to have good sensitivity. Circular plate couplers have proved useful, and are more readily accepted in GIS because they are similar to capacitive dividers, and can be seen not to cause stress enhancement. Circular couplers are themselves resonant structures at UHF frequencies, and the effect of design parameters has been investigated [33], showing how their sensitivity can be enhanced within the design constraints.

A more detailed description of UHF theory and the generation and transmission of UHF signals will be found in the next section.

propagation

PD source source parameters:

– location of the PD – length of discharge path – shape of current pulse

excitation

Figure 2.9 Transfer functions involved in the UHF detection of PD in GIS 2.8 The generation and transmission of UHF signals in GIS

2.8.1 Introduction to UHF theory

Detection of PD by the UHF method involves the stages of energy transfer that are shown in Figure 2.9.

To take full advantage of the UHF technique, an understanding of the basic processes involved is important. In the following notes, a represents the radius of the inner conductor of the GIS, and b represents the radius of the outer conductor. A system of cylindrical coordinates (r, φ, z) will be used to describe the electromagnetic field components.

2.8.2 Excitation

The shape of the streamer current pulse i(t) at the PD source is most important in determining the characteristics of the UHF signal. The energy radiated in the UHF range is highly dependent on the rate of change of PD current. However, for a given pulse shape, the UHF signal amplitude scales linearly with the current flowing at the defect.

For small defects, the UHF signal amplitude is proportional to the product ql when the pulse shape is constant [34]. Here, q represents the charge contained in the PD current and l is the length over which it flows. Because the length of the streamer itself rarely exceeds 1 mm, l is predominantly a function of the defect size (e.g., particle or protrusion length).

The UHF signal excited by a PD source depends on the position of the defect in the transverse plane. This is because the coupling coefficients to each of the waveguide modes vary across the coaxial cross-section of the GIS.

–10 –5 0 5 10

amplitude, mV

0 50 100 150

time, ns

200 250

Figure 2.10 Typical UHF signal excited by PD in a 400 kV GIS, as measured at the output of a UHF coupler

2.8.3 Propagation

The electromagnetic waves radiated from the defect region begin to propagate in the GIS chamber. Different frequency components of the PD pulse propagate at different velocities, causing dispersion of the pulse. The overall effect of dispersion is to cause the signal to appear as a long, oscillating waveform with a somewhat random appearance (Figure 2.10). The UHF signals obtained from a GIS coupler typically have a duration of 100–1000 ns. Some of the signal is rapidly attenuated because it is below the cut-off frequency of the mode in which it is propagating. The highest frequency components travel along the coaxial lines with a velocity approaching c.

Propagation through barriers takes place at a lower velocity, c/√

ε_r, where ε_r is the relative permittivity of the insulating material (typically 5–6). The relative arrival times of the wavefronts at couplers on either side of the PD source can often be used to locate the defect.

Any non-uniformities in the GIS will cause partial reflections of the UHF signals.

Most discontinuities inside the GIS have a complicated reflection pattern, because they do not reflect the signal at a plane, but over a distributed volume (e.g., a conical gas barrier). The attenuation of the UHF signal along the GIS duct (between one coupler and the next) is mainly due to the confining of signal energy within the chambers by the partially reflecting discontinuities. These effects cannot be analysed theoretically, except in a greatly simplified form [31]. However, numerical techniques and experimental measurements have resulted in guideline figures for the attenuation caused by reflecting obstacles in the GIS.

2.8.4 Extraction

Internal UHF couplers are normally mounted at a recess in the outer conductor.

Because of the boundary conditions in this region, the radial component of electric field is predominant. The intensity of this electric field is therefore the primary factor affecting the signal level that can be obtained from the coupler. Externally mounted couplers (e.g., at an inspection window) will be affected by the field patterns in the structure on which they are mounted. In this case, the mounting arrangement should

be considered as part of the coupler, so that the reference plane is still the inner surface of the outer conductor. The coupler’s function is to maximise the output voltage for a given radial component of UHF electric field [35].

The frequency response of the coupler should be suitable for the frequency range of the UHF signal. In a 400 kV GIS, the UHF energy is normally concentrated between 500 and 1500 MHz. UHF signals from low level PD can only be detected if they are of sufficient amplitude to be distinguished from electrical background noise. Internal couplers are best from this point of view, as the noise levels are low, especially if the GIS is cable fed. Where external couplers must be used, they should be screened from interfering signals [36].

2.8.5 Waveguide modes and UHF propagation

This section introduces the basic field patterns that can exist in a coaxial waveguide, and describes their relevance to the UHF detection of PD.