OSM-Longshot PD Tester

Scope

This document describes the OSM-Longshot PD Spot Tester which can be used for both short-duration PD spot-tests (typically 5 to 10 minutes per test) and also short-term PD monitoring (typically overnight or for a few days). The OSM-Longshot unit is thus a very flexible technology suitable for most on-line PD testing applications.

The OSM-Longshot PD Test technology provides an 'early warning system' to pick-up early stage PD activity in HV Plant to which the PD sensors are attached. The PD Spot-Tester captures the waveforms of PD pulses and other electrical activity using high speed data acquisition technologies and differentiates between types of PD activity and electrical 'noise' such as thyristor switching etc. The Longshot unit provides on-the-spot assessment of the insulation condition including source and magnitude of PD activity whilst filtering out noise from other sources.

The effectiveness of the technology and management approach proposed herein in providing 'closed loop' Partial Discharge Monitoring and Management of HV plant is dependent on a close working relationship between the equipment supplier, inspection company and customer. Success of the approach depends on combining the operations knowledge held by the customer's engineers with the new technology and diagnostic knowledge provided by IPEC and HVSL.

Summary Specification

The OSM-Longshot Unit ('the system') is based on a large-memory, wideband digital waveform data acquisition and analyser and has been designed for both on-line and off-line PD acquisition, processing, separation and identification of partial discharge signals originating from insulation defects in all types of high voltage electrical apparatus, such as cables, switchgear, transformers, generators, motors, etc.

The system has the following (summarised) functionality:

• The unit's PDGold© Software can capture signals synchronously across the 50Hz (or 60Hz) power cycle with the software dividing the power period into slots. The peak signal is then detected for each slot, allowing the user to observe familiar phase-related patterns of discharge.
• The Software provides a PD Threshold Level for recording the number of PD pulses (count) per power cycle.
• PD levels are measured in pC (picoCoulombs) as standard without the need for an external pC Calibrator.
• The unit can be used both as a PD 'Spot Tester' and a PD Monitor by simply changing the period between signal acquistions.
• The unit contains 'Event Recogniser' Software which can achieve signal/noise ratios of up to 20 times better than conventional PD detectors, with an average of 5 times better being the norm.
• Discrimination of PD and electrical noise is carried out using adaptive software algorithms, and does not require any external hardware for this whilst noise gating is also possible.
• The Software includes an RF noise reduction function which uses single frequency spectral subtraction.
• The new technology is compatible with the On-Line Cable Mapping Package from IPEC Engineering which provides for the location of PD Sites along the length of HV Cables of up to 5km in length (refer to IPEC Engineering for details).

The scope of Supply includes: -

1. Supply of a dual use on-line/off-line partial discharge (PD) measurement and analysis system as described herein.
2. Supply of Calibrated PD Sensors for measuring PD in pC on-line.
3. Site training on the operation, maintenance and calibration of the system using IPEC's dedicated PD Sensors (HFCT's and TEV's) and/or The Customer's built-in PD Sensors e.g. High Voltage Capacitive Couplers installed on generator/motor. The PD measurement and analysis system to be supplied shall be compatible for use with both the IPEC PD sensors and built-in sensors such as capacitive couplers in GIS.

1. The OSM Longshot system is suitable for both on-line and/or off-line PD, testing, monitoring, measurement and analysis.
2. The OSM Longshot system is based on a large memory, large bandwidth (350MHz) digitizer for acquisition purposes. The system can acquire the entire PD waveforms of a large number of detected signals and can be provide representation (2D pattern) of PD phenomena.
3. The sampling rate of the system is up to 2GS/s, the frequency upper cut-off is 100MHz to ensure all high frequency signals such as 'local' Transient Earth Voltage (TEV) are measureable. The lower frequency cut-off is 100kHz in order that as large a number of lower frequency PD events such as those emanating from long cables will be detected.
4. The system is able to efficiently separate PD phenomena and noise on the basis of the pulse shape and frequency using it's 'Event Recogniser' software module. The system allows different PD phenomena to be classified on the basis of their characteristic pulse shape and split in different clusters. Further analysis can be carried out on each dataset separately if required.
5. The system has on board processing and data storage capabilities.
6. The system can evaluate PD magnitude in picoCoulombs (pC) while using large bandwidth acquisition system by means of a software procedure. The system complies with the requirements of IEC 60270 STD for PD measurement as an option. In particular, two main acquisition modes are integrated in the system.
1. Standard Mode: a built-in, narrow band filter narrows the band of the system so that the PD detection measurement is in compliance with the requirements of IEC 60270 STD. The system can directly evaluate the PD apparent charge in pC, upon calibration.
2. Diagnostic Mode: PD pulses are properly sampled and processed with a large bandwidth acquisition unit, so that an efficient separation and identification of the different phenomena can be carried out. Full calibration of PD in pC is possible with a PD Calibrator.
7. The system has an efficient, software-based RF Noise Rejection capability without the need for hardware filters or disturbance suppression devices (these can limit the significance of acquired data when used).
8. The system can provide distributions of quantities related to pulse shape (pulse features) for classification purposes, such as equivalent time length, pulse risetime/falltime, pulse width and equivalent bandwidth.
9. The system is able to provide quantities related to phase, amplitude and time between discharge distributions to enable the user to observe and record typical PD patterns in real-time and also to review the signals through a 'play-back' feature in the software.
10. The system can be controlled remotely via RS232 communication interface if required.
11. The acquisition software is compiled for a 32-bit system and operates under a standard WindowsTM operating system.
12. The system has been designed primarily for on-line PD Testing with PD sensors attached to the earth/neutral points of the plant under test.
13. The system can synchronise the PD signal with either an external voltage synchronisation signal or an internal (line) synchronisation derived from the unit's AC power supply.
14. The system has two different software modules for data acquisition (PD Gold©) and post processing/analysis (PD Reader©). The system is also supplied with PDReader© PC Software which can be installed in the any PC owned by the customer, thus enabling post-test analysis of the data to be carried out at the office with the unit remaining in the field.
15. The OSM Longshot's advanced PD diagnostic system is able to perform the following functions:
1. Classification: Unsupervised clustering of the recorded data, which is performed in order to separate PD pulses into shape-homogenous classes. Shape homogenous classes contain pulses relevant to a single type source (cable PD, switchgear PD, noise etc).
2. Separation: The original PRPD pattern is divided in sub-patterns, each one relevant to a sub-class containing a single type source.
3. RF Noise Rejection: this is achieved using single frequency spectral subtraction and analysis of some stochastic characteristics of sub-classes.
4. Identification of Defects: Identification and the severity of defects is realised by adaptive software algorithms which are built into the PDGold© software. The adaptive algorithms are able to recognise defects belonging to macro-categories, such as cable PD, switchgear PD and noise.
16. All processed data is provided in an 'Open File' Based Format to allow the processed data to be transferred to a customer database as required.
17. A SMART gating option is available aimed at realising pulse discrimination and rejection on the basis of either phase or pulse features. As regard pulse features, the operator can choose a PD area in the plane T-W (equivalent time-length and equivalent band-width of the pulse) thus rejecting all the pulses whose features are out of the defined area, thus achieving immediate noise rejection based on pulse shape.
18. The system has a spectrum analyser option (ScopeControl© Software) aimed at analysing the frequency spectrum of the detected signal (Fast Fourier Transform - FFT analysis)

Acquisition hardware

The OSM Longshot unit comprises a data acquisition unit and a PC built into one portable, self-contained unit. Remote control is possible via RS-232 if necessary. It is recommended that the user uses a 256MB+ USB Memory Stick or other portable storage device to back-up software files from the unit.

Software - Data Acquisition and Post-Processing modules

Data Acquistion Software - PDGold©

1. This module contains all the functions that allow control of the instrument, to set the correct values of the acquisition parameters to acquire and visualise the PD dataset in order to get an immediate diagnostic response.
2. The acquisition interface is provided with different probing/recording modes - PRPD pattern, scope, analog meter, pulse window, Lissajous figure.
3. Different quantities are recorded during the acquisition phase. The system can either record the entire waveform of each detected pulse or execute a on board signal conditioning phase in order to provide phase, amplitude, time and pulse features of each signal.
4. Different alarm systems can be made available to warn the operator when the input signal exceeds given thresholds (either in mV or in pC) or saturates with respect to the current gain level.

Data Post Processing module - PD Reader©

1. This module allows the acquired data files to be processed for in-depth analysis of the detected PD phenomena.
2. Since all processing can be effective only if it is performed on a homogenous dataset and on acquisition can contain signals originated by more than one PD phenomena or due to noise activity, an adaptive algorithm program is available to separate these different contributions on the basis of the pulse shape.
3. A feature extraction procedure allows data size reduction for the classification purpose while keeping the pulse shape information. By classifying pulse features in the system it is able to classify properly all the pulses in the different clusters and derive the relevant sub-patterns.
4. Statistical analysis can be carried out on each separated data-set in order to calculate quantities useful for identification purpose.
5. A multilevel identification tool is able to infer the main nature of the defect (among internal, surface and corona discharge categories), to evaluate some defect characteristics (such as the location of the defect with respect to the electrode system) and finally to indicate the defect originating PD as belonging to one of the typical rotating machine defect categories.

Dynamic range : 1:10,000
Input impedance : 20kOhm/20pF
Pre-trigger recording : 0-100% of horizontal timescale
Dead time min : 25us

Sensor Compatibility

The system is compatible for use with sensors designed to acquire partial discharge signals from Cables (using High Frequency Current Transformers) and Switchgear (using Transient Earth Voltage Sensors). It is recommended that any sensors used with the system are calibrated in order that accurate measurements can be made in pC. The system is also compatible with built-in coupling capacitors on generators/motors. IPEC Engineering and HVS supply calibrated PD sensors as standard.

Communication System

Hardware

The OSM Longshot unit is a combined unit with data acquisition and PC built into one unit. Remote control is possible via RS-232. It is recommended that the user uses a USB Memory Stick or other portable storage device to back-up software files from the unit.

Software

Operating system: Win 2000, Win XP (home and professional), compatible MS Office for report generation All PD acquisition and processing programs are included in the unit price.

Case

A carrying case is included in the scope of supply.

The complete package includes all accessories required for the satisfactory operation of the system and shall include the following

1. Calibrated On-Line PD Sensors
2. BNC Data Cables
3. 50Hz Filters/Decouplers

A minimum of 3-days of on-site training on the operation, maintenance and calibration of the system is recommended. The training shall be conducted by IPEC and HVSL by a competent person in the use of the equipment, the training language being English.

The system is guaranteed to be free from defects in workmanship or material for a period of one calendar year (12 months) from the date of final acceptance.

All software supplied is original and is supplied complete with a User Licence. Any upgrades/updates to the acquisition software shall be provided at no charge to the purchaser within a period of 12 months from date of final acceptance.