was applied. An overview of large-scale tests or live-line applications. ' >70 kV XLPE cables and the sensitivities is given in Table 1. 2.3 NON-CONVENTIONAL.
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Vol. 6 No. 6, December 1999
Discrimination between Internal PD and Other Pulses Using Directional Coupling Sensors on HV Cable Systems D. Pommerenke Hewlett Packard, Roseville, USA
T. Strehl, R. Heinrich, W. Kalkner Technical University of Berlin, Germany
F. Schmidt, W. Weinenberg Siemens, Berlin, Germany
ABSTRACT On-site partial discharge (PD) measurement is required to ensure proper installation of extra high voltage (EHV) cable systems accessories. To achieve high sensitivity and good localization, two problems have to be overcome. First, the strong high frequency attenuation in long XLPE cables requires that the sensors be located along the cable, preferably directly at the accessories. Secondly, the detection system must be able to distinguish internal PD from other pulses. This paper describes a solution based on directional coupling sensors and a data visualization system, which displays phase-amplitude diagrams for individual PD sources which are identified by the direction of pulse propagation. It has been applied to on-site measurements, type and routine testing of HV cable joints and stress cones. Due to the reliable discrimination between internal PD from the accessory measured and from other pulses, testing can be done in unshielded rooms even using terminations with internal P C Iand corona. The method works independently well on line voltage, resonance sources, oscil1,atingvoltages and 0.1 Hz. cosine-square voltage. It has been used to verify the cable accessories installed in the 6.3 km long 380 kV cable system in Berlin, Germany
1
INTRODUCTION
along HV cables, standard PD detection and decoupling methods often will not achieve t10 pC PD detection for cables longer than 2 km. In HE crosslinked polyethylene (XLPE) insulated ac power cables and addition, noise from feeding voltage sources, (e.g. gas-insulated system components like pre-fabricated joints and field-grading inserts for (GIS) and others) will couple into the cable, further reducing the sensiterminations are often tested before installation. This is done especially tivity. To be able to monitor cable system performance, it is desirable at the extra high voltage (EHV) range for each individual cable length to achieve a sensitivity of tl pC. Due to the consequences of missing or component leading to a rejection of any cable or component with de- a PD source before breakdown or erroneously shutting down a PD-free tectable PD. In well-shielded rooms using low frequency techniques to cable system, a rehble method to distinguish between PD from critiovercome the attenuation of PD pulses propagating along the cable, such cal sources and other noise is needed. Such methods should not rely on tests achieve noise levels of t0.5 pC. This statistical evidence [I, 21, and engineering judgment, because this would complicate automated monthe mechanical robust design of the HV cables in strictly controlled pro- itoring severely. duction environments, supports the often used [l,3-51 assumption that Different sensor types and methods to distinguish noise and PD on cable failures are unlikely to happen. Most breakdowns of XLPE insucables have been proposed and partially put into service. Placing dilated EHV cable systems are caused by their accessories (joints, terminarectional coupling sensors (DCS) inside or close to the accessories is an tions), which have been mounted on-site. attractive option. I’reviously, directional coupling sensors have been To guaranty the quality of the installationwork and to ensure reliabil- applied to stator bars [6], HV transformers [7] and to power cables by ity, more and more sensitive on-site PD tests and on-line PD monitoring the authors [8]. In Section 2 the paper will review non-conventional PD are requested. However, due to attenuation of PD impulses propagating coupling methods and analyze methods to distinguish internal PD from
T
1070-9878/99/ $3.00 01999 IEEE
IEEE Transactions on Dielectrics and Electrical Insulation other pulses, hereafter referred to as ’noise’. Section 3 characterizes the sensors. The data analysis system is described in Section 4 and results are given in Sections 5 and 6.
2
NON-CONVENTIONAL PD COUPLING METHODS FOR CABLE ACCESSORIES
2.1
REQUIREMENTS FOR PD MEASUREMENT SYSTEM ON EHV CABLES
The requirements for a cable PD measurement system determined by the authors are: .
1. The sensors need to be placed outside the active dielectric (above the outer semicon) in order to avoid any degradation of the insulation. 2. The current handling ability of the cable shield must remain in order to sustain short circuit currents. 3. Pulses from other sources (noise) shall not be interpreted as internal PD or vice uersa. For automated monitoring, no engineering judgment can be used in identifying a critical PD source.
Any evidence of PD in EHV XLPE cable systems may point to a potential insulation breakdown. No PD robustness threshold is known. But it can only be shown that an accessory is free of PD relative to the measurement noise floor. Thus, the measurement system needs to be as sensitive as possible. The sensitivity can be optimized by enlarging the coupling from the cable into the measurement system and by matching the measurement bandwidth to the PD pulse bandwidth [9]. The often seen sub-nanosecond current rise time of PD in XLPE leads to an optimal bandwidth of 100 to 500 MHz [9]. Driven more by practical availability and experience on installed systems than derived from any PD threshold considerations, a sensitivity of 70 kV XLPE cables and the sensitivities is given in Table 1.
2.3 NON-CONVENTIONAL COUPLING METHODS 2.3.1 INDUCTIVE COUPLING This method couples energy from the magnetic field of the PD pulse into the measurement system. Typical configurations are shown in Figure 1. Inductively, a signal can be coupled from the cable by attaching a coil such that the magnetic field of the PD pulse induces a voltage in the loop. For cables having a wire sheath, this can be done on the outside of the’ cable, because the magnetic field is not perfectly confined within the interior of the cable. The spiral winding of the wire causes an axial magnetic field component that can be measured [7,19,20,22-261 as shown
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Table 1. PD in large-scale tests of HV XLPE cables. V kV $00
$00 275 275 275 275 275 275 170
150 77 77 77
Freq.range Sens. Sensoring Comment principle MHz pC capacitive 2-20 1 directional coupler 10-500 ’ 0.1 capacitive (REDI) 10-50 5 dual capacitive 1 Cross bonding joints 10 With sheath connection dual capacitive 3 5 lot known 1-10 capacitive 1 5-50 dual capacitive
