Partial Discharge Measurements on 110kV Current Transformers. Case Study. Results

Transcription

1 2016 International Conference on Electrical Engineering and Automation (ICEEA 2016) ISBN: Partial Discharge Measurements on 110kV Current Transformers. Case Study. Results Cristian DAN 1 and Roman MORAR 2 1 Electrica Transilvania Sud Power Distribution Subsidiary Sibiu, Romania 2 Technical University of Cluj Napoca, Cluj Napoca, Romania Keywords: Partial discharge, PD, Instrument transformer, High voltage, 110kV, 3PARD three phase amplitude relation diagram, PRPD phase resolved PD, MPD600 partial discharge measuring system. Abstract. The case study presents a series of partial discharge measurements, reflecting the state of insulation of 110kV instrument transformers located in Sibiu county substations. Measurements were performed based on electrical method, using MPD600: an acquisition and analysis toolkit for detecting, recording, and analyzing partial discharge, product of Mtronix Precision Measuring Instruments. MPD600 consists of three acquisition units (to separate, this way, the DP signal from electromagnetic disturbances, based on the three Phase Amplitude Relation Diagram 3 PARD), an optical interface and a computer with dedicated software. The system allows measurements of partial discharge on site, even in presence of strong electromagnetic interferences because it provides synchronous acquisition from all measurement points. Measurements, with the ability to be calibrated, do render: a value subject to interpretation according to IEC : IEC : IEC :2011 and IEC : 2011; the possibility to determine the nature of the fault by identifying defective winding. Considering the results, the 110kV instrument transformer was directed to the repair shop and had the fault identified. Introduction Electrical utility industry all over the world faces a tough challenge with an ageing of the power grid component population as failure of these assets may cause interruption of power supply and also revenue losses. Insulation condition is an essential aspect for the operational reliability of high voltage equipment. Considering this, it is very important to identify the healthy units in the ageing population of equipment, as not extending service to these units will result in substantial cost savings for the power company [5]. Modern technologies and developments in signal acquisition and analysis techniques do provide new tools for diagnostic of electrical power equipment. Instrument Transformers To safely operate a power system are indispensable appliances capable of measuring electrical quantities (current, voltage, power, energy, frequency, power factor (cosφ), and so on) and also those that ensure correct functioning of the network and / or do limit proportions of possible damage to it. First are called meters, and other, protective devices. The order of magnitude of voltage and current in a power system is very diverse. Adapting both voltage or current metering systems and protective devices to this wide range of values is neither technically nor economically justified. Therefore, these devices are connected to the electrical circuitry indirectly, via current and voltage transformers instrument transformers whose construction and operation is tailored specifically to this purpose. The instrument transformers possessing a primary winding and a certain number of secondary windings, performing an isolating function (isolating the utilization current or voltage from the supply current/voltage for safety to

2 both the operator and the end device in use), change currents and voltages from one magnitude to another, suitable for power metering and protection devices. For each instrument transformer, insulation issues are fundamental and determine a priori the possible limits of their functional properties [20]. In a CIGRE survey also the used maintenance techniques prior instrument transformers failure are listed [1] in table I. Table 1. Maintenance strategy used prior to failure from 3004 reported failures. Maintenance strategy How often used Regular visual inspection 95% Check of oil level and/or pressure gauge 61% Secondary voltage monitoring for CVTs 15% Insulation resistance checks 11% DGA and/or moisture of oil 7% Thermovision inspection 4% DF measurement at mains frequency 2% PD measurement 1% Partial Discharge Measurement Partial Discharge (PD) measurement is a worldwide accepted tool for quality control of high voltage (HV) equipment [2] [3]. PD indicate partial loss of the insulating capacity and is, thereby, considered a measure of electrical ageing of the insulation system [4]. The classical measuring circuit, according to IEC60270: High voltage test techniques Partial discharge measurements, requires a coupling capacitor for the connection of the PD measurement instrument and works at a measuring frequency of several hundred khz up to one MHz [7], detecting therefore, the line based pulse propagation. Otherwise, measurements in higher frequency range (UHF) do determine the field base wave propagation. Interference Free Measurement Setup Outside screened laboratories, PD signals are very often superposed by noise pulses, a fact that makes a PD data analysis more difficult for both human experts and for software expert systems [6]. Therefore the handling of disturbances is one of the main tasks when measuring PD. Additionally, with the ongoing development of permanently installed PD monitoring systems the PD data analysis needs to become more effective to be done automatically. The effects of external interference in the measuring circuit are minimized through electrical isolation between the measuring system and the control unit. Frequency Selective, Narrow Band and Wide Band Measurement The measuring systems used today extend the measuring range historically limited to ~ 1MHz, as far as 20MHz. Within this wide frequency range, the user can select a band that has only marginal superimposed interference or none at all. 3PARD Evaluation Procedures A new field of evaluation methods is opened by fully synchronous multi-channel PD acquisition in order to gain more reliable measuring results combined with effective noise suppression. Therefore the 3-Phase-Amplitude-Relation-Diagram (3PARD) was introduced [20] as a new powerful analysis tool to distinguish between different PD sources and noise pulses when measuring 3-phase high voltage equipment [7]. Through suitable superimposition of the amplitudes, the traditional phase resolved PD (PRPD) pattern can be broken down into individual components, allowing individual sources to be singled out.

3 Analysis of the Frequency Spectrums With these processes, it is not the amplitudes measured at various locations that are superimposed, but rather the spectral components measured selectively but still synchronously at one single location. The frequently noisy PD patterns can also be broken down into individual sources using these methods. PD Measuring System The MPD 600 as a modern type of a fully digital PD measuring system is capable of performing synchronous multi-channel PD measurements. A brief overview of the system is given below (more detailed information in the product specification [8] and type test description [9]). The measurement system consists of one or more acquisition units (Figure 1), an optical interface (FO bus-controller) and a PC including the measuring software. The PD signals are filtered, amplified and digitized. An optimum frequency band can be chosen to avoid continuous-wave disturbances and to reach a high SNR (signal-to-noise-ratio) even under noisy conditions on site. For multi-channel PD measurements several 100 acquisition units can be connected to one distributed PD system [7] (Figure 2) while a maximum number of 64 units can be operated in a fully synchronized mode. Figure 1. Components of a MPD system: measuring impedance (1), PD acquisition unit (2), optic-usb converter (3), notebook (4). Figure 2. Upgradeable to a multi -channel PD measuring system. Up to 25 phase-resolved patterns can be displayed by the software at one time. With powerful adequate accessories, as the special inductive PD sensor MCT 100 [10], a measurement range extension UHF 608 [11], the MPD system can easily be adapted to various measuring conditions. Based on PD measurements that have been performed on the instrument transformers functioning in Sibiu county substations, it has allowed Electrica Transilvania Sud to assess the condition for some of their instrument transformers and to avoid any catastrophic failures (Figure 3). Units with higher PD level were subsequently replaced.

4 Case Study Figure 3. Example of a catastrophic failure. The 110kV instrument transformers were tested due to failure of previous instrument transformer in MEDIAS / Aurel Vlaicu substation (Figure 3). With a synchronous PD acquisition system it is possible to combine the benefits of conventional PD measurements according to IEC with the advantages of PD measurements in the UHF range. With the UHF sensor able to detect the relevant PD events, the information of the IEC method can be restricted to the relevant PD signals in order to assess the pc charge value. In this, a first stage, on-site field-measurements were taken on three (3) 110kV current transformers (Figure 4) during their normal operation. Figure 4. PD testing of 110kV instrument (current) transformers The transformers were taken out of service to install the PD equipment and for charge calibration. One of the main benefits of the PD systems used is that the PD acquisition units can be placed very close to the transformer. This minimizes the required length of electrical connecting cables and allows PD signal acquisition with high bandwidth, while minimizing electromagnetic interference. Figure 5. PRPD pattern obtained, electrical PD measurement according to IEC 60270, centre frequency: 1MHz, bandwidth: 1.5MHz, all PD data. After using the built-in frequency sweep function a measuring frequency of 1MHz (1.5MHz bandwidth) was selected. The results of the PD measurements on phase 2 CT (later identified as defective), are shown in the following figures 5-7.

5 As usual for conventional on-line PD measurements the PRPD diagram shows a significant 3- phase corona cross-talk from the substation and the near overhead transmission lines. No PD activity from the transformer could be determined as the interference was in excess of 2nC. Figure 6. UHF PD measurement, center frequency: 1MHz, bandwidth: 1.5MHz. Figure 7. Gated PRPD pattern, electrical PD measurement, centre frequency: 1MHz, bandwidth: 1.5MHz. Figure 7 shows the PD activity measured with the UHF 608 extension. With the near overhead transmission lines de-energized, the recorded PD pattern showed no external interference. In order to obtain a valid PD charge information the PD results from the IEC-method had to be used [7] [13]. The external noise was successfully gated with the information from UHF PD acquisition unit. Figure 7 shows the corrected PD pattern recorded. Relevant charge level of the PD pulses was determined. As the result, exceeded from a far the permissible PD level [15], according to IEC :2012 (10pC), the 110kV instrument transformer was taken out of service, pending servicing or inspection. The 110kV current transformer was directed to the repair shop and had the fault identified (figure 8). Conclusions The state of insulation of 110kV instrument transformers can be accurately assessed by PD measurements. The presented result pointed out the high sensitivity of the PD measurement, the modeling tool developed especially on detecting incipient faults inside power equipments. The digital PD measuring system performs PD measurement even under noisy on-site conditions. Separating multiple PD sources / noise, a clear PD data analysis will be possible. Handling single PD faults, PRPD patterns and pulse sequence analysis will deliver reliable results. Synchronous multi-channel PD measurements provide new and advanced options of PD evaluation.

6 Figure 8. PD fault. Primary connexion. References [1] State of Art Instrument Transformers, CIGRÉ Committee A3, Technical Brochure 394, Paris, [2] J.J. Smit, Trends in PD-diagnostics for Asset Management of Aging HV Infrastructures, 14th International Symposium on High Voltage Engineering, Bejing/P.R.China, August, [3] D. König, Y.N. Rao, Partial Discharges in Electrical Power Apparatus, VDE, [4] J.C. Montanari, Insulation diagnosis of high voltage apparatus by partial discharge investigation, Liu-Yeda Memorial Lecture, in Proc. IEEE ICPADM, Bali, Indonesia, [5] M.A. Talib, N. Miswan, G.K. Supramaniam, Condition Assessment of HV Equipments in Tenaga Nasional Berhad Malaysia, Instrument Transformer Measurement Forum, Salzburg, Austria, 13 October, [6] S. Tenbohlen, M. Koch, Significant parameters for determining ageing of insulation system in power transformers Diagnostic Measurements on Power Transformers Workshop, Salzburg, Austria, 13 October, [7] A. Kraetge, K. Rethmeier, M. Krüger, P. Winter, Synchronous Multi-Channel PD Measurements and the Benefits for PD Analyses, OMICRON Training, Klaus, Austria, [8] Omicron electronics, MPD600 Product Brief and Specification, Austria, [9] R. Holle, R. Plath, K. Schon and W. Lucas, Type testing a digital PD measuring system according to IEC 60270, ETG Conference on Diagnostic, Kassel, Germany, [10] Omicron electronics, MCT100 Product Brief and Specification, Austria, [11] Omicron electronics, UHF608 Product Brief and Specification, Austria, [12] Wikipedia article on Ageing, available on 31 March 2013 at: en.wikipedia.org/wiki/ageing. [13] IEC 60270, High-voltage test techniques Partial Discharge Measurements Third Edition, [14] IEC (cancels and replaces the first edition of IEC , and its Amendment 1 (2000) and Amendment 2 (2002), Instrument transformers General requirements, edition 1.0, [15] IEC , Additional Requirements for Current Transformers, Edition 1.0, [16] IEC , Additional Requirements for Inductive Voltage Transformers, Edition 1.0, [17] IEC , Additional Requirements for Combined Transformers Edition 1.0,

7 [18] IEC , Additional Requirements for Capacitive Voltage Transformers, Edition 1.0, [19] T.K. Saha, Review of modern diagnostic techniques for assessing insulation condition in aged transformers, IEEE Transactions on Dielectrics and Electrical Insulation, vol. 10, no. 5, October, [20] K.D. Plath, R. Plath, H. Emanuel, W. Kalkner, Three-phase synchronous PD measurements on transformers on site and in the laboratory, ETG Conference on Diagnosic, Berlin, Germany, 2002.