Ef Fred Steennis

Experiences with continuous condition monitoring of in-service MV cable connections

A new technology for assessing the condition of MV cable connections is described. The technique measures partial discharges (PDs) from joints and cables on-line, i.e. while the cable connection remains in-service. The system (called PD-OL) is able to locate the origin(s) of PDs from a complete cable connection by using only two inductive sensors, each at one cable termination. These sensors can often be installed while the cable remains in-service. Data is measured continuously and measurement results are collected in a centralized Control Center, in which the results are analyzed and interpreted. Results from this are made visible on a secured web-site for the network owners. The first field experiences from ca. 100 installed systems show some very interesting phenomena, not seen before. Furthermore, various (verified) weak spots have already been detected before leading to an outage. With that various outages have already been prevented.

Partial discharge trends in medium voltage cables measured while in-service with PDOL

This paper presents partial discharge trends of various developing faults such as drying out of impregnated paper cables, moisture ingress, earth screen damage, etc. These trends were measured over either weeks, or months, by PD-OL (PD Online with Location), which measures and locates PD activity. The presented cases are results from continuous measurements on more than 150 cable connections. The paper also briefly discusses the principles behind the measurement technique, the centralized data processing, and the internet portal. PD-OL is a mature and unique diagnostic and monitoring tool for medium-voltage power cable connections, up to several kilometers length for any desired period of time (hours, weeks, years). The network owner can monitor the actual health of their cable systems via the internet, backed up by a warning service. The results show that PD-OL is an economical and reliable alternative for off-line PD measurement methods, while offering so much more.

Propagation of PD pulses through ring-main-units and substations

Online partial discharge (PD) monitoring systems are traditionally installed at a single mediumvoltage (MV) cable connection between two ring-main-units (RMUs). It is more efficient to monitor two or more consecutive cables using a single monitoring system. Moreover, practical experience with the PD-OL system [1], shows that for substations, with many parallel MV cables, and RMUs installing the inductive sensor may be hampered or even impossible. In this paper the influence of RMUs and substations on the propagation of PDs is studied. An RMU or substation can be modeled as a combination of complex impedances representing switchgear, transformer and MV cables. A PD pulse from a cable encounters a load impedance that does not match the cables characteristic impedance, resulting in partial reflection and partial transmission transmission to other cables. Models for RMUs and substations are proposed and verified by measurements. Feasible options for online PD monitoring through RMUs or substations are determined.

Guarding MV cables on-line: With travelling wave based temperature monitoring, fault location, PD location and PD related remaining life aspects

On-line partial discharge (PD) detection and location has proven to be a valuable tool for assessing the condition of medium-voltage (MV) power cables in service. A few advances are discussed in this paper. It is of importance to know all factors affecting PD activity in order to judge its severity. Cable temperature, more specifically its variation due to load cycling, is one key parameter. From the variation of the signal propagation velocity along the cable it is shown that the temperature can be monitored within one degree centigrade accuracy. The PD activity was found to respond upon temperature cycling caused by load variation. Moreover, data gathered from years of field experience has led to more insight in the predictability of upcoming faults based on (trends in) PD activity. It is possible now to identify the time between the moment partial discharges appear (or reach a certain level) and a final breakdown for different types of cable insulation. Occasionally, cable faults occur without being preceded by PD activity. For distribution network operators, a quick location of a MV cable fault, either a direct complete fault or a self-healing (or intermittent) fault, will contribute to reduce the outages both in duration and frequency. The earlier applied PD location technique is extended to capture also faults based on first arrival of the associated transients. This fault pinpointing is possible for any MV power cable type within 1% of the cable length, independent of the network grounding and whether a fault is a permanent one or a self-healing one. This paper presents the various techniques and results from test cases and a real captured field fault.

Temperature dependent signal propagation velocity: possible indicator for mv cable dynamic rating

Insulation temperature is the critical parameter for dynamic rating of underground power cables. Power cables on medium voltage (MV) level are too numerous and widespread to economically justify solutions like optical fibers as temperature sensors, often employed for high voltage power cables. This paper proposes to utilize the propagation velocity of high frequency signals as indicator for MV cable dynamic rating. Laboratory scale tests are performed for both PILC and XLPE cables. Test results show that the high frequency signal propagation velocity for XLPE insulation increases with the temperature rise while PILC cable shows opposite behavior. The variation of propagation velocity with temperature is consistent with the variation of permittivity measured on test samples of both materials. The variation of propagation velocity of XLPE is confirmed by data of a power cable in service subjected to strong load cycling monitored over a week. Field data on propagation velocity also matches load variation observed for a PILC cable for load cycling recorded over one week. From laboratory tests and field measurements, it seems feasible to extract thermal information from MV cables for dynamic rating by means of high frequency propagation velocity.

Single-sided partial discharge location method based on impedance discontinuities along power cable

Time domain reflectometry (TDR) applied to live cables is hampered by the absence of a clear reflection from the far end. Instead, location of partial discharge (PD) can be extracted from small structures in recorded patterns arising from any impedance variation along the signal propagation channel. This paper explores two approaches for a single-sided PD location system. The first method compares similar signal sequences occurring in PD pattern and pattern obtained from an injected signal. The second method exploits the change of impedance at a far end Ring Main Unit (RMU) by introducing ferrite material, whose effect can be switched on and off. Experiment on medium voltage (MV) cable with about 560 m length shows that both methods can locate the PD within 2% uncertainty.

Risk-controlled application of current MV cable feeders in the future by intelligent continuous diagnostics

In future (smart) grids different load flows and patterns will occur. This will also have impact on the cable feeders between all smart appliances. Especially old (but also new) cables and accessories may not have the required reliability anymore and additional degradation mechanisms can arise due to the different cable load (patterns). Instead of replacing the cable feeders, there is now also a possibility to monitor the condition of complete cable connections continuously. Thereby intelligence is brought to the cable feeders. Furthermore, this enables the use of the current state of condition of each feeder for operational purposes (i.e. worsening condition may not tolerate increased load). This also enables the use of the current cable feeders in the future with controlled risks. This paper describes a technology to monitor the cable feeders condition continuously, thereby providing insight in risk of operation. Field experiences are shown, together with overall findings of the use of this system up until now.

Experiences with diagnostics on complete in-service MV cable feeders

By using an on-line continuous condition monitoring system, many problems in cable connections can be detected before leading to an outage. The system used for the experiments in this paper is capable of detecting and locating weak spots in complete cable feeders, continuously and while the cable remains on-line, by using only two sensors for the complete cable connection. The system uses partial discharges (PDs) that originate from weak spots in the cable as its diagnostic principle. For MV cables, joints en terminations, detection of partial discharges is proven to be a reliable method. However, this statement is only valid if the PDs are measured correctly and interpreted reliably. The paper summarizes the measuring principles that guarantee the correct detection of partial discharges. The main part of the paper discusses field experiences and results. The system was designed for application on existing, sometimes old, cable connections. However, some interesting phenomena have been observed in new cable feeders too. This, together with the basic principles behind the technology, makes the system extremely applicable in the majority of practical field situations. From the first field experiences important lessons have been learned to increase the reliability of the interpretation. Partial discharge signals are being measured, but risks on failures are produced.

Smartening cable connections by an intelligent health monitor system

A new technology for smartening existing cable connections by means of implementing an intelligent heath monitoring system is described. The system is called PD-OL and monitors the condition of power cable connections continuously by means of partial discharge detection and location while the cable connection remains in-service. The system (called PD-OL) is able to locate the origin(s) of PDs from a complete cable connection by using only two inductive sensors, each at one cable termination. Data is measured continuously and measurement results are collected in a centralized Control Center, in which the results are analyzed and interpreted. The paper describes the system and gives field results. The effectiveness in which the system was able to detect weak spots shows to be over 80%. This means the system can help significantly to prevent outages, have continuous insight in risks and determine optimal replacement moments. Thereby intelligence is brought to cable connections.

Influence of ring-main-units and substations on the propagation of PD pulses

Partial discharge (PD) location in online diagnostics on medium voltage cables is achieved using a sensor at each cable end. Monitoring consecutive cables with a single monitoring system would, however, be more efficient. Moreover, substations and ring main units (RMU) along a cable connection without possibilities for sensor installation can be circumvented by installing the sensor at the next RMU. This paper studies the influence of RMUs and substations along the cable under test on online PD monitoring, including their influence on detection sensitivity, location accuracy and charge estimate accuracy. Models for RMUs and substation are proposed and verified by measurements. The performance of online PD monitoring is studied for a number of network configurations.