This paper discusses the recently developed analysis method for service aged power transformers equipped with an OLTC: dynamic resistance measurements (DRM). Because the on-load tap changer contributes to many power transformer failures the emphasis is on OLTC diagnosis. Off-line diagnosis of on-load tap changers during regular maintenance can detect maintenance errors and assess the condition of OLTC parts not accessible for inspection. DRM can be valuable to on-load tap changer (OLTC) condition assessment because of the large variety of defects and OLTC degradation mechanisms that can be found. This paper deals with the results that can be measured with DRM. Besides undisturbed DRM-data, most common deviations originating from degraded and defect tap changers will be discussed. It is explained how these different types of defects appear in the DRM results. The location and shape of the deviation can be used to find the cause and importance of the defect. A subdivision is made between OLTC's in good condition, OLTC's with increased contact resistance, OLTC's with open contacts and OLTC's with deviating switch times. By doing so, an overview of possible DRM results is gained. The seriousness of the defects is also discussed in this paper. Differences between the DRM results of different types of OLTC's will be discussed by showing results of DRM tests. Most graphs presented in this paper are recorded on service-aged OLTC's in the Dutch utility grid. Some measurement results are performed during experiments in a high voltage laboratory.

Resistance measurements on power transformers are offline diagnostic methods that are performed without opening the transformer main tank. In case the power transformer is equipped with an on-load tap changer (OLTC), this resistance measurement can be used to find degradation and defects inside the OLTC. Many OLTC defects as well as contact degradation can be measured by means of dynamic resistance measurements, which is a resistance test during OLTC operation. A low (DC) test current through the on-load tap changer is used during dynamic resistance measurements. This method is in particular sensitive to maintenance errors, contact degradation and contact timing problems. In general, distinction can be made between defects that interrupt the current through the OLTC, defects that affect the timing of the OLTC contacts and contact degradation resulting in abnormal contact resistance.

It is known that many failures of power transformers are related to aging effects of the on-load tap changer contacts. This paper discusses the recently developed analysis method for service aged power transformers. In particular, the effect of several aging stages of the tap changer change-over selector contacts on dynamic resistance have been considered. As a result, to assess on-site the condition of a power transformer, the following approaches are combined: - The dynamic resistance measurement, to detect deviant contact resistances inside an on-load tap changer; - The velocity measurement to the driving axis of the tap changer drive mechanism and the motor power measurement on the drive motor of the tap changer, to detect mechanical wear or damage inside the drive mechanism; - Transformer turns ratio measurement and DC winding resistance measurement, to diagnose the transformer windings. This paper also presents decision support rules for dynamic resistance measurements to make an unambiguous decision about the status of the tap changer contacts.

Performing on-site non-destructive testing and diagnosis of HV power cables is becoming an important issue to determine the actual condition of the cable systems and to determine the future performances. Therefore in this contribution based on field application of on-site testing and available advanced diagnostics overview of experiences will be discussed with regard to on-site testing methods: energizing, diagnostic possibilities and to non-conventional PD diagnosis of new and service aged power cables.

For advanced, non-destructive on-site condition monitoring of HV power cables up to 150kV by partial discharge detection and dielectric losses measurement it is necessary to energize the disconnected cable system. One of the methods available for this purpose is based on applying damped AC voltages up to 150kV. In this paper, the use of modern technological solutions in power electronics and signal processing as well as in technical design and production methods will be discussed on the basis of the ultra light system (300kg) which is able to test cables up to 20km lengths.