On-site modernisation of power transformers

On-site modernisation of power transformers

1. Introduction

At present, there are numerous technological possibilities related to carrying out advanced repair works at the place of installation of a transformer. Repair and modernisation operations include, among others: internal inspections of transformers, treatment or regeneration of transformer oil, sealing, modernisation of cooling systems, fitting of valves and gates, replacement of accessories, cleaning as well as painting, and application of anti-corrosion protection of transformers. An important part of the modernisation process is also the repair or replacement of the on-load tap changer (OLTC) as well as the replacement of bushings [1-4].

On-site service or repair works can extend the trouble-free lifetime of transformers by up to 15 years

However, the cost-effectiveness of modernising transformers after long service must be evaluated individually for each unit, with the assessment of the technical condition being an essential component. On its basis, the scope of the repair and the expected service life with the assumed parameters are determined [5].

The paper presents the technical possibilities of extending the lifetime of transformers by performing a repair or modernisation. Based on many years of experience of Energo – Complex, a decision-making algorithm has been developed to determine the profitability of modernisation, the scope of work and the technical feasibility of performing it on site. Issues related to the verification of the quality of the work performed before placing the unit in operation were also addressed.

2. Assessment of the technical condition of the unit

The main components of a reliable evaluation of the technical condition of a transformer are modern methods for diagnosing the condition of insulation, windings, bushings and the tap changer (Fig. 1).

2.1. Testing transformer oil

The degree of ageing and moisture in the insulation and the occurrence of many undesirable physicochemical processes in a transformer are assessed based on physicochemical tests of the properties and content of gases dissolved in oil. The determination of the electrical strength of the oil, the dielectric loss factor (tgδ) and the content of furans (especially 2FAL), makes it possible to assess the degree of ageing of the cellulose and oil [6, 7].