Transformer Oil Testing (TOT)

Our TOT aims to prevent transformer failure ensuring consistent electricity supply for network providers and consumers. It is a proven loss prevention technique that can form a key part of any condition-based predictive maintenance programme. It works by testing the condition and quality of the transformer insulating oil to provide information on the condition of the transformer, providing an early warning system for preventative action.

Given the nature of the industries we serve, we have a 24-hour emergency support hotline for urgent customer testing requirements:

VPS Transoil: 24-hour Emergency Tel Number: (+44)-7584-069059

Our TOT Service

Analysis of transformer oils and fluids aids provides an early indication of fault detection which can help reduce unplanned power outages and improve plant reliability. Our TOT is performed to internationally recognised test methods and standards using the most up to date and innovative analytical techniques. Our technical team provide technical support to customers and advise on the most suitable testing requirements for your electrical assets including Transformers, Shunt Reactors, Tap Changers & Switchgear. Test suites cover fluid condition, gas, insulating paper, environmental and investigative assessments.


Our TOT improves your asset management by:

  • Protecting life by preventing explosion/fire.
  • Reducing downtime & unnecessary maintenance reducing costs.
  • Reducing environmental impact.
  • Protecting your reputation.
  • Increasing life expectancy of your asset.
  • Minimizing disruptions to your network and supply.
  • Estimating end of life & probability of failure.
  • On-line access to reports with data trending via customer portal
  • Bespoke sampling kits and Samplogic, a well-developed logistics network accessible from our website.
  • 24-hour emergency hotline for urgent customer testing requirements (+44)-7584-069059

Test Methods & Reporting

Our TOT reports contain the analysis results and diagnostic commentary from our diagnostic engineers that provides guidance. To make things simpler, we include a traffic light coding system for the reports. The reports can be emailed, uploaded to your database or reviewed directly on the VPS customer portal. Our service combines the expertise of diagnostic professionals with the latest analytical and diagnostic techniques to examine the sample. By interpreting the test data, VPS delivers a trended report, visibly tracking data over time with expert commentary and advice.

Our TOT is performed to internationally recognised standards as detailed below:

  • IEC 60296 – Fluids for electrotechnical applications – Mineral insulating oils for electrical equipment (Unused and recycled oils).
  • IEC 60422 – Mineral insulating oils in electrical equipment – Supervision and maintenance guidance.
  • BS 148 – Recycled mineral insulating oil for transformers and switchgear - Specification.
  • IEC 62770 – Fluids for electrotechnical applications – Unused natural esters for transformers and similar electrical equipment.
  • IEEE C57.147 – Guide for acceptance and maintenance of natural ester insulating liquid in transformers.
  • IEC 61099 – Insulating liquids – Specifications for unused synthetic organic esters for electrical purposes.
  • IEC 61203 – Synthetic organic esters for electrical purposes – Guide for maintenance of transformer esters in equipment.
  • IEC 60836 – Specifications for unused silicone insulating liquids for electrotechnical purposes.
  • IEC 60944 – Guide for the maintenance of silicone transformer liquids.

Fluid Quality Analysis

In our TOT, each parameter is analysed to provide diagnostic information on the transformer insulating oil as detailed below:

Colour and Appearance- Changes in colour and the physical appearance of the insulating fluid can highlight changes within the system. The presence of physical particles, changes in colour, the presence of free water or the turbidity of the fluid can all be identified.

Water Content- elevated levels of water can damage the mechanical strength of the paper insulation and deteriorate the insulating fluid. Ingress can be from external sources (poor maintenance activities) or internal degradation of the paper or fluid. Once lost, the paper does not recover its mechanical properties. It is worth noting that doubling the water content can half the working life of a transformer.

Dielectric Strength- Inservice insulating fluids must withstand electrical stress without failure. Contaminants such as water, conducting particles, polar compounds, can reduce the dielectric strength of the fluid. 

Acidity -Oxidation of the insulating fluid will occur during its service and form acidic products. Often noticed with a darkening of the fluid colour and if left unchecked can cause internal corrosion, degradation of paper and ultimately the formation of sludge.

Interfacial Tension (IFT)- Determines the presence of soluble polar material and products of fluid degradation. Changes to IFT values suggests fluid degradation, or perhaps incompatibility with other fluids through top-up, or other materials (gaskets etc).

Fluid Quality Analysis continued

In our TOT, each parameter is analysed to provide diagnostic information on the transformer insulating oil as detailed below:

Dielectric Dissipation Factor (DDF) -Measure the current leakage in the insulating fluid. Directly affected by the level of polar contaminants in the fluid. During service the DDF value tends to increase.

Resistivity-The fluids ability to resist the flow of direct current. Directly affected by the level of polar contaminants in the fluid. During service the resistivity value tends to reduce.

Furanic Compounds -Organic compounds formed during degradation of the paper insulation. Tool to determine the remaining useful life of the paper insulation via a relationship with the degree of polymerisation value of the cellulose. 

Degree of Polymerisation (DP) - The predicted DP value indicates the average paper condition across the whole of the transformer. Provides an indication on the remaining useful life of the insulation and consequently the transformer.

Polychlorinated biphenyls (PCB) - Organic chlorinated compounds, previously used as transformer insulating fluids but subsequently banned due to health and environmental concerns. Now classified as Persistent Organic Pollutants and controlled as part of the Stockholm Convention. All transformers should be measured to determine the PCB content.

Transformer Health Analysis

Dissolved Gas Analysis (DGA) -Under electrical or thermal stress insulating fluids breakdown generating gases. The amount and type of gas is related to the type and severity of fault allowing for specific faults (thermal, arcing, partial discharges) to be detected, allowing for early maintenance actions.

We use Duval’s Triangle method for our expert interpretation using the standards detailed below:

  • IEC 60599 – Mineral oil filled electrical equipment in service – Guidance on the interpretation of dissolved and free gases analysis.
  • IEEE C57.104 – Guide for the interpretation of gases generated in mineral oil immersed transformers.
  • IEEE C57.139 – Guide for dissolved gas analysis in transformer load tap changers.
  • IEEE C57.146 – Guide for the interpretation of gases generated in silicone immersed transformers.
  • IEEE C57.155 – Guide for the interpretation of gases generated in natural ester and synthetic ester immersed transformers.

Additional Analysis

Particle Counting– Insulating fluid cleanliness is affected by paper degradation and external contamination. Particle counting measures particles present in the fluid often unseen by the human eye, reducing dielectric strength and catalysing further oil degradation.

Metals Analysis– Pumps and fans can undergo wear and can contaminate the insulating fluid with wear metals. These can catalyse further fluid degradation and reduce the dielectric strength.

Corrosive Sulphur– Metallic sulphides can be formed at high temperatures on hot surfaces. Propagating gas generation and particulates in the insulation fluid, reducing dielectric strength and catalysing further oil degradation.

TOT Sampling Equipment

Effective sampling is paramount to the delivery of a quality analytical and diagnostic service. We recommend that International standard IEC60475 (Method of sampling insulating liquids) is followed as closely as possible. This will produce a representative sample allowing our experts to provide world leading guidance, supporting your business and keeping your assets in optimum health.

Our innovative Transformer Oil Testing kits contain all the necessary items to take, label and safely return samples to our dedicated laboratories. Whilst we can supply a standard kit, we can also provide custom kits based on your requirements. Contact us for details.

Transformer Sampling kit -Contains all the necessary items to allow samples to be taken from a transformer. Contents include various sized nitrile bungs, valve, lint free cloth, tubing, digital thermometer, together with how to guides all packaged in a case for repeat usage.



TOT Sampling Equipment continued

Insulating Fluid sampling, bottle kits - Contains all the necessary items to provide representative samples from your transformers. Various kit options are available. Contents include 500mL clear or brown glass bottles, black plastic bags for uv protection, sample labels, protective packaging, how to documentation and the hard outer case for safe carriage return.

Insulating Fluid sampling, syringe kits - Contains all the necessary items to provide representative samples from your transformers. Various kit options are available. Contents include 50mL clear glass syringe, black plastic bags for uv protection, sample labels, protective packaging, how to documentation and the hard outer case for safe carriage return.

Insulating Fluid sampling, Aluminium bottle kits -Contains all the necessary items to provide representative samples from your transformers from glass free locations. Various kit options are available. Contents include 500mL aluminium bottles, sample labels, protective packaging, how to documentation and the hard outer case for safe carriage return.

Other sampling options are available on request.