Megger VLF SIN-45 - 5.3.3.2 Manual Evaluation

Operating Modes
44
5.3.3.2 Manual Evaluation
An automatic evaluation of the measurement results made by the software should be
understood as a valuable tool, however by no means should it be used as a sole
decision-making criterion.
Criteria such as deviating measurement results within a cable system, the influence of
leakage currents as well as outside influencing parameters can only be analysed to a
limited extent by software. The technician performing the test is therefore urged to
scrutinize the evaluation critically and, if necessary, to conduct own analyses to avoid
incorrect conclusions.
After completing a test, an overview of the following evaluation criteria derived from the
individual tanδ values can be called up by selecting the menu item :
Criterion Description
tanδ at xUo
The mean of the measured tanδ values is calculated separately for
each voltage level.
It is not recommended to perform the evaluation by means of the
absolute values only, as they can be influenced by several factors:
• Number of joints in the cable stretch
• Type of joints
• Temperature of the cable
• Air humidity
• Leakage current along the terminations / isolators
Nevertheless, important information can be derived from the mean
value. For example, a comparison can be made of the values for all
three phases of a cable system under identical conditions. As a rule, all
three phases of a cable stretch are subjected to the same conditions.
They have the same number of accessories and are subject to the same
environmental influences. By taking the measurements within a short
time frame, an almost uniform cable temperature can also be ensured.
Consequently, the mean values of the three phases should be almost
identical. Substantial deviations upwards indicate that the condition of
the affected phase is poor. In this case, further study (e.g. a PD
measurement) is advised.
σ
The standard deviation σ is specified separately for each voltage level
and is a measure of the distribution of the individual tanδ values around
the mean value of the respective level.
1.5Uo – 0.5Uo
(Δtanδ)
The most important criteria for a meaningful evaluation of the insulation
condition is the Δtanδ, which reflects the voltage dependency of the
tanδ.
The Δtanδ is calculated from the difference between the tanδ mean
value of voltage levels 0.5Uo and 1.5Uo.
Δtanδ = tanδ
1.5Uo
– tanδ
0.5Uo
The Δtanδ can only be calculated for measurements with at least 3
voltage levels.
Introduction
Evaluation criteria