Megger VLF SIN-45 - Page 45

Operating Modes
45
An insulation in good condition is indicated in PE-based cables (e.g. XLPE cables) by a
low Δtanδ, which corresponds to a nearly constant tanδ over increasing test voltages.
For an aged insulation, the tanδ value increases slightly with increasing voltage. For a
critically aged insulation, the tanδ value clearly increases with increasing voltage.
Using the relevant literature as an aid, the absolute tanδ values measured on a
PE-based cable can also be used to derive conclusions about the condition (with the
limitations described on the previous page).
Depending on the condition, different measures are recommended for PE-based cables
according to the following table (taken from IEEE 400.2):
Mean value
at Uo
[10
-3
]
σ at Uo
[10
-3
]
Δtanδ
(1.5Uo – 0.5Uo)
[10
-3
]
Condition assessment
<4 and <0.1 and <5 No action required
4 to 50 or 0.1 to 0.5 or 5 to 80 Further study advised
>50 or >0.5 or >80 Action required
In comparison, the following (lower) limits for PE-based cables have already been
specified in the older IEEE 400-2001 standard:
Mean value at 2Uo
[10
-3
]
Δtanδ (2Uo – Uo)
[10
-3
]
Condition assessment
<1.2 <0.6 Good
1.2 bis 2.2 0.6 to 1.0 Aged
>2.2 >1.0
Highly degraded
The interpretation of the loss factor in evaluating the condition of PILC cables has not
yet been thoroughly investigated. An exact, qualitative evaluation can therefore only be
derived to a limited extent from the measurement results received, as compared to PE-
based cables.
In principle, it can be said that the dielectric loss factor of a PILC cables is always
considerably greater than that of a PE-based cable. Even a vulnerable PE-based cable
will reflect lower tanδ absolute values compared to a fault-free PILC cable.
The condition of the paper insulation can at least be roughly evaluated with the aid of
the following limit values (taken from IEEE 400.2):
Mean value
at Uo
[10
-3
]
σ at Uo
[10
-3
]
Δtanδ
(1.5Uo – 0.5Uo)
[10
-3
]
Condition assessment
<85 and <0.1 and <5 No action required
85 to 200 or 0.1 to 0.5 or 5 to 80 Further study advised
>200 or >0.5 or >80 Action required
Evaluation of PE-based
cables
Evaluation of PILC
cables