Megger INGVAR, User Manual

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BH0654BE ZP-BH05E 45
7 HOW TO USE INGVAR
Test object impedance can be estimated
from experience, some examples:
▪
Low Voltage air breaker rated 4 kA:
0.09 - 0.2 mΩ
▪
Low Voltage circuit breaker rated 630 A:
0.3 - 1mΩ
▪
Outdoor breaker pole or disconnector:
0.2 - 0.5 mΩ
In general test objects rated for higher currents have
lower impedance.
Regarding current transformers, the impedance in the
secondary circuit has a direct impact on the imped-
ance. Shunting secondary circuits not included in the
test reduces but some users dislike to do that. For out-
door current transformers current cable impedance is
usually the problem, not impedance of the CT.
7.18 Selecting output
configuration and cables /
conductors
Current output – Series or Parallel
You can choose to connect the Current Unit output
in serial or parallel. Using serial connection reduces
current but delivers higher voltage. This is useful when
you cannot use short cables and it will do with lower
current
1. Load time - Current & Output Voltage
See Specifications chapter for data.
2. Calculate maximum allowed test circuit
impedance
Test circuit impedance = Output voltage / Current
(m
W
= V / kA)
3. Calculate maximum allowed cable set
impedance
Subtract test object impedance from test circuit im-
pedance.
(Simplification. Both cable set and test object are
inductive)
4. Select current cables / conductors
You know the length. Select from tables in chapter
6.8, ”Current cables and conductors”.
▪
Impedance may not exceed maximum allowed value. It
should be as low as possible but cable set should not be
unnecessarily clumsy or heavy.
▪
Impedance can be reduced by:
a) twisting cables is important since it reduces the
reactance.
b) using sets with more cables
c) using several cable sets in parallel. This is especially
useful when Current Unit is in parallel.
▪
At continuous or long term load:
Check that the current per cable not will be too high (a
120 mm2 cable can withstand 350 A continuously). If so,
use a set with more cables in parallel.
▪
Weight. Overhead test object must be able to carry the
weight of the cable set. Divide weight by 2 to get load at
one side of the test object or check if the cables can be
supported by other method.
▪
Solid bars can be a better solution than cables at high
currents and short distances.
▪
If distances to test object terminals are different, using a
set with different cable lengths can reduce impedance.
This also makes twisting easier