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© IEC 2021
INTERNATIONAL ELECTROTECHNICAL COMMISSION
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IEC 61869-11
Edition 1.0 2017-12
INSTRUMENT TRANSFORMERS –
Part 11: Additional requirements for
low-power passive voltage transformers
INTERPRETATION SHEET 1
This interpretation sheet has been prepared by IEC technical committee 38: Instrument
transformers.
The text of this interpretation sheet is based on the following documents:
DISH Report on voting
38/663/DISH 38/672/RVDISH
Full information on the voting for the approval of this interpretation sheet can be found in the
report on voting indicated in the above table.
IMPORTANT – The "colour inside" logo on the cover page of this document indicates that it
contains colours which are considered to be useful for the correct understanding of its
contents. Users should therefore print this document using a colour printer.
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1 Introduction
IEC 61869-11 was published in 12-2017 and since then experience with the application of the
document has been gained. During this period, it became visible that the type test "Test for
impact of electric field from other phases" as required in 7.2.6.1101 and outlined in Annex 11A
creates ambiguities in the execution of the test and the interpretation of its results.
ICS 17.220.20
IEC 61869-11:2017-12/ISH1:2021-09(en-fr)
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7.2.6.1101 Test for impact of electric field from other phases
The purpose of this test is to verify the influence of the electric fields at rated frequency emitted
by other phases.
The test shall be performed in a configuration representing the real installation. The test can
be performed in three-phase or single-phase. Test arrangement and procedure are given in
Annex 11A.
Annex 11A (normative)
Tests for impact of electric field from other phases
11A.1 General
Adjacent phases in a three-phase power system can influence the accuracy of passive LPVT.
To evaluate the impact of electric fields effects at rated frequency generated by adjacent phases
in the power system the following test shall be performed.
2 Background
2.1 General
The type test is intended to evaluate the impact of horizontal and vertical stray capacitances
that the equipment is exposed to in service, which is typically different to the situation in the
laboratory. In order to estimate this impact on the ratio of the LPVT, Annex 11A describes a
test layout and procedure where:
Figure 1 (Figure 11A.1 from IEC 61869-11, annotated, with stray capacitances when busbar is
grounded) shows the general layout of the test setup in which the influence of stray
capacitances is evaluated through a two-step test process. The setup consists of the equipment
under test (EUT, coded 1 in Figure 1 and Figure 3 (Figure 11A.1 from IEC 61869-11, annotated,
with stray capacitances if busbar is energized), a second LPVT (coded 2 in Figure 1 and
Figure 3), a metallic busbar with a length equal to twice the distance between the second LPVT
and EUT, a switch to connect the busbar to either ground or high voltage, a grounded metallic
wall with 1,5 times the height of the EUT, a reference VT, measuring equipment and an HV
generator, see Figure 1 and Figure 3. The distance D between the EUT to the metallic wall as
well as between the EUT to the second LPVT is equal to the distance between phases of a
power system operating at U of the EUT.
m
The stray capacitances of the setup originate from the horizontal capacitance between the two
LPVTs as well as to the metallic wall and the vertical stray capacitances of the LPVTs to ground.
The horizontal stray capacitances depend on the height of the LPVT and the distance to parallel
objects. The vertical stray capacitances depend on the height of the LPVT over ground
potential, i.e., height of a pedestal, specified by the system design supplier. Depending on the
dimensions of the test laboratory hall, the distance between the top of the EUT to the laboratory
roof may pose restrictions due to necessary insulation clearances and add additional stray
capacitances to the grounded roof.
© IEC 2021
2.2 First step
In the first step of the evaluation the busbar is grounded, and the following stray capacitances
are effective:
: Stray capacitance between EUT and ground
C
E1_E
C : Stray capacitance between EUT and grounded metallic wall
E1_MW
C : Stray capacitance between EUT and grounded busbar
E1_E’
C : Stray capacitance between EUT and second LPVT
E1_2
C : Stray capacitance between second LPVT and ground (can be disregarded, only
E2_E
shown for completeness)
Figure 1 – Test setup with stray capacitances when busbar is grounded
(Figure 11A.1, annotated)
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Figure 2 – Equivalent circuit of the test setup in Figure 1 with current flow direction
In this test configuration, the stray capacitances act to decrease the primary capacitance C of
the EUT leading to an increase of the transformation ratio, K > K . The stray capacitances with
r
the largest impact on the ratio are marked with dotted blue circles. The red marked path
represents the EUT, the green marked path represents the second LPVT shown in Figure 2.
2.3 Second step
In the second step of the evaluation, the busbar and hence the second LPVT are energized to
the same high voltage source as the EUT and the following stray capacitances are effective:
C : Stray capacitance between EUT and ground;
E1_E
C : Stray capacitance between EUT and grounded metallic wall;
E1_MW
C : Stray capacitance between EUT and energized busbar;
E1_HV
C : Stray capacitance between EUT and second LPVT (0 pF if identical units);
E1_2
C : Stray capacitance between second LPVT and ground (can be disregarded, only
E2_E
shown for completeness).
© IEC 2021
Figure 3 – Test setup with stray capacitances when busbar and
second LPVT are energized (Figure 11A.1, annotated)
Figure 4 – Equivalent circuit of the test setup in Figure 2 with current flow direction
In this test configuration, the stray capacitances act to increase the primary capacitance of the
EUT leading to a decrease of the transformation ratio, K < K . The colour coding is the same as
r
described for Figure 2.
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© IEC 2021
2.4 Result of the two steps
IEC 61869-11, Annex 11A requires the following limits:
The transformation ratios as well as the phase displacements evaluated in step 1 and in step 2
are then compared. The difference between the actual transformation ratios, evaluated in step 1
and step 2 divided by the actual transformation ratio evaluated in step 1, shall be lower than or
equal to 1/5 of the ratio error associated with the specified accuracy class. The difference
between the phase displacements shall be below 1/3 of the phase displacement associated
with the specified accuracy class.
2.5 Open topics – Analysis of the test
This two-step test procedure suffers from the following issues:
a) The first step in the process is not representative of service conditions since it measures
the impact of stray capacitances to a grounded parallel busbar. In three-phase system
all 3 busbars are either grounded or energized.
b) The test layout needs a lot of space especially for HV and UHV LPVTs and available
test laboratories are of limited size with varying dimensions. This leads to different
spacing to other grounded objects and walls in different laboratories. These spacing
differences and the influence of the additional stray capacitances may lead to different
test results from one laboratory to another making the comparison of results and their
reproducibility difficult.
c) HV and UHV LPVTs are typically installed in a substation on a pedestal. This elevation
has an influence on the stray capacitance to ground which can vary by more than 10 %
depending on voltage class. No pedestal is foreseen in the test setup. However, if
included, the overall geometric structure would then exacerbate the spacing situation
and in some cases test may not be possible.
d) The test evaluates in-phase contributions to the ratio accuracy whereas in typical
three-phase applicatio
...