•	latest date by which the existence of this document has to be announced at national level	(doa)	dav + 6 months
•	latest date by which this document has to be implemented at national level by publication of an identical national standard or by endorsement	(dop)	dav + 12 months
•	latest date by which the national standards conflicting with this document have to be withdrawn	(dow)	dav + 36 months (to be confirmed or modified when voting)

This document will supersede EN 50209:1998 and all of its amendments and corrigenda (if any).

prEN 50209:2026 includes the following significant technical changes with respect to EN 50209:1998:

— expanding the voltage range up to 27 kV;

— addition of some tests as routine tests (interturn test, surface resistance, voltage withstand test and partial discharge measurement);

— modification of the criteria for the dissipation factor measurement;

— included the requirement to record the capacitance during the dissipation factor measurement.

Introduction

The purpose of this document is to assess the uniform quality of manufacturing and determine the dielectric behaviour of the insulation of rotating electrical machines having rated voltages from 5 kV to 27 kV. It applies to stator bars and stator coils.

1.0 Scope

This document applies to rotating electrical machines with rated voltages (U_N) from 5 kV to 27 kV inclusive and with rated output from 5 MVA upwards for generators and from 5 MW upwards for motors.

This document is also applicable to machines with rated outputs between 1 MVA (1 MW) and 5 MVA (5 MW) and with rated voltages of 5 kV and above, provided its use has been agreed beforehand.

Requirements for machines with a rated voltage above 27 kV are the subject of individual agreement.

In the case of machines whose windings are cured in the stator, tests on the separate winding elements are not possible; for these machines the requirements in Clause 6 apply.

Converter fed machines are excluded from the scope of this standard. The described tests can be used for this type of machines however the criteria are mutually agreed upon between user and manufacturer.

2.0 Normative references

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

EN 10204:2004, Metallic products - Types of inspection documents

EN 60034‑1:2010, Rotating electrical machines - Part 1: Rating and performance (IEC 60034-1:2010)

EN IEC 60034‑15:2025, Rotating electrical machines - Part 15: Impulse voltage withstand levels of form-wound stator coils for rotating a.c. machines (IEC 60034-15:2025)

EN IEC 60034-27-1:2018, Rotating electrical machines - Part 27-1: Off-line partial discharge measurements on the winding insulation (IEC 60034-27-1:2017)

EN 60034-27-3:2016, Rotating electrical machines - Part 27-3: Dielectric dissipation factor measurement on stator winding insulation of rotating electrical machines (IEC 60034-27-3:2015)

EN IEC 60034-27-4:2018, Rotating electrical machines - Part 27-4: Measurement of insulation resistance and polarization index on winding insulation of rotating electrical machines (IEC 60034-27-4:2018)

3.0 Terms and definitions

For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp/

— IEC Electropedia: available at https://www.electropedia.org/

3.1

winding set

collection of winding elements needed to produce a complete stator including additional elements intended for (destructive) quality assessment and spares

3.2

coil side

either of the two, normally straight, parts of a coil which lie essentially in the axial direction of the machine

[SOURCE: IEV 411‑38‑06]

3.3

multiturn coil

physical assembly of one or more electrical coil sections generally surrounded by common insulation comprising a conductor or group of conductors formed into two or more conducting loops

[SOURCE: IEV 411‑38‑01, 411-38-03, modified]

3.4

conductive slot coating

OCP

conductive paint or tape layer in intimate contact with mainwall insulation in the slot portion of the coil/bar side, often called semi-conductive coating

Note 1 to entry: The purpose is to prevent partial discharge from occurring between the coil/bar and the stator core.

4.0 Routine tests

4.1 Number of elements to be tested

The routine test shall be carried out on the number of bars or coil sides, including any spare bars or coils, as shown in Table 1. Exception is the test on the parallel conductor insulation; this test shall be performed on all elements.

Table 1 — Number of elements to be tested

Number of poles	Rated output MVA (MW)	Number of test elements
All pole numbers	< 5	10 % of bars or coil sides with a minimum of 20
Two (2) and four (4)	≥ 50	All bars or coil sides (100 %)
Two (2) and four (4)	< 50, ≥ 5	At least 60 bars or coil sides and, in addition, 10 % of all bars or coil sides
Six (6) and more	≥ 5	At least 60 bars or coil sides and, in addition, 10 % of all bars or coil sides

4.1.1 Tests to be performed

4.1.2 Voltage withstand test between parallel conductors (bars versus coils)

The parallel conductor insulation of Roebel bars shall be tested using an AC voltage of minimum 110 V, 50 Hz before applying the main wall insulation. Alternatively, a DC voltage of 250 V may be used, particularly in case the conductors are interconnected to the inner corona protection (when present). In that case a minimum insulation resistance of 200 Ω is required.

In case of multiturn stator coils, the parallel conductor insulation shall be tested using an AC voltage sufficiently high enough in relation with the nature of the single conductor insulation. The minimum level is 400 V, 50/60 Hz.

NOTE For half-coils without Roebel transpositions reference is made to the procedure for multiturn coils.

4.1.3 Interturn test

This test is only performed on multiturn coils. As a default the test is carried out as a steep-front impulse test according to the provisions set in EN IEC 60034‑15:2025, Clause 6 and Annex D. In case the coil is built up from turns with parallel conductors stacked in the height and no dedicated turn insulation is present, an AC voltage withstand test is preferably used as an alternative.

NOTE A multiturn coil can be built from parallel conductors positioned in the width of the stack and/or in the height of the stack. Only in the latter case the AC test can be used to assess the interturn insulation capability.

The steep-front impulse test level is depending on whether the coil end winding insulation is in its final state or not. When the coil insulation is in its final state the default test levels as given by EN IEC 60034‑15:2025, Table 1, shall be used for the steep-front test. For more details, see EN IEC 60034‑15:2025, Annex D. In case of an AC voltage withstand test the levels shall be agreed upon, recommended is a level of 0,3 × U_N for 1 min between turns.

4.1.4 Measurement of the surface resistance of the conductive slot coating (OCP)

The surface resistance of the OCP shall be measured and recorded at a minimum of two positions along the straight portion of the bar or coil and at least at every meter for long bars/coils over 2 m length. The manufacturer shall state the criteria and measurement method beforehand.

4.1.5 Measurement of the dissipation factor (and capacitance)

The dissipation factor (tanδ) and capacitance shall be measured on the elements at room temperature in relation to voltage (see Figure 1) over the range of 0,2 × U_N at least up to 1,0 × U_N at intervals of 0,2 × U_N with increasing voltage.

Figure 1 — Curve of tanδ against the ratio U/U_N

In case of coils, both coil sides shall be measured separately.

The measurements shall be carried out by means of a dedicated instrument, using a guard ring arrangement to exclude the influence of the end winding (for instance due to the stress grading) and thus only determining the dielectric losses of the main wall insulation.

The outer surface of the straight part of the element shall be properly connected to the earth (or the sensing terminal of the test equipment) over a length equal to that of the stator core. The guard ring arrangement shall lie outside this zone. Refer to EN 60034‑27‑3 (grounding: 5.2.1, guard rings: 5.2.2) as well as IEEE 286 (driven guard technique) for further details.

The initial value of tanδ_0,2, the tip-up (tanδ_0,6 - tanδ_0,2) and the increment Δtanδ per measuring step of 0,2 × U_N shall not exceed the values in Table 2 for rated voltages up to 27 kV irrespective whether the voltage is increased or decreased during the measurement.

If more than 5 % of the samples show test results in the ranges between columns 2 and 3 or between columns 4 and 5 of Table 2, or in a different range of agreed values, the testing shall be continued with an equal number of further elements, if necessary, up to the total number of bars or coil sides. If during these tests the values of Table 2 or the agreed values are not exceeded, the test of the insulation shall be regarded as satisfactory. In case of non-conforming bars or coils they shall be restored of replaced to comply with the above criteria.

Table 2 — Highest permissible values of dissipation factor

tanδ_0,2	tip-up (tanδ_0,6 - tanδ_0,2)		Δtanδ per step of 0,2 × U_N
All elements	95 %	Remaining 5 %	95 %	Remaining 5 %
	Elements		Elements
15 × 10⁻³	5 × 10⁻³	6 × 10⁻³	3 × 10⁻³	4 × 10⁻³

It is recommended to collect and report the results of the dissipation factor measurements in a log-normal cumulative distribution graph to show the “smoothness” of the production and to identify possible outliers. Figure 2 gives an example of such a graph for the starting value tanδ_0,2.

Figure 2 — Example of a cumulative normal distribution graph [1]

It is acknowledged that a few insulation systems exist (and may be engineered in future) that exceed the here specified criteria. In those cases, the manufacturer is required to provide cumulative normal distribution graphs (Figure 2) of the measurement results to show the evenness of the production and the absence of outliers. In this case the requirements as specified in EN 60034-27-3 shall be met. It is preferred that the manufacturer provides graphs on the typical results found in the past in advance of the production for a specific project as a reference. Clear outliers shall be restored or replaced.

NOTE A survey carried out by CIGRE in 2019 published in TB769, comprising the results on more than 20000 elements has shown that the starting value tanδ_0,2 in most cases (99,45 %) can be expected to be less than 15 × 10⁻³ provided that adequate guarding is used while measuring.

4.1.6 Voltage withstand test on the main wall insulation

The main wall insulation of the bar or coil shall be tested with an AC voltage of industrial frequency at a level of 3 × U_N for 1 min. It is allowed to take measures to prevent overstressing of the end corona protection.

4.1.7 Partial discharge measurement

On the number of samples determined using Table 1 a partial discharge measurement shall at least be performed at a voltage between 0,8 × U_N and 1,0 × U_N and preferably at two levels including 0,6 × U_N. The test shall be performed according to the standard EN IEC 60034-27-1. Criteria can be defined by contract specifications. The voltage withstand test according to 4.2.5 serves as the conditioning of the insulation.

4.1.8 Black out test

An optional corona test may be carried out, upon agreement, to check the performance of, amongst others, the stress control system (ECP) and/or in case of anomalies found during the partial discharge measurement (4.2.6). See IEEE 1799 for more details regarding test configuration. It can involve single bars, or an assembly simulation prepared in a stator core dummy involving bottom and top bars. Absence of corona can be checked with a UV camera. Alternatively, a dark room can be used instead. Voltage levels to be used and criteria have to be agreed upon between manufacturer and customer.

4.2 Test reporting

The results of the described tests shall be incorporated into the EN 10204:2004, 3.1 certificate that shall be delivered as part of the quality documentation together with the machine concerned.

Besides the measured values for the dissipation factor and the capacitance at each voltage step themselves, also the temperature, humidity, characteristic values (tanδ_0,2, tip-up (tanδ_0,6 - tanδ_0,2), Δtanδ per measuring step of 0,2 × U_N, and the relative capacitance variation over the voltage (Formula (1)) shall be recorded in the test report.

(1)

where

ΔC is the capacitance variation;

C_1,0 is the capacitance at 1,0 × U_N;

C_0,2 is the capacitance at 0,2 × U_N.

5.0 Random sample test

5.1 Sampling

If a random sample test has been agreed, it shall be carried out on two samples (two bars or two coils) chosen at random from the winding set in the sequence shown below. In case of multiturn coils an additional coil shall be chosen to be subjected to the steep-front impulse test resulting in a total of three (3) samples for multiturn coils.

Prior to the random sampling test the objects shall have been routine tested as described in Clause 5. The customer may request a witnessed repeat of the routine tests.

The random sample test is regarded as a destructive test. Therefore, after being tested, these elements shall be discarded and not be used in the stator winding.

5.1.1 Tests to be performed

5.1.2 Insulation resistance measurement

The measurement of the insulation resistance for 10 min, including determination of the polarization index, shall be according to EN IEC 60034-27-4.

5.1.3 Thermal stability test

The dissipation factor shall be measured as a function of the voltage (voltage increasing from 0,2 × U_N up to at least 1,0 × U_N, in steps of 0,2 × U_N) at room temperature directly before heating to at least 90°C for 4 hours. After cooling the samples to room temperature, the dissipation factor shall be measured again in relation to voltage.

The test samples may be fitted into a model of the slot. The temperature of the insulation, which shall be as uniform as possible, shall be measured at its surface.

For voltages equal to or below 27 kV, the maximum value of Δtanδ per measuring interval of 0,2 × U_N measured at room temperature after heating the sample, shall not exceed the maximum value measured before heating by more than 2 × 10⁻³ (Formulae (2) and (3)).

(2)

with

(3)

NOTE The aim of this test is to assess whether the insulation is thermally stable. It is acknowledged that the dissipation factor often drops due to post curing of the insulation resin. However, the test focuses on a potential increase of the steepness of the tanδ per step of the curve against voltage which can be an indication of delamination occurring in the main wall insulation. A shift of the curve downwards or upwards is not critical, an increase in tip-up and/or Δtanδ per step of 0,2 × U_N can be critical (depending on the level of increase).

In the case of rated voltages above 27 kV the maximum values per measuring interval of 0,2 × U_N shall not exceed values fixed by special agreement.

5.1.4 PD measurement

A PD test shall be performed with increasing voltage from 0,2 × U_N up to 1,0 × U_N, in steps of 0,2 × U_N. where at each step the PRPD pattern is recorded (minimum 1 min). Before the measurements, the samples shall be conditioned at 1,0 × U_N for 5 min, followed by the determination of the PDIV and PDEV. The background noise shall be determined including the recording of the PRPD pattern. Normalization shall be carried out with 1 nC (up to 5 nC). Refer to EN IEC 60034‑27‑1 for further details.

5.1.5 AC Voltage test on end winding insulation

The end winding insulation of one coil or bar shall be tested for 1 min with (2 × U_N +1) without breakdown. Immediately after this test, the voltage shall be increased at the rate of 1 kV/s, up to breakdown, flashover or limitations of the test setup. The test voltage shall be recorded including the circumstances that led to the end of the test (breakdown, flashover, etc). Both end windings to be tested separately.

Preparation of test for coils: conductive braided copper or aluminium foil (covered by glass/polyester fabric to ensure a sufficient contact) or self-adhesive copper/aluminium tape (with conductive adhesive) to be applied over the entire end winding insulation at non-connection end of the coil from one coil side to the other coil side. Sufficient contact with the OCP shall be ascertained or the conductive layer shall be separately earthed.

In case of bars, at least half the length of the end winding shall be covered with the conductive electrode while also a temporary stress grading (ECP) shall be applied. In case that the end winding length is too short to apply a temporary ECP the test may be omitted.

5.1.6 AC Voltage test on main wall insulation

The slot insulation of the second coil or bar shall be tested for 1 min with the test voltage prescribed for the rated voltage of the winding according to EN 60034-1:2010, 9.2. Immediately after this test, the voltage shall be increased at the rate of 1 kV/s, up to 2 × (2 × U_N +1) without breakdown. Followed by further increase of the voltage till breakdown, flashover or limitations of the test setup. The test voltage shall be recorded including the circumstances that led to the end of the test (breakdown, flashover, etc). In case of a coil, both slot sections to be tested separately up to breakdown.

To make it possible to force a breakdown it is preferred that the test equipment is able reach a voltage of at least 5 × U_N.

NOTE Measures to prevent overheating of the coils/bars (such as forced air cooling) during the test are allowed.

5.1.7 Steep-front impulse test

To check the ability of the coil to withstand steep voltage transients a steep-front impulse test shall be performed according to the stipulations in EN IEC 60034‑15 on the third coil. After reaching the specified test level (5 impulses at the default level or 5 impulses at the enhanced level), the amplitude of the voltage impulse shall be increased for information purposes, in increments of 5 kV until breakdown (or flashover) of the coil is reached or until the impulse level reaches twice the required impulse test level as specified in EN IEC 60034‑15.

The breakdown voltage, being the prospective impulse level of the test where the breakdown occurred, shall be recorded.

NOTE In case explicitly the application of EN IEC 60034‑15 is agreed apart from the use of EN 50209, obviously more sample coils will be produced to satisfy the requirements in EN IEC 60034‑15 (at least two sample coils).

5.2 Reporting

The results of the described tests shall be incorporated into the EN 10204:2004, 3.1 certificate that shall be delivered as part of the quality documentation together with the machine concerned.

5.2.1 Result of the tests

If the random sample test in accordance with 5.2.1 to 5.2.6 on one of the samples is not successful, the unsuccessful part of the test shall be repeated on six (6) further bars or three (3) further coils. The random sample test shall be considered satisfactory if the part of the test repeated on the number of samples mentioned above fulfils the requirements.

In case the repeated tests are not successful, customer and manufacturer shall discuss the consequences.

6.0 Globally impregnated windings

6.1 General

Since stator windings that are produced in a global impregnation technique only reach their final insulation properties after the impregnation and curing of the complete stator including the windings, the separate coils or bars that are inserted in the stator core cannot be tested in the same manner as described in the previous clauses. To overcome this issue a possibility exists to produce additional sample coils together with the concerned stator. If the manufacture of such sample coils has been agreed upon, the manufacturing quality can be assessed on these sample coils.

At least two additional winding elements (coils or bars) shall be manufactured at the same time and under the same conditions as the rest of the winding. In case of multiturn coils a third sample coil is necessary for the steep-front impulse test. These winding elements shall be inserted in appropriate slot models that simulate the original stator. Reference is made here to EN IEC 60034‑15:2025, Annex C for further details. In particular, the use of additional release agents or release materials (or both) that are not used in the stator is prohibited.

All tests shall be performed with the coils or bars still mounted in their slot model.

6.1.1 Tests to be performed

In principle, the tests to be performed on these sample coils are the ones that are already described in Clauses 5 and 6 where the tests in Clause 6 are mandatory. Basic difference is that the order of the test is slightly different. Therefore, here only the order of the tests is listed without an extensive description of the tests. For the description reference is made to the appropriate sections in the Clauses 5 and 6. Some of the tests are only performed on a particular coil, this will be indicated in the list.

All individual stator winding coils shall be tested during the production process of the stator winding, before insertion of the coils in either the stator or the slot models.

These pre-winding tests comprise the following:

— voltage withstand test between parallel conductors (4.2.1);

— interturn test (4.2.2).

After insertion of the coils in the stator core or slot model, the following test shall be performed:

— voltage withstand test on main wall insulation at reduced voltage (level to be stated by the manufacturer).

After finalization of the impregnation and curing of the insulation in the stator and the accompanying sample coils the following tests shall be performed on the sample coils:

— insulation resistance measurement on the main wall insulation (5.2.1);

— voltage withstand test on the main wall insulation (4.2.5);

— measurement of the dissipation factor (4.2.4);

— thermal stability test (5.2.2) to be performed with the coils still mounted in the slot model;

— partial discharge measurement (5.2.3);

— voltage withstand test on the end winding insulation until breakdown (5.2.4) on one coil;

— voltage withstand test on the main wall insulation until breakdown (5.2.5) on a second coil;

— steep-front impulse test until breakdown (5.2.6) on the third coil.

After completion of the test program as listed, the slot models shall be removed from the coil sides. Dismounting the coils from the slot models may be difficult and appropriate measures shall be taken to prevent personal injury. When the OCP of the coil has survived the dismounting, the surface resistance of the coil sides shall be measured (4.2.3), otherwise the damage to the OCP shall be noted in the test report.

Complementary to the tests on the sample coils, also the insulation resistance, dissipation factor and PD activity of the completed stator winding shall be measured and recorded for future reference and comparison with the results obtained at the sample coils or bars. If possible, the separate phase windings shall be measured.

The results of the tests shall be reported as described in 5.3. Table 3 gives an overview of the tests to be carried out in the various situations.

Table 3 — Overview of tests

	Clause	Routine tests	Random Sample tests	GVPI sample tests
Voltage withstand test between parallel conductors	4.2.1	X	X	X
Interturn test	4.2.2	X	X	X
Measurement of the surface resistance of the OCP	4.2.3	X	X
Measurement of the dissipation factor	4.2.4	X	X	X
Voltage withstand test on the main wall insulation	4.2.5	X	X
Partial discharge measurement	4.2.6	X	X
Insulation resistance measurement on the main wall insulation	5.2.1		X	X
Thermal stability test	5.2.2		X	X
Partial discharge measurement	5.2.3		X	X
Voltage withstand test on the end winding insulation until breakdown	5.2.4		X	X
Voltage withstand test on the main wall insulation until breakdown	5.2.5		X	X
Steep-front impulse test until breakdown	5.2.6		X	X

Bibliography

[1] CIGRE Technical Brochure TB769. “Dielectric dissipation factor measurements on new stator bars and coils”, June 2019

[2] IEEE Std 286^TM-2024, IEEE Recommended Practice for Measurement of Power Factor Tip-Up of Electric Machinery Stator Coil Insulation

[3] IEEE Std 1799^TM-2012, IEEE Recommended Practice for Quality Control Testing of External Discharges on Stator Coils, Bars, and Windings

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