Experience on Off-line Partial Discharge Measurement

for Hydrogenerator with Capacity of 280 MVA

Panawat Udomthong, EGAT, +6624366277, Panawat.u@egat.co.th

Chainarong Surinkon, EGAT, +6624366254, Chinarong.su@egat.co.th

Nuttapong Suppakomonan, EGAT, +66234366253, Nuttapong.su@egat.co.th

Abstract

Off-line partial discharge measurement (Off-line PD) is one of reliable tools used to diagnose partial discharges’

activities in the generator. This paper shows an example of using off-line PD measurement as a part of generator

maintenance works of two units of 280 MVA, 18 kV hydrogenerator including finding the partial discharges (PDs),

comparing them with visual inspection results, and measuring the improvement of PDs’ condition after maintenance

works. Interpretation of the test results is mainly based on pattern recognition method which is achieved by comparing

measured recorded phase-resolved partial discharge patterns (PRPD patterns) with typical PRPD patterns illustrated

in IEEE std 1434-2014 [1] and IEC 60034-27 [2]. It was found that recorded PRPD patterns corresponded with

visual inspection results where most of PDs were surface discharges in stator end winding areas. After maintenance,

recorded PRPD patterns also corresponded with maintenance works done where some PRPD sub-patterns were

reduced according to PD sources that were really mitigated.

Keywords: Off-line, Partial discharge, PD, Generator, Hydrogenerator, PRPD, Experience

1. Introduction

In order to evaluate the most real insulaion condition of stator windings of hydrogenerator, basic diagnostic

tools, for example, insulation resistance and insulation power factor tip-up measurement, may be unable to fully

reveal some important insulation defects. In absense of on-line PD measurement system, the off-line PD measurement

is very effective tool that can provide information about the defects, for example, contamination/deterioration of

insulation surface in stator end-winding areas, large voids or delamination between layers of the insulation, foreign

objects near or on insulation surface, and so on.

This paper shows the advantage of using off-line PD measurement as a part of maintenance work for two

units of hydrogenerator that on-line PD measurement system had not been installed.

2. Off-line partial discharge measurement in generator maintenance work

2.1 Maintenace overview

In January 2016, EGAT performed maintenance work for six units of hydrogenerator at a large hydro power

plant. Four of them were rated 280 MVA, 18 kV and the other two were rated 50.2 MVA ,11 kV. However, this paper

only focuses on two units of 280 MVA, 18 kV generator which will be named in this paper as generator H1 and H2.

The scope of maintenance for the two generators was minor inspection which consisted of working steps as

following:

1) Remove floor plate & air baffle

2) Disconnection of electrical connections and instruments (Stator leads, rotor carbon brush, RTD)

3) Electrical tests (including off-line PD) – before maintenance

4) Stator and rotor visual inspection & bore scope inspection

5) Reconditioning works (including maintenance works for PDs)

6) Electrical tests (off-line PD only) – after maintenance

7) Reconnection of electrical connections and instruments (Stator leads, RTD)

8) Assembly air baffle & floor plate

9) Reconnection of carbon brush and final checking

The purposes of off-line PD measurement in step no.3 (before maintenance) and step no.6 (after

maintenance) are described in table 1

Off-line PD in step 3 (before maintenance) Off-line PD in step 6 (after maintenance)

- To help maintenance operators find PD,

especially surface discharges in end winding

areas

- To check how much PDs’ condition, especially

surface discharges in end winding areas, were

improved after maintenance works

- To confirm that PD clues found from visual

inspection were real PD

- To find for internal PDs and other PDs that

generally cannot be seen by visual inspection

Table1 - The purposes of off-line PD measurement

2.2 Off-line partial discharge measurement

2.2.1 Test procedure and circuit

The off-line partial discharge measurement for the two generators was complied with IEC 60270 [3] and

IEC 60034-27 [2]. The test procedure and circuit was as following:

1) All electrical leads of the generator must be fully isolated

2) Perform insulation resistance measurement (1 minutes) at 5000 VDC (depends on generator rated kV)

for each phase to check if stator windings are ready for application of AC high voltages

3) Perform off-line measurement for each phase by applying voltage from 1 kV to 10 kV (1.0Un ) using

external AC high voltage source. Voltage level is increased 1 kV per step and PRPD pattern is recorded

at each step. Noted that other untested phases must be grounded during the tests

Figure 1 – Off-line PD measurement test circuit

2.2.2 Interpretation of test results (concentrate on interpretation of PRPD pattern)

The recorded PRPD pattern at each voltage step of each phase will be compared with typical PRPD patterns

illustrated in Annex C of [1] and Annex E of [2]. Sketchs of some important typical PRPD patterns based on [1] and

[2] are shown in figure 2.

PRPD patterns recorded from before and after maintenance of both generators were compared together to

check the improvement of PDs’ condition.

Internal discharges (micro voids) End winding discharges (surface

discharges/tracking)

End winding discharges (gap type

discharges)

End winding discharges (PD at

stress control coating)

Slot discharges Delamination discharges (between

conductor and main insulation)

Firgure 2 – Sketchs of typical PRPD pattern based on [1] and [2]

3. Measurement results – comparison between before and after maintenance

3.1 Generator H1

Off-line PD measurements were performed both before and after maintenance. Figure 3 shows recorded

PRPD patterns of before and after maintenance. The improvement of PDs’ condition in each phase was recognized by

disappearance of some sub-patterns of PRPD which indicate in black circles. Type of PDs are already described in the

figure. Figure 4 and 5 show maintenance works done relating with PD findings. It can be seen in figure 3 that sub-

patterns of end winding surface discharges (surface/tracking discharges) in every phase were mostly mitigiated

corresponding with maintenace works done for this type of PD (figure 4 and 5).

Figure 6 shows real PDs that could not be corrected because their location were inaccessible. For external

PD at stress control coating, its sub-pattern is in position D of phase W (after maintenance). For end winding surface

discharges (surface/tracking discharges), its sub-pattern cannot be determined since maintenance operators did not

have enough time to check which phases that the PD were on.

3.2 Generator H2

Like generator H1, off-line PD measurements were also performed both before and after maintenance. Figure

7 shows records of PRPD patterns for before and after maintenance, and figure 8 shows maintenance works done

relating with PD findings. More description in each figure are in the same way as generator H1.

Figure 9 shows real PDs that could not be corrected. For external PD at stress control coating, its sub-pattern

is in position C of phase W

Figure 3 – PRPD pattern of generator H1 (Bipolar)

Before maintenance

After maintenance

Figure 4 PD maintenance works done for generator H1

Figure 5 PD maintenance works done for generator H1 (continued)

Figure 6 Uncorrected PD of generator H1 (locating in inaccessible area)

Figure 7 – PRPD pattern of generator H2 (Bipolar)

Before maintenance

After maintenance

C

Figure 8 PD maintenance works done for generator H2

Figure 9 Uncorrected PD of generator H2 (located in inaccessible area)

4. Conclusion

The using of off-line PD measurement as a part of maintenance work for generator H1 and H2 is a good

example of how off-line PD measurement can benefit the maintenance work for hydrogenerator where on-line PD

measurement system had not been installed. The before maintenance results provide maintenance operators the rough

locations on stator windings that should have more focus while the after maintenance results help evaluate the effective

of maintenance works done. This does not only give feedback information about PD maintenance results to

maintenance operators, but also guarantee the quality of maintenance work to the customer.

5. References

[1] IEEE std 1434-2014 IEEE Guide for Measurement of Partial Discharges in AC Electrical Machinery

[2] IEC 60034-27 Off-line partial discharges measurement for stator winding insulation of rotating electrical

machines

[3] IEC 60270 High-voltages test techniques -partial dischage measurements