ON-SITE TESTING WITH COMPACT AC TEST-SYSTEM AT THE FIRST 500 KV XLPE CABLE PROJECT IN SOUTH AMERICA

C.4.1 8th International Conference on Insulated Power Cables C.4.1

Jicable11 19 23 June 2011, Versailles - France

ON-SITE TESTING WITH COMPACT AC TEST-SYSTEM AT THE FIRST 500 KV XLPE CABLE PROJECT IN SOUTH AMERICA

Andreas WEINLEIN, Gero SCHRDER, Sebastian EBERT, Sdkabel GmbH, Mannheim (Germany), [email protected], [email protected], [email protected], Harald GEYER, agea kull ag, Derendingen (Switzerland), [email protected] ABSTRACT The first 500 kV XLPE cable project in South America was delivered, installed and commissioned in Colombia as turnkey project. Due to the given local logistical and technical requirements a new compact on-site AC high voltage resonant test system was designed.

The test-set consists of 4 pieces cylinder reactors all placed transport locked in a 20 ft container suitable for a fast and reliable erection of the test system on-site. The compact test-set is rated for 540 kV.

The on-site test was carried out successfully in September 2010 including both, high voltage and PD tests.

KEYWORDS 500 kV XLPE cable system, compact on-site AC HV resonant test system, PD measurement on-site, compact plug-in sealing end, mobile spare phase

INTRODUCTION The first 500 kV XLPE cable project in South America was delivered, installed and commissioned in Colombia in September 2010.

The entire job was handled as turnkey project. Due to the given local logistical and technical requirements a compact on-site AC HV resonant test system was designed to carry out the AC HV commissioning tests.

The XLPE cable connection consists of two systems (additionally one spare phase) rated 500 kV with individual cable length between 750 855 m. The conductor cross section is 800 mm2 aluminium. Both circuits are laid in flat formation in a vertical saddle arrangement inside the cavern of the Porce III Hydro Power plant. Total 7 pieces of factory pre-tested compact plug-in SF6 sealing ends and 7 pieces of factory pre-tested and pre-assembled compact plug-in outdoor sealing ends were installed.

The test-set consists of 4 pieces cylinder resonant reactors (3 t each) all locked for transport in a 20 ft container with removable roof suitable for a fast and reliable erection of the test-system on-site. The step-up transformer and frequency converter including control unit for voltage and PD measurement are placed in a 10 ft container. The entire transport weight of the test-system is less than 24 t. Extension up to 6 resonant reactors is considered and easy to implement. Because of the module-like set-up the handling procedure on-site is optimized. The test-set is rated for 540 kV, 11.6 A, max.170 nF cable capacitance (set-up resonant reactors: 2 serial, 2 parallel) or for 280 kV, 23.2 A, max. 680 nF cable capacitance (set-up resonant reactors: 4 parallel).

A full scale commissioning test of the test-set was carried out in the cable manufacturers factory under full load conditions with 530 kV for the duration of 2 hours at a cable drum of a capacitance of 170 nF. Additionally, a PD measurement at both plug-in test cable sealing ends of test set-up was carried out. The test frequency for this set-up was 20 Hz, representing the minimum allowable frequency for AC voltage tests after installation acc. to IEC 62067 [11].

The on-site after installation test with 493 kV / 1 h was carried out successfully at all cables including the spare phase in September 2010 comprising both, AC HV tests and PD measurements.

The testing of the spare phase was carried out by phase-plugging of the regular cable phase inside the GIB cable enclosure which was performed in less than one day. This was possible because of both, the modular set-up of the test-set and the advantages of the compact plug-in sealing end system.

PROJECT DETAILS / REQUIREMENTS A turnkey project with 5.4 km 500 kV XLPE cable of type A2XS(FL)2Y 1x800 RM/150 290/500 kV was planned, manufactured, delivered, installed and commissioned for the hydropower project Porce III near Medelln / Colombia (Empresas Pblicas de Medelln E.S.P.).

The road distance to the next Atlantic sea port is more than 600 km with very frequent unpaved road quality in very high mountain regions.

Fig. 1: map of region Porce III in Colombia

The project consists of 2 systems with an approximate phase length of 760 m. Additionally one spare phase of 855 m length was supplied and installed. In total 7 pieces of factory pre-tested 500 kV compact plug-in SF6 sealing ends and 7 pieces of factory pre-tested and pre-assembled 500 kV compact plug-in outdoor sealing ends were installed:

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Fig. 2 (left): 500 kV compact plug-in SF6 sealing end

Fig. 3 (right): 500 kV compact plug-in OSE

design AC test voltage to ground: 580 kV design impulse voltage: 1550 kV design switching impulse voltage: 1175 kV

Fig. 4: cable design

The two circuits connecting the main transformers with the overhead lines are fixed in the cavern by means of a special short-circuit approved cable saddle system where the cables are laid in flat formation in vertical arrangement at 400 mm distance.

Fig. 5 (left): 500 kV compact outdoor sealing ends

Fig. 6 (right): 500 kV cables fixed with saddle system

The concept of using the spare phase leads to a heightened final position of the outdoor sealing end (middle phase) and a mobile fixation of the concerning compact plug-in SF6 sealing end which can be connected with any main transformer inside the transformer cavern via the associated GIB enclosure. This concept allows to replace a phase with the spare phase within a few hours. During HV commissioning test this plug-in procedure has to be demonstrated.

According to customers specification an AC HV test of the insulation after installation acc. to IEC 62067 with a voltage of 1.7U0 = 493 kV for the duration of 1 h had to be carried out at each phase including spare phase/s. For after-installation tests, PD measurements are actually not required by IEC standards [11].

Although each single cable and accessory is subject to routine tests at the manufacturers lab, incidents during transport, cable laying and installation can lead to unnoticed defects. In consequence, after-installation tests of the insulation focus on defects in cable accessories, e.g. interfacial problems, improper positioning, cuts or scratches, contaminations etc. Such defects do not necessarily lead to a breakdown during the HV test, bearing the risk of breakdown later in service. Sensitive on-site PD measurement significantly reduces this risk [4, 6, 8].

At each installed EHV accessory (Um 362 kV) a PD measurement is carried out during AC voltage test of the insulation as activity of quality assurance by Sdkabel GmbH [6, 7, 10].

Because of the required high test voltage and the secluded region in the high mountains of South America it was nearly unfeasible to organise AC HV test without final confirmed completion date. As there is no appropriate test-set available in South America and in order to be independent from test institutes it was decided to specify an own compact AC HV test system with the main focus on testing cable systems used as power plant links to overhead lines, switchyard or GIS in regions difficult to reach. The typical cable lengths of the power plant leads in EHV systems are 200 - 1500 m, the conductor cross sections are typically 1200 mm2. For lower test voltages even longer cables can be tested.

Acc. to IEC 60840 for cables with Um 170 kV and IEC 62067 for cables with Um > 170 kV requirements for the shape of suitable AC test voltages and for the time of its application are defined:

substantially sinusoidal waveform frequency between 20 and 300 Hz time of voltage application equal to 1 hour Both standards define the AC test voltage level for on-site tests of newly installed cable systems which depends on the cable rated voltage with 1.1 - 2.0U0 [11, 12, 13]. Test voltages up to 1.2U0 can be reached with insulated operation and voltage supply by a generator which can increase the voltage step by step from approx. 20% up to 120% for short time duration (increasing of excitation voltage). Acc. to both standards alternatively, a voltage of U0 may be applied for 24 h. For EHV systems this method should be accompanied by a PD measurement [4].

To cover all international and local requirements a test system for a maximum capacitance of 170 nF at AC test voltage 540 kV was specified. Because of limited availability of cranes on-site and limited access routes the weight and design of one single resonant reactor was specified to be as minor as possible also considering the minimal allowed frequency of 20 Hz. To carry out a PD measurement at each accessory of a cable system on-site this compact mobile test system has to be free of internal PD and free of corona up to the maximum test voltage of 540 kV (PD 5 pC).

1 conductor aluminium, round stranded

2 conductor screen conductive XLPE-compound

3 insulation XLPE

4 insulation screen conductive XLPE-compound

5 bedding swelling tape, semi-conducting

6 wire screen copper wires

7 bedding swelling tape, semi-conducting

8 bedding fabric tape, semi-conducting

9 metallic sheath copolymer-laminated aluminium

10 outer sheath HDPE, outer conductive and flame-retardant layer

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DESIGN OF TEST-SET A system was designed with following characteristics:

power supply (50/60 Hz): 3 x 400 V power: max. 150 kVA output voltage: 0 540 kV output current: max. 11.6 A nominal power: 6265 kVA frequency range: 20 144 Hz capacitance range: 3.3 nF 170 nF duty cycle (at 20C / 100%): 2 h ON / 10 h OFF quality factor: > 100 weight reactor: approx. 3000 kg nos. reactors: 4 pieces weight exciter transformer: approx. 1400 kg weight frequency converter: approx. 330 kg humidity (no condensation): < 95% system weight incl. containers: approx. 18 t + 6 t erection time of test-set on-site: < 4 h

Fig. 7: 540 kV layout of test system

High test power can only be efficiently generated by mobile resonant test systems, where the weight to-power ratio and feeding power demand is relatively low and the transport volume is manageable [2]. When operated at resonance, the feeding power is reduced to the real power loss in the test circuit. Only the actual power loss has to be supplied in order to maintain the test voltage. The quality factor Q is the ratio of reactive power and real power. The losses in the resonant reactors are by far higher than inside the XLPE cable system. Frequency tuned resonant circuits (ACRF) consist of constant inductance(s), the capacitive load and a control module with frequency converter unit [5].

Fig. 8: design of resonant reactor size DSH6W

The design of the reactor is based on the experience with smaller units which were developed for on-site testing of GIS installations since 1979 [01]. The reactors consist of two high voltage windings in series. Both windings enclose a common bar core at half potential. In difference to most other reactors, the DSH6W has no closed iron core with divided air gaps. The magnetic field generated in the iron core closes back via the air. This has to be taken into consideration during erection of the test circuit.

The result of this design is a compact reactor with low weight compared to its testing power. Each resonant reactor is designed for 280 kV and 5.8 A (inductivity: 360 H). The entire height of the 540 kV set-up is approx. 5.1 m. The HV divider is of three parts type.

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Fig. 9: 540 kV set-up of 4 reactors

The concept of the design considers a 20 ft transport container with a removable roof suitable for sea transports and fast and reliable erection of the test-system on-site. All resonant reactors are fixed inside the container with special holding devices locked for transport. All toroidal corona rings are dismantled and stored volume optimized.

Fig. 10: 20 ft transport container with removable roof

All corona rings are made of conductive hard plastic. This technique avoids perturbing buckles as noticed very often on metallic hollow surfaces. To prevent from corona at test voltages of 300-540 kV the diameter of the plastic pipes is 300 mm with an outer ring diameter of 1.5 m.

Between resonant reactor(s) and HV divider a HV filter is integrated to protect the set-up in case of breakdown of the test object and to improve the quality of the PD measurement at the test object.

The step-up transformer and the frequency converter including the control unit for voltage and PD measurement are placed in a 10 ft container.

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Fig. 11: conductive hard plastic corona rings

Further accessories of the test-set are a full size safety circuit, a 50 m long connection cable of 200 kVA (5 x 400 V), a 12 m robust telescope HV connection (diameter 250 mm), a 20 m long medium voltage cable (1 x 25 kV) and for each resonant reactor an oil-sump including post insulators for isolated installation.

Fig. 12: 10 ft control container

The entire test-set is upgradeable with two further resonant reactors type DSH6W to be stored in a second 10 ft container. With this upgrade cables with a maximum capacitance of 255 nF can be tested up to 540 kV.

Alternatively the entire test-set can be built up for 280 kV use. Test voltage 280 kV covers most of the tests (approx. 90%). For this application all 4 pieces of resonant reactors have to be connected in parallel:

Fig.13: 280 kV layout of test system

output voltage: 0 280 kV output current: max. 23.2 A frequency range: 20 206 Hz capacitance range: 5 nF 680 nF

duty cycle (at 20C / 100%): 2 h ON / 10 h OFF quality factor: > 100 nos. of reactors: 4 pieces erection time of test-set on-site: < 3 h The entire height of the 280 kV set-up is approx. 3.8 m. The HV divider consists of two parts. With the upgrade by two further reactors cables with a maximum capacitance of 1020 nF can be tested up to 280 kV.

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Fig. 14: 280 kV set-up of 4 reactors

REAL HEATING TEST The factory acceptance test of the AC HV test-set without load capacitance at manufacturers test bay was carried out for each single resonant reactor with 336 kV / 1 min. and 280 kV / 30 min. at fTest = 112 Hz (including PD measurement). Additionally the 540 kV set-up and the 280 kV set-up with 4 reactors each was tested at 540 kV / 1 h (at fTest = 130 Hz) resp. 280 kV / 1 h (at fTest = 125 Hz) without load.

Fig. 15: set-up 4 x 280 kV / 1h

As final acceptance test the entire set-up was tested under real heating conditions connected with a HV XLPE cable. A cable length of approx. 1000 m was prepared with 2 pieces of 500 kV SF6-plug-in compact sealing ends. One end was connected via a gas filled bushing with the test-set. The other side of the cable was closed with a gas filled dead-end pipe. The entire capacitance of cable including sealing ends, bushing and HV divider was approx. 170 nF.

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test voltage: 530 kV test duration: 2 h test capacitance: 170 nF test frequency: 20.2 Hz test current: 11.5 A At both cable sealing ends a synchronous PD measurement was carried out with inductive sensors.

Fig. 16: heating test with 530 kV / 2 h / 170 nF

ON-SITE TESTS Besides the transport logistic of shipping the test-set to site a concept was created to test all 7 phases with minimal alteration works of resonant reactors considering 11 m phase distance at outdoor sealing ends. Only one alteration of the resonant reactors had to be performed.

Fig. 17: sequence of testing 7 phases

After the 3rd test (spare phase) the spare phase was un-plugged from GIB at transformer no. 4 and the primary compact plug-in SF6 sealing end was plugged into GIB no. 4 (4th test). With all 6 pieces of 500 kV transformers approx. 6 m GIB pipe was HV tested together with the cable. All links to transformer were removed during the tests.

The test set-up for on-site PD measurements should be corona-free. Therefore, corona toroids and connections of suited diameter have to be used. Sufficient clearance from HV connections to any part of the construction should prevent PD from earthed or potential free components. At both sealing ends an inductive sensor (HFCT) was used connected with a multi-channel PD detection system whose acquisition units were connected with optical fibres to controller and laptop in the control container. The voltage signal for synchronization of PD measurement was directly taken from the frequency converter. The voltage was increased step by step PD controlled.

Fig. 18 (left): sensor and PD acquisition unit at OSE Fig. 19 (right): sensor and PD acquisition unit at GIB

Fig. 20: voltage time characteristics of all tests

test voltage: 493 kV test duration: 1 h test capacitance: 102 - 117 nF test frequency: 24.5 - 26.2 Hz test current: 8.2 - 8.8 A

Fig. 21: set-up test phase no. 1

Fig. 22 (left): PD measurement at 493 kV at OSE Fig. 23 (right): PD measurement at 493 kV at GIB

Synchronized two-side PD measurements enable time-of-arrival PD localization. In contrast, single side PD detection is restricted to time domain reflectometry for PD localization, which results in lower sensitivity and accuracy [9, 4].

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Fig. 24: set-up test phase no. 7

ON-SITE RE-PLUGGING PROCEDURE For this project a special spare phase design and concept was developed. The spare phase is mobile fixed at a special carriage system under the ceiling of the transformer cavern.

Fig. 25: spare phase carriage fixation in cavern

This design allows to reach each of the 6 pieces of horizontal GIB cable enclosures to plug-in the 550 kV compact plug-in SF6 sealing end of the spare phase. As part of the commissioning tests this procedure shall be demonstrated. First the spare phase was tested, subsequently the regular phase in the same GIB cable enclosure (transformer) was tested with 493 kV. The re-plugging procedure was performed in less than one day. This was possible because of both, the modular set-up of the test-set and the advantages of the plug-in sealing end system.

Fig. 26 (left): carriage system for spare phase

Fig. 27 (right): ready installed spare sealing end

CONCLUSIONS For EHV projects in secluded regions a compact AC test system has a lot of benefits. The typical cable lengths in such projects are less than 1500 m. A mobile test system as the described one meets the specification in all matters. AC resonant testing in combination with distributed multi-channel PD measurements using PD sensors at each accessory ensures best efficiency for EHV XLPE extruded cable systems. The modular upgrading of the test system with two further resonant

reactors for extensive projects allows for more flexibility regarding capacitance, set-up and above that it provides redundancy. The on-site test was carried out successfully in September 2010 including both, HV and PD tests.

REFERENCES

[1] F. Bernasconi, W.S. Zaengl, K. Vonwiller, 1979, A new HV-series resonant circuit for dielectric tests, 3rd ISH Milan, paper 43.02

[2] W. Hauschild, W. Schufft, W. Spiegelberg, 1997, Alternating voltage on-site testing of XLPE cables: The parameter selection of frequency-tuned resonant test systems, 10th ISH Montreal, Vol. 4, pp 75-78

[3] W. Schufft, W. Hauschild, 2000, Resonant test systems with variable frequency for on-site testing and diagnostics of cables, HV Testing Monitoring and Diagnostics Workshop, Alexandria, Virgina

[4] U. Hermann, A. Kluge, R. Plath, 2007, After-installation testing of HV/EHV extruded cable systems procedures and experiences, Jicable07

[5] W. Hauschild, W. Schufft, R. Plath, U. Herrmann, K. Polster, 2002, The technique of AC on-side testing of HV cables by frequency-tuned resonant test systems, CIGRE Session, paper 33-304

[6] R. Plath, U. Herrmann, K. Polster, R. Heinrich, W. Kalkner, J. Spiegelberg, P. Coors, 1998, On-site PD measurement on an extra-high voltage XLPE cable line, Jicable98

[7] S. Sutton, R. Plath, G. Schrder, 2007, The St. Johns Wood - Elstree experience testing a 20 km long 400kV XLPE-insulated cable system after installation, Jicable07

[8] CIGRE WG 21.16, 2001, Partial discharge detection in installed HV extruded cable systems, CIGRE technical report no. 182

[9] R. Plath, 2005, Multi-channel PD measurements, 14th ISH, Beijing, China

[10] J. Kaumanns, E. Plieth, R. Plath, 2003, On-site AC testing and PD measurement of 345 kV / 2500 mm2 XLPE cable systems for bulk power transmission, Jicable03, paper A8.4

[11] IEC 62067 Ed.1.1 2006-03, Power cables with extruded insulation and their accessories for rated voltages above 150 kV (Um = 170 kV) up to 500 kV (Um = 550 kV) - Test methods and requirements

[12] IEC 60840 Ed.3 2004-4 Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV) - Test methods and requirements

[13] IEC 60060-3 Ed.1.0, 2006-02, Definitions and requirements of on-site tests

GLOSSARY

AC Alternating Current ACRF Frequency Tuned Resonant Circuits (E)HV (Extra) High Voltage GIB/GIS Gas Insulated Bushing / Switchgear HDPE High Density Polyethylene HFCT High Frequency Current Transformer OSE Outdoor Sealing End PD Partial Discharge(s) Q Quality Factor XLPE Cross-Linked Polyethylene

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ON-SITE TESTING WITH COMPACT AC TEST-SYSTEM AT THE FIRST 500 KV XLPE CABLE PROJECT IN SOUTH AMERICA