ABB, American Electric Power (AEP) and EPRI documented the impact of high-frequency switching on cable terminations used for the converter-based Eagle Pass Back-to-Back (BtB) installation located at Eagle Pass Substation on the AEP (Columbus, Ohio, U.S.) transmission system. Fast-acting switching of high-power semiconductor switches results in steep voltage steps, which have high-frequency harmonic-voltage contents. Although these high-frequency harmonics have low-voltage amplitudes and low-energy contents, they may cause stress on certain insulation materials. These harmonics also may excite local resonance and thereby may cause voltage harmonics with appreciable amplitudes. Rapid degradation of specific insulation materials has been detected and investigated through site measurements and laboratory tests. Considerable data have been collected to understand and avoid this type of insulation degradation.

The Installation

A BtB asynchronous tie at the Eagle Pass facility, located near the Mexican border, connects the U.S. AEP Texas Central Co. (AEP-TCC) transmission grid with the Mexican Comision Federal de Electricidad (CFE) grid. The BtB provides control of the real power transferred between the two networks.

A voltage-sourced converter (VSC), using IGTB-based valves with pulse width modulation (PWM) switching technique, was used for the BtB installation. This installation has 42 cables, with lengths ranging from 4 to 32 m (13 to 105 ft), and a set of two parallel XLPE main-phase cables. Filter banks and charging-resistor cables used EPR insulation. Of the 84 cable terminations installed for the short runs, 48 were of the indoor type and 36 of the outdoor type. Terminations were originally of the resistive/refractive stress grading type, which is common for voltages up to 36 kV.

Cable Termination Failures

Three cable failures, occurring shortly after energization, were all phase to ground within the cable terminations. After the third failure, the cause of the failures was thought to be improper workmanship. Therefore, all cables were replaced with the same basic equipment that originally was used but from a different manufacturer.

Three days after the new cables were energized, a fourth failure occurred. Extensive field measurements were conducted to analyze the actual voltage stress on the cable and on the insulation of the terminations. At several locations in both the United States and Mexico, voltage measurements were made on the main-phase cables between the conductor and the cable screen at the ends of the cables, where the screens were grounded. The cables were connected to ground at the phase reactors and at the transformers.

During BtB back-to-back operation, very high frequencies and high-amplitude harmonics were observed to be superimposed on the power frequency. The dominating harmonics were 1.26 kHz (21^st, IGBT switching frequency) and 12.4 kHz (207^th, about ten times the switching frequency). Other main harmonics were 180 Hz (3^rd) and 3.78 kHz (63^rd, three times the switching frequency). The 12.4-kHz harmonic, which was unexpected, varied between 13% and 40% of the power-frequency voltage, depending on the operating mode of the BtB and the angle difference between the two grids. The maximum measured amplitude, including all harmonics, was about 37-kV peak. The 3^rd, 21^st and 63^rd harmonics had fairly constant amplitudes for different operating modes and were in accordance with design values.

No significant difference existed between the three phases, between parallel cables in each phase, or between the cables connected to the reactors or the transformers. Measurements of cable screen currents and calculation of the voltages confirmed the voltage amplitudes and the harmonic spectrum. Proper current balance between the two parallel cables in each phase was confirmed by measurements. Transients from the operation of external breakers on the CFE side were also measured, revealing that the transients were low and could not have been the cause of any cable insulation breakdown. Rather, on the basis of the measurements, it was clear that the system during BtB operation had a local resonance near 12 kHz, which was caused by the IGBT switching.

Laboratory Testing

It appeared that the high-frequency overlay on the power-frequency voltages was responsible for the cable failures. To verify this observation, laboratory tests were carried out at the ABB Corporate Research Center in Vasteras, Sweden. Three test objects were prepared, each consisting of a short 1.5-m (5-ft) length of cable with terminations at each end. One termination was the same as those that had failed at Eagle Pass, and the other was an APIT (ABB brand name), which is generally called a geometric type with an insulation characteristic not dependent on frequency. The cables came from the same manufacturing runs as the replacement cables that had been installed at Eagle Pass. The test objects were exposed to voltages, using both 50-Hz power frequency and a high frequency of 7.5-10.6 kHz. Both supplies were connected to the test object at the same time to obtain a high-frequency voltage superimposed on the power-frequency voltage. The voltage amplitudes were controlled independently for the two supplies. The testing room was a heat chamber simulating the high outdoor temperature condition at Eagle Pass. All three cable terminations of the resistive/refractive type failed within 470 hours at the same physical location inside the terminations, as was the case for the failed terminations at Eagle Pass. None of the APIT terminations suffered any damage.

Discussion of Failures

The resistive/refractive stress-graded termination is of the slip-over termination (SOT) type. It is easy to apply to the prepared end of the cable. The interface between the cable and the termination is filled with a high-quality silicon grease to avoid cavities that could lead to partial discharges. For outdoor use, the terminations are provided with integrated sheds.

The SOT stress-graded termination has a stress-grading layer molded into the silicon rubber, defining the stress distribution along the termination, together with the local stray capacitance inside the cable. The length of the stress-grading layer, its distribution of resistivity and its dielectric constant are selected to provide a linear-voltage distribution outside of the termination at power frequencies. At higher frequencies, the coupling between the resistance of the stress-grading layer and the stray capacitance results in a high electric-field strength near the edge of the outer semiconducting layer of the cable.

Calculations to reflect the worst conditions measured at the Eagle Pass site confirmed that at the highest frequency both the electrical-field strength and the losses were high near the edge of the semiconducting layer. Further field amplification, due to differences in dielectric constants, will cause high field strength in any small cavity, exceeding 3 kV/mm, the approximate level for starting partial discharges in air. Local high-temperature levels will increase and eventually lead to break down of the insulation. The analysis clearly demonstrated the increase in stress that was observed in all of the failures observed at the demonstration site and in the laboratory tests.

Corrective Action

APIT terminations, the geometric type not considered to be frequency-dependent below the megahertz range, replaced all terminations. This type of termination is a classical design and generally used for high- and extra-high voltages, although it also can be used for medium-voltage applications. The electrical-field distribution is determined only by the stray capacitance in the cable and in the termination itself. Because there is no resistive grading interacting with the capacitances, voltage distribution is mainly independent of frequency, with highest field strength occurring inside the homogeneous high-quality EPDM (Ethylene Propylene Diene Monomer) insulation used for the terminations. The geometric type of termination has a larger cross-section than the resistive type and requires more space for its installation. In addition to replacing all of the cable terminations, RC filters were installed to damp the 12.4-kHz oscillation.

Summary

The sequence of failure developed when a local high temperature was produced by the high-frequency losses in the stress-grading layer, resulting in local dry-out of the silicon grease in the interface between the cable insulation and the stress-grading layer of the termination. At high temperatures, the grease will migrate through the stress-grading layer and into the silicon rubber. As soon as any cavity becomes present, partial discharges are generated, further increasing temperature. This high temperature results in electrical treeing, providing a conducting channel through the XLPE insulation in the cable.

Since the installation of the APIT terminations, the facility has operated without incident, providing support to the AEP and CFE networks. The knowledge gained regarding the generation of high-frequency voltages has emphasized the necessity for taking into account the strong possibility that insulation systems may be exposed to a voltage stress for which they have neither been designed nor tested.

Abdel-Aty Edris received the BS degree with honors from Cairo University in 1967, the MS from Ain-Shams University in 1973 and the Ph.D. from Chalmers University of Technology, Gothenburg, Sweden, in 1979. He is presently manager of flexible ac transmission systems (FACTS) at EPRI. Previously, Edris spent 12 years with ABB in the development and application of reactive power compensators and high-voltage dc systems. He is a member of several IEEE and CIGRE working groups.
aedris@epri.com

Ben Mehraban received the BSEE and MSEE degrees from the University of Missouri, Columbia, in 1969 and 1971, respectively, and the MBA degree from Ohio University in 1985. He has more than 30 years of experience in engineering, design, coordination and project management for HV/EHV, GIS, HVDC and FACTS projects. Mehraban is a member of several IEEE-PES working groups.
bmehraban@aep.com

Lars Paulsson received the MSEE degree from Chalmers University of Technology, Sweden, in 1968. He is currently with ABB in development and application of FACTS controllers. Having joined ABB in 1969, Paulsson has held various engineering positions within the company. He has also been technical and marketing manager for IFO Electric High Voltage AB, Sweden, and worked in private business and consulting.
lars.h.paulsson@telia.com

Per Halvarsson received technical education from Malardalen University in 1987. Presently, he is manager of development and new projects at ABB Power Technologies, Power System FACTS. Halvarsson has worked since 1982 at ABB in the development and application of FACTS systems. HE is a CIGRE member.
per.halvarsson@se.abb.com

The Eagle Pass VSC BtB

The Eagle Pass Back-to-Back (BtB) consists of two separate 36-MVA VSCs connected to a common dc capacitor link. Via phase reactors and step-up transformers, each VSC is connected to the AEP-TCC and CFE 138-kV grids. Due to space limitations, the BtB station is compact with phase reactors and harmonic filters located outdoors, connected via cables to the indoor switchgear.

The BtB is designed to provide independent control of the real power transferred between the two grids. The rating of the medium-voltage ac sides is 17.9 kV, and the dc side is ungrounded. The ac sides are high impedance grounded via 50 Mohm voltage dividers, used for measurement purposes. All equipment on the 17.9-kV ac sides is rated 24 kV or higher and in accordance with standards for ac applications.