Type Testing the 420-kV, 63-kA, 50 and 60 Hz EconiQ Circuit-breaker Based on C4-FN/CO₂/O₂ Gas Mixture Technology

Introduction

Dielectric tests

The SF₆ dielectric design process involves optimization of the various breaker parts to fulfill internally developed and widely verified guideline limits. During this process, the electric field (E-field) at each different breaker location is compared to the correct limiting value. The same well-established tools and processes were applied to the new SF₆-free breaker, by proper adaptation of the limits to the C4-FN mixture. The internal knowledge built over several years during the study of this and other alternative gases was the key to easily extending the design methodology to cover also C4-FN mixtures⁽⁴⁾⁽⁵⁾.

The E-field shielding design of the two-unit concept, as well as the arcing region dielectric design and verifications, were accomplished with the help of both 2D and 3D simulations and optimizations.

Following this process, the first design was achieved extremely efficiently and fast, and already fulfilled the requirements for both lightning impulse (LI) and chopped wave (CW) tests. The type tests confirmed all impulse and AC test duties including partial discharge measurements, where no partial discharge signal above the noise level could be recorded at all required voltage levels.

Capacitive and Shunt Reactor Current Switching Tests

Terminal Faults

The SF₆-free 420-kV circuit breaker successfully passed all the terminal fault tests for 63 kA (T10, T30, T60, T100s, and T100a) defined by the IEC and IEEE standards. The tests were performed in a manner that permitted the requirements of both the IEC and the IEEE standards to be met. The IEC requires a more complex sequence of test operations to be performed, including an open-close-open (O-CO) operation followed within three minutes (if not prevented by the limitations of the test laboratory) by a close-open (CO) operation. The IEEE standard, on the other hand, requires higher transient recovery voltage (TRV) peak values than the IEC standard for the T10 and T30 test duties. Since the IEC standard considers a test with higher voltage than specified to be valid, a test with the IEEE TRV also covers the requirements of the IEC standard. The TRV requirements for the T60, T100s, and T100a tests are the same in both the IEC and IEEE standards.

The minimum arcing times of the circuit breaker were found in all terminal faults to be rather low (comparable or lower than SF₆)—illustrating the strong performance of the circuit breaker. Given the short minimum arcing times and opening times provided by the fast drive, interruption can be achieved within two power-frequency cycles.

During the T100s and T100a tests, the maximum short-circuit current must be interrupted by the circuit breaker. As mentioned above, both test duties were successfully passed, for both 50 and 60 Hz. Concerning T100s, part (a) consists of making tests and the breaker demonstrated successfully asymmetrical making with peak factor 3.1, corresponding to 195 kA. Figure 3 shows two examples of successful T100a interruptions of the major loop current at rated asymmetry (134 kA peak, corresponding to a DC time constant of 75 ms) and at intermediate asymmetry during the type tests performed at 50 Hz.

The 420-kV SF₆-free circuit breaker has low post-arc currents, less than about 1 A and with a duration of only 10-20 µs⁽¹⁾. These post-arc currents were measured during the L90 test duty, i.e., the worst case, since the steep rise of the line-side voltage after current-zero leads to even higher post-arc currents than during T100s and T100a tests. Recently, it has been shown that vacuum circuit breakers struggle under such conditions⁽¹⁰⁾, in that, even after T100s current interruption, higher post-arc currents (of order 10 A) are measured, which create issues with voltage distribution between chambers in series. It should be noted that in vacuum circuit breakers the dielectric withstand voltage does not scale linearly, but only very slowly, with increasing contact distance⁽¹¹⁾. Therefore, for vacuum circuit breakers, employing multiple interrupting chambers for higher ratings (above 145 kV) becomes a necessity, rather than an option, with the intrinsic difficulty represented by the high post arc currents.

Figure 3—Example of T100a Test Shots at 50 HzLeft-hand plots correspond to first-pole-to-clear, major loop, intermediate asymmetry and right-hand plots to second-pole-to-clear, major loop, rated asymmetry. The top panel illustrates the applied short-circuit current, including the synthetic current injected before the final zero-crossing. The middle and bottom panel show the TRV across the breaker (the recovery voltage phase and a close-up view).

Short Line Faults

The breaker successfully cleared the SLF75 and SLF90 test duties for 63 kA covering IEEE and IEC standards and the initial transient recovery voltage (ITRV) requirements (with less than 100 ns time delay). From the type tests, since all were successful, it would not be possible to say which test, SLF75 or SLF90, has the larger margin. During development tests, though, margins were actively determined by increasing the current derivative at current zero (di/dt) and performing several additional breaking operations, concluding that SLF75 has a larger margin than SLF90. It has been speculated—based on a comparison to air-blast circuit breakers—that the SLF75 test duty may be more severe than the SLF90 test duty for CO₂-based circuit breakers⁽⁶⁾. However, in the case of the SF₆-free 420-kV breaker based on puffer technology, this is contradicted by the obtained results. This is similar to SF₆ and in line with expectations, since the 420-kV SF₆-free circuit breaker design is based on the reliable puffer SF₆ technology and current interruption physics. During type tests, shorter minimum arcing times and higher extinction peaks (see examples at 50 Hz in Figure 4) have been obtained for the SLF75 with respect to the SLF90, for both 50 and 60 Hz. This is also an indication of the better performance of the former.

During all type tests, the breaker showed consistently very short minimum arcing times. This is mostly due to the reliable puffer technology adopted and is an important indication of the breaker performance and clearing capability. A short minimum arcing time allows having shorter medium and maximum arcing times, with less energy input, less ablation, and finally, less wear of the breaker at the end of the tests. This allows a high number of short-circuit switching operations, as also demonstrated by the service capability test.

Figure 4—Arc Voltages during Similar OperationsArc Voltages during Similar Operations in SLF90 and SLF75 at 50 Hz, the latter having larger extinction peaks.

Conclusions

The first eco-efficient 420-kV gas circuit breaker based on a 3.5 mol% C4-FN, 10 mol% O₂, and 86.5 mol% CO₂ gas mixture was developed at Hitachi Energy and fully type tested, showing excellent performance. The breaker covers both 50 Hz and 60 Hz and was type tested according to both IEEE and IEC standards, covering the more severe requirements for both. The SF₆-free breaker also fully demonstrated the extended electrical endurance capability (E2). The breaker is based on a reliable gas circuit breaker technology utilizing the puffer principle. The same technology and tools are currently used to develop the next higher voltage rating aiming the release of world’s first eco-efficient 550-kV circuit breaker in 2024.