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Direct test circuits can be either fed by specially designed short-circuit generators or supplied from the network. A one-line diagram showing the principal layout of a high-power laboratory, equipped with generators, is depicted in Figure 10.2. Directly behind the specially designed three-phase short-circuit generator (Figure 10.4), the master breaker is placed (Figure 10.5). This master breaker has the duty to clear the short-circuit current in the case of a failure of the test object. After the master breaker, it is the make switch (Figure 10.6), which makes the short-circuit current to flow when it is closed. The current-limiting reactor (Figure 10.7) is used to add extra reactance in the circuit (if required) to match the current with the driving voltage. Because the terminal voltage of the short-circuit generators is relatively low (between 10 and 15 kV), specially designed short-circuit transformers (Figure 10.8) are necessary to transform the short-circuit power to a higher voltage level (at KEMA high-power laboratory till 245 kV to ground). In the case of a circuit breaker as a test object, the TRV-adjusting elements are usually connected at the high-voltage side

Figure 10.4 Two of the four short-circuit generators with their auxiliary machines in KEMA’s generator room (courtesy of KEMA)

Figure 10.5 Single-phase master breaker. This air-blast breaker has an operating pressure of 80 bar and is capable of clearing 160 kA_rmsin 7 milliseconds at 15 kV. Each generator phase has its own master breaker (courtesy of KEMA)

of the transformers. At the end of the one-line diagram, we find the test object that is usually solidly grounded. The test-object can not only be a switching element, such as a high-voltage circuit breaker, a load break switch, or a disconnector, but also a bus bar, a high-voltage fuse, a surge arrester, or a transformer. The high-power laboratory contains the hardware to simulate the electromagnetic switching transients as they occur in the distributed environment of the real network.

Apart from the hardware, adequate measurement equipment and sophisticated measuring techniques are essential while performing tests in the high-power laboratory. As mentioned earlier, improvements in test techniques and measurements make higher interrupting ratings and the application of new extinguishing media possible. For any interruption test, a number of records of current and voltage are required and they must be recorded at different timescales. Other time-varying parameters, such as the position of the breaker contacts, the pressure inside the interrupting

176 TESTING OF CIRCUIT BREAKERS

Figure 10.6 Single-phase make switch. Each generator phase has its own make switch, capable of making 270 kA_peakwith an accuracy of 3^°. The maximum short-time current is 100 kArmsfor 3 s (courtesy of KEMA)

chamber, and the temperature of the extinguishing medium describe the environment of the current-breaking process. For many years, magnetic oscillographs have been used, providing a rather slow but nevertheless rather complete record over the complete duration of a test, usually lasting a few cycles of current and voltage. With a time resolution of about 0.2 ms, the magnetic oscillograph gives an overall record of current, voltage, and the position of mechanical parts for adjusting the timing and the contact travel of the breaker contacts. The rather high-voltage insulation level of the current and voltage channels on the magnetic oscil-lograph has always been one of its strong advantages. The application of computer data acquisition and processing systems has resulted in the position of the traditional analogue magnetic oscillograph being replaced by the digital transient recorders in recent years. For very fast recordings, specially designed digital transient recorders are now available, which can sample every 25 ns with a resolution of 12 bits. In spite of the fact that

Figure 10.7 Current limiting reactors for one short-circuit generator. (courtesy of KEMA)

the very sensitive electronic circuits make the measurement setup prone to electromagnetic disturbances, the great advantage is that once the data are stored in digital form they can be analysed by the computer and stored permanently for future study.

Although the overall accuracy of a measurement system cannot be better than the weakest link, which is usually the transducer, recent improvements in the instrumentation have made it possible to overcome many traditional problems. Optical isolators and fiber-optic transmission make it less cumbersome to eliminate electromagnetic interference in the rather unfriendly electromagnetic measurement environment of the test bay in the high-power laboratory.

In the test circuit depicted in Figure 10.9, the test breaker TB is the high-voltage breaker under test. While performing a break test, TB is in a closed position. In addition, the master breaker MB is closed, but the make switch MS is open. After the generator is spun up to the nominal power frequency and the rotor is excited to a voltage, giving the required rated testing voltage at the high-voltage side of the short-circuit transformer,

178 TESTING OF CIRCUIT BREAKERS

Figure 10.8 Short-circuit transformers. (courtesy of KEMA)

Ls

u_s

i_s

C_d

C1

R1

C₂₂ R₂₂ L22

TB

Figure 10.9 Single-phase test circuit for a short-circuit test on a high-voltage circuit breaker

u_s i_s

TRV

0 0.01 0.02 0.03

Time [s]

−4

−3

−2

−1 0 1 2 3 ×10⁵

Figure 10.10 Current and voltage traces of a single-phase current interruption

the make switch is closed and the short-circuit current flows through the TB. When the mechanism of TB receives an opening command, the breaker contacts move apart and the TB interrupts the current. After a successful interruption of the short-circuit current, the TB is stressed by the transient recovery voltage, coming from the oscillation of the TRV-adjusting elements R₁, C₁, C_d, R₂₂, L₂₂, and C₂₂ together with the inductance formed by the current-limiting reactor L_S, the synchronous reactance of the generator, and the leakage reactance of the short-circuit transformers. When the transient recovery has damped out, the TB faces the power frequency–recovery voltage. In Figure 10.10, the current and voltage traces of a single-phase current interruption are shown.

When the circuit breaker has to perform a make–break test, the breaker must close in on a short circuit. Before a make–break test, the TB is in open position and it closes after the MS has closed. Therefore, TB senses the power frequency supply voltage, and when the contacts of TB close the short circuit, current flows through the test circuit. In Figure 10.11, the current and voltage traces of a single-phase make–break test are shown.

The DC component of the current is determined by the instant of closing of the TB. The supply circuit is mainly an inductive circuit and this implies that if TB closes when the supply voltage is at its maximum, the DC component is zero and the current is called symmetrical. When TB closes at voltage zero, the current starts with maximum offset and is called asymmetrical. Because of the ohmic losses in the test circuit, the DC

180 TESTING OF CIRCUIT BREAKERS

0 0.02 0.04 0.06 0.08 0.10

Time [s]

−3

−2