Thyristor based devices

In the solid-state FCLDs, semiconductor AC switch (two GTO thyristors connected in inverse parallel) is placed in series with each phase of the power line. In parallel with each switch is current limiting impedance as shown in figure 2-13. When a fault is detected, the normally conducting switch is turned off and the current is diverted to the parallel impedance which limits the current. A voltage arrester (Varistor) and a snubber circuit are connected in parallel with a switch to limit the level and the initial rate of rise of the transient voltage across the thyristors [2.10]. GTO thyristors now available can block forward and reverse voltages of up to 4500 V, carry continuous currents of up to 1100 A (rms) and interrupt peak currents of up to 3000 A [2.10].

26 Z Arrester Reactor SNBR GTO switch SNBR _SNBR

Figure 2- 13 GTO based CLD for a 15 KV circuit

Different circuit configurations based on the same principle were suggested. The parallel impedance was replaced by an inductor. Under normal load conditions, the GTO’s are gated continuously and maintained in full conduction. When a fault occurs, the fault current initiates a turn-off for the GTO’s. The GTOs of this type were connected in series to block the voltage, as the one used previously for 15 kV distribution system voltage class [2.20].

Then the inductor was replaced by a resistance, and in both cases the FCL does not utilize the full switching capabilities of GTO thyristors employed. This is because the current limiting action is essentially carried out by the parallel high-power impedance when the GTOs turn off at the event of the fault current [2.10]. Another development of FCL of this kind was the fault current interrupting device (FCID) for use at power distribution voltage levels. They have the advantages of very fast operating speed, produce no arc and incur very little maintenance [2.10].

Different types of FCIDs employing SCRs (silicon-controlled rectifier) or GTO thyristors have been investigated and employed in some applications [2.10] such as the main incoming transformer source leads, feeder applications, and bus tie position [2.20].

27 Varistor

SNBR GTO switch Snubber circuit

Figure 2- 14 Fault current interruption devise (FCID)

The current limiter interrupting device should have the following requirements: - 1. Interrupt the current in less than 1/2 cycle.

2. Reset only when both buses are healthy or by operator command.

3. Include isolating switches to keep it open when it is needed to be isolated.

4. Include bypass switches to carry current when no interruption by the CLID is desired. 5. Have continuous and overload ratings which match the capabilities of the circuit to

which the CLD is applied.

6. Meet the frequency, maximum continuous voltage and dielectric strength ratings prescribed by ANSI/IEEE standards for the voltage class to which the CLD is applied. 7. Limit transient voltages to the same level as permitted by other protective devices,

such as circuit breakers or reclosers.

Basically, the circuit layout of a FCID, shown in Figure 2-14, is like that of a FCLD except that the parallel current limiting impedance is not used. This enables the FCID to completely interrupt the current [2.10]. The FCID in existing power distribution network, although possible and advantageous as far as speed and maximum through current are concerned, suffers from the disadvantage of lacking coordination with downstream devices. On the contrary, the FCLD provides the possibility of limiting the fault current to safe limits and enable coordination with downstream devices. However, due to their inherent structure (the parallel low impedance), they do not have the capability to completely interrupt the fault

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current. As a result, a hybrid solid-state breaker using a combination of GTO thyristors, SCR's and current limiting impedance was proposed. The fault current limiting and interrupting device (FCLID) is very similar to FCLD, instead of switching the GTO thyristors off for the complete period of overcurrent and leaving the parallel element to bear whole limiting duty, the GTOs thyristors is continuously switched on and off. Although the fault current can be shared by both GTOs thyristors and the parallel element, this device still does not utilize the full capability of GTO and incorporates more losses. Hence, for the sake of a better performance, the parallel element was replaced by a nonlinear resistor (Varistor). During normal operation the Varistor is short circuited by the GTO thyristors, which are in the on state. On detection of fault current, the conducting GTO thyristor is switched off and the fault current is diverted to Varistor. When the transient overvoltage is suppressed and the Varistor current decreases, the GTO switch is turned on again. When the current reaches the pre-set value, the GTO thyristor is turned off again, and so on. The state of operation is maintained for a specific period. If the overcurrent continuous for a longer period, the GTOs thyristors are permanently switched off. The FCLID can limit the fault current as well as being able to interrupt it. It has a simple structure and fast response. Contrary, the FCLID suffers from the thyristors operating losses and the fault current takes several cycles prior to a complete interruption [2.10].