Switch fault

In the previous section, each discrete switch state has been taken into account by a row in the FSM. System mode changes can be viewed as faults in discrete switch states. This means that there are far more fault candidates than sensors. Accordingly, discrete switch states will share the same component fault signature so that a switch fault cannot be isolated. If multiple simultaneous faults have happened only in parameters that change continuously with time then parameter estimation can be used to isolate them. However, if there are discrete switch state faults among the multiple simultaneous faults then parameter estimation may result in meaningless real values for the discrete switch states. Therefore, in [6], Alavi and her co-authors propose to chose a combination of switch states, to insert them into the functional to... 

Finally, the knowledge of the current values of discrete switch states can be used to detect and to isolate switch faults by rule-based reasoning. If switches toggle then-state at a high frequency little time is left for mode identification. However, again, parallel processing can help to cope with the time constraints. 

The previous section considers a simple DC to DC boost converter with two switches controlled by two complementary signals. The healthy boost converter thus may be in one of two feasible modes. Reference [18] studies switch faults in a simple single phase half-bridge inverter. In [17], bond graph-based FDI is applied to a single phase H-bridge inverter. Both works represent switches by means of controlled junctions, i.e. use hybrid bond graphs. 

S.2.4.2 Open Switch Fault Detection by Means of Park s Vector Method... 

One conventional method to detect open switch faults in voltage source inverters is to transform the three line currents ia, i, ic into a domain with two currents id, iq and to study the shape of the trajectory iq vs id- An advantage of the method is that no further sensors are required. 

Figure8.46a displays the diode current 14 in the healthy system and the diode current 14 in the faulty system. As the two currents are identical when D4 is on in the healthy system, the current t4 has been scaled to better distinguish it from the faulty one. A comparison with Fig. 8.42 shows that a short-circuit fault in diode D4 affects the output voltage Ud less than an open switch fault. The values of Ud are lower by about 10 V whenever the faulty diode current t4 is negative.

The structural information of the ARRs has been captured in an all-mode FSM. With the sensors used, all faults can be detected but cannot be isolated by analysing ARRs. It depends on how a system to be considered is built up whether further sensors can be added so that voltages across switches and currents through them can be measured. For instance, if there is a fault in one leg of the three-phase inverter or the rectifier, it can be due to a fault in one of the two switches in that leg. As to open circuit faults in switching cells, characteristic trajectories can be obtained after application of the dq-transformation that allow to isolate open switch faults. This technique not based on ARRs but often used in power electronics has been illustrated in Sect. 8.2.4 for an open circuit fault in the lower switch Sw4 of inverter leg a . 

The studies of fault scenarios in the power electronic systems considered in this chapter focus on a single switch fault and its impact on the load. The switch fault is achieved by a modification of the control signal. It is assumed that a switch changes instantaneously from a healthy state into a faulty one. The latter one is accounted for by a modification of its discrete state variable. 

Pearman, G. R. Automatic Busbar Transfer Switch Fault Tree Analysis. Marine Periodica Polytechnica Mechanical Engineering 36, no. 1 (1992) 395—407. 

FIG. 29-5 Siiiiplificd schciiiatic dia raiii of a coiiilunation line starter with a circuit lirciLker as the fault interrupter and disconnect. Alternative fuses and disconnect switch are shown as siilistitiites for the circuit lirciLker,... 

Cuncnt imiisieiits A similar situation will arise wheti a switching ON operation of the rectifier unit occurs hen it is a thyristor rectifier. Under load conditions, the stored magnetic energy in the incoming supply system, which can be the feeding transformer and the line reactances similar to a fault condition discussed earlier, may cause a current transient which can be expressed by... 

For oil-filled control equipment such as autotransformer starters or oil circuit breakers (BOCBs or MOCBs, Chapter 19), insulating oil should be checked periodically for its insulating properties. Leading manufacturers of this equipment indicate the number of switching operations under different conditions of load and fault, after which the oil must be replaced and these must be followed. 

Thermal capacity to perform the required switching duties and sustain the fault conditions, at least up to the cut-off time of the short-circuit protective device, say, the HRC fuses. 

The type of duly defines the capacity of a switch or a contactor by the value of the current and the p.f. of the associated circuit, it can make or break on fault. For value.s of currents and p.f. for different duties, refer to the relevant standards as noted above. 

The rated momentary peak value of the fault current, /y, will relate to the dynamic rating of an equipment. It is also known as the making current of a switching device and defines its capability to make on fault. 

An isolator, such as at locations C and Ct in Figure 13.29, may sometimes be used to isolate the circuits it is feeding, say, for maintenance or repairs. This isolator is simply a switch and provides no protection to the circuits. For a fault on the outgoing side, the individual outgoing feeders must have their own protection. Fora severe fault elsewhere in the system, there must be a protective feeder closeby, in the upper stream. 

For a switching device (which has not been previously tested for a short-circuit test). This should be closed and held in the normal service position. The test voltage (that would generate the required level of fault current) may be applied on one set of terminals, the other terminals being shorted. The test may be continued until the short-circuit device operates to clear the fault, but in no case for less than 10 cycles. In LT assemblies the point where the short-circuit is created should be 2 0.4 m from the nearest point of supply. 

This is not material in voltage transformers, as neither the voltage measuring instruments nor the protective relays will carry any inrush current during a switching operation or a fault. No short-time rating is thus assigned to such transformers. 

Ammeter selector switch 32. Over current ground fault... 

Tripping an inductive or capacitive circuit immediately after a switch ON as a result of a momentary fault or for whatever reason and even during a fast bus transfer (temporary tripping and reswiiching). 

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