The multiple impulse method (SIM/MIM)
Cable fault pre-location method
The multiple impulse method (SIM/MIM) is one of the most successful fault pre-location methods. It briefly converts a high-resistive or intermittent fault into an easy-to-locate low-resistive fault, thus achieving the desired result in around 80% to 90% of all cases. It is therefore worthwhile first applying this method when pre-locating cable faults.
In 1999, BAUR refined the secondary impulse method (SIM) and in combination with the multiple impulse method (SIM/MIM), launched a solution onto the market which enables the better pre-location of all types of cable fault.
Not only is this method easy to apply and makes measurement results easy to interpret but in most cases, it also quickly leads to the desired objective. It is possible to locate almost all cable faults that occur during a typical cable fault location process using the SIM/MIM pre-location method.
In BAUR systems, a reflectogram is generated prior to the actual SIM/MIM measurement to obtain a picture of the cable's "health". The length of the cable and the position of the joints are visible on this reflectogram. A high-voltage surge pulse is then discharged from the time domain reflectometer via a surge voltage generator that is controlled by a coupling unit, and this pulse briefly converts the high-resistive fault into a low-resistive fault. For the brief duration of the breakdown (the short-circuit), it is possible to emit a secondary measurement pulse to pre-locate a fault using time domain reflectometry. The measurement device automatically uses the reflection from the fault location to calculate the distance to the fault location.
However, the time period during which the fault is triggered is very short. How short and for how long the short-circuit occurs after the high-voltage surge pulse is emitted depends on various influencing factors. For example, in damp conditions or if there is water in a joint, the time behaviour is different in terms of measurement compared to a faulty oil-and-paper-insulated cable. It can therefore be the case that the secondary measurement pulse for the time domain reflectometry is emitted too soon or too late to obtain a useable reflectogram. In this case, it is necessary to repeat the measurement with a different set of parameters. However, because of the many high voltage surge pulses, the cable experiences an unnecessary load and it requires more time. This is why, with the multiple impulse method, BAUR has developed an even more universal method, whereby a short series of up to 20 pulses is emitted instead of a single measurement pulse. This enables reflection measurements to be taken just prior to the fault being triggered, during the short-circuit and afterwards. At least one of the reflectograms is usually suitable for identifying the fault position.
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