This field probe unit measures system harmonics. Since it is not connected to the LA circuit at all. It only deals with system voltage and measures system harmonics. And these system harmonics are used as a reference and are deducted from the total harmonics measured by the CT unit. Leaving behind only those harmonics, which are generated by ZnO blocks that is LA. In some measurement kits, system harmonics compensation is not done using the field probe unit. And 3rd harmonic resistive leakage current is measured without system harmonic compensation. But for more accuracy, it is better to use system harmonic compensation.
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The next obvious question is, why do we need to do system harmonic compensation for better accuracy? We know that non-linear loads and equipment, such as transformers, motors, wave-traps, etc. are connected in electrical substations. Harmonics are generated due to these non-linear loads, which include the third harmonic also. On the other hand, harmonics are also generated in the LA ZnO block, due to the non-linear resistive properties of ZnO elements. Which are mostly comprised of 3rd harmonics. Since 3rd harmonic component is predominant in the resistive leakage current of ZnO blocks, which is roughly around 10% to 40% of resistive leakage current. Hence, we focus, mainly on calculating the third harmonics due to ZnO blocks only, and not the system harmonics, for the exact health assessment of LA. Because this 3rd harmonic resistive leakage current is the main cause of deterioration of ZnO blocks. And subsequently LA failure. That’s why we do system harmonic compensation for better accuracy.
To brief it, simply. We can say, the CT unit measures total leakage current along with system-generated harmonics and harmonics generated due to ZnO blocks since it is clamped directly to the LA counter cable. Whereas the Field probe unit measures only system harmonics since it is not connected to the LA circuit. Hence CT unit measured values minus field probe unit measured values, gives ZnO generated harmonics values.
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Now, once the connections and measurement set-up are ready, we press the test button on the kit. We get this reading. Total leakage current and resistive leakage current. Now, to decide, whether LA is healthy or not. We need to see the resistive leakage current value (Ir). This (Ir) value ranges from 10 microamperes to 600 microamperes, depending upon the rating, design, and age of LA. In this example, the total leakage current (It) is 755 microamperes. And resistive leakage current (Ir) is 31 microamperes. We can say, the condition of this LA is ok, based on the (Ir) value.
The least common multiple is the smallest number that two or more numbers will divide into evenly. This means that the LCM must contain all the prime factors of each number in question. Suppose we wanted to find the LCM of the numbers 15 and 12. 15's prime factorization is 3 times 5, whereas 12's prime factorization is 2 times 2 times 3. So to find the LCM, we will simply move from left to right putting the missing primes directly into the LCM. Looking at this number 15 here, notice that we have the 3 and the 5 being the primes of 15 so we simply place those into the LCM. Now that we are done with the 15 and we have both the 3 and the 5 in there, we now move to the 12. So notice, in this 12 we've got this 2 times 2 times 3. Notice in the LCM here, the 3 is already there so we can discard the 3. What's missing in this LCM here are these 2's, so we simply place the 2's in the LCM. Now that we are done with both numbers, all we have to do is multiply them all together which, now, that equals 60. But remember the LCM is the smallest number that two or more numbers will divide into evenly. To check ourselves, all we have to do is say 60 divided by 15 equals 4 and 60 divided by 12 equals 5. Both numbers divide evenly, therefore, the definition of LCM is satisfied.
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