Transients are divided into two categories which are easy to identify: impulsive and oscillatory. If the mains signal is removed, the remaining waveform is the pure component of the transient. The transient is classified in the impulsive category when 77% of the peak-to-peak voltage of the pure component is of one polarity. Each category of transient is subdivided into three types related to the frequencies contained. Each type of transient can be associated with a group of phenomena occurring on the power system.

The impulsive low-frequency transient rises in 0.1 ms and lasts more than 1 ms.. Its companion, the oscillatory low-frequency transient, contains frequency components up to 5 kHz. These types are the most common transients recorded on a power system. They are not only easily propagated but they can also be amplified by a power-system resonance phenomenon. Measurement of these types of transients should be useful for all classes of application (benchmarking, legal, trouble shooting and laboratory)

The medium-frequency impulsive transient lasting between 50 ns to 1 ms and oscillatory transients between 5 and 500 kHz are less frequent than the low-frequency types but have much higher amplitude. These transients may not propagate as easily as the low-frequency types but may cause arcing faults on the power distribution system which result in voltage sag on many user power systems. It is most appropriate to measure these types of transients for trouble shooting and laboratory classes.

High-frequency types with high amplitude can be observed only near where the phenomenon occurs. The high-frequency impulsive transient has duration below 50 ns and the frequency of the high frequency oscillatory type ranges between 0.5 and 5 MHz. These measurements are useful for laboratory and trouble shooting classes of application.

A very severe transient can result in a negligible overvoltage if a surge arrester installed nearby diverts its energy off line to protect the equipment, although this can cause damage to arresters. To ensure that useful information is gleaned from measurements, a technique for assessing transients and recommendations for the window width and sampling rate are given below.

The basic reason for measuring transients is to compare measured values with those used to verify the immunity of electrical and electronic equipment connected to the distribution system.

It should be kept in mind that the transients-immunity test aims to predict the performance of electrical equipment over its normal service life. To certify an electrical appliance as acceptable for use, manufacturers employ transient generators that comply with the standards applicable to the equipment to be tested. These standards specify an open-circuit voltage Ug which decreases at the terminals of an impedance Zs at the moment the generator injects a current into the equipment under test. This impedance Zs is known as the artificial mains network or line impedance stabilization network (LISN) which is specified as a function of the range of frequencies contained in the transient, as follows:

In immunity tests, the reproducibility of transient severity is ensured by the LISN which supports the idea of associating the measured transient current to the transient voltage drop across the LISN. The source voltage corresponds to the sum of the actual measured transient voltage and the assessed voltage drop across the LISN produced by the measured transient current. This approach has the advantage of offering a common basis for comparison by fixing the supply impedance and offers the possibility of standardizing the load impedance by connecting a non-linear load.

The LISN assessment approach allows transient severity to be assessed for any type of load. Therefore, the addition of non-linear load at the point of measurement limits the voltage scale of the instrument without affecting the of the transient severity assessment.

In summary, the LISN assessment approach gives adequate source voltage waveforms U_aS, U_bS, and U_cS so that a transient generator could reproduce transients U_a, U_b, and U_c as recorded if connected to the same load. These voltage waveforms include the measured voltage and the low-, medium- and high-frequency voltage components U_a-LF, U_b-LF, U_c-LF, U_a-MF, U_b-MF, U_c-MF, U_a-HF, U_b-HF, U_c-HF across the impedance of the assumded system which is produced by the transient current at the PCC. Variables U_a-LF, U_b-LF, U_c-LF, U_a-MF, U_b-MF, U_c-MF, U_a-HF, U_b-HF, U_c-HF can be calculated from the measured current.

The transient voltage analysed should include the fundamental and all harmonic components of the power-system voltage. However, for classification purposes, the transient voltage should be extracted from the mains voltage.

The transient current should be filtered using a digital filter to supply the low-frequency (below 9 kHz) component I_a-LF(t), I_b-LF(t) and I_c-LF(t) and the high-frequency (above 9 kHz) component I_a-MF(t), I_b-MF(t) and I_c-MF(t). The filtered current then serves to rebuild two transient voltage waveforms using components below and above 9 kHz as needed in Equations. 1 to 5^§ which estimate the voltage drop across the source impedance of the assumed system:

[1]

where

[2]

where

[3]

where

[4]

where

[5]

where

[6]

where

Lastly, the source voltages U_aS, U_bS, and U_cS to be compared to the values recommended in the standards for susceptibility tests are

[7]

[8]

[9]

The measurement instrumentation therefore needs a four-cycle window width of recorded data so as to analyse capacitor switching transients with a frequency component below 5 kHz [IEEE 62.41]. Tests have shown that a minimum sampling rate of 20 kilosamples per second (ksamples/s)is needed to record this type of low-frequency transient which is 4 sampled values per period of 5 kHz signal. However, 4 ksamples/s sampling rate records properly most low-frequency transients which are produced by capacitor-switching transients characterised by frequency components below 1 kHz.

- for legal class : sampling rate should be above 36  ksamples/s

- for benchmarking class : sampling rate should be above 4 ksamples/s,.

A power distribution system comprises multiple interconnected elements. At each connection, the surge impedance changes, with the result that the transient can be reflected and increase as much as twofold at the point of reflection, becoming a local ring wave. The sampling rate for recording this type of transient should be higher than 2 megasamples per second(Msamplse/s) but IEC 816 recommends 150 mega-samples per second.

- for laboratory class: sampling rate should be above 150 000 ksamples/s

- for trouble shooting class: sampling rate should be above >2 000 ksamples/s,.

Transient  recorded waveforms do usually not contain exactly the frequency components composing the reference transient used to test equipment. However, the severity in terms of energy can be assessed by comparing the rms voltage envelope of the recorded transient to the rms voltage envelope of the reference transient.

The voltage envelope consists of a curve showing the severity of the disturbances as momentary rms voltage deviations from the declared voltage that lasts a certain period of time.

The rms voltage assessment is used to assess the rms voltage envelope for a duration exceeding a half cycle. However, the voltage envelopes of transients lasting less than a half cycle are more complex to assess and need a special assessment method.

When the supply voltage U(t) includes a short transient detected at time a , the percent voltage Vp of interval T related to the voltage envelope is given by:

% [10]

where:

The aim of transient analysis is to identify a suitable time a which yields the maximum Vp calculated with Eq. 10.

The following procedure is used to assess the Vp maximum values related to the interval T for all incremented a :