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【Sonel】注意!暫態! - 絕緣破壞! 電容器穿透! 電力電子裝置故障!

ATTENTION! Transients!
Insulation damage! Capacitors breakthrough! Failures of power electronics!
What truly are transients?
Transients are instantaneous voltage disturbances with significant amplitudes, high ramp speeds and short duration. The primary cause of transient disorders is the fact that the wires, cables or overhead lines have their finite perfection, which makes other factors important i.e. their resistance, serial inductance, capacity of individual cores and earthing capacity. These parameters are very predictable and taken into account during the normal transmission of electric power at a frequency of 50 or 60 Hz. However, they form a much more complex structure (described by surrogate parameters of long lines) in case of very fast changes in potential or electrical charge flow. It turns out that electric charges, flowing fast through such a long and multi-segment line, will energize local sections causing instantaneous voltage changes in individual components of this structure, complementing the basic parameters of voltage and current both during power supply to devices and during energy generation. These events may have the shapes of single pulses, e.g. 8/20 µs up to several kV, as is the case with atmospheric discharges. They also occur as resonant events with frequencies of 30 kHz ... 200. The last characteristic of transients is the fact that they are caused by one-time ‚boost’ of power, which causes additional effects and is systematically dissipated by them, resulting in the end of disruption(s). Therefore, these interferences are called transients.
Transients are instantaneous voltage disturbances with significant amplitudes, high ramp speeds and short duration.
Three examples of negative effects of transients
The first source of transients, mainly in voltage, includes typically the connection effects. They cause a very rapid equalization of charge distribution, i.e. short-term current flow, even with quite large values. This current causes additional voltage effects, overlapping its basic waveform. When a transformer is connected to a MV network of 15 kV, a few characteristic events may be observed (Fig. 1). In case of medium and high voltages, the connection process is accompanied by arc breakthrough occurring before connecting contacts of the connector. Due to the high energies of this event, the connection process is accompanied by dangerous over-voltages measured relative to the earth. The presented example shows that they can considerably exceed the amplitude of nominal phase-to-phase voltage of 21 kV, reaching even -48 kV. In this case, total duration of the transient caused by the connection is approx. 2µs. Despite the very short duration, the instantaneous values of the voltages have a significant impact on the insulation and the possible risk of a breakthrough causing network failure. In this example, the second transient is also shown. Activation of a three-phase transformer is accompanied by a short-term flow of high inrush currents and overfluxing of the transformer core.

Fig. 1a. Waveforms of connection process: a.) phase-to-phase voltages and phase currents.

Fig. 1c. Waveforms of connection process: b.) and c.) actual instantaneous values of voltages relative to earth captured by a high-speed recorder.

Currents of initial core overfluxing (Fig. 2a) may cause significant voltage distortions (Fig. 2b) and result in unjustified tripping of over-current protection devices, interrupting the normal activation process due to improper setting.

 Fig. 2. a.) and b.) Transient voltage distortions caused by activation of a high-power transformer.
Another frequent source of transients includes increasingly common power electronics. Modern semiconductor solutions offer very high switching speed that causes high speed of current increase. Transience in this case should be understood as a fading interference, which is repeated however at every key switching. It often occurs in commonly used method of generating analogue signals by keying with adjustable content. This requires the use of properly selected EMC filters on the network side to prevent interferences penetrating the power supply (Fig. 3). The switching frequency of 3 kHz is relatively low and may result in low efficiency of output filters or, in extreme cases, their absence caused by cost-saving solutions. The risk caused by this type of events is the transfer of interferences from power supply cores through parasitic coupling and electromagnetic waves. These interferences disturb the operation of sensors, transmitters, control signals for industrial automation and IT transmissions. Certain 3-phase interferences, even sporadic, may reach dangerous values relative to the earth (Fig. 4).

Fig. 3. Permanent interferences caused by switching power electronics.

Fig. 4. Sporadic disturbances of UL1 approx. 460 V.

Another example of a dangerous transient may be activation of output voltage in a high-power reserve UPS (Fig. 5). In the first moments of the start-up, hazardous transients may be generated with voltages to the earth (Fig. 5b) reaching 580 V in a 230 V network.

Fig. 5. Hazardous activation of UPS output: a.) waveform, b.) quick recording of transients (UL3 approx. -580 V).
Krótkotrwałe stany przepięciowe stanowią główne zagrożenie dla ochrony przeciwprzepięciowej oraz dla izolacji  kondensatorów, powodując w niesprzyjających warunkach przebicia i stany awaryjne.
The third case of transients may be surprising, as it concerns a typical low voltage power supply and transient disturbances caused by short-circuits on overhead lines. Examples shown in Fig. 6 to Fig. 8 are taken form LV networks in very harsh weather conditions, mainly gusty winds.

Fig. 6. Voltage shape disturbance.

A clear beginning and end of the disturbance, close to voltage peak, may indicate air insulation piercing by an object close to the power line. Such an object may be e.g. tree branches.
Transient surges, which significantly exceed the values of voltage amplitude should be suppressed by the surge arresters in efficient power systems. However, very frequent disorders of this type, especially with high-power discharges, may lead to „wear out” of surge varistor and the lack of effective surge protection. Therefore, it is important to detect and eliminate transients, before they cause any damage to the protection systems and consequently to inevitable system failure.

Fig. 8   Lightning discharge on LV line.
A few simple actions for performing diagnostics and measuring transients
he features described above, i.e. the rate of changes and their amplitude, may prevent noticing all transients using a standard power quality analyzer. The main limitation is the sampling frequency, which defines the frequency of reading out the instantaneous values that form a set of points that determine shape of the disturbance. Typical sampling frequency of standard analyzers are in the range of 10 ... 50 kHz, which is insufficient to efficiently record changes presented in the examples above. The only reliable and effective solution is an additional function of transient recorder, offered for example by PQM-703/711 analyzers. They allow to record transients with the sampling frequency up to 10 MHz and a bandwidth of 1.6 MHz. This means that the lightning discharge of 8/20 µs will consist of 200 instantaneous values.
In order to record transients using PQM-703/711 analyzers:
1. In voltage recording settings activate recording transient events (Fig. 9):

  • Set the sampling frequency depending on the expected events:
    • 10 MHz for lightning discharges - recording 2 ms of the waveform,
    • 1 MHz for industrial disruptions from switchboards - recording 20 ms of the waveform (1 period)
    • 0.1 MHz for low-frequency events – recording 200 ms of the waveform.
  • Select the expected threshold for record triggering.
  • Turn on the recording of waveforms and transient plots.
  • In additional settings of the configuration, the user may set the recording buffer at maximum.

2. Connect the analyzer, remembering about PE conductor and connect voltage test leads L1, L2 and L3.
3. Perform recording (START/STOP
4. Read the data for analysis using Sonel Analysis software.
5. In case of problems with the operation of the analyzer, use „Quick Start” and „Quick Manual”

Fig. 9. Configuration of transients.

Analysis of transient measurement results
After loading the measurement file containing the recorded transients, using Sonel Analysis:
1. Select ‚ Events’ tab and then select transients from other event types
2. Search the list of events for the event you want to analyse and by clicking the waveform icon open oscillographic waveforms:

  • The ‚Waveform’ window shows recording of disruption voltages and currents with the basic sampling frequency of 10.24 kHz,
  • The ‚Transient’ window shows oscillographic image of voltages to earth with a higher selected sampling frequency

3. By changing the time scale and the vertical scale, you may zoom-in sections of the recording that interest you,
4. By setting markers 1., 2., 3. in characteristic point, as shown in Fig. 1 to Fig. 8,  you can:

  • Read time and parameter value for each of the 3 markers presented by circle icon with number, 
  • On the basis of the differences, the time interval and values between single points of the same signal may be determined.

Author:Krzysztof Lorek