Figure: Understanding transients

A power line transient (electrical transient) is a temporary sudden increase in voltage and/or current on an electrical power line or electrical system that can disturb or damage the electrical & electronic products connected (& near) to the power lines. It has the potential to impact the AC and DC systems. The electrical transients can also be propagated via data or communication lines. Common causes of power line transients are load switching, capacitor bank switching, device faults, power grid switching, interruption of inductive loads, relay contact bounce, AC/DC switching operation, circuit breakers switched, lightning discharges, etc.

The power line transients are characterized as destructive high voltages (a few volts to several thousand volts) that drive large amounts of current, even in a low-voltage electrical system/circuit, for a few microseconds to several milliseconds. These transients are undesirable and momentary in nature. For example, when current flow through an inductive load is disrupted suddenly (by switching operation), the magnetic field associated with the current will collapse, resulting in voltage impulses or voltage transients across the inductive load, as per equation E = - N × dϕ/dt = - L × di/dt. The higher the rate of change of current, the larger the transient voltage spike can be. The transient voltage level typically is in KV, which can severely damage electrical/ electronic devices connected (&near) to the same line. 

Figure: Lightning transient phenomenon

 The power line transients (switching & lightning surges) appears as traveling wave on transmission lines, cables, bus sections, etc, and travels with a velocity of light (3 x 10⁸ m/s). The transients that propagate along the line are called conducted electromagnetic interference/disturbances (conducted EMI). These transients can also couple with other nearby power lines, data cables, and communication lines by means of capacitive coupling, inductive coupling, and radiated electromagnetic disturbance (radiated EMI). In these ways, the power line transients can interfere (i.e., induces overvoltage) with connected & nearby electrical & electronic devices, such as computers, telecommunication equipment, sensitive electronic circuits, and other devices, and can cause disruptions or damage to the normal operation of these devices, leading to malfunction or failure.

EMC, which stands for electromagnetic compatibility, is a field of study that deals with the ability of electrical and electronic devices to operate properly in their electromagnetic environment without interfering with other devices or being affected by interference from other devices. Power line transients are one of the sources of EMI that can affect the electromagnetic compatibility of electrical & electronic devices connected to the power line/system.

To ensure electromagnetic compatibility, devices need to be properly designed and tested to withstand the effects of electromagnetic interference (EMI), such as power line transients. This may involve using surge protection devices, proper grounding and shielding techniques while designing & producing devices, and compliance with relevant EMC standards and regulations. Power line transient related to EMC is an important consideration in the design, installation, and operation of electrical and electronic systems to ensure reliable and interference-free operation.

Table shows internal and external sources of electrical transients: 

 Types of transients:

According to IEEE 1159, transients can be classified mainly into two categories: impulsive transients and oscillatory transients.

Impulsive transients: 

Fig: Impulsive transient waveform nature (as per IEC 61000-4-5 document)

As per IEEE 1159, the impulse transient is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both that has unidirectional polarity (either positive or negative). The transient due to lightning is an example of an impulsive transient.  

The impulsive transients are described by their rise and decay times. Also, they can be characterized by their spectral content. For example, the meaning of impulsive transient 1.2 X 50-μs 1000-V is; it increases from zero to its peak value of 1000 V in 1.2 μs, then, it will reduce/decay to 50% of its peak magnitude value in 50 μs. 

 Oscillatory transients: It is defined as a sudden, non–power frequency change in the steady-state condition of voltage, current, or both that has bidirectional polarity values (i.e., has both positive and negative polarity values). This transient is also called the ring wave transient. This type of transient can occur on sub-transmission and distribution systems, especially due to capacitor bank energization. 

Fig: Oscillatory transient due to back-to-back capacitor switching

Transients waveform characteristics: 

The electrical transient waveforms are characterized by factors such as rise time, decay time, duration, front time, polarity, and peak value or test level. These are not the only factors used to describe these waveforms; generally speaking, a particular standard (or requirement) will also include further describing the transient pulse to a particular requirement.

Rise time (Tr): The time interval between the instants at which the instantaneous value of a transient waveform first reaches 10 % value and then 90 % value, is called rise time (Tr).

Figure: Rise time of common transient waveform (for Surge, Ring wave, and EFT-Electrical fast transients) 

Duration: It is the time interval between the instant at which the surge/transient current rises to 0.5 of its peak value and then falls to 0.5 of its peak value (Tw).

Fig: Transient waveform duration

Peak value: It is the peak value of the transient waveform; the peak value of the waveform typically varies by test level.

Polarity: The transient pulses may be negative and positive in polarity.

Fig: Shows negative polarity spike

Decay time: It is the time interval between the instants at which the surge/transient waveform magnitude decays to 50 % from its peak value.

Several factors decide the effect/severity of transients on equipment: The overall severity (effect) of the electrical transients on equipment is determined by several factors such as the rise time of the transient pulse, the peak value of the transient pulse, size of the transient source, location of transient in the system, and equipment operating environment (e.g., equipment in an industrial environment have a severe transient effect).

Associated standards for power line transients:

After manufacturing the electrical & electrical products, in order to ensure that the products can withstand the power line surges and operate properly in their intended electromagnetic environment, it is necessary to test the ability of the devices/products to withstand these transients/surges. EMC immunity tests are performed to ensure that the product can withstand the power line transients due to switching & lightning discharges & other causes that the device will encounter while in normal usage.

In the EMC test, a test signal is applied to equipment under test (EUT) to analyze the performance of EUT against the applied signal. The test signal (generated by a signal generator) simulates the electrical transients that the product will encounter in its normal indented operating environment. If the EUT withstands the applied surge/transient test signal voltage/current and operates properly, the EUT is considered to pass the test; otherwise, the EUT is considered as failed. The EMC immunity test is typically conducted in a laboratory environment as per the EMC immunity standard.

There are a variety of both international and manufacturer-based standards used to describe, define, and test the impact of electrical transients on equipment. Some commonly used standards related to power line transients are listed below. 

IEC 61000- 4-4: It is an international EMC immunity standard that specifies requirements and test procedures related to electrical fast transient (EFT)/bursts immunity test on electrical & electronic equipment, developed by IEC (International Electrotechnical Commission). 

IEC 61000 4-5: It is an international EMC immunity standard that specifies requirements and test procedures related to surge (switching & lightning transients) immunity tests on electrical & electronic equipment, developed by IEC.

IEC 61000-4-18:   It is an international EMC immunity standard that specifies requirements and test procedures related to damped oscillatory wave immunity tests on electrical & electronic equipment developed by IEC.  

EN 61000-4-4: It is a European standard for Electrical Fast Transient (EFT), equivalent to IEC 61000-4-4. 

ANSI/IEEE C37.90.1: It is the IEEE standard for surge withstand capability (SWC) tests for relays and relay systems associated with electric power apparatus. 

ANSI/IEEE C62.41: Provides information on surge voltages in low-voltage ac power circuits.