An electrical LV device can serve its desired function without getting impacted by the power surges if proper LV insulation coordination has been done during the design phase.
LV insulation coordination refers to an electrical study and design exercise that culminates in selecting properly rated industrial or domestic equipment with respect to an electrical power surge. In this exercise, the surge levels or voltages likely to appear on an electrical system or installation are evaluated and matched with the surge withstand capability of the electrical devices.
Thus any insulation coordination control exercise requires the following:
- Evaluation or estimation of the surge level and energy.
- Understanding the characteristics of the installed devices and the impact of their locations.
- Selection of appropriate surge protectors.
You must consider that for a surge protection device, there may be only one surge withstand.
What is a Power Surge?
A power surge is simply defined as an unexpected sudden increase in the voltage or the current levels of an electrical system or both. It has the ability to cause damage to electrical appliances or electronic devices due to dielectric or insulation breakdown.
The impact of the power surges depends on the following main characteristics,
- The duration,
- Steepness of the rising edge or frequency.
- Damping of the surge with distance.
Types of Power Surges.
Based on the cause of power surges, you can classify them into the following four types.
- Lightning Strikes.
- Electrostatic Discharges.
- Switching Surges.
- Power Frequency Surges.
The table below shows the main characteristics of these types of surges as per IEC 1000 – 4.
|Type of Surge||Duration||Rising Edge Steepness or Frequency||Damping with Distance|
|Lightning Strikes||In Micro - seconds||Very High (In 1000kV/μs)||Strong|
|Electrostatic Discharges||in Nano - seconds||Approx. 10 MHz.||Very Strong|
|Switching||In Milli - seconds||1 to 200 MHz||Average|
|Power Frequency||In Seconds or Hrs.||At Network Frequency||Zero|
The power surges get superimposed on the network voltages and can manifest themselves in two different ways, known as their modes.
- Common Mode: The common mode surges appear between the live conductors of the power lines or equipment and the earth.
- Differential Mode: The differential mode surges appear between the different live conductors of a power line or an electrical appliance.
How Often Do Power Surges Occur?
Power surges are more common than you think. In France alone, more than 2 million lightning strikes happen each year, resulting in more than 50,000 disruptions in the power and telephone networks.
Countless transformers and household appliances are rendered unserviceable because of this.
Causes of Power Surges and their impact?
Instead of a general discussion, let us understand how power surges happen type-wise and the impact or damage they can cause.
A lightning strike is usually preceded by severe storms and the formation of storm clouds. These charged clouds form a dipole with the ground leading to electrical fields in excess of 20 kV/m.
The stroke can be positive or negative depending on the polarity of the cloud with respect to the ground. If the cloud is positive, the stroke is positive. Positive strokes usually have more energy than negative ones. Stokes can also be classified as ascending or descending.
The lightning strike is called direct lightning if the object of interest directly receives the stroke. It is termed indirect if the element under study is not directly hit but suffers the effects of the strike.
Direct Lightning Strikes
Hence, if electrical installations like the overhead transmission or distribution lines or the substations are directly hit, they are subjected to considerable energy, as highlighted below.
- It is estimated that 50% of the lightning strokes have more than 25kA peak electrical current, 10% have more than 73kA peak, and 1% have more than 180kA peak current.
- The steepness of a lightning discharge can be more than 100kA/μs.
- Usually, there are multiple discharges separated by a few milliseconds following a stroke. The magnitude of the subsequent arcs may be less than the original.
- They can lead to electrocution, melting of electrical and electronic components, and fires in buildings. The lightning rod in the buildings or guard wires in the overhead lines may limit the damage or risks.
A lightning stroke may manifest itself at a distance by means of a conducted surge, rise in the earth’s potential, or radiation. Let us understand them briefly.
The surges are conducted through the overhead lines and may have a magnitude of several hundred kVs. Suppose direct lightning struck the MV network. The capacitive coupling of the transformer transfers it to the LV side. Usually, less than 4% surge amplitude on the MV side gets transferred to the LV side. In some statistical studies, it is found that in less than 98% of cases, the LV surges are below 6 kV, and in less than 91% of cases, it is below 4 kV.
Rise in Earthing Potential:
The heavy lightning current flowing through the ground causes a ground potential variation. These variations impact the earthing connections of the installations. The Potential U at any point at a distance D from the point of a direct strike is given by the equation:
U = 0.2 I ρS / D
where I = Lightning Current,
ρS = soil resistivity in Ohm – m.
D = distance in meters.
If the lightning current is 10,000 A or 10 KA, ρS = 500 ohm-m, the two points under consideration are D1 = 50 m (earthing of an electrical motor installation), and D2 = 100 m has a transformer neutral earthing.
The potentials at D1 and D2 are 20 KV and 10 kV, respectively. Hence a potential of 10 kV exists between the two earthing connections. The surge, in this case, depends on the soil resistivity.
This is usually the reason for the electrocution of four-legged animals in lightning, whereby the potential between their front and rear exceeds the tolerable potential.
A lightning storm and consequential indirect stroke can produce rapid variation in the electromagnetic field, which induces a very high voltage in the loop. These voltages may exceed seven hundred volts per square meter of the loop.
As stated earlier, the steepness of the rising edge exceeds 1000 kV/μs, and the resultant current is in the form of a pulse. Solid-state electronic equipment and electronic circuit boards are extremely vulnerable to the voltages induced by lightning and related current surges.
The studies of lightning-related power surges and overcurrents have resulted in the standardization of waveforms to the following:
- Current 8/20 μs wave, and
- Voltage 1.2/50 μs wave
The equipment is characterized according to these lightning waveforms withstand.
A person in very dry conditions can get electrostatically charged to very high voltages (in kV) due to friction. The effect is more pronounced if he is using a synthetic carpet. The discharge of this accumulated charge can exceed several tens of amperes and cause perforation of electronic devices.
The switching surges are typically caused by the rapid opening of the protection devices in the event of short circuits or the opening and closing of control devices like circuit breakers or contactors. These surges travel on the network as high-frequency waves but dampen out very fast.
Switching of Inductive Circuits
The making or breaking of an inductive alternating current can produce a high-amplitude impulse with a very low rise time. These are differential mode surges with peak voltage in excess of thousand volts and a rising edge of a few microseconds. The devices involved may include a switch or contactor of an electric motor or LV transformer.
Switching of Capacitive Circuits
Switching capacitor banks or overhead lines without load may result in the resonance of LC circuits and consequential oscillating surges. The consequential surge may cause the flashing of the spark gaps on the same circuit and tripping of the circuit breakers.
If re-arching happens after breaking the capacitive currents, the surge factor can reach a value of three.
Breaking of Large Currents by the Breaking Devices.
When large short circuit currents are interrupted very quickly by the circuit breakers, and the arc has not consumed or dissipated the energy, power surges may get generated. Another such case is when a circuit breaker interrupts an arc welding.
The waves encountered during the switching surges are classified into,
- Long damped – 250/2500 μs wave.
- Recurrent pulse – 5/50 ns – usually occurs on blowing of fuses.
- Damped sinusoidal 0.5 μs/100 kHz wave.
Surges at Power Frequency
It is another category of internal power surges, as the cause is an internal one. They are named “Surges at Power Frequency” because they assume the frequency of the network at 50 Hz, 60 Hz, etc. Some of the reasons may include
Holding currents of the MV Spark Gaps
Let us assume some external power surges from lightning impacted the MV line. This will lead to arcing of the MV spark gaps. Or there is an MV frame insulation breakdown in the MV/LV transformer.
If the spark gaps or MV frame are connected to the same earthing connection as the LV neutral, the holding current of the spark gap will cause a rise in the earthing potential of the LV network, causing a surge until the MV protection acts.
There can be multiple surges if any reclosure of breakers happens while the insulation fault still exists.
The Insulation Faults in the Network
If the neutral is unearthed or is earthed with an impedance, and one phase suffers from an insulation failure. The phase voltage of the other two phases with respect to the earth will rise to the phase-to-phase value.
Breaking of Continuity of Neutral
If any LV consumer has a single-phase supply, and somehow, there is a break in the neutral of the three-phase supply it is getting power from, the single-phase voltage it normally receives may rise to the phase-to-phase voltage.
We hope that this article on the causes and impact of a power surge was informative. Our next article will cover the principles of the design of a surge protector, different surge protection devices like Surge arrestors, filters, wave absorbers, power strips, etc, that may help you limit the power surge damage.