Lightning strikes on power systems are common. A storm is overhead and lightning attempts to find the easiest pathway to ground. This could be through a transmission line or tower, through a lightning rod and down conductor, or through a number of more unconventional paths. When lightning strikes, there's a possibility that serious damage can occur. Even with the best protection systems, lightning is famously unpredictable (think "lightning never strikes the same spot twice"). So, what can we do?
Figure 1: A Lightning Storm Near an Overhead High Voltage Transmission Line
What is Lightning? - Well, the first and easiest thing we can do is understand just what lightning is. This might sound obvious, but in order to protect against lightning strikes to our electrical system, we need to know how to model the strike itself. Lightning is modeled with an 8/20 microsecond current wave. This means that the lightning rises to its maximum current value in 8 microseconds and returns to half its peak value in 20 microseconds. The IEEE definition of this waveform uses a pair of exponentials, but for high level design it's suitable to use a triangular wave shape (See Figure 2 below).
With a pulse this quick, usual steady-state power system analysis is far from suitable. Changes in current values on the order of microseconds means that the dominant frequencies in the system are on the order of MegaHertz. The effectives impedance from capacitances in the power system are no longer trivial (since capacitive reactance is proportional to the inverse of the source frequency) and the effective impedances from inductances are massive (since the inductive reactance is proportional to the source frequency). In order to analyze the effect of a lightning strike in detail, it's necessary to know the values of stray capacitances in the system (transformer winding to ground, cable to ground, etc.). Moreover conductors and buses usually need to be modeled as transmission lines to capture the transient behavior of the pulse moving through them, including voltages developed from surge impedance (also known as characteristic impedance) of the line. Unfortunately, all of this detail can be difficult to obtain and may significantly affect the accuracy of results.
Figure 2: The 8/20 microsecond Lightning Current Waveform
The 8/20 microsecond waveform describes the shape of the lightning strike, but what about the magnitude? According to the National Weather Service, a median lightning strike is about 30,000 Amps with a Norton Equivalent resistance of about 10 kiloOhms. IEEE standards provide additional estimates on the value of strike currents and the probability distributions.
Figure 3: Model of a Lightning Strike in a Circuit
Basic Lightning Insulation Level (BIL) - Although we model lightning as a current source, the bigger problem is usually the voltages developed from lightning. Conductor withstand for such a short duration is generally not an issue, but when the current passes through the system's impedance to ground, overvoltages will develop in accordance with Ohm's Law. They will be brief but put incredible strain on the electrical system. Equipment is rated to withstand these events by having proper electrical insulation, designed to not break down over short time frames of immense electric field strength. The ability to withstand these induced voltages is characterized by a piece of equipment's basic lightning insulation level (BIL). Various IEEE standards prescribe the minimum requirements of BIL based on the system operating voltage and the type of equipment. The NEC also lists blanket minimum requirements for BIL on systems up to 230kV (See Article 490 of the 2020 Code). As long as the lightning-induced voltage is below the equipment BIL rating, the equipment should be protected.
Surge Arresters - Having the minimum BIL for a piece of equipment isn't usually sufficient protection. Without a low impedance path to ground, overvoltages will develop on equipment and may exceed the system's limitations. Surge arresters solve the problem of providing a low impedance path from phase to ground without compromising normal operation. A surge arrester can be thought of as a nonlinear resistor (also known as a varistor). During typical operating voltages, the resistance to ground is so high that the system behaves like normal. However, the surge arrester offers a very low resistance when a large current passes through, limiting the voltage developed across the arrester to something on the order of 2-3x the arrester's continuous operating voltage rating.
Figure 4: Surge Arrester Curve Example
Generally, surge arresters are applied between the phase conductor and earth. On an ungrounded system, such as a delta or ungrounded wye, the surge arresters need to have a maximum continuous operating voltage (MCOV) equal to the line-line voltage of the system. This is because a single-line-ground fault in one phase can lead to elevated voltages in other phases due to reference point shifting. On solidly grounded systems, the MCOV must be equal to or greater than the line-neutral voltage of the system. NEC Article 242 in the 2020 Code lists these requirements and more. IEEE C62.11 provides even more detailed information.
Summary - With the right surge arrester and BIL we'll go a long way to making sure our equipment is protected, even if there is a lightning strike. But, as mentioned at the beginning of this article, lightning is very unpredictable and there's always a possibility that a strike with a fast wavefront (higher induced voltage on inductances) and a large peak current (higher induced voltages on resistances) will lead to excessive overvoltages. There's no way to protect a system 100%, but it's possible to bring it close!
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