Preventing Electrical Overstress in Power Systems

Multi-column high-energy surge arresters: when and why they are needed In modern power systems, the continuous expansion of long AC and DC transmission networks, the integration of renewable generation, and the deployment of FACTS and HVDC converter stations have dramatically increased the energy content of transient overvoltages. Under such operating conditions, single-column surge arresters often reach their limits in thermal energy absorption. When system studies or simulations reveal multi-megajoule energy levels, the solution is to use multi-column high-energy surge arresters, which distribute the energy through several parallel-connected columns. These arresters are typically applied in: - Series and shunt compensation systems (FACTS) – to absorb energies released during capacitor or reactor switching and to protect thyristor or IGBT-controlled components. - Switching control of long HVAC transmission lines – where line lengths above 500 km lead to high switching surge factors (2.0–2.7), resulting in total energy stresses up to 20 MJ for 550 kV systems. - HVDC converter stations – where overvoltages originate from commutation events, load rejection, or cable switching, requiring arresters capable of 10–100 MJ. -HVDC circuit breakers – where the entire load circuit energy must be dissipated during current interruption, a case demanding extremely high energy reserves. Designing for such conditions is not only about the total energy rating but also about ensuring equal current sharing among parallel columns. Even small differences in the voltage–current characteristics of individual columns can lead to unbalanced energy absorption and thermal overstress. For this reason, multi-column arresters undergo current distribution tests to verify symmetrical current flow under impulse conditions—a topic that will be discussed in detail in the next post of this series. Multi-column high-energy arresters are therefore not simply scaled-up versions of standard arresters—they represent a different design philosophy, combining electrical symmetry, thermal coordination, and mechanical integrity to handle the most demanding overvoltages in today’s AC and DC grids. References: Philipp Raschke. (2025). Guidelines for Specification, Selection & Lifetime Management of Multi-Column High-Energy Surge Arresters for HVAC & HVDC Systems. Tridelta Meidensha GmbH, INMR World Congress 2025, Panama City.

Imagine a fault occurs, but it's outside the transformer differential (87T) zone. The transformer has nothing to do with it, right? So, why bother? Well, the truth is more interesting than that: even if the fault is external, the transformer still feels the consequences. External faults or system conditions, can create thermal, electrical, or mechanical stresses that directly impact aging, reliability and protection. Let's start with the simplest one: overload. An overload forces the transformer to work hotter than it was designed for. The heating time constant is long, so the danger isn’t instantaneous, but persistent exposure shortens insulation life. In many utilities, overload protection is not applied on large transformers. Operators get an alarm and must act before the long-term damage accumulates. A common cause of overloads is unequal loading of parallel transformers or unbalanced loading in 3 phase banks. Then we have overvoltage and overexcitation. Overvoltages often appear after sudden load rejection on an isolated section of the system. When voltage increases, the V/f ratio rises and so does the core flux. This drives iron losses higher and causes the exciting current to surge. This causes lamination insulation, core steel, and winding insulation to face rapid heating. This is why utilities rely on dedicated Volts/Hz protection (ANSI 24) to trip before the transformer enters damaging overfluxing. Underfrequency brings a similar risk. Even if voltage stays normal, a drop in frequency increases the flux and pushes the core into overexcitation. The most severe condition  occurs when both high V and low f happen simultaneously. This is why most transformers are not allowed to exceed roughly 1.1 to 1.2 pu V/Hz for steady-state operation, with short duration limits slightly above that. And of course, we have external short circuits. A heavy external fault usually does not electrically damage the transformer (if cleared quickly), but it delivers very high mechanical forces to the windings. These forces scale with the square of the current and peak within the first half-cycle and relays can't operate fast enough to mitigate that initial shock. The transformer must be mechanically designed to withstand these through-fault stresses. Protection only limits how long the fault lasts, not the intensity of that first cycle. So, it is worth noting that some externally caused stresses cannot be eliminated by protection alone. They must be addressed by transformer design, system design, and operating practices. ______ For the protection engineers and transformer specialists reading this: How do you approach V/Hz limits, external fault stress, and overload alarms in your projects? What practices have you seen utilities or manufacturers adopt to manage these external conditions? _____ Add your perspective in the comments or share this post with your network so the thread can gain momentum without heading into overfluxing!!