What is an electrical power surge in medium and high voltage networks?

A power surge is a temporary increase in voltage in an electrical installation, power network, or device that exceeds its rated operating voltage. Because permissible voltage levels vary depending on the system and electrical devices, there is no single universal threshold that defines a surge. Instead, surges are described using the surge factor (k_p), which expresses the surge amplitude as a multiple of the highest operating voltage relative to grounded components.

What types of power surges exist? How can they be prevented, and how can infrastructure be protected against their effects? These questions are addressed in this article.

What types of power surges are there?

The most common classification distinguishes them based on where the power surges occur. There are internal (system-generated) surges and external surges, also known as lightning surges. As the names suggest, internal surges originate within the electrical system itself, while external surges are caused by atmospheric phenomena.

Let’s take a closer look at each category.

Internal surges – Temporary (slow-developing) surges

These surges are caused by sudden changes in network configuration or load conditions and are therefore classified as slow-developing. Temporary surges are typically divided into three subtypes:

1. Ground faults – caused by a sustained short circuit to ground (lasting from approximately 0.1 seconds up to several hours), for example between a phase conductor and earth.
2. Dynamic – resulting from a sudden loss of load, such as the abrupt disconnection of a large consumer from a generator at the end of a transmission line.
3. Resonant/ferroresonant surges – caused by disturbances affecting one phase of the system.

For this category, the surge factor (k_p) typically ranges from 1.1 to 1.5.

Internal surges – Switching (fast-transient) surges

Switching surges occur as a result of switching operations within the network, such as turning equipment on or off. Because their characteristics can vary widely, this group is further subdivided.

In practice, switching surges are classified as:

1. Operational surges – caused by intentional switching operations.
2. Emergency surges – resulting from intermittent ground faults. 

Switching surges have a higher surge factor than slow-developing surges, typically in the range of k_p = 2 to 4.

External surges – Direct and induced lightning surges

Direct surges are caused by electrostatic discharges, most commonly lightning strikes. Specifically, this refers to a direct lightning strike on live conductors or structural components of the power network, or on equipment connected to it.

This is the most serious threat to medium- and high-voltage installations. From the moment of impact, a lightning current can reach peak values of up to 200 kA within a few microseconds, and the resulting surge propagates along the power line in both directions. Such voltages far exceed normal operating levels and can overload installations or, in extreme cases, cause severe damage to electrical and electronic equipment.

Induced surges occur when lightning strikes near power lines or infrastructure rather than directly hitting them. The intense electromagnetic fields generated during the strike induce a short-duration voltage increase that may exceed the insulation strength of the system.

In both direct and induced cases, the surge factor (k_p) typically exceeds 5.

What happens after power surges occur depending on network type?

A sudden, short-term rise in voltage poses a serious risk in any electrical network. However, failures in medium- and high-voltage systems usually have far more severe consequences than those in low-voltage installations.

Examples of the damage caused by power outage and surges

For example, if a direct surge causes transmission cables to overheat, power transmission must be interrupted until the damaged components are replaced. This results not only in material and repair costs but also in losses associated with potentially extended power outages.

While a power surge in a residential installation can certainly cause serious inconvenience and damage, its impact is relatively minor when compared to disruptions affecting socially or strategically critical facilities such as hospitals, industrial plants, or key infrastructure.

Selecting appropriate surge protection devices 

Lightning strikes and switching operations cannot be completely eliminated. For this reason, any well-designed medium- or high-voltage power system must include appropriate surge protection measures.

In low-voltage systems, protection often takes the form of surge protectors or power strips used in homes and offices. In contrast, medium- and high-voltage networks rely on specialized surge arresters of various energy classes, installed on poles and in substations. The selection of these devices must always be based on an analysis of system parameters and operating conditions.

If you are looking for surge protection solutions for your project, please contact our sales department. Our specialists will be happy to help you select devices tailored to your technical requirements.

FAQ

What is the meaning of power surge?

Power surge is a sudden voltage spike above safe operational levels, caused either externally or internally. Power surges happen within a system, as a result of switching operations or due to lightning-induced voltage spikes. 

How common are power surges in the UK?

Electrical surges that cause power grid failures do not occur very often in Poland, but they can generate high repair costs. For this reason, it is crucial to properly protect the grid.

Is power surge different in low-voltage networks compared to medium- and high-voltage ones? 

Although this phenomenon is the same regardless of the type of network, it differs in terms of its effects and the damage it causes. In the case of low-voltage systems, it only affects household appliances. In the case of medium- and high-voltage networks, there is also a risk of damage to the infrastructure at the level of substations or transmission lines in a given region.

How to choose surge protection devices for medium and high voltage networks?

Electrical devices operating in medium and high voltage networks require different surge protection than household appliances (offering greater protection). In their case, a surge means a sudden voltage spike of hundreds or even thousands of volts, which ordinary surge arresters suitable for home installations would not be able to withstand.