November 30, 2021
4 min read

Pretact®: Resilience for U.S. power plants

Siemens Energy successfully tested three mobile resilience generator-step-up units (GSU) for leading U.S. wholesale energy provider Southern Power. These versatile, mobile transformer units, a part of the Siemens Energy Pretact® concept, will increase power security at generating stations.


A long-term interruption in power supply, whatever the cause, can have disastrous consequences. In case of a failure of a power transformer a replacement within a timeframe of a few days might not be possible by conventional approach. This is where the ‘REACT’ part of the Pretact® concept comes in. When a transformer is damaged despite outage prevention by monitoring, fleet maintenance and despite protective features, the priority is to react as soon as possible. In order to do this, it is advisable to have a resilience unit which is versatile, mobile and easy to install.

Mobile generator-step-up (GSU) resilience transformer

Especially in the case of generator-step-up transformers, that are key to transmit the generated energy into the grid, the outage of such a unit not only has tremendous impacts with regard to power delivery, but also with regard to the financial situation of a power generating plant. While the mobile resilience transformer concept is the state-of-the-art solution in the area of substations, the resilience GSUs become utterly important to power plants as well.

Ease of transportation is one of the most important criteria when it comes to transformer replacements: only if a transformer is lightweight and compact enough, it can be deployed smoothly without long waiting times for transportation approvals. Therefore, the resilience GSUs are designed as single-phase units with a transport weight of just 49.8 tons (109,790 lbs) each, making them highly lightweight by using high temperature insulation material like Nomex®. Its plug-and-play design reduces the installation time significantly: this 150 MVA bank of transformers can be installed in approximately 24 working hours, reducing the installation times from already optimized seven working days to a single day. Another benefit of the solution: Mobile resilience units, like the ones for Southern Power are also versatile regarding their voltage. The transformer can operate in multiple configurations, either at 230 kV or 115 kV and a low voltage of 34.5 kV. This means the operator can make use of the units in different Southern Power locations and can use cables to connect the generator to the transformers, without having to store different spare units for each and every single power plant location.

Ensuring a secure power supply

This new GSU resilience unit has a lot to offer. It enables the customer to have a solution to a problem that shouldn’t happen. If a disaster should strike, and a transformer is put out of operation, the mobile resilience unit can be rapidly transported, installed and energized. With this preventive solution, the effects of a prolonged outage can be significantly reduced.

The mobile GSU unit that we will supply to Southern Power is a great example of how our Pretact® resilience concept can allow our customers to react in advance. It will be ready to secure the power supply at any time, thereby making an invaluable contribution to overall grid resilience.

Eduardo Terzi

Head of the Non-Switching segment of Siemens Transmission Products


The Pretact® concept of Siemens Energy was introduced to maximize the resilience of modern power grids. It comprises a comprehensive set of modular features to prevent and protect transformers and high-voltage products better from operational issues as well as against natural disasters or forced outages. In addition, highly flexible “mobile resilience units” provide a chance to react quickly in case of an emergency or planned maintenance. 

Siemens Energy Transmission Products

Transmission products play a vital role in the energy value chain. Siemens Energy Transmission Products offers all key elements including power transformers, gas-insulated and / or air-insulated switchgear and components, individually or bundled, with related engineering. With our global factory network and leading innovations we provide the highest level of quality and reliability to support our customers in achieving their objectives.