Retrofit of Bullet Resistant Transformer

Presented By:
Thomas Schneider

Helmut Pregartner
Justin Case
Mario Leitgeb – Presenter
Siemens AG Austria

TechCon 2020


Transformers are complex assets that are designed to withstand many operational risks. Due to their significance in the power network, transformers are designed to strict national or industry standards, manufacturer’s guidelines and customer specifications. One aspect, however, which is not well covered, is the vulnerability of transformers to intentional human assaults.

Customer X recently identified the risk of malicious attacks on a number of their assets. To mitigate these risks several conventional options, such as concrete walls and ballistic barriers had been investigated. However, as opposed to the conventional options, Customer X has implemented an innovative concept that can easily be retrofitted onto the transformers and does not require any additional civil works.

Key factors in the solution required by Customer X were a high level of protection from physical attacks and a significant reduction in the transformer noise levels. Additionally, in order to eliminate the risk of fire, all oil insulated condenser type bushings (OIP) were to be replaced with composite resin impregnated synthetic (RIS) bushings. Siemens Transformers, with their Pretact® concept and expertise in low noise transformer applications, was chosen as a partner to develop, manufacture and retrofit the bullet resistant, low noise solution.

The significant features of the solution offered by Siemens included bullet resistance up to ballistics standard UL 752 level 10, representing the maximum likely threat posed by a firearm attack. The design has also been optimized such that there is negligible impact on the cooling system, with the fans and radiators receiving the same airflow as the original design values. In addition, the solution features sound reduction measures, which have decreased sound pressure level of the transformer by 20 dB(A). This is significantly more than a standard sound wall design and is comparable with a sound enclosure.

Two in-service 448 MVA, 345/138/13.2kV auto transformers were selected by Customer X to feature the bullet resistant, low noise solution developed in collaboration with Siemens. The transformers, which have been in service for approximately five years, are located in an outdoor substation and are easy visible from outside the security fence. This made them an accessible target for high-powered ballistics. Furthermore, the substation is close to a wooded area with local roads, makings it simple for saboteurs to hide and escape. Substation configurations similar to this are very common across the United States of America.


Transformers have been targeted by hand held firearms in the United States of America for several decades. In most reported cases these attacks have focused on the oil filled porcelain condenser bushings, with the resulting risk of an external flashover event, leading to a fire and a catastrophic failure. To mitigate such an occurrence, transformers are usually tripped within milliseconds by electrical transformer protection equipment, for example differential relays.

In addition to shooting at the bushings, there are known instances where a transformer’s cooling system or control cabinets have been targeted during malevolent attacks. In these cases, traditional protection equipment is not able to recognize the predicament and may therefore not react as quickly as required. Specifically, targeting the cooling equipment may result in a significant loss of oil over a longer period of time, being in the magnitude of minutes to hours.

Discounting the auxiliary equipment and bushings, attackers may also fire directly at the main transformer tank. The bullet, in such an instance, is likely to penetrate through the tank walls and may cause a flashover between the internal insulated components. As with firing at the bushings, this would lead to an immediate tripping of the unit by the transformer protection equipment. This paper describes a complete solution of how to protect existing transformers against damage caused by the abovementioned ballistics attacks with Pretact® Technology. Additionally, the paper addresses the noise reduction potential of the Pretact® solution offered by Siemens Transformers, which is an additional benefit.

Problem Statement

The risk of damage to critical transformers in the power network, or even catastrophic failures, due to intentional attacks with firearms has increased. As a consequence, the NERC CIP-014-2-Physical Security requirements were brought into effect on October 2015, with US utilities being asked to conduct a risk assessment of Transmission assets and thereby take appropriate actions to protect their assets from the risk of a physical attack. Customer X, being a leading utility which operates a large number or power transformers throughout the north east of the United States, has identified a number of transformers which may be at risk from such an attack. As such, two 448 MVA, 345/138/13.2kV power transformers were chosen for a retrofit project with bullet resistant panels.

The transformers were designed to meet the ANSI standard noise requirements, with one unit at 78.0 dB(A) and the sister unit at 77.9 dB(A). In addition to the ballistics protection required by Customer X, it was decided due to additional needs to reduce the sound pressure levels of the units during retrofit process. A noise reduction of 10 dB(A), excluding the impact of the cooling equipment, was specified during the planning and guaranteed by Siemens.


To meet the variable site-specific requirements set by Customer X, Siemens carried out a detailed feasibility study accounting for the transformer designs, existing foundations, site location, condition of the units, and the infrastructure around the transformers. Several options were assessed on how best to retrofit bullet resistance panels to the transformer. Considerations that were important in the design phase also included finding the correct level of ballistics protection for the units while maintaining efficient operation of the cooling equipment. Furthermore, the retrofit solution needed to allow access to the relevant gauges, valves, protection equipment, bushing test taps and others equipment needed for service works and inspections.

As Customer X was interested in a total protection of the transformer, including the cooling system and conservator, a design was chosen that completely surrounded each transformer. Over the course of several studies, Siemens found that a 3/8” steel sheet, which is a common transformer tank design, can be easily penetrated by a UL 752 Level 8 bullet (see Table 1). Furthermore, the testing conducted illustrated that even ballistic rated concrete is not an adequate material, as required by ballistics standard UL 752.

Table 1 UL 752 ballistic standards – level 8 and level 10 projectiles

Although more advanced materials are available, steel is still dominant in the design of protective structures and is the preferred material for safeguarding critical transmission equipment. This is due to the high absolute strength and hardness of the material, combined with high ductility, formability, and durability. Furthermore, steel often provides a more cost-effective bullet resistant solution than other materials. As a result of these factors, a high-grade steel material was selected and tested that is able to withstand impacts from UL 752 Level 10 rounds (see Table 1) – being the highest class of rifle projectile according UL 752.

The bullet resistant panels that have been retrofitted onto the Customer X units are attached to a support structure and protect the tank cover, turrets, relevant piping above the cover and bushing flanges. To utilize the existing foundations, whereby avoiding additional civil works, and in order to reduce amount of bullet resistant material a tight fit to the transformer was chosen. These design factors also reduced the overall cost of the solution.

As the Customer X transformers are equipped with an ONAN/ONAF cooling system, the bullet resistant solution had to be design in such a way to allow sufficient air flow to the radiators and fans. For this purpose, the area underneath the radiators and fans remains open and the panels feature an offset to the cooling system, allowing the required air flow. Nevertheless, the design of the structure prevents a direct line of sight to the transformer, cooling system and auxiliary equipment, providing total protection from ballistic attacks. The complete protection of the unit also entails that the solution does not require any additional electronic components, such as shut off valves or leakage detectors.

In addition to protection of the tank, cover, turrets and cooling equipment, the conservators have been protected. This has been achieved by surrounding the conservator, which is located above the tank on the narrow side of the transformer, with bullet resistant panels. From no angle is there a direct line of sight to the original conservator and thus it also is fully protected against UL 752 Level 10 bullets. The oil level indicator of the conservator is accessible through a small hole on the side of the bullet resistant panels. Retrofit of an electronic remote oil level indicator would have been an alternative but was not relevant in this specific case.

As stated in the scope of work, Customer X required a significant noise reduction in parallel with the bullet resistant properties of the solution. Detailed research was therefore conducted into materials with a high sound absorption coefficient and durability, to withstand environmental factors over the lifetime of the transformers. Design details, including vibration isolation of the bullet resistant structure from the transformer tank were also taken into account. Furthermore, the top of the transformer was covered with sound panels to additionally reduce sound radiation to nearby urban areas. As a result of the noise reduction measures, a sound pressure level decrease of 20 dB(A) was made possible, considerably exceeding the 10 dB(A) guaranteed value. The sound level reduction achieved is significantly more than a standard sound wall design and is comparable with a sound enclosure. It should also be noted that the sound reduction measures may be provided without the bullet resistant characteristics of the solution provided to Customer X, or with a lower resistance rating. This may be an option if a transformer is not considered to be at risk from a physical attack and would decrease the costs of the retrofit project.

Both Customer X transformers fitted with the bullet resistant panels were originally equipped with oil impregnated paper (OIP) bushings and porcelain insulators. This configuration creates a problem in case of a shooting event as bullets can easily penetrate and break the porcelain and catastrophic failures involving ignition of the oil are a realistic consequence. Resin Impregnated Paper (RIP) or Resin Impregnated Synthetic (RIS) bushings substantially decrease the likelihood of secondary damage (fire from oil), as bullets will merely lodge in the RIP/RIS bushing assembly. Taking these advantages into account, replacement of the original bushings with RIS bushings was added to the concept and project scope.

For servicing requirements, the control cabinet and all relevant valves, gauges and other important auxiliary items along the transformer are accessible through doors that are attached on hinges. These doors can easily and quickly be opened and closed and allow technicians to access the relevant areas of the transformers when there is a need.

All materials used for the retrofit were designed at Siemens Transformers in Weiz, Austria and to minimize the risk of misalignment plus extensive field modifications, the complete bullet resistance construction was fully pre-installed at the manufacturer’s location. Before shipping to site all components were marked so the team in the field could easily identify the components, aiding the assembly process. Assembly of the first retrofit took 13 days, while assembly of the second unit took 8 days, excluding weather-related delays.

Assembly was executed by the Siemens Industry Inc. transformer service group. Due to the design and materials used for the bullet resistance retrofit, the on-site assembly project was a natural fit for the Siemens transformer service technicians. With the technicality of the field assembly, some specialized tooling was required for small modifications, as ARMORED Rated steel is not easily drilled nor bent. The biggest challenge during the assembly phase was to install the inner frame construction so that it fit with the position of the transformer, allowing installation of the bullet resistant panels without any misalignment.

Replacement of the bushings has not yet been performed at the time this paper was written.


In summary, the Siemens bullet resistance, low noise retrofit solution developed for Customer X incorporates many significant benefits. These include maximum protection against UL 752 Level 10 rounds, a sound pressure level reduction of 20 dB(A), and the use of maintenance free materials that have a similar lifecycle to the transformer. Furthermore, the solution does not require any additional electronic components, such as shut off valves or leakage detectors, and has been designed to have no influence on the efficiency of the cooling system.


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