Controlled switching with the Siemens phase synchronizing device (PSD)

Author: Aasim Atiq


Controlled switching is a reliable and economical technique to mitigate switching stresses produced on a network from energizing or de-energizing substation equipment. On three phase networks, this is achieved on Siemens circuit breakers by using the Phase Synchronizing Device (PSD) to send three separate trigger commands to time the moment of make or break so that it happens at a specific point on the AC voltage waveform.

Figure 1: Three phase energization showing staggered switching instants with controlled switching. Reference voltage shown in orange and currents shown in red, blue, and green.


Siemens circuit breakers equipped with individual spring drives that allow isolated control of each pole are called independent pole operated breakers, abbreviated as IPO. Siemens offers controlled switching solutions with our entire portfolio of IPO breakers. These breakers are still capable of performing protection duty and are equipped with an additional set of trip coils that are activated in the event of a fault. Trip commands through grid protection bypass the PSD and are forwarded directly to the protection trip coils.


The most prevalent applications for controlled switching in North America are for capacitor and shunt reactor banks but Siemens PSD02 can also be used for transformer switching. Customers have the option to have the PSD02 installed in the breaker cabinet or in their control house. The device is robust and comes with Siemens’ proprietary PSD control software which allows the flexibility to change settings such as phase rotation with a few simple clicks. Some common applications for controlled switching are outlined in Table 1.

Type of load
Switching operation
Avoided stress
Inductive load: transformer or shunt reactor
Re-strike, overvoltages
Inductive load: transformer or shunt reactor
Inrush current
Capacitive load: capacitor bank, unloaded lines, cables, or filter
Re-strike, overvoltages
Capacitive load: capacitor bank, unloaded lines, cables, or filter
Overvoltages, inrush current


Table 1: Stresses avoided by using controlled switching under various types of loads. 


The PSD builds on the reliability and consistency of our spring-spring drives to deliver repeatable performance. The controller senses and accounts for changes in operation time due fluctuations in control voltage and temperature in real time and adjusts the command to hit point on wave at every operation regardless of external influence. The device also has dry alarm contacts that can tie into the customer’s monitoring so they can be assured that the breaker is always performing within their specified range of parameters.

Figure 2: The circuit breaker issues a synchronous closing command after a calculated delay (tdelay) based on the closing time (tbreaker) thus ensuring that the contacts make at zero voltage.


The PSD uses station bus voltage as a reference as shown in Figure 2. It detects zero crossings on the bus voltage input and calculates the instant to trigger the coils based on programmed values for operating time and compensation due to temperature and control voltage fluctuation. This enables the breaker contacts to make or break at a certain target, relative to zero potential. The reference voltage is customizable to any one phase and the other two phases are shifted by the controller based on the 120o offset for three phase voltage. As the breaker is energized, current input from the breaker current transformer comes into play and helps to determine the exact moment of make or break. Our commissioning engineers are trained to analyze data from current and voltage inputs to fine tune the breaker to optimized performance settings.


The PSD is designed to seamlessly tie into Siemens IPO breakers but each successful energization ties back to our team and core values. We work closely with the customer to design and execute synchronous controlled breakers for their project.  Several key inputs such as control voltage, CT ratios, device location, etc. are customizable and need to be decided early on in the project. Our engineering team works with the customer to outline needs and provide the best solution for their application and commissioning engineers are also involved in the product development phase early on so they are familiar with the project details throughout its life cycle.


We engage with the customer to provide trainings on topics ranging from device application to hands on training on setup and configuration when it is time to execute the project in the field. At the same time, Siemens provides on-site support to energize each individual breaker and fine tune it for its specific setup. Once set, the PSD’s compensation picks up to provide repeated successful operations regardless of conditions and frequency of operation.


From order to execution

  1. Customer's order is engineered
  2. Breaker is built and factory tested along with synchronous controller.
  3. Breaker is installed on site.
  4. Customer performs testing of PSD with remote support from Siemens engineering (optional)
  5. Energization date is scheduled.
  6. Siemens personnel join customer team on site for breaker energization to fine tune and optimize settings.

Siemens has been manufacturing synchronous controlled circuit breakers in USA since 2009 with application on capacitor banks being the most prevalent. For further information, your sales representative can initiate contact with our applications engineering team.