Following the dispute between Nikola Tesla and George Westinghouse over the technology for supplying electricity to the USA in the 1880s, alternating current became widely accepted. However, the cables used to transmit alternating current over long distances act like a capacitor, which leads to transmission losses and a need for compensation through reactive power. When transmitting direct current, however, this reactive power requirement is negligible, meaning that the current can be transported with significantly lower losses. For this reason, direct current under high voltage is now being used again to transmit high power.

Direct current transmission

For such a transmission by means of direct current, two power converters with a common DC link are used - in a very simplified way. Each power inverter can transfer the energy from the grid with alternating current to the DC link with direct current and also feed the energy from the DC link back into the grid as alternating current. In this way, electrical energy can be transferred in any direction between the two grid connections. Direct current with a very high voltage is used for transmission in the DC link, which is how the system got its name high-voltage direct current transmission (HVDC). Bipolar transistors with an insulated gate electrode - insulated-gate bipolar transistors (IGBTs) - are used to convert the current and act like valves. These can allow the current to pass through or block it and thus generate the desired current curves using pulse patterns.

However, the power converter of an HVDC system is dimensioned differently to conventional converters. This is because modular multi-level converters (MMCs) consisting of hundreds of IGBTs are used and installed on an area of 10 to 15 hectares. The intermediate circuit uses a voltage between 100 and 800 kV and transmits power between 500 and 6400 MW over distances of hundreds of kilometers.

A new control concept

As a manufacturer of systems for energy transmission and stabilization of the power grid, Siemens Energy will in future rely on PC-based control technology from Beckhoff. Embedded PCs and EtherCAT I/O terminals as well as TwinCAT automation software in conjunction with Model-Based Design are used for the control and protection of large power converters. Among other things, these power converters form the basis of such HVDC systems, but are also used for systems for compensating reactive power or for supporting and stabilizing electrical energy networks (Flexible AC Transmission Systems, FACTS).

In order to achieve a high degree of reliability for such an important part of the energy grid, redundant systems are often used. The control and protection systems in hardware and software are permanently in a hot standby mode so that they can immediately switch over to the redundant system in the event of any malfunction. To this end, redundant communication is established via several separate Ethernet networks using the TwinCAT Parallel Redundancy Protocol (PRP) in accordance with IEC 62439-3. This is used for communication between the Embedded PCs via EtherCAT Automation Protocol (EAP) as well as via MMS and GOOSE in accordance with IEC 61850 to external systems such as circuit breakers.

Fast response times

The test cabinet with the embedded PC and the directly connected EtherCAT Terminals.

© Siemens Energy

The requirements for fast response times for current control are realized by EtherCAT and high-performance Embedded PCs. The AMD Ryzen CPU in the Embedded PCs makes it possible to execute the control in TwinCAT with cycle times of 250 µs and minimal jitter. A total of twelve such Embedded PCs are used per power inverter, which exchange fast signals in redundant segments via the EtherCAT Bridge terminal.

The TwinCAT/BSD operating system is used to ensure safe operation of the systems as part of the critical infrastructure. It offers a stable Unix platform for the TwinCAT 3 Runtime, which also meets the currently increasing security requirements. TwinCAT modules are executed in the TwinCAT 3 real-time environment. TwinCAT modules developed directly in C/C++ are used for basic functions or special communication stacks. This makes it possible to abstract the control software from the details of the hardware or communication via various protocols such as EtherCAT or IEC 61850. Specific functions and controls of the system are then configured with model-based development in MATLAB and Simulink and transferred to the embedded PCs using code generation.

Consistent and open software

Since such HVDC systems are not available for development and verification as a physical system, early testing via simulations is of central importance. In the past, these tests were carried out in different simulation environments, for which the control and protection software had to be manually translated into each environment. This manual process was too error-prone and time-consuming to achieve a comparable behavior of the control system in all environments. To be able to use a single source for the software, Siemens Energy relies on model-based design or engineering of the processes using Matlab and Simulink. By developing the control and protection software in Simulink and then generating the code with TwinCAT 3 Target for Simulink, the aforementioned manual steps are no longer necessary. Instead, the developers can concentrate on their core task. The fact that the same software runs in different simulation environments as well as on the final control hardware makes it easier to compare the behavior.

Another advantage is the time saved in the event of errors or when extending the models. Whereas in the past it was necessary to rectify errors in the respective target system or to extend the functions there, this is now carried out in the source model in Simulink. In conjunction with TwinCAT, the software modules that have already been tested can then be ported to the powerful, highly real-time-capable embedded PCs with little effort and only need to be connected to the physical interfaces. In this way, both HIL tests (hardware in the loop) and tests with the control cabinets later installed in the real system can be carried out with the control system in order to deliver a control system that is optimally adapted to all scenarios in the power grid.