CompactRIO controls and measures low voltage offshore substation at the Lysekil wave power research station in Sweden

Industry: Energy/Power, Scientific Research

Products: CompactRIO, LabVIEWChallenge

: Develop a control and measurement system for a low-voltage offshore substation at the Lysekil wave power research station in Sweden.

Solution: Develop a control and measurement system at the Lysekil wave power research station with the help of four NI CompactRIO systems, three of which are located on the seafloor and one on the coast, and NI LabVIEW software.

"We successfully implemented a control and measurement system based on the CompactRIO platform. The system is placed in a switchgear and placed on the seafloor with it."

In the summer of 2009, the Lysekil wave power research station consisted of three WECs (wave energy converters), one LVMS (low-voltage offshore substation), and one ground measurement station. An overview of the research station is shown in Figure 1.


LVMS low voltage offshore substation control

The control system consists of three CompactRIO devices located inside the LVMS, one CompactRIO, and one computer located at the ground measurement station. The communication structure is shown in Figure 2.


The first CompactRIO system is a fuse, which is an on/off system that controls contactors and relays in the substation. The second system controls the conversion of DC to AC voltage. The third system is a dedicated data acquisition system that records WEC data and environmental data from sensors inside the LVMS. Figure 3 shows the first CompactRIO system, a signal conditioning module, and a modem. The fourth CompactRIO system is used to control resistive power loads outside the measurement station and measure voltage and current uploaded to shore.


Fuse and relay control system

The first CompactRIO system was developed using only field programmable gate arrays (FPGAs) to increase system stability. A real-time program consists of many processes that are interdependent and there is always a risk that one process will block another. In general, three approaches are used to overcome deadlock: deadlock prevention, deadlock avoidance, and deadlock detection. If only a portion of the computing resources are utilized, the possibility of deadlock is reduced, but real-time systems cannot be 100% stable. The first CompactRIO system switches the WECs (wave energy converters) for rectification purposes, or it connects one WEC to a ground measurement station and the other WECs to their resistive loads. It also measures voltage and current values ​​and disconnects the WEC from the LVMS if they exceed the limit.

Inverter control

The second CompactRIO system is responsible for converting the DC voltage into 50Hz AC voltage. The inverter inside the LVMS consists of a CompactRIO and six IGBTs (insulated gate bipolar transistors) with drivers. Based on the measurements of the DC bus and AC output, the inverter performs PWM (pulse width modulation) on the IGBT (insulated gate bipolar transistor). We put the high-speed switching algorithm in the FPGA and communicate with the real-time controller to perform correction calculations, and then send the pulse width information back to the FPGA. CompactRIO also sends the measurement results to the ground station computer and stores the data on the computer's hard drive. The final test results of the inverter are shown in Figure 4. The control interface is shown in Figure 8.


Dedicated Data Acquisition System

The third system is a dedicated data acquisition system that measures voltage and current from each WEC and two WEC internal sensors. These include encoder position, generator flux, and stator temperature inside WEC2 and WEC3. Strain gauges are also placed on the metal structure of WEC2, as well as laser sensors that measure the horizontal movement of the piston. The system also measures water leaks, temperature, pressure, and humidity inside the LVMS.

Placement of the Data Acquisition System

Because the electronics will eventually need repair and calibration, we placed the measurement CompactRIO system inside the switch. This allows us to lift the switch from the seafloor to the surface and tow it into the harbor, but lifting a WEC is more expensive.

In evaluating the measurement data, there was a challenge with time synchronization. Most data logging system clocks are accurate only to the second. To evaluate the data from the WEC, the sensors must be synchronized to the millisecond level, which can be achieved using the IEEE-1588 clock synchronization protocol. However, if the data logging system is used to achieve synchronization, the sensor data inside the WEC will be synchronized with the voltage and current signals generated by the WEC. Therefore, it is better to directly transmit the analog signals generated by the WEC and then collect all the signals in this data acquisition system.

Results

We have successfully implemented a control and measurement system based on the CompactRIO platform. We put the system in a switch and then put the switch on the seabed. We can control the DC to AC conversion with a frequency converter designed based on CompactRIO.

Acknowledgements

The Lysekil project was funded in 2009 by Vattenfall AB, Statkraft AS, Fortum oy, the Swedish Energy Agency, Draka Cable AB, Gothenburg Energy Research Fund, Falkenberg Energy AB, Wallenius Foundation, Helukabel, ProEnviro, Seabased AB, Olle Engkvist Foundation, The J. Gust. Richert Foundation, ?ngpannef?reningen Research and Development Foundation, CF Environmental Foundation, G?ran Gustavsson Research Foundation and Varg?ns Research Foundation.