Using CompactRIO for Control and Measurement in the Low-Voltage Marine Substation at the Lysekil Wave Power Research Site

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"We successfully implemented a control and measurement system based on the CompactRIO platform. We placed the system inside the switchgear, which we placed on the ocean floor."

- Olle Svensson, Division of Electricity, Uppsala University

The Challenge:
Developing a control and measurement system for the low-voltage marine substation at the Lysekil wave power research site.

The Solution:
Using four NI CompactRIO systems, three subsea and one on the shore, and NI LabVIEW software to develop a control and measurement system at the Lysekil wave power research site.

Author(s):
Olle Svensson - Division of Electricity, Uppsala University

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

Figure 1: The controlled research site in April 2009; WEC 3 is red, WEC 2 is blue, and WEC 1 is grey. The LVMS is located between the resistive generator loads and land-based measurement station.

Control of the LVMS

The control system consists of three CompactRIO units inside the LVMS and one CompactRIO and one PC at the land-based measurement station. The communication structure is shown in Figure 2.

Figure 2: Communication Structure Including the Point-to-Point Communication between the LVMS and the Measurement Station through the Communication Cable

The first CompactRIO system is a safety and on/off system and controls the contactors and relays in the substation. The second system controls DC-to-AC voltage conversion. The third system is a dedicated data acquisition system that logs WEC data and environmental data from sensors inside the LVMS. Figure 3 displays the first CompactRIO system, one signal conditioning module, and the modem. The fourth CompactRIO system controls the resistive power loads placed outside the measurement station and measures the voltages and currents brought to shore.

Figure 3: Safety System and Modem with Two Programmable Automation Controllers (PACs) Mounted behind CompactRIO

The Safety and Relay Control Systems

The first CompactRIO system uses only the field-programmable gate array (FPGA) to increase system stability. Traditionally, we use three methods to overcome deadlock: deadlock prevention, deadlock avoidance, and deadlock detection. The probability of deadlock decreases if only a fraction of the computer power is used, but a real-time system can never be 100 percent stable. The first CompactRIO system either switches the WECs to rectification or connects a single WEC to the land-based measurement station and the other WECs to their resistive loads. It measures voltage and current and disconnects the WEC from the LVMS if defined values are exceeded.

Inverter Control

The second CompactRIO system controls the conversion of the DC voltage to 50 Hz AC voltage. The inverter inside the LVMS consists of one CompactRIO and six IGBTs with drivers. Based on the measurements on the DC bus and the AC outputs, the inverter performs PWM of the insulated gate bipolar transistors (IGBTs). We place the fast-switching algorithms in the FPGA, which communicates with the real-time controller to make regulating calculations. Information about the pulse width is sent back to the FPGA. It also sends the measurements to the land-based PC that stores the data on a hard disk drive. The results from the final test of the inverter are shown in Figure 4. The control interface is shown in Figure 8.

Figure 4: Currents and Voltage Measurements at the Final Test in Uppsala: a) AC Voltage Measured, Load 107 Ω b) AC Voltage Measured, Load 107 Ω c) AC Voltage Measured after the Transformer, load 36 µF//107 Ω d) DC Voltage Measured before the Inverter

A Dedicated Data Acquisition System

The third system is a dedicated data acquisition system that measures the voltages and currents from each WEC and from sensors inside two of the WECs. The position of the translator, the magnetic flux in the generator, and the temperature on the stators are measured in WEC 2 and WEC 3. Also, WEC 2 is equipped with strain gage sensors on the metal structure and laser sensors that measure the horizontal movement of the piston. The system can measure water leakage, temperature, pressure, and humidity inside the LVMS. 

Placement of the Data Acquisition System

We placed the measurement CompactRIO system inside the switchgear because the electronics will eventually need service and calibration. We can raise the switchgear to the ocean surface and tow it to a harbor, but the cost to lift one WEC is more expensive.

The process of evaluating measured data presents a time synchronization challenge. Most data-logging system clocks are accurate to within seconds. To evaluate data from a WEC, the sensors must be synchronized on a millisecond level, which is possible to implement with the clock synchronization protocol IEEE-1588; but if the same data-logging system is used, the sensors inside the WEC are synchronized with the WEC voltage and current. Therefore, it was a good choice to transfer analog signals from the WECs and sample all signals in the same data-logging system.

Results

We successfully implemented a control and measurement system based on the CompactRIO platform. We placed the system inside the switchgear, which we placed on the ocean floor. We can control DC-to-AC conversion with an inverter design based on CompactRIO.

Acknowledgements

The Lysekil Project was 2009 supported by Vattenfall AB, Statkraft AS, Fortum oy, The Swedish Energy Agency, Draka Cable AB, The Gothenburg Energy Research Foundation, Falkenberg Energy AB, The Wallenius Foundation, Helukabel, ProEnviro, Seabased AB, The Olle Engkvist Foundation, The J. Gust. Richert Foundation, Ångpanneföreningen's Foundation for Research and Development, CF Environmental Fund. The Göran Gustavsson Research Foundation, Vargöns Research Foundation.

Author Information:
Olle Svensson
Division of Electricity, Uppsala University
Box 534
Uppsala SE-751 21
Sweden
olle.svensson@angstrom.uu.se

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