Testing wind power turbines doesn’t just require megawatts of drivetrain power, it requires sophisticated hydraulic control systems and high performance actuators to simulate blade loads. Today’s wind turbines are rated at 4 megawatts with new wind farm projects being proposed using 6 megawatt and larger units. New wind turbines exceed the capacity of any known testing capability. While these larger turbines are being built on the premise that they will perform more effectively than smaller systems, the history of the industry so far has shown that many turbine components are subjected to severe conditions. At this enormous scale, there is little experience to guide the design process, making the creation of testing facilities a critical issue.
Anticipating this trend, the Department of Energy circulated requests for proposal to create a test facility to support testing at 7.5 and 15 megawatts. In 2009, Clemson University Restoration Institute, bid and won the project based on a sophisticated concept designed by RENK Test System.
The power of horizontal wind turbines is difficult to grasp. For comparison, think about a small car at 100 horsepower or 75 kW. A small school bus might be 450 HP or 336 kW. The biggest locomotive engines currently operating are approximately 3 MW. Testing at 7.5 and 15 MW might seem like a stretch, but working with an engineering team that has done many programs at 3-4 MW is what made RENK Test System the ideal partner for Clemson to team with.
RENK Test System of Augsburg, Germany is a supplier of high power, custom engineered test systems. The company specializes in complex testing of critical equipment like tank transmissions, locomotive engines, and helicopter drive trains. Each test system is engineered to deal with the unique operating conditions required for the article under test.
Producing the equivalent input power
To provide input power to the 15 MW test bench, two 8500 kW, water cooled motors are fed into a special dual input gearbox to achieve the necessary input power. Since wind turbines only operate at low speeds, the high-speed motor input is gear reduced to provide output at 0-17 RPM. This special gearbox is the largest and highest torque, ever built by RENK. With 1.5 million Nm of torque capability and weighing in at 300 tons, it could only be transported by ship. Since the Clemson Restoration Institute is located in the shipyard by the Cooper River, the gearbox could be shipped directly to the site and unloaded by crane. Due to its massive size it was brought into the building while the facility was still under construction and open on one side.
Creating a Definition of Testing
Not only is wind turbine testing a challenge because of the power levels involved, it is impossible to do testing with the turbine blades attached. Fixturing the test system at the height required for a blade to clear the ground would make it impossible to build a sufficiently rigid structure.
Testing without blades requires the creation of a system to apply equivalent inertial loads to the shaft of the turbine generator system, and at the same time, produce the on- and off-axis loading, as it would occur in the real world. Not only does the test system have to produce the forces needed, but precision testing requires a measurement system to insure that applied forces are exactly as programmed by the test engineer.
Engineers at RENK Test System solved the feedback problem by creating a 6-axis strain gauge measuring flange that is integrated into the coupling that connects to the turbine input. This provides input to the RENK RDDS software system, which manages the testing and gathers data about the actual forces acting on the coupling.
The problem of required inertia was solved by creating a Load Application Unit, a complex system using a massive cast iron flywheel to act as the equivalent inertia mass instead of actual blades. Three blade propellers present an oscillating load to the horizontal axis, so precision hydraulic actuators are mounted radially and axially to present forces and deflections that simulate real world blade behavior.
Simulating blades that aren’t there
The massive inertia wheel is pushed on by an array of hydraulic cylinders. Each of the cylinders supplied by Storz is designed to produce 150 metric tons of force. By distributing the cylinders to 24 and 72 points of contact respectively for the 7.5 and 15 megawatt systems, extremely high forces can be generated much more easily than trying to engineer a smaller number of larger actuators.
Precision servo valves from Moog control flow so that actuator starting and stopping is smooth. Position feedback devices from Balluff provide position reference to +/-0.00004 in. accuracy to the control system. The addition of feedback and precision flow control provides the basis for the RENK RDDS controller to coordinate the motion of the hydraulics.
Software becomes a strategic element in the design of the test systems. There is a complex interaction required between the large number of distributed actuators in order to make the inertial flywheel react as in three orthogonal axes as real world blades react. Custom algorithms of control were created by RENK engineers, which perform the calculations and translate the user-defined test case into the motion control of the distributed actuators. The RDDS can exercise the dyno and gather performance data for acceptance test and endurance test cycles.
The test configuration screen allows the test engineer full flexibility in defining the test loads that will be applied to the unit under test. The values of forces and deflections are entered in the Renk RDDS software using a graphical interface and “fill in the blanks” approach. The test configuration screen allows for positive or negative loads in any of the X, Y or Z axes, and the forces can be described as oscillating sinusoidal loads with user defined frequency and amplitude.
There is a further complication for the control system software. The pistons that are perpendicular to each other at any point on the flywheel must operate in exact mirror image motion to maintain contact of both pistons with the surface of the flywheel. If the pistons are not properly coordinated, there could be a gap in position resulting in unbalanced forces with respect to the flywheel load.
Bend It, Don’t Break It
All the normal rules of large machinery design still have to be observed, things have to line up, the support system must be stiff enough that machinery is not flexing. But the simulation of wind loading requires bending the rules without breaking anything. Both Load Application Units are designed to undergo +/- 50 mm excursions at the edge and 0.8° of angular displacement.
In order to provide hydraulic fluid for the test facility, the requirement for both systems was combined into a single fluid reservoir of 50,000 liters, which is easily the volume of a two-lane lap pool. Six pumps, each driven by 500 kW alternating current motors with a common manifold, provide pressure and flow. The hydraulic reservoir, delivered by Hainzl, is managed by a Siemens PLC and responds to commands from the main control system when hydraulic pressure is required.
Housing the giant “Test Bench”
The challenge of mounting the giant 7.5 and 15 MW “test benches”, as they are referred to, is every bit the technical challenge that the test systems themselves are. Heavy industrial machinery, like large CNC machines, requires special foundation work to be done to support the mass of the machine without shifting. The test systems components from RENK are 800 tons of weight and require very stable foundations.
The foundation piers begin 15 ft below the floor where massive rebar reinforcements were created to handle the loading. The entire building and all the structure needed required 3,600 tons of concrete and 650 tons of rebar to support the two dynamometers.
Building construction schedules had to be coordinated with equipment manufacturing to insure that the right pieces would show up at the right time. This process becomes even more complex since many of the suppliers were in Europe and mass of the components would require shipment by boat.
Engineering requirements that might be manageable in any other situation become incredibly complex when scaled up to the massive scale of 15 MW. The inertia wheel for the 15 MW dynamometer is 160 tons by itself. Fabrication of the flywheel by IAG Magnum had to be done as two halves with considerable precision in order to insure good balance. Piping of the hydraulic lines and lubrications systems by Tube Mac were incredibly complex.
Fortunately, the detailed planning of the project is paying off and everything is on site and on time. The commissioning of the 7.5 MW test system is almost complete and the 15 MW is well under way.
Renk Systems Corporation
Filed Under: Fluid Power World Magazine Articles