Testing Wind Turbines: Controlling a Mobile Voltage Dip Generator Using NI PXI
Print
Author(s):
Ana Morales - Energy To Quality S.L.
Xavier Robe - Energy To Quality S.L.
Industry:
Energy/Power
Products:
LabVIEW, PXI-1052
The Challenge:
Controlling a medium-voltage dip generator using one system to record the signals and postprocess the data for full-scale tests of wind turbines with the capability of closing a circuit breaker on a specific phase angle.
The Solution:
Developing a control system using the NI PXI-1052 PXI/SCXI chassis with a data acquisition device and HV digital inputs and outputs combined with a digital protection relay for safety and operated for control and signal acquisition using a user-friendly interface developed in the NI LabVIEW graphical programming environment.
"The PXI chassis transmits the states of the circuit breakers and executes orders given by the user. If communication is lost or if the application crashes, the system keeps the switches and circuit breakers in a safe position."
Energy To Quality S.L. (E2Q) has been testing wind turbines in the field for more than two years. The equipment, mounted in a trailer, generates short circuits in medium-voltage networks. Using a voltage divider, short circuits are generated by the actuation of a circuit-breaker. Due to the requirements of a new customer and the need for an easy-to-use interface, we developed a voltage dip generator controlled by the PXI-1052 chassis.
Dip Generator Control System Requirements
Generating deliberately short circuits in medium-voltage networks is not common. At medium and high voltage levels, the main consideration is the safety of the staff, followed by protecting the equipment. Besides the dimensioning of the components and the standard measures of protection, the control system must be reliable and prevent any operation that puts people in danger or damages equipment. The interface should also be user friendly and display recorded signals from the test in real time. The system must be able open and close four medium-voltage switch and circuit breakers as well as give the position of all the elements at any time. Our customer requested additional controls as well, such as a circuit breaker that generates the voltage dip and closes on a specific value of the voltage phase. The closing angle determines the behavior of the power electronics converter installed in the wind turbines.
System Implantation
The circuit breakers are controlled at 125 VDC by a rectifier with a back-up battery. The voltages and currents are measured on the secondary voltage and current transformers at 110 VAC. To successfully complete these operations, we built a system based on the PXI-1052, a 4-slot PXI chassis with eight slots for SCXI modules, so that we could make inputs at higher levels than with the standard PXI system. To overcome the high power consumption of the tripping coils, the digital outputs controlling the breakers interface with a programmable digital protection relay designed to control medium voltage elements. Digital outputs of the PXI connect to digital inputs of the relay. When the protection relay is activated, the protection functions can be removed from the control system leaving computation power for the second challenge: quickly estimating the phase angle. We used a LabVIEW program to optimize the performance of the system. This program runs on a remote computer at a safe distance from the trailer, which communicates with the chassis by TCP protocol. The chassis transmits the states of the circuit breakers and executes the order given by the user. If the communication is lost or if the application crashes, the system puts all the switches and circuit breakers into a safe position.
The design of the new user interface makes for easy operation of the voltage dip generator. The operator can maneuver each switch or circuit-breaker by pushing the open or close button; undesired operations are automatically prevented by interlocking functions included in the program. To generate voltage dips, the operator can set different parameters such as the duration, the active power range, the reactive power range, and the phase angle. Once the sequence is started, the system waits for the defined conditions to be met before closing and opening the short circuit breaker. Active and reactive powers and phase angle are computed in real time by a specifically designed algorithm. Closing very accurately on a specific phase angle requires a relatively high sampling rate (several kHz).
The same system is also used to record the signal during the voltage dip tests, and the system saves the time series in IEEE COMTRADE format. The postprocessing and automatic generation of the report are interfaced with a stand-alone application. In less than one minute after test completion, the customer can know if the wind turbine is compliant with the requirements and low-voltage ride through capabilities set by the transmission network operator.
Conclusion
The control system for a mobile voltage dip generator has been tested and implemented in the field with three main advantages over its predecessor: a friendly graphical user interface, logic programming to avoid undesired operations, and new capabilities such as phase angle control. The customer can know, less than one minute after the test, if the wind turbine complies with the requirements of the grid operator. The versatility of this design will adapt easily to new customers’ requirements in the future.
|
|