Electronic Transformer Calibration System Based on LabVIEW

Abstract: This article establishes an electronic transformer calibration system based on the NI PCI-4474 data acquisition board combined with the LabVIEW development environment. The field test results show that the system has the characteristics of high measurement accuracy, stable and reliable performance, and meets the requirements for the calibration of electronic transformer ratio and angle deviation, and has certain practical value.

  1 Introduction

With the acceleration of the construction of digital substations and smart grids, electronic transformers have developed rapidly.

Electronic transformers include electronic current transformers and electronic voltage transformers. In order to ensure the accuracy of the electronic transformer and ensure the safety and stability of the system, it is necessary to verify the transformer. The electronic transformer calibration system is used to verify the ratio and angle differences of the electronic transformer.

Compared with traditional transformers, electronic transformers have undergone fundamental changes in the measurement principle, structure and output signal method. The verification principles and methods are completely different from traditional transformer verification principles and methods, so traditional transformers The calibration method cannot be applied to the calibration of electronic transformers.

LabVIEW is a graphical programming language (G language), which is different from traditional text-based programming languages. It encapsulates various functions into function modules, which can quickly establish a graphical user interface for the system, with high development efficiency and development cycle. The short feature is widely used in test measurement and signal processing. The program written using LabVIEW development environment is called Virtual Instrument program (VirtualInstrument program), referred to as VI.

Based on the NI PCI-4474 data acquisition board and LabVIEW development environment, this paper establishes an electronic transformer verification system, and combines the synchronization signal card and digital signal processing to achieve the verification of the transformer. For calibration of electronic voltage transformers with a voltage level of 10~500kV and electronic current transformers with a rated current of 5~5000A, the accuracy can reach five ten thousandths. It has the characteristics of high accuracy, stable and reliable performance, and meets the requirements of electronic The requirements for the calibration of the transformer ratio difference and angle difference. The field test shows the validity of the verification system established in this paper.

2. System composition

The verification system consists of two parts: hardware and software. The system hardware mainly includes the following parts:

(1) Standard voltage and current (electromagnetic) transformers (including boosters and current boosters). The purpose of using a standard transformer is to provide a standard output signal for the calibration system, and it must be traceable. The accuracy of the transformer is selected to be 0.01 level, the purpose is to improve the measurement of the system.

(2) PCI-4474 data acquisition board. The board is a dynamic signal acquisition card for high-precision dynamic and transient measurement. It has 4 simultaneous sampling analog input channels, and the input signal is -10~10VDC. In addition, it has a 24-bit resolution and a sampling rate of 102.4kS/ s.

(3) Sampling resistance. Select a sampling resistor with a resistance value of 80Ω, a rated current of 0.05A, and an accuracy of 100ppm.

(4) Voltage transformer 100V (100/√3V)/4V, precision 0.01 grade; current transformer 5A/0.05A, precision 0.01 grade.

(5) Synchronization card: used to synchronize the combined unit of the transformer under test and the standard loop acquisition card.

(6) Merging unit: The main function of the merging unit is to synchronously receive the digital signals output by the 12-channel electronic transformer (the 12-channel signals are defined in accordance with the DATASET data channel defined in the IEC60044-8 standard), and the frame format is specified in the IEC60044-8 standard And send it to relay protection, measurement and control equipment.

Combining unit generally provides a total of 12 current and voltage signals (all optical fiber transmission), including 7 current signals and 5 voltage signals, which are packaged according to certain rules and output via Ethernet. The merging unit is generally provided by the user and follows the GBT-20840-7/8 and IEC61580-9 standards.

The connection between the merging unit and the verification system mainly includes two parts: one is the synchronization pulse signal connection, and the other is the Ethernet connection. The connection block diagram of the merging unit and the verification system is shown in Figure 1:



(7) Industrial computer. The industrial computer is connected to a standard A/D converter through a PCI-GPIB card with a communication protocol of IEEE488.2.

(8) Voltage regulator.

The system software is the LabVIEW development environment, which is composed of three parts: the front panel, the block diagram program, and the icon/connector.

(1) The front panel is the graphical user interface of LabVIEW, which can monitor various parameters in the system. It consists of input controls and display controls, as shown in Figure 4.

(2) The block diagram layer sequence is written in a graphical programming language, which is equivalent to a program statement in a text-based programming language and can realize specific functions.

(3) The icon/connector can generate a program (VI) written in the LabVIEW development environment into a subVI for other VIs to call, which improves the readability and development efficiency of the program.

3. Principle of calibration system

The working principle of this calibration system can be divided into two parts: the principle of electronic current transformer calibration and the principle of electronic voltage transformer calibration.

3.1 Calibration principle of electronic current transformer

The standard electromagnetic current transformer is connected in series with the electronic current transformer under test, the current transformer is provided with current for the transformer side, and the voltage regulator is adjusted to change the output current of the current transformer. The secondary side of the standard CT is connected to a standard current transformer 5A/0.05A, and its secondary side is connected to a standard resistor, and the voltage on the resistor is taken and input to the data acquisition card (PCI-4474). The current signal obtained by the secondary side of the electronic current transformer is sent to the merging unit on the low-voltage side by optical fiber. The computer obtains the measurement results of the standard CT through the data acquisition card and the measurement results of the electronic current transformer through the Ethernet. The calculation and the clock synchronization signal are used to synchronously sample the electronic current transformer and the data acquisition card under test.

After digital filtering, DFT calculation and other signal results, the ratio error and phase error of the electronic current transformer are calculated.

The principle of verification is shown in Figure 2.



3.2 Calibration principle of electronic voltage transformer

The calibration principle of electronic voltage transformer is similar to that of electronic current transformer. The difference is that the standard electromagnetic voltage transformer is connected in parallel with the electronic voltage transformer under test, and the booster provides voltage to the transformer side. The output voltage is adjusted by a voltage regulator. The secondary side of the standard PT is connected to a standard voltage transformer 100V (100/√3V)/4V and input to the data acquisition card (PCI-4474). The principle is shown in Figure 3.



4. System features and performance indicators

This calibration device is suitable for laboratory and field accuracy calibration of electronic transformers that comply with IEC61850-(9-1,9-2) standard digital output and IEC60044-7/8 standard output.

4.1 System features

(1) Users do not need to know IEC61850 or IEC60044-8, nor need to set MAC address to verify the transformer error.

(2) Verification of non-traditional transformers (small signal output).

(3) Built-in 4 channels of 24Bit sigma-delta AD can meet the large amount of computing and other various needs that may appear in the future.

(4) It can be traced to the source with an eight-and-a-half-digit high-precision digital multimeter (Agilent3458A).

4.2 Technical indicators and performance (as shown in Table 1) #1

       

5. The interface design of the calibration system based on LabVIEW

This calibration system is designed based on the LabVIEW8.6 development environment. The software interface includes three parts: system calibration interface, detailed parameter analysis interface and system verification interface.



The system verification interface mainly displays the results after the software calculation is completed. As shown in Figure 4, this interface is the entire system. After the parameter configuration is completed, click the start button and the system will complete the entire inspection and testing according to the configuration parameters "one-click", and can display the standard source amplitude percentage, system rate, ratio difference (value, value and average), phase difference, value , Value and average value, detailed parameter list for each comparison (comparison times, amplitude percentage, ratio difference, angle difference, phase difference and whether the comparison data is self-effect, etc.).

6. Summary

Aiming at the working principle and structure of electronic transformers, this paper establishes a calibration system for transformers based on NI's data acquisition board and LabVIEW development environment. It has the characteristics of high measurement accuracy and simple operation, and has certain practical value. (Author: Li Dongfang)