Numerical Relay Development Environment

M. P. Vinod K. R. Satheesh K. Madhusoodana Guha Somnath Power Research and Development Consultants Pvt. Ltd., Bangalore.

vinod@prdcinfotech.com and somnath@prdcinfotech.com

Abstract—Advancements in application of digital signal

processors in measurement and protection relaying schemes has evolved from many decades. New adaptive techniques are being developed for better power system protection. These techniques need to be tested on numerical relay test bench to analyze its accuracy and reliability under real time power system transient behavior. This paper presents generic numerical relay development environment comprising of newly developed relay and computer based hardware-in-the-loop test bench setup. The hardware is designed to accept data in IEEE COMTRADE format. Different power system operating conditions are simulated in MiPower software package and injected to the relay using National Instrument’s NI 9263 converter modules and LabVIEW software package. Relaying scheme performance is analyzed in computer by event capture capability of the relay and serial communication with computer. Percentage bias generator differential scheme is implemented and tested in the setup. Results show stable and accurate performance of the hardware.

Keywords— EMTP, hardware, power system protection, relays

I. INTRODUCTION Power system has observed tremendous advancements in

application of digital signal processors in power system monitor, control and protection. One of the major advancements in protection is the development of numerical relay and its usage over the conventional electromechanical and static relays. Many research and developmental activities are observed in the form of publications over previous decades in the areas of measurement and protection relaying schemes [1-6]. Most of the publications realized the developed algorithms on hardware platform for testing its accuracy and performance. Since the recorded data handling design depends on the manufacturer which is generally proprietary, fault data recorded by the disturbance recorder or any numerical relay is exchanged with others in IEEE COMTRADE format.

Numerical Relay Development Environment (NRDE) provides a standard platform for researchers and working professionals to develop and test measuring and protection relaying schemes. NRDE accepts data in IEEE COMTRADE 1991 and 1999 format [7] so that newly developed schemes can be tested with the existing recorded data or simulating the power system condition in Electro Magnetic Transient Analysis (EMTA) module of MiPower simulation package. With the option of handling the signal samples directly, NRDE provides wide option for the user to develop, debug and test measuring and protection relaying algorithms. New technique for current transformer saturation blocking scheme

for percentage bias differential protection for generator is developed and tested on NRDE [6].

II. ENVIRONMENT As shown in Fig.1, NRDE is computer based hardware-in-

the-loop test bench setup constituting National Instruments make digital to analog (D/A) module cDAQ NI 9263 mounted on chassis NI 9174, newly developed numerical relay board with trip contactors and computer loaded with power system simulation package MiPower, Microchip Technology, Inc product MPLAB integrated development environment (IDE) and LabVIEW.

Fig. 1. Numerical Relay Development Environment schematic

Different power system operating conditions are simulated in MiPower and fault discriminants are stored in COMTRADE format. The stored data is processed in interface software and converted to LabVIEW readable measurement files. Transient behavior of fault discriminants is not lost as the values are only scaled by a factor. Real time analog signals are injected to the relay hardware using two modules of cDAQ NI 9263 and LabVIEW virtual instrument files. The analog signals are fed to the relay board where test relaying algorithms are preloaded using MPLAB IDE. The relaying algorithm stored in the controller takes suitable decisions based on the input analog signals and stores the event data in flash memory. Data in flash memory are retrieved to the computer using serial communication and results are analyzed at computer level for algorithm performance and accuracy. NRDE design sectioned as hardware and software elements are presented separately.

III. HARDWARE DESIGN AND IMPLEMENTATION Fig.2 shows the block diagram of the numerical relay

hardware. Numerical relay comprises of Analog to Digital

Numerical Relay

Trip Contactors

cDAQ NI 9263

MiPower LabVIEW MPLAB IDE

Converters (ADC), digital signal processor, flash memory and serial communication sections.

Fig. 2. Block diagram of relay hardware

Analog signals from the converter modules NI 9263 are fed to the relay hardware. Differential signaling is employed as it is highly immune to outside electromagnetic interference and crosstalk from nearby signal conductors [8]. At relay hardware board, each analog signal channel is separately conditioned. Signal conditioning circuit scales the analog voltages to the ADC operating range and filter using active low pass filter with cutoff frequency around 700Hz considering design limit for measurement of 5th harmonic content of power frequency signals. The conditioned analog signals are fed to ADC from where equivalent digital signals are fed to the processor serial buffer. Analog Devices make AD73360 is used in this design as it employs sigma delta conversion technique for analog to digital (A/D) conversion. Sigma delta conversion technique in AD73360 employs oversampling, filtering, decimation and has better noise immunity as oversampling reduces the in-band quantization noise [9]. Two AD73360 are cascaded in daisy chain configuration to simultaneously acquire 12 analog input channels as each ADC supports 6 individual A/D conversion channels. Microchip make dsPIC33FJ256GP710A processor is interfaced serially with Atmel make AT25DF161, 16 mega bit flash memory and cascaded ADCs with inbuilt serial peripheral interface (SPI) modules [10]. ADCs are configured to extract analog signal data at a sampling rate of 1.6 kHz. Three wire serial communication with computer for data exchange is achieved with Maxim make MAX3232 based RS232 interface. The complete hardware is realized in two layer printed circuit board as shown in Fig.3.

Fig. 3. Developed numerical relay

IV. FIRMWARE AND INTERFACE SOFTWARE Interface software comprises of program for conversion of

IEEE COMTRADE files to the LabVIEW readable

measurement files and LabVIEW virtual instrument file for generating analog signals at defined rate as per the created measurement files. Program modules in processor for synchronous data handling, accepting runtime update of relay settings, configuration data and event capture capability are developed as standard firmware.

A. Interface Software Interface software reads the selected IEEE COMTRADE

files and provides the user with the provision to map the channel ID in the configuration file as voltage or current channel. Once configured the interface software reads the configuration, data files and extracts the instrument transformer data, analog and digital channel information with limit values. The data values are converted to actual using the ‘a’ and ‘b’ values form configuration file. The actual values are scaled suitably to NI 9263 module output voltage range and save the data files and sampling information in LabVIEW measurement file format. Since NI 9263 module outputs voltage signal, current channels are imitated as respective voltage signals at the instrument transformer secondary level across the burden.

To analyze the relaying scheme behavior for instrument transformer transients, COMTRADE files should be in terms of instrument transformer secondary.

Virtual instrument blocks are used to interface the data files stored in measurement files to produce analog voltages at defined sampling rate. Fig.4 shows the virtual instrument block to read the interface software generated measurement files and generate the analog signals [11].

Fig. 4. LabVIEW software measurement blocks to generate voltage signals

from measurement data files

B. Firmware package Firmware consists of mainly two sections of programs,

configuration programs and user programs. Configuration programs can be accessed only in factory mode of operation which is used to assign ports and variables during design stage of NRDE. Under user mode of operation, configuration programs are directly loaded before the user programs and the complete code is programmed in controller.

Configuration programs also consist of preloaded signal measuring functions such as recursive Discrete Fourier Transforms (DFT), non-recursive DFT [12], 3 sample techniques, and least square error technique [13] which can be directly evoked by the user programs. Latest ten cycle sample values obtained from ADC are stored in cyclic buffer for the user program to develop and test signal measuring techniques.

To check for synchronous twelve channel data acquisition, time interval between each channel data acquisition is checked within a sample period. Complete sample data set for all twelve channels is discarded if any one channel data is found missing from the data set acquired. This check helps in maintaining accurate synchronized phase difference between each channel which would reflect in relaying scheme performance.

User program section consists of user written functions for testing the relaying algorithms. User can either evoke the standard measuring functions or use the latest sample values present in cyclic buffer as their program input for developing the relaying scheme. With event capture functionality of relay, totally 310 sample values out of which 5 post fault cycles are stored per field of signal in flash memory where the storage field is user defined.

With the user enabling the data send signal, relay sends the event data recoded in the cyclic buffer to computer serial port via RS232 communication, where data and results of the event are analyzed for relaying scheme performance and accuracy. Fig.5 shows the developed NRDE setup.

Fig. 5. Numerical Relay Development Environment setup

V. TESTING NRDE Percentage bias generator differential protection [12] is

programmed in the user program space. Fig.6 shows the test system built in MiPower EMTA module [14]. Fault discriminants are stored in IEEE COMTRADE 1999 format.

Fig. 6. Test System

Test system of 15MW, 11kV generator with 0.26 per unit (p.u.) transient reactance, 2.116 p.u direct axis reactance and 0.01587 p.u. armature resistance is modeled as shown in Fig.6. Generator is modeled as a source behind constant transient reactance and armature resistance for relay testing. Stator core saturation is neglected. Generator reactance and resistance is separated into two units to create internal fault condition. Percentage bias differential relay with bias setting of 0.2 (in terms of normalized difference current), first and second slope points of 1.5 and 3.5 respectively (in terms of normalized average current), first and second slope of 25% and 90% respectively and highset setting of 10 (in terms of normalized difference current) is loaded to the relay. Case studies presented in Table I are simulated in MiPower and performance and accuracy of NRDE is tested.

TABLE I. SIMULATED CASE STUDIES

Case 1 Generator at rated load.

Case 2 Fault at the generator terminals (through fault)

Case 3 Fault at the midpoint of the generator winding. Fault feed from the terminal side exists.

Case 1: Fig.7 and Fig.8 shows the LabVIEW output and

NRDE relay event capture data transferred to computer. It is observed the relay secondary current is measured correctly and relay does not operate.

Fig. 7. LabVIEW output for case 1.

Fig. 8. Relay event capture buffer data for case 1.

Case 2: Fig.9 and Fig.10 shows the LabVIEW output and NRDE relay event capture buffer data. It is observed that the relay secondary current is measured correctly and relay does not operate.

Computer Relay

Trip Contactor NI 9263

Fig. 9. LabVIEW output signal for case 2.

Fig. 10. Relay event capture buffer data for case 2.

Case 3: Fig.11 and Fig.12 shows the LabVIEW output and NRDE relay event capture buffer data. It is observed the relay secondary current is measured correctly and relay operates. Time delay taken by the contactor trip circuit to operate after fault initiation and voltage spike due to non-simultaneous single pole contactor trip for differential input is observed. Fig.13 shows the trajectory of differential and average current on the relay characteristics for this case.

Fig. 11. LabVIEW output signal for case 3.

Fig. 12. Relay event capture buffer data for case 3.

Fig. 13. Trajectory of differential and average current for case 3 on percentage

bias differential characteristic.

VI. CONCLUSION NRDE provides a stable platform for researchers,

academicians and working professionals to develop and test measuring and protection relaying algorithms. Test case results show high degree accuracy of the data acquisition system. Using NRDE and past site experiences where fault discriminants are recorded in relays and disturbance recorders, value additions to existing relaying schemes with adaptive techniques can be achieved.

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