Numerical algorithms for power system protectionProf. dr. sc. Ante Marui, doc. dr. sc. Juraj HavelkaUniversity of ZagrebFaculty of Electrical Engineering and [email protected], [email protected]
2010/2011.
Introduction Quality of digital relays depends on:Numerical algorithm quality (software)Hardware qualityGeneral digital relay characteristics: selectivity, stability, satisfactory trip time and sensitivity
Lecture partsFirst partTypes of signalsSampling theorySampling and A/D circuitsNumerical methods: Interpolation formulas numerical integration and differentiation, curve fitting, Fourier analysis and digital filtering
Lecture partsSecond partSinus wave based algorithmsFourier based algorithmsLeast squares based algorithmsDifferential equation based algorithms
Lecture partsThird part: Real time algorithm testing50 Hz SignalSimulated short circuitReal short circuit
Signal classification hierarchyDigital signals1-0 (On Off or TTL) signalPulse train (counters, timers)Analog signalsDC signal (slow)Signal in time domain (fast)Signal in frequency domain
Basic elements of digital protectionAD converter resolutionNyquists theoremAnalog filtersTransducersSample and hold circuit
Let us assume that numerical values of some function x(t) are given at equally spaced intervals every t seconds. 1/t is then called sampling frequency. Signal can then be represented by discrete set of samples: [x(0), x(t), x(2t), , x(kt),]
AD converter resolutionEvery sample of analog signal is converted in to digital value with final number of bitsConversion is preformed in AD converter
3 bit resolution; 23=8 combinations, which means 8 discrete divisions that analog signal can be represented with
Nyquists theorem Sampling frequency (how often is AD conversion preformed)
Nyquists theoremTo avoid signal alias sampling frequency must be at least two times higher then maximum frequency component in analog signalFor accurate waveform representation sampling frequency should be at least 5 to 10 times higher then maximum frequency component in analog signal
Analog filtering
Transducers and surge protection circuitsReduce voltages and currents (10 V and 20 to 40 mA) to suit hardware requirementsProtect hardware from overvoltagesSignal distortion is the problem (current transducers saturation)
Sample and Hold circuit
Basic components of digital relay
Numerical differentiationDerivatives in point k is
Numerical integrationLagrange interpolation formula
Curve fittingLinear fit:
Exponential fit:
General polynomial fit:
General linear fit:
Levenberg-Marquardt fit:
Least square method
Fourier analysis
Fourier series Fourier transform
Discrete Fourier transform DFTSamples of signals from AD: time domainNo need for curve fittingUse DFT: frequency domain
Smoothing Windows
Smoothing Windowsn=0, 1, , N-1
Digital filtersInput signal is discreteThey are software programmableThey are stable and predictableThey do not drift with temperature or humidity and do not require precision componentsThey have superior performance to cost ratioThey do not age
Digital filters
Signal generator
Control loopt=1/fs
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ALGORITHM
ALGORITHM
Measure
Calculate
Measure
Calculate
Measure
Sine wave based algorithmsWaveform is assumed to be sinusoidalThey predict amplitude at every momentThey can be used for impedance calculationSix are presented:Sample and first derivative with two pointsSample and first derivative with three pointsFirst and second derivativeTwo sample techniqueThree sample techniqueR i X calculation with three sample technique
Sample and first derivative with two points
Sample and first derivative with two points
Sample and first derivative with three points
Sample and first derivative with three points
First and second derivative
First and second derivative
Two sample technique
R i X calculation with three sample technique
R i X calculation with three sample technique
Fourier algorithmsWaveform does not have to be pure sineThe basic assumption is that the waveform is periodicThe principle of work is moving frameMoving frame is constant in size which means that it always contains the same number of points
Fourier series with whole periodIf fs is 600 Hz then in one period of 20 ms there are 12 samples.Weighting factors are calculated in advance for fixed samplin frequency.
Fourier series with whole period
Fourier series with whole period third harmonicn=3
Fourier series with whole period third harmonic
Fourier series with half period
FFT algorithm
Least squares based algorithmsAll components of measured waveform must be predicted in mathematical model.After curve fitting data about amplitude, harmonics, angle, etc. are obtainedDownside is large number of calculationsThey are complexFour of them are presented:Algorithm with general polynomial fitLSQ 1, 3 multivariable algorithmLSQ 1, 3, 5 multivariable algorithmLSQ 1, 3, 5 ,7 multivariable algorithm
General polynomial fit algorithm
General polynomial fit algorithm
LSQ 1, 3 multivariable algorithm
LSQ 1, 3 multivariable algorithm
LSQ 1, 3, 5 multivariable algorithm
LSQ 1, 3, 5 multivariable algorithm
LSQ 1, 3, 5, 7 multivariable algorithm
LSQ 1, 3, 5, 7 multivariable algorithm
Differential equation based algorithmThere is no need to assume that the waveform is sineThe fundamental approach is based on the fact that all protected equipment can be represented by differential equations of first or second order.The methods are described by reference to transmission lineThree algorithms are presented:Integration algorithmThird harmonic filtration algorithmDifferential algorithm
Integration algorithm
Integration algorithm
Third harmonic filtration algorithmFor fs=600 Hz :
Third harmonic filtration algorithm
Differential algorithm
Differential algorithm
Differential algorithm
Algorithms and Real-Time operationSystem is operating in real-time if it can guarantee fulfillment of various tasks in specific timeOS in real-timeHardware and software in real timeControl loop time
Algorithms and Real-Time operation
Power system signal
Sine wave based algorithms
Fourier based algorithms
Least squares based algorithms
Differential equation based algorithms
Short circuit
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P
380 V
380 V
Z=R+jX
N1:N2=2
SC
Sinus wave based algorithms
Fourier based algorithms
Least squares based algorithms
Differential equation based algorithms
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