Jun-Zhe Yang

An adaptive PMU based fault detection/location technique for transmission lines. I. Theory and algorithms

An adaptive fault detection/location technique based on a phasor measurement unit (PMU) for an EHV/UHV transmission line is presented. A fault detection/location index in terms of Clarke components of the synchronized voltage and current phasors is derived. The line parameter estimation algorithm is also developed to solve the uncertainty of parameters caused by aging of transmission lines. This paper also proposes a new discrete Fourier transform (DFT) based algorithm (termed the smart discrete Fourier transform, SDFT) to eliminate system noise and measurement errors such that extremely accurate fundamental frequency components can be extracted for calculation of fault detection/location index. The EMTP was used to simulate a high voltage transmission line with faults at various locations. To simulate errors involved in measurements, Gaussian-type noise has been added to the raw output data generated by EMTP. Results have shown that the new DFT based method can extract exact phasors in the presence of frequency deviation and harmonics. The parameter estimation algorithm can also trace exact parameters very well. The accuracy of both new DFT based method and parameter estimation algorithm can achieve even up to 99.999% and 99.99% respectively, and is presented in Part II. The accuracy of fault location estimation by the proposed technique can achieve even up to 99.9% in the performance evaluation, which is also presented in Part II.

A precise calculation of power system frequency and phasor

A series of precise digital algorithms based on discrete Fourier transforms (DFT) to calculate the frequency and phasor in real-time are proposed. These algorithms, called the smart discrete Fourier transforms (SDFT) family, not only keep all of the advantages of DFT but also smartly take frequency deviation and harmonics into consideration. These make the SDFT family more accurate than the other methods. Besides, the SDFT family is recursive and very easy to implement, so it is very suitable for use in real-time. The authors provide the simulation results compared with conventional DFT method and second-order Prony method to validate the claimed benefits of SDFT.

A new method for power signal harmonic analysis

With the increasing use of nonlinear loads in power systems, the harmonic pollution becomes more and more serious. It is well known that fast Fourier transform (FFT) is a powerful tool for power signal harmonic analysis, but leakage effect, picket fence effect, and aliasing effect make FFT suffer from specific restrictions. In this paper, we proposed a new method for power signal harmonic analysis. The major components of this method are a frequency and phasor estimating algorithm, a finite-impulse-response comb filter, and a correction factor. We also combine other methods to enhance our performance, such as discrete Fourier transform and least square error (LSE) method. To verify this method, we provided the comparisons of this method with FFT.

A Fault Location Technique for Two-Terminal Multisection Compound Transmission Lines Using Synchronized Phasor Measurements

This paper presents a fault location technique for two-terminal multisection compound transmission lines, which combine overhead lines with underground power cables, using synchronized phasor measurements acquired by global positioning system (GPS) based phasor measurement units (PMUs) or digital relays with embedded PMU or by fault-on relay data synchronization algorithms. The technique is extended from a two-terminal fault location method with synchronized phasor measurements as inputs. A novel fault section selector is proposed to select the fault line section in advance. The proposed technique has the ability to locate a fault no matter where the fault is on overhead line or underground power cable. The adopted technique has a solid theoretical foundation and is direct and simple in terms of computational complexity. Both extensive simulation results and field test results are presented to demonstrate the effectiveness of the proposed scheme. The proposed technique has already been implemented in the Taiwan power system since the year 2008. Up to the present, the proposed technique yields excellent performance in practice.

A Hybrid Method for the Estimation of Power System Low-Frequency Oscillation Parameters

A hybrid method which combines four different algorithms for low-frequency oscillation parameter estimation using wide-area measurements is presented in this paper. The first algorithm is the discrete Fourier transform which is used to start the low-frequency oscillation estimation process. The second one is the finite-impulse response (FIR) window filter which is employed in filtering unwanted components, and window correction factor is provided to eliminate the side effect of FIR window filter. The third one is the prototype algorithm which is proposed to estimate low-frequency oscillation parameters, and the last one is the least-square error method which is employed to enhance the computational accuracy by varying the window size. This hybrid method has been implemented in wide-area phasor measurements system by Java language and is under test in Taiwans power system. Matlab/Simulink simulation and field test results are provided in the present paper.

An adaptive fault locator system for transmission lines

This paper presents an adaptive fault locator system which utilizes two-terminal Global Positioning System (GPS) based synchronized phasor measurements for on-line estimation of line parameters in steady-state and calculation of fault location in the occurrence of faults. Uncertainty about the line parameters, which usually results in a significant error in the calculated fault location, can be resolved in the proposed parameter estimation algorithm. A novel fault location index in terms of Clarke components of the synchronized voltage and current phasors is also proposed to calculate the fault location in the accuracy of up to 99.9%. This paper also proposes a new discrete Fourier transform (termed as smart discrete Fourier transform, SDFT) based algorithm to eliminate system noise and measurement errors. The field tests of two GPS based phasor measurement prototypes demonstrate that a synchronization error between two remotely spaced prototypes is small enough for a fault locator. The performance of the parameter estimation algorithm and index is illustrated with simulation results from EMTP.

A new family of measurement technique for tracking voltage phasor, local system frequency, harmonics and DC offset

A series of precise digital algorithms based on discrete Fourier transforms (DFT) to calculate the frequency and phasor in real-time are proposed. These algorithms, we called the smart discrete Fourier transforms (SDFT) family, succeeded in overcoming several problems of DFT, which include frequency deviation, harmonics and DC offset. Moreover, if using smoothing windows to filter noise, the SDFT family is not affected by phase shift and amplitude decay. Factional cycle computing is allowed. Also, the advantages of DFT can still be reserved in SDFT family. These make the SDFT family more accurate than conventional DFT. Besides, SDFT family is very easy to implement, so it is very suitable for use in power systems. We provide the general form of SDFT family and simulation results compared with conventional DFT method to verify the claimed benefits of SDFT family.

An effective method for power system frequency estimation

Frequency is one of the most important quantities in power system operation because it can reflect the dynamic energy balance situation between load and generating power. Therefore, an effective method for frequency estimation is an important task in the operation of power systems. The proposed method combines a prototype algorithm with other algorithms like FIR window filter and discrete Fourier transform (DFT). The prototype algorithm is developed for frequency estimation in the absence of harmonics and noise. FIR window filter enhances the resistance of the prototype algorithm to the noise, and discrete Fourier transform boosts the immunity of the prototype algorithm to harmonics. To demonstrate the effectiveness of the proposed method, we provides the computational results of field measurement data with comparison to DFT based algorithm.

A new optimal aim scheme for TCSC contingency control

After the contingent event of power system, if the permanent fault occurs, the structure of power system will be changed. Or, the structure of power system will remain unchanged for temporary fault. Since the structure of post fault power system can be prior known. For ease of design, the transient stability controller always aims to stabilize the angular speed. Thus, the post fault power angle may reach a new value for temporary fault. In addition, the post fault steady state value of transient stability controllers, such as FACT devices, may be close to its boundary value. If there is another contingent event occurs, the transient stability controller will no longer have enough capability to handle that event. This work develops a new optimal aim scheme to overcome the above problems. The proposed scheme is based on a self-correction two-machine equivalent model and very suitable for stability control of FACT devices. Shaping of the self-correction two-machine equivalent model uses only the real-time phasor signals available from the synchronous phasor measurement units (PMUs). Using the proposed scheme, the transient stability controller does not need the information of power system post fault structure. Meanwhile, the FACT devices can retain its maximum capability to handle the upcoming contingent event. Since the parameters and model uncertainties are also considered in the proposed scheme, this approach is adaptive and robust. The proposed optimal aim scheme is successfully applied to a three-areas six-machine test system that installed two thyristor controlled series capacitors (TCSCs) controllers. Simulation results indicate that the proposed scheme is effective and robust in stabilizing the transient swings between interconnected systems under various system conditions and the occurrence of severe faults.

A hybrid method for the estimation of power system low-frequency oscillation parameters

A hybrid method which combines four different algorithms for low-frequency oscillation parameter estimation using wide-area measurements is presented in this paper. The first algorithm is the discrete Fourier transform which is used to start the low-frequency oscillation estimation process. The second one is the finite-impulse response (FIR) window filter which is employed in filtering unwanted components, and window correction factor is provided to eliminate the side effect of FIR window filter. The third one is the prototype algorithm which is proposed to estimate low-frequency oscillation parameters, and the last one is the least-square error method which is employed to enhance the computational accuracy by varying the window size. This hybrid method has been implemented in wide-area phasor measurements system by Java language and is under test in Taiwans power system. Matlab/Simulink simulation and field test results are provided in the present paper.