Lingwei Zhan

Wide-Area-Measurement System Development at the Distribution Level: An FNET/GridEye Example

Summary form only given. Electric power grid wide-area monitoring system (WAMS) have been extended from the transmission to distribution level. As the first WAMS deployed at the distribution level, the frequency monitoring network FNET/GridEye uses GPS-time-synchronized monitors called frequency disturbance recorders (FDRs) to capture dynamic grid behaviors. In this paper, the latest developments of monitor design and the state-of-the-art data analytics applications of FNET/GridEye are introduced. Its innovations and uniqueness are also discussed. Thanks to its low cost, easy installation and multi-functionalities, FNET/GridEye works as a cost-effective situational awareness tool for power grid operators and pioneers the development of WAMS in electric power grids.

Dynamic Single-Phase Synchronized Phase and Frequency Estimation at the Distribution Level

This paper proposes a method for estimating synchronized phase and frequency at the distribution level under both steady-state and dynamic conditions. The discrete Fourier transform-based method is widely used for phasor and frequency estimation, thanks to its low computational burden. However, errors arise when the power system is operating at off-nominal frequency, especially under dynamic conditions such as phase modulation. Additionally, the power grid signal at the distribution level contains more noise and harmonics, which cause phase and frequency estimation errors. In this paper, a synchronized phase and frequency estimation algorithm suitable for measurement at the distribution level is proposed and tested under noise and harmonic conditions, as well as various conditions in the phasor measurement unit Standard (C37.118.1-2011 and C37.118.1a-2014), to verify its measurement accuracy at the distribution level.

Universal Grid Analyzer design and development

This paper promotes a better understanding of the power grid quality and dynamics through the introduction of a newly developed Universal Grid Analyzer (UGA). The UGA is a real-time, highly accurate, and GPS synchronized power grid monitoring device used at distribution level. They can function as a power quality analyzer by performing harmonics measurement along with voltage sag and swell detection. They can also be used as a phasor measurement unit (PMU) at distribution level. Accurate synchronous sampling is challenging and it is the hardware core of PMUs, thus a new synchronous sampling method is proposed to achieve accurate synchronous sampling. More importantly, the UGA can analyze power grid signal in a wide-frequency range through the “noise analysis” function, and analyze true synchrophasor measurement errors when the UGA is connected to the power grid. A prototype UGA is built to evaluate functions and measurement accuracy of the UGA.

Measurement accuracy limitation analysis on synchrophasors

This paper analyzes the theoretical accuracy limitation of synchrophasors measurements on phase angle and frequency of the power grid. Factors that cause the measurement error are analyzed, including error sources in the instruments and in the power grid signal. Different scenarios of these factors are evaluated according to the normal operation status of power grid measurement. Based on the evaluation and simulation, the errors of phase angle and frequency caused by each factor are calculated and discussed.

Improved WLS-TF algorithm for dynamic synchronized angle and frequency estimation

This paper proposes an improved method for estimating synchronized angle and frequency under both steady-state and dynamic conditions based on the weighted least squares Taylor expansion Fourier (WLS-TF) method. The WLS-TF method using classical windows as weighting factors assumes the input signal is a narrowband band-pass signal, and it has efficient performance under most conditions. However, error arises when the signal is not a band-pass signal, which is the case for the waveforms of power system. This paper extends the band-pass signal model to a more general signal model that considers 2nd order harmonic and proposes a new method to estimate the frequency. Examples of the proposed method are illustrated following different steady-state and dynamic conditions considered in power system.

A Clarke Transformation-Based DFT Phasor and Frequency Algorithm for Wide Frequency Range

Despite its wide applications in power grid monitoring, the classic discrete Fourier transform (DFT)-based synchrophasor estimation algorithms suffer from significant errors when the power system operates under off-nominal frequency conditions. This phenomenon is caused by spectral leakage of DFT and becomes even more severe for single-phase synchrophasor estimation. To address this issue, a theory to eliminate the spectral leakage-caused errors is proposed and a Clarke transformation-based DFT synchrophasor estimation algorithm is proposed to implement the theory in this paper. The Clarke transformation constructs a second signal that has exactly 90° phase angle difference from the original single-phase input signal and helps eliminate the estimation errors for a wide frequency range. The proposed algorithm is tested under the conditions required in the phasor measurement unit standard C37.118.1-2011 and C37.118.1a-2014, as well as the harmonic and noise conditions not required in the standard to verify its performance. More importantly, the idea of using Clarke transformation can be used for other DFT-based synchrophasor algorithms in order to achieve higher synchrophasor measurement accuracy under dynamic conditions. An example is presented at last to demonstrate the expandability of the proposed idea.

Improvement of timing reliability and data transfer security of synchrophasor measurements

Synchronized phasor measurements are becoming one of the key measurement elements of wide area measurement systems in advanced power system monitoring, protection, and control applications. Availability of global positioning system (GPS) provides the possibility of wide-area deployment of Phasor Measurement Unit (PMU) and Frequency Disturbance Recorder (FDR) in power system. GPS is the only timing source for PMU and FDR so far, and they will cease to work when GPS signal is lost or unstable. In addition, phasor data is transferred over the Internet without any encryption, which exposes data to cyber-attacks. The purpose of this paper is to develop an alternative GPS independent timing synchronization method for PMU and FDR, and to implement an encryption system without disrupting real-time data delivery. Primary test results confirm improvement of timing reliability and data transfer security of synchrophasor measurements.

Highly accurate frequency estimation for FNET

Frequency disturbances is one of the most important indicator that reflects the state of an electrical power system. Making accurate frequency measurement from low voltage distribution systems through the wide deployment of Frequency Disturbance Recorders (FDRs) has been the major innovation for the Frequency monitoring network (FNET). Currently, the frequency calculation algorithm based on the phasor angles (FPA) of the measured voltage signal in the FDR, has achieved high accuracy. However, this algorithm is very sensitive to noise which is inevitable in the signal sampling process. As a result, the frequency estimation accuracy will be degraded if the measured signal is not clean enough. Therefore, to achieve even more accurate and robust frequency estimation from the measured voltage signal, a novel algorithm is proposed in this paper consisting of two stages of operations. The first stage applies a band pass filter that eliminates the irrelevant harmonics; while the second stage removes the noise within the pass band and estimates the frequency fluctuation simultaneously based on the least squares nonlinear curve fitting. The experiments based on synthetic data and real data validates the effectiveness of the proposed method for improving the accuracy of frequency estimation.

A Microgrid Monitoring System Over Mobile Platforms

Real-time awareness of the phasor state, including the volatile frequency and phase angle, is critical to maintain reliable and stable operations of the power grid. However, the high cost and low accessibility of current synchrophasors restrict their large-scale deployment over highly distributed microgrids. In this paper, we present a practical system design for monitoring the microgrid frequency and phase angle over mobile platforms and significantly reduce the cost of such monitoring. Being different from current synchrophasors, our system does not rely on continuous GPS reception and hence it is highly accessible and applicable to heterogeneous microgrid scenarios. We develop various techniques to provide the timing signal that is necessary for precise microgrid monitoring. For frequency monitoring, the network time protocol is exploited for time synchronization. For phase angle monitoring which requires a higher timing accuracy, 200 Hz primary synchronization signal being embedded in the 4G LTE cellular signal is harvested for time synchronization. We implemented our system over off-the-shelf smartphones with a few peripheral hardware components and realized an accuracy of 1.7 MHz and 0.01 rad for frequency and phase angle monitoring, respectively. Although the accuracy of the prototype is lower than that of the GPS-based systems, the system could still satisfy the requirements of microgrid monitoring. The total cost of the system can be controlled within

Power grid frequency monitoring over mobile platforms

100 and no installation cost is required. Experiment results compared with the traditional frequency disturbance recorders verify the effectiveness of our proposed system.