28 research outputs found
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Methodology for characterizing electric power system response and locating the energized capacitor banks using conventional power quality data
textA relatively small harmonic current with frequencies near or at the
power system parallel resonant frequencies could excite the power system into
a resonance condition. While a capacitor bank is not the root cause of the
condition, it facilitates and helps cause the problem. This is because when the
capacitor bank is energized, the system resonant frequency could shift closer
to existing harmonic frequencies produced by nonlinear loads. Therefore, the
objective of this dissertation is to quantify the power system characteristics
corresponding to the parallel resonant frequencies and system damping. Additionally,
since a capacitor bank actively facilitates the resonance condition,
the relative or exact location of the involved bank must be determined.
This dissertation first presents a practical and accurate methodology to
estimate system parallel resonant frequencies by performing spectral analysis
of the voltage and current transient data immediately after the capacitor bank
switching. The proposed method is also robust in that the accuracy of the
resulting estimates is not affected by prevalent harmonics in the system.
This dissertation provides two efficient algorithms for estimating the
system damping parameters using the Hilbert and analytic wavelet transforms.
These algorithms take advantage of the principle of an asymptotic or weaklymodulated
signal, for which the signal phase varies much more rapidly than the
amplitude. The zero-input voltage response or free response of the capacitor
bank energizing can be categorized into these asymptotic signals, and one can
assign a unique time-varying amplitude with the system damping information
and phase pair by building analytic signals. System model reduction theory
also allows us to interpret or quantify this damping as an effective X/R ratio.
This dissertation defines two fundamental signatures of shunt capacitor
bank energizing. It demonstrates that these two signatures can be utilized
to accurately determine the relative location of an energized capacitor bank
whether it is upstream or downstream from the monitoring location. This dissertation
also presents an efficient and accurate methodology for finding the
exact location of an energized capacitor bank. Once a PQ monitor is found
to be upstream from the capacitor bank by the relative location finding algorithm,
the proposed algorithm can further determine the exact location of
the switched capacitor bank by estimating the distance between the PQ monitor
and the energized capacitor bank. Thus, one can pinpoint the energized
capacitor bank causing the resonance.Electrical and Computer Engineerin
Load Forecast Model Switching Scheme for Improved Robustnessto Changes in Building Energy Consumption Patterns
This paper presents a new, accurate load forecasting technique robust to fluctuations due to unusual load behavioral changes in buildings, i.e., the potential for small commercial buildings with heterogeneous stores. The proposed scheme is featured with two functional components: data classification by daily characteristics and automatic forecast model switching. The scheme extracts daily characteristics of the input load data and arranges the load data into weekday and weekend data. Forecasting is conducted based on a selected model among ARMAX (autoregressive moving average with exogenous variable) models with the processed input data. Kalman filtering is applied to estimate model parameters. The model-switching scheme monitors the accumulated error and substitutes a backup load model for the currently working model, when the accumulated error exceeds a threshold value, to reduce the increased bias error due to the change in the consumption pattern. This switching reinforces the limited performance of parameter estimation given a fixed structure and, thus, forecasting capability. The study results demonstrate that the proposed scheme is reasonably accurate and even robust to changes in the electricity use patterns. It should help improve the performance for building control systems for energy efficiency
Incorporating Charging/Discharging Strategy of Electric Vehicles into Security-Constrained Optimal Power Flow to Support High Renewable Penetration
This research aims to improve the operational efficiency and security of electric power systems at high renewable penetration by exploiting the envisioned controllability or flexibility of electric vehicles (EVs); EVs interact with the grid through grid-to-vehicle (G2V) and vehicle-to-grid (V2G) services to ensure reliable and cost-effective grid operation. This research provides a computational framework for this decision-making process. Charging and discharging strategies of EV aggregators are incorporated into a security-constrained optimal power flow (SCOPF) problem such that overall energy cost is minimized and operation within acceptable reliability criteria is ensured. Particularly, this SCOPF problem has been formulated for Jeju Island in South Korea, in order to lower carbon emissions toward a zero-carbon island by, for example, integrating large-scale renewable energy and EVs. On top of conventional constraints on the generators and line flows, a unique constraint on the system inertia constant, interpreted as the minimum synchronous generation, is considered to ensure grid security at high renewable penetration. The available energy constraint of the participating EV associated with the state-of-charge (SOC) of the battery and market price-responsive behavior of the EV aggregators are also explored. Case studies for the Jeju electric power system in 2030 under various operational scenarios demonstrate the effectiveness of the proposed method and improved operational flexibility via controllable EVs
Reference compensation method for enabling dispatchability of the wind power generation using battery energy storage system
Due to intermittent characteristics of wind power generation, battery energy storage system (BESS) has been exploited for decreasing the adverse impact of wind power output on the grid. This paper focuses on the BESS operation strategy called reference compensation for dispatchable wind. By adaptively compensating a reference signal that is typically set to be an average forecasted wind power for certain duration, the BESS maintains its SOC within a proper range, avoiding the non-compliant BESS when it is required to be charged or discharged because it is already fully charged or discharged, respectively, due to the unavoidable forecast errors. The proposed method has been applied to the real world wind farm data which is scaled down for the simulation in order to demonstrate its effectiveness of the proposed method. Simulation results demonstrate that the proposed method can decrease the operation suspension due to non-functioning BESS and keep the BESS on, and help thus enable the wind dispatchability
Incorporating State-of-Charge Balancing into the Control of Energy Storage Systems for Smoothing Renewable Intermittency
Incorporating State-of-Charge Balancing into the Control of Energy Storage Systems for Smoothing Renewable Intermittency
This paper proposes an effective control methodology for the Energy Storage System (ESS), compensating for renewable energy intermittency. By connecting generation variability and the preset service range of the State of Charge (SOC), this methodology successfully secures the desired SOC range while smoothing out power fluctuations. Adaptive to grid conditions, it can adjust response time (control bandwidth) of the ESS via energy feedback coefficients subject to the ESS capacity and its SOC range. This flexibility facilitates the process of developing ESS operation and planning strategies. Mathematical analysis proves that the proposed method controls the ESS to perform best for specific frequency bands associated with power fluctuation. Time-domain simulation studies along with power-spectrum analysis using PSCAD and MATLAB demonstrate the excellent power-smoothing performance to the power grid
Harmonious Integration of Faster-Acting Energy Storage Systems Into Frequency Control Reserves in Power Grid With High Renewable Generation
Generic Analysis Framework for Modular Multilevel Converter HVDC with Multi-Infeed Line-Commutated Converter HVDC System
This paper proposes a generic analysis framework for a grid supporting modular multilevel converter (MMC)-high voltage DC (HVDC) in a multi-infeed of line commutated converter (LCC) and MMC (MILM) system. MMC-HVDC can support the grid by compensating for the exact reactive power consumptions within the MMC-HVDC system and the varying power system conditions in the MILM system. Maximum active/reactive power capability (MPQC) curve and PQ loading curve comparison process is introduced to properly design a grid supporting MMC-HVDC. While the MPQC curve presents the maximum PQ range of the MMC-HVDC system based on the submodule capacitance value and the modulation index, the PQ loading curve presents the reactive power requirement from the power system that MMC-HVDC needs to compensate. Finally, the comparison of these two curves yields the proper value of submodule capacitance and the modulation index for sufficiently supporting the MILM system. The proposed framework is validated with detailed PSCAD/EMTDC simulation; it demonstrated that it could be applied to various power system conditions
Impedance-Based Stability Analysis in Grid Interconnection Impact Study Owing to the Increased Adoption of Converter-Interfaced Generators
This study investigates the emerging harmonic stability concerns to be addressed by grid planners in generation interconnection studies, owing to the increased adoption of renewable energy resources connected to the grid via power electronic converters. The wideband and high-frequency electromagnetic transient (EMT) characteristics of these converter-interfaced generators (CIGs) and their interaction with the grid impedance are not accurately captured in the typical dynamic studies conducted by grid planners. This paper thus identifies the desired components to be studied and subsequently develops a practical process for integrating a new CIG into a grid with the existing CIGs. The steps of this process are as follows: the impedance equation of a CIG using its control dynamics and an interface filter to the grid, for example, an LCL filter (inductor-capacitor-inductor type), is developed; an equivalent impedance model including the existing CIGs nearby and the grid observed from the point of common coupling are derived; the system stability for credible operating scenarios is assessed. Detailed EMT simulations validate the accuracy of the impedance models and stability assessment for various connection scenarios. By complementing the conventional EMT simulation studies, the proposed analytical approach enables grid planners to identify critical design parameters for seamlessly integrating a new CIG and ensuring the reliability of the grid