Nilanjan Ray Chaudhuri

Adaptive Droop Control for Effective Power Sharing in Multi-Terminal DC (MTDC) Grids

Following a converter outage in a Multi-Terminal DC (MTDC) grid, it is critical that the healthy converter stations share the power mismatch/burden in a desirable way. A fixed value of power-voltage droop in the DC link voltage control loops can ensure proper distribution according to the converter ratings. Here a scheme for adapting the droop coefficients to share the burden according to the available headroom of each converter station is proposed. Advantage of this adaptive (variable) droop scheme for autonomous power sharing is established through transient simulations on an MTDC grid with four bipolar converters and DC cable network with metallic return. Results for both rectifier and inverter outages under two different scenarios are presented. Post-contingency steady-state operating points obtained from transient simulation are shown to be consistent with those derived analytically. Impact of varying droop coefficients on the stability of the MTDC grid is established. An averaged model in Matlab/SIMULINK which has been validated against detailed switched model in EMTDC/PSCAD is used for the stability and modal analysis.

Stability Analysis of VSC MTDC Grids Connected to Multimachine AC Systems

Interaction between multimachine ac systems and a multiterminal dc (MTDC) grid and the impact on the overall stability of the combined ac-MTDC system is studied in this paper. A generic modeling framework for voltage-source converter (VSC)-based MTDC grids, which is compatible with standard multimachine ac system models, is developed to carry out modal analysis and transient simulation. A general asymmetric bipole converter configuration comprising positive and negative pole converters and dc cable network with a positive, negative, and metallic return circuit is considered to enable different types of faults and dc-side unbalance studies. Detailed dynamic representation of the dc cables with distributed pi-section models is used along with the averaged model and decoupled control for the converter stations. An averaged model in Matlab/SIMULINK is validated against the detailed switched model in EMTDC/PSCAD by comparing the responses following small and large disturbances (e.g., faults on the dc side). Modal analysis is performed to identify the nature and root cause of the dynamic responses. Interaction between a multimachine ac system and an MTDC grid is examined following faults on the ac and dc sides and outage of converters. It is shown that the cause of instability in certain cases could only be attributed to the dc-side state variables. An averaged model of the converter along with the dc cable network is shown to be essential to analyze the stability and dynamics of combined ac-MTDC grids.

System Frequency Support Through Multi-Terminal DC (MTDC) Grids

Control of the converter stations in a multi-terminal DC (MTDC) grid to provide frequency support for the surrounding AC systems is the subject matter of this paper. The standard autonomous power sharing control loop for each converter is modified with a frequency droop control loop. The objective is to minimize the deviation from nominal AC system frequency and share the burden of frequency support among the converter stations of the MTDC grid. The effectiveness of the frequency support is demonstrated through nonlinear simulation of a test system consisting of three isolated AC systems interconnected through an MTDC grid with four converter stations. An averaged model of the MTDC grids is developed to carry out modal analysis of combined multi-machine AC-MTDC systems. Modal analysis is used to characterize and substantiate the time domain behavior in presence of frequency droop control. It is established that appropriate droop control loop for the MTDC grid converters could be effective in reducing the deviation from nominal AC system frequency provided the sensitivity of the system eigen-values to changes in control parameters (e.g., droop coefficients) is accounted for a priori through modal analysis.

A New Approach to Continuous Latency Compensation With Adaptive Phasor Power Oscillation Damping Controller (POD)

Latency or delay in remote feedback signals can adversely affect the closed-loop damping performance. Accurate time-stamp information at both (PMU location and control center) ends offers a possibility to continuously compensate for time-varying latency. In this paper, an adaptive phasor power oscillation damping controller (APPOD) is proposed wherein the rotating coordinates for phasor extraction are adjusted to account for the change in phase caused due to the delay. The oscillatory component of the original signal is thus retrieved out of the delayed signal received at the control center. Unlike conventional phasor POD, which uses a fixed phase shift to generate damping control signal, an adaptive phase shift algorithm is used here to suit varying signal locations and operating conditions. Case studies confirm the effectiveness of the proposed technique, both in terms of robustness and handling continuously varying delays. A comparison with a conventional gain scheduled POD (CGPOD) and a Unified Smith Predictor (USP) approach is also presented.

Dynamic Modeling of Electric Springs

The use of “Electric Springs” is a novel way of distributed voltage control while simultaneously achieving effective demand-side management through modulation of noncritical loads in response to the fluctuations in intermittent renewable energy sources (e.g., wind). The proof-of-concept has been successfully demonstrated on a simple 10-kVA test system hardware. However, to show the effectiveness of such electric springs when installed in large numbers across the power system, there is a need to develop simple and yet accurate simulation models for these electric springs which can be incorporated in large-scale power system simulation studies. This paper describes the dynamic simulation approach for electric springs which is appropriate for voltage and frequency control studies at the power system level. The proposed model is validated by comparing the simulation results against the experimental results. Close similarity between the simulation and experimental results gave us the confidence to use this electric spring model for investigating the effectiveness of their collective operation when distributed in large number across a power system. Effectiveness of an electric spring under unity and non-unity load power factors and different proportions of critical and noncritical loads is also demonstrated.

An Architecture for FACTS Controllers to Deal With Bandwidth-Constrained Communication

Bandwidth constraints could have an adverse impact on the flexible AC transmission system (FACTS) controllers relying on signals communicated from distant locations. An observer-driven system copy (OSC) architecture is adopted here to deal with low data rates caused by the limited bandwidth availability. The basic idea is to use the knowledge of the nominal system to approximate its actual behavior during the intervals when data from the remote phasor measurement units (PMUs) are not available. This is corrected whenever the most recent states are obtained from the reduced-order observer at the PMU location. The closed-loop behavior deteriorates as the operating condition drifts away from the nominal. Nonetheless, significantly better response is achieved under limited bandwidth availability compared to the conventional approach of communicating the measured outputs. The deterioration is quantified in terms of the difference between the nominal and the offnominal condition.

Modeling and stability analysis of MTDC grids for offshore wind farms: A case study on the North Sea benchmark system

Modeling of VSC-based multi-terminal DC (MTDC) grids for modal analysis and stability studies is reported considering bipolar converters and DC cable network with metallic return path. The proposed model is flexible enough to accommodate different converter grounding options and unbalance on the DC side due to cable and/or converter outage. A simplified version of the benchmark test system for the envisioned offshore MTDC grid in the North Sea is studied. Modal participation factor analysis under nominal and converter outage conditions is used to ascertain the nature and root cause of the dynamic responses. Comparison of the time-domain simulation results using the proposed averaged model in Matlab/SIMULINK against a detailed switched model in EMTDC/PSCAD confirm the accuracy of the modeling approach.

Damping and Relative Mode-Shape Estimation in Near Real-Time Through Phasor Approach

A technique for estimating damping and electromechanical mode-shape in near real-time as oscillations develop under transient condition is presented. At each sampling instant, measured signals are expressed as phasors using corrected values of modal frequencies. Damping is obtained from the exponential variation of estimated phasor magnitude using a moving window least squares (LS) algorithm. The relative mode-shape is computed directly from the magnitude and phase angle of the phasors. Random variations in loads are considered to examine possible impact on phasor estimation, especially the frequency correction loop. Accuracy and speed of convergence is validated by comparing the time variation of estimated dampings and relative mode-shapes against the actual values obtained from the linearized models under respective operating conditions. Besides the well-known four-machine, two-area test system, a 16-machine, five-area system is considered for illustration of the concept. Monte Carlo simulations are used to capture the statistical variability in estimation as a result of persistent disturbances (e.g., random fluctuations in loads) leading to different signal-to-noise ratios (SNRs). Results from a commercial real-time simulator illustrate the practical feasibility of the proposed approach.

Multi-Terminal Direct-Current Grids: Modeling, Analysis, and Control

A multi-terminal DC (MTDC) grid interconnecting multiple AC systems and offshore energy sources (e.g. wind farms) across the nations and continents would allow effective sharing of intermittent renewable resources and open market operation for secure and cost-effective supply of electricity. However, such DC grids are unprecedented with no operational experience. Despite lots of discussions and specific visions for setting up such MTDC grids particularly in Europe, none has yet been realized in practice due to two major technical barriers:

Damping Control in Power Systems Under Constrained Communication Bandwidth: A Predictor Corrector Strategy

Damping electromechanical oscillations in power systems using feedback signals from remote sensors is likely to be affected by occasional low bandwidth availability due to increasing use of shared communication in future. In this paper, a predictor corrector (PC) strategy is applied to deal with situations of low-feedback data rate (bandwidth), where conventional feedback (CF) would suffer. Knowledge of nominal system dynamics is used to approximate (predict) the actual system behavior during intervals when data from remote sensors are not available. Recent samples of the states from a reduced observer at the remote location are used to periodically reset (correct) the nominal dynamics. The closed-loop performance deteriorates as the actual operating condition drifts away from the nominal dynamics. Nonetheless, significantly better performance compared to CF is obtained under low-bandwidth situations. The analytical criterion for closed-loop stability of the overall system is validated through a simulation study. It is demonstrated that even for reasonably low data rates the closed-loop stability is usually ensured for a typical power system application confirming the effectiveness of this approach. The deterioration in performance is also quantified in terms of the difference between the nominal and off-nominal dynamics.