arXiv subjects

Venkataramana Ajjarapu

Publications and source records attributed to Venkataramana Ajjarapu.

25 records · Page 2Linked to original sources

arXiv2018

Monitoring Long Term Voltage Instability due to Distribution & Transmission Interaction using Unbalanced μPMU & PMU Measurements

This paper extends the idea of the Thevenin equivalent into unbalanced 3ϕ circuits and proposes a 3ϕ long-term voltage stability indicator (VSI), that can identify critical loads in a system. Furthermore, in order to identify whether the voltage stability limit is due to the transmission network or a distribution network, a transmission-distribution distinguishing index (TDDI) is proposed. The novelty in the proposed indices is that they can account for the unbalance in the lines and loads, enabling them to use unbalanced phasor measurements naturally. This is supported by mathematical derivations and numerical results. A convex optimization formulation to estimate the 3ϕ Thevenin equivalent using PMU & μPMU measurements is proposed, making it possible to calculate VSI and TDDI in an online model-free manner. Numerical simulations performed using co-simulation between Pypower and GridlabD are presented for the IEEE 9 bus and the 30 bus transmission networks combined with several modified IEEE 13 node and 37 node distribution networks. These case studies validate the proposed 3ϕ-VSI and TDDI over a wide range of scenarios and demonstrate the importance of μPMU measurements in identifying the regions causing long term voltage instability.

math.OC↗

arXiv2018

Identifying Long Term Voltage Stability Caused by Distribution Systems vs Transmission Systems

Monitoring the long term voltage stability of the power grid is necessary to ensure its secure operation. This paper presents a new phasor based methodology that distinguishes between long term voltage stability caused by distribution systems versus transmission systems. From a conceptual understanding of a simplified system, a Transmission-Distribution Distinguishing Index (TDDI) is proposed to distinguish between the two scenarios. A methodology to calculate the TDDI for multi-bus systems using quasi-steady state phasor measurements is described and validating results are presented for the IEEE 9 Bus system with a load replaced by various distribution feeders. The results verify that the TDDI can indeed be used to distinguish between transmission limited and distribution limited systems. This information can be utilized by the operator to effectively choose controls in distribution and transmission systems to improve the system margin.

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arXiv2018

On Information Transfer Based Characterization of Power System Stability

In this paper, we present a novel approach to identify the generators and states responsible for the small-signal stability of power networks. To this end, the newly developed notion of information transfer between the states of a dynamical system is used. In particular, using the concept of information transfer, which characterizes influence between the various states and a linear combination of states of a dynamical system, we identify the generators and states which are responsible for causing instability of the power network. While characterizing influence from state to state, information transfer can also describe influence from state to modes thereby generalizing the well-known notion of participation factor while at the same time overcoming some of the limitations of the participation factor. The developed framework is applied to study the three bus system identifying various cause of instabilities in the system. The simulation study is extended to IEEE 39 bus system.

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arXiv2018

PMU based Monitoring and Mitigation of Delayed Voltage Recovery using Admittances

This paper analyzes the delayed voltage recovery phenomenon by simplifying the Western Electricity Coordinating Council (WECC) composite load model using the load admittance and the thermal relay dynamics. From this analysis, a closed form expression approximating the recovery time is derived and the key load parameters impacting the behavior of voltage recovery are identified. A monitoring scheme based on the measured load admittance is then proposed to detect the onset of stalling, even in the presence of voltage oscillations, and estimate the duration of the delayed voltage recovery. A mitigation scheme utilizing smart thermostats and offline learning is also derived to ensure that the voltage recovers to the pre-contingency voltage within a specified time. Both the monitoring and mitigation schemes only need local measurements at a substation making them promising for online applications. Results for the monitoring and mitigation schemes are described in detail for the IEEE 162 bus system validating the various assumptions used for the analysis and establishing the connection between the delayed voltage recovery phenomenon and load admittance.

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arXiv2018

A Framework to Utilize DERs' VAR Resources to Support the Grid in an Integrated T-D System

Increasing penetration of inverter-based distributed energy resources (DERs) opens up interesting opportunities for the transmission systems. We present a hypothesis that the numerous DERs in var control mode can be seen as geographically distributed var devices (mini-SVCs) and if controlled properly, can be exploited to increase system flexibility by providing local var support to the grid as an ancillary service. Based on this premise, a var support framework is proposed in this paper. It utilizes a novel D-OPF formulation for unbalanced three-phase feeders enabling the estimation of the maximum var support that can be provided by the DERs to the grid at different operating points without compromising the distribution network performance. Further, a co-simulation method is developed to investigate the true impact of the proposed DER var support on the grid in an integrated Transmission-Distribution (T-D) system.

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arXiv2018

Dynamic Co-Simulation Methods for Combined Transmission-Distribution System and Integration Time Step Impact on Convergence

Combined Transmission and Distribution Systems (CoTDS) simulation for power systems requires development of algorithms and software that are numerically stable and at the same time accurately simulate dynamic events that can occur in practical systems. The dynamic behavior of transmission and distribution systems are vastly different, especially with the increased deployment of distribution generation. The time scales of simulation can be orders of magnitude apart making the combined simulation extremely challenging. This has led to increased research in applying co-simulation techniques for integrated simulation of the two systems. In this paper, a rigorous mathematical analysis on convergence of numerical methods in co-simulation is presented. Two methods for co-simulation of CoTDS are proposed using parallel and series computation of the transmission system and distribution systems. Both these co-simulation methods are validated against total system simulation in a single time-domain simulation environment. The series computation co-simulation method is shown to have better numerical stability at larger integration time steps. The series computation co-simulation method is additionally validated against commercial EMTP software and the results show remarkable correspondence.

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arXiv2017

Long-Term Voltage Stability Assessment of an Integrated Transmission Distribution System

Long-term voltage stability assessment (VSA) of independent transmission (T-VSA) and distribution (D-VSA)systems have been studied since long to estimate load margins. However, their impacts on each other have been neglected due to simplified assumptions i.e. in transmission systems, loads are assumed to be aggregated, and in distribution systems, substation bus voltage is assumed to be constant. This work investigates the VSA of integrated transmission-distribution (TD-VSA) using PV curve superimposition approach and reveals the possibility that the overall system loadability may be limited by the distribution system rather than the transmission system. In this paper, we analyze why T-VSA and D-VSA are not reliable enough to estimate true load margin of the overall system. The analysis has been verified on an integrated test system in different scenarios with and without DER penetration. Overall, the paper points out the need for integrated analysis and further builds a case why it is essential to develop a realistic co-simulation framework for a reliable long-term VSA of large-scale coupled T-D systems.

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