Evangelos Polymeneas

Advanced Distribution Management System

This paper proposes an advanced distribution management system (DMS) that a) monitors each component and performs protection functions using a dynamic state estimation, b) the estimated states are transmitted to the DMS where the real time model of the entire feeder is synthesized, c) uses the real time model to perform upper level optimization (operations planning) and lower level optimization (real time control) via a hierarchical optimization procedure; and d) applies proper controls to operate the system at optimal points. The proposed approach for protection, operations planning, and real time control of the system provides the infrastructure for additional important applications. As an example, the paper presents a novel application for monitoring available reserves from all resources in the system. We propose the concept of Reserve-O-Meter that monitors in real time the available reserves from all resources (utility and customer owned).

Grid Modernization: Seamless Integration of Protection, Optimization and Control

The objectives of smart grid and grid modernization are to increase automation and seamlessly integrate data, models, protection, optimization and control of the power grid. This effort is affected by technological advances. One such technology is the numerical relay which has increased its domination to the point that today has almost completely displaced electromechanical and solid state relays and the most recent technology of merging units that has separated the data acquisition function from protective relays and SCADA systems. The capabilities of the numerical relays are not fully utilized today, specifically, by and large, they simply mimic the logics that were developed for the electromechanical relays but with much more flexibility. Recent developments towards substation automation are utilizing the numerical relays for SCADA, communications and in general an integrated system for protection and control. These approaches indicate the recognition that numerical relays offer much more than simply mimicking protection functions of the past. They also offer the ability to form the basic infrastructure towards a fully automated power system, the subject of this paper. In previous work, we presented a new protection scheme that is a generalization of differential protection. The approach is based on dynamic state estimation. Specifically, the protection scheme is based on continuously monitoring terminal voltages and currents of the component and other possible quantities such as tap setting, temperature, etc. as appropriate for the component under protection. The monitored data are utilized in a dynamic state estimation that continuously provides the dynamic state of the component. The dynamic state is then used to determine the health of the component. Tripping or no tripping is decided on the basis of the health of the component. The present paper takes the above concept one step further. Using the dynamic state estimation of a protection zone as the basic technology, it builds an integrated automation system that performs the protection functions, validates models, transmits the models to the control center, integrates monitoring and control, enables optimization and provides automated disturbance playback capabilities. The system provides the infrastructure and real time models for any application along the spatial extend of the power system.

Integration & automation: From protection to advanced energy management systems

This paper proposes an integrated and seamless infrastructure for protection, control and operation of an electric power system. At the lower level we propose a on dynamic state estimation of a protection zone for the purpose of providing protection for the zone. This scheme simplifies the protection approach for the zone by not requiring coordination with other protection zones (setting-less protection). The scheme provides the real time dynamic model of the zone as well as the real time operating conditions. The scheme can be also implemented in present day numerical relays with GPS synchronization. Using this basic protection infrastructure, we propose that the real time model of substation be autonomously created, send to the control center where the real time model of the system is also autonomously created. The system wide real time model is used to perform system optimization functions, and then send commands back through the same communication structure to specific power system components. Since protection is present in any power apparatus the proposed approach is realizable with very small investment. The availability of the real time dynamic state of the system enables the seamless integration of applications in the proposed system. Three applications are discussed in the paper: (a) setting-less protection, (b) stability monitoring, and (c) voltage/var control.

Multi-agent coordination of DG inverters for improving the voltage profile of the distribution grid

Increasing penetration of distributed energy sources in distribution grids introduces new challenges, including regulation of voltages in the distribution system, particularly voltage rise issues in cases of low load and high generation. This paper proposes a methodology to achieve cooperation between the inverters that interface the DG units to the grid, so that the voltages across all the nodes of the distribution system maintain an acceptable profile. Each node in the distribution system acts as an agent, measuring its deviation from nominal voltage. Subsequently, all the nodes engage in a multi-agent consensus algorithm, to share their measurements. The algorithm is decentralized, and each node needs to communicate exclusively with its neighboring nodes. Utilizing the feedback of the observed total deviation of all the nodes from their desired voltage, each inverter adjusts its local reactive power injection, through a local I controller. Control design of this local controller is also discussed in this work. Simulation results for the IEEE 123-node test feeder verify that the approach results in significantly improved voltage profile compared to the unity power factor control and it addresses the issue of voltage rise in the distribution grid by utilizing each units reactive power margin.

Methodology for monitoring, control and operation of power systems with wind farms

This paper presents an infrastructure and a monitoring and control approach for systems with wind farms that enables coordination of the operation of wind farms with the bulk power system and maximizes the utilization of the available wind energy at the wind farm locations. A hierarchical optimization scheme is proposed in this work. In particular, a system level optimization defines the optimal operating point for the wind farms, the thermal generating units and other controllable devices in the system. At each wind farm, a lower level optimization procedure is performed which defines the operating condition of each wind turbine at the wind farm. The proposed infrastructure is enabled by a distributed state estimation procedure that evaluates the real time model of the system and the wind farms, used by the hierarchical optimization scheme, continuously with speeds of 60 times per second.

Aggregate modeling of distribution systems for multi-period OPF

Distribution systems with active components, such as responsive load, distributed storage and renewables, supplemented with thermostatically controlled loads have the capability to support the transmission grid and provide part of the required capacity reserve. Including distribution system resources in transmission level multi-period economic dispatch is challenging due to the large number of devices. In this paper, a two-level scheme is proposed for optimally dispatching the distribution systems active & reactive support to the grid, over a look-ahead horizon using an aggregate model. Initially, a semidefinite programming problem is solved in order to obtain a maximal dynamic ellipsoidal model of the feasible region for the power consumption of the distribution system. Subsequently, a look-ahead AC-OPF problem is solved for the optimal look-ahead dispatch of the transmission grid, using the aggregate distribution model. The distribution dispatch is then disaggregated to individual devices. The procedure is repeated in a receding horizon basis. Results on the accuracy and benefits of the approach are demonstrated on standard IEEE systems.

Distributed dynamic state estimation: Fundamental building block for the smart grid

The changing face of the electric power system due to new power apparatus, the proliferation of customer owned resources and smart devices creates the possibility of coordinated control of all these resources for the benefit of the power grid and its customers. Coordinated control requires real-time monitoring of models and resources and subsequent model-based analysis, optimization and control. Legacy state estimation is limited to the transmission system only and provides only the bus voltage values at transmission buses. This paper proposes a distributed dynamic state estimation (DDSE) technology that (a) extracts the dynamic real-time model of all components in substations as well as transmission lines, and (b) executes each cycle thus providing the real time model each cycle of the system. The DDSE uses all available measurements in the system, PMU data (GPS-synchronized), SCADA data, smart meter data, etc. The DDSE provides the real time model of the system which is the fundamental building block for all smart grid applications. The representation of the real-time model is object-oriented that allows interoperability among various applications. As an example, the paper discusses the seamless application of the optimal power flow using the real-time model provided by the DDSE.

Flexible resource optimization to mitigate operational problems from variable generation

Increased penetration of variable resources mainly in the form of wind and PV plants creates new challenges to power system operation, in the form of increased thermal unit cycling, ramping requirements and increased capacity reserve requirements. One approach is to address these issues by utilizing the increased flexibility offered by energy storage devices (utility and customer owned), distributed responsive load and thermally controlled loads. The flexible resources come with their own set of physical and customer convenience constraints to their flexibility. In this paper, the flexible power system operation problem is cast as a multi-step OPF problem which includes transition constraints of flexible dynamic resources and customer requirements. The flexible resource constraints are derived from their physically-based model and imposing the “no customer inconvenience” requirement. Model, objectives and constraints are expressed with quadratized models (objects) leading to an object-oriented quadratic flexible OPF. An efficient solution algorithm with good convergence properties is presented. Initial results in a sample system are also presented.

Margin-Based Framework for Online Contingency Selection in Unbalanced Networks

A framework for performing fast online contingency selection in unbalanced power systems is presented in this paper. Selection methods are typically based on the principle of identifying the effect of contingencies on multiple normalized performance indices and ranking them using the results. Presently used performance indices are highly nonlinear and they are known to mask the effects of single contingencies leading to misclassifications. In this paper, we propose two new methods, one relying on margin-based performance indices and another based on state sensitivity. New performance indices are proposed based on margins of 1) circuit loading, 2) bus voltages, and 3) reactive power. In addition, a state sensitivity method is proposed which estimates a systems post contingency operating state via a single iteration of the quadratized power flow model and provides estimates of post contingency line loading, bus voltages, and reactive power levels. Numerical experiments on a three phase version of the IEEE Reliability Test System show that the proposed performance indices yield more accurate results, at a computational cost comparable to a single power flow iteration. The state sensitivity method is more accurate in identifying critical contingencies but its computational cost is higher. The method has been also demonstrated in the larger PEGASE systems.

Aggregate equivalent models of flexible distribution systems for transmission-level studies

Increased penetration of variable resources in the power grid have resulted in variable power flow patterns, volatility in system stress and higher reserve capacity requirements. New approaches are emerging for dealing with these problems. Specifically, flexible assets, such as storage, Thermostatically Controlled Loads, smart appliances, flexible load and other flexible customer resources are expected to provide practical solutions for mitigating the variability. In this paper, a state-transition model of aggregate flexibility of a distribution feeder with responsive loads and flexible generation is derived. The method is general enough to handle any feeder with flexible resources. It yields a concise and tractable “aggregate virtual plant” model of feeder flexibility. Results using simulation data verify the models predictive capabilities and its applicability to the transmission level multi-period dispatch problem.