Marija Prica

A Decision-Making Framework and Simulator for Sustainable Electric Energy Systems

In this paper, we propose a new framework for the organization of the electric power industry, based on extensive use of information technology (IT) and on interactive decision making, where consumers and distributed producers join the traditional actors, utilities in particular, in making decisions. While many ideas considered in this paper have been put forward in recent years, such as the need to manage intermittency of renewable resources by means of proactive forecasting, and coordination with responsive demand and storage, we introduce a possible systematic IT-enabled mechanism necessary for the actual implementation of these technologies. We point out that in order to achieve a long-term sustainable energy utilization, it is essential to provide on-line information to internalize the value of just-in-time, just-in-place, and just-in-context distributed adaptation across the entire supply chain, ranging from the smallest consumers and energy providers, through their aggregators and system coordinators. We illustrate using our model-based novel simulator, how a carefully designed multidirectional and multitemporal information exchange could enable sustainable decision making while accounting for unique needs and capabilities of various resources and users. At the same time, information incentivizes the resources and users to contribute to system-wide sustainability objectives at value. We illustrate the dependence of such decisions-driven industry evolution on the industry rules (choice of performance objectives), as well as on the operating and planning practices for implementing the industry rules (temporal and spatial factors). Our model-based simulator could be used as a means of designing novel regulation defining rules, rights, and responsibilities regarding the type and rate of information to be exchanged in support of sustainable industry evolution.

Optimal Distribution Service Pricing for Investment Planning

Distribution network development planning is a complex task due to the size of a typical distribution network, the interconnectivity of its elements and the presence of various uncertainties. This planning process becomes even more complicated due to the constant pressure to reduce distribution network capacity reserve, operation and maintenance cost, and to postpone the capital investments as far as possible. Recent technological and organizational changes in the electric industry sector have had an influence on the operations and planning objectives of distribution network systems. New technologies and customer demand types require new paradigms of distribution network planning and operations. A corresponding model in support of these new paradigms should not include only the load forecast, but economic analysis, risk assessment and the impact of the new technologies. In this paper, we propose a model for distribution development planning with direct load control, distributed generators and optimal distribution capacity expansion.

Calculation of relay currents in active weakly-meshed distribution systems

This paper deals with short-circuit calculation, as one of the basic Distribution Management System (DMS) applications. It is common practice in short-circuit calculations to neglect loads, line and transformer shunt parameters as well as influence of the pre-fault state. This paper shows that some of these approximations could lead to incorrect results when calculating currents at relay locations (relay currents) in distribution systems. Consequently, an efficient short-circuit calculation procedure for active weakly-meshed distribution systems is proposed. Procedure is based on back/forward sweep with compensation for loops applied to faulted network models. The proposed procedure is robust and easy for programming and the results are significantly more accurate than those obtained by the oversimplified methods commonly used. Results show that errors made by listed approximations could be as high as 16% for a 20 KV test system, which can seriously affect the setting and coordination of protective equipment.

A novel fault-dependent-time-settings algorithm for overcurrent relays

A typical structure of electric power distribution networks is radial or a normally open-loop structure with a single supply path between the high-voltage supply substation and the end-consumers. The selectivity of a protection scheme for the radial network is generally achieved through time-coordination of different protection devices (relays, reclosers and fuses). However, future distribution networks may become more meshed (normally closed-loop) in order to integrate distributed generators (DGs) and more active demand-side response. This will, in turn, require novel protection schemes since the current time-coordinated protection which assumes radial network structure may no longer be effective. Furthermore, it is also recognized that todays practice of automatically disconnecting DGs when a fault occurs in their vicinity may cause un-necessary generation shortages. It is, instead, desired to adjust protection so that a DG unit remains connected to the system when nearby faults occur as long as there are no related safety problems. This paper introduces a novel fault-dependent time settings algorithm for overcurrent relays which is capable of overcoming these problems. The implementation of the proposed algorithm on the existing protection scheme is enabled by using communications among the relays. The proposed algorithm ensured a reduced relay tripping time in the primary protection zone by enhancing the selectivity of the existing protection scheme. Consequently, it becomes possible to operate distributed generators during times when nearby faults occur. While the use of the proposed algorithm is illustrated in the distribution network, the same algorithm is also applicable to the transmission networks.

Imbedding Smart Relays in Large Electric Power Networks: The Scalability Problem and a Possible Solution

As the need to meet the high security and reliability requirements of the electric power grids intensify, it is essential to make the existing relays smarter and more adaptive to changing system conditions. A successful implementation of enhanced relay performance requires both improved logic and the ability to manage computing and communications in near-real time. In this paper, we first summarize our recently introduced hypothesis-testing-based improved logic for over-current relays. The new contribution concerns the computational and communications problem associated with the large combinatorial burden of decision making underlying this new logic when relays are embedded in large electric power networks. We refer to this problem as the scalability problem. In this paper, a possible approach to solving the scalability problem is presented. In particular, this approach leads to: 1) Shorter simulation and training time prior to installing new relays; 2) Less data requirement for training and updating each relay; and 3) Faster dynamic updating with lower communication traffic load. These features are essential for a physical cyber system (PCS) to effectively meet the goals of secure and reliable critical infrastructures, such as a large electric power system.

Generalized Δ-Circuit Concept for Integration of Distributed Generators in Online Short-Circuit Calculations

In this paper, a novel concept of Generalized Δ-circuit is proposed, that enables integration of distributed generators (DGs) based on contemporary technologies into the short-circuit calculations of large-scale distribution systems. Modern DG models, such as doubly fed induction generators (DFIGs) and three-phase inverter based DGs (IBDGs) differ from the classical synchronous and induction generator models. It is shown that their models cannot be integrated in traditional short-circuit calculation procedures because they include a large number of possible fault current control strategies. Therefore, the concept of Generalized Δ-circuit is proposed and allows for any control strategy implemented in modern DGs to be integrated in the short-circuit calculation procedure. An improved backward/forward sweep procedure is developed for calculation of the Generalized Δ-circuit state. The faulted system state is calculated by the superposition of the known prefault state and calculated Generalized Δ-circuit state. Results show that unlike previously developed online short-circuit calculations, the proposed method can handle DFIGs and IBDGs with arbitrary selected fault current control strategies.

Real-time short-circuit analysis of active distribution systems

In this paper a real-time short-circuit calculation method for large-scale distribution systems with a high penetration of distributed generators is proposed. Recent years have witnessed increasing penetration of distributed generators. Thus, distribution systems have become active, where the fault current flow is no more straightforward. Additionally, modern distributed generators models differ from the classical synchronous generators models. Hence, traditional short-circuit calculation methods are not applicable in todays systems. The proposed method is based on the superposition theorem. An improved back/forward sweep in the sequence domain is used for calculation of the faulted system state. Results show that the proposed method is highly accurate and particularly suitable for real-time applications in large-scale systems, where the calculation time is of critical interest.

Short-circuit modeling of inverter based distributed generators considering the FRT requirements

In this paper short-circuit models for inverter based distributed generators (IBDGs) are proposed, based on the fault ride through (FRT) requirements of existing distribution codes. Currently, the IBDGs do not have generally accepted short-circuit models as the traditional synchronous and induction generators do. However, as their usage in the power industry is constantly increasing, there is an urgent need for developing the strictly defined short-circuit models for the IBDGs. In this paper, first FRT requirements of several distribution codes are presented and their mutual characteristics are derived. Then, based on these requirements, the IBDG models suitable for real-time short-circuit calculations are proposed. To validate the proposed models, they were integrated in the recently developed real-time short-circuit calculation procedure for active distribution systems. The IEEE 13 test feeder and several large-scale distribution systems consisted of as many as 5200 three-phase busses and 50 IBDGs were used as the cases studies. The simulation results showed that the proposed models provide desirable results for short-circuit calculations in active distribution systems in which the FRT requirements are strictly imposed.

On the fundamental importance of relating operating and planning objectives in the changing electric power industry

In this paper we emphasize the fundamental importance of relating the short-term efficiencies in utilizing the existing equipment to the capital cost of candidate new investments. We suggest that the systematic assessment of candidate investments based on understanding the break-even cumulative short-term system performance with the capital cost of the investments is the key to selecting the investments which enhance system-wide utilization of the existing and the new resources. The two-part tariff underlying guaranteed revenue recovery is compared with the peak-load-pricing-based approach in both the regulated and the deregulated industry. We point out that the economic policy regulation is needed to give incentives for near-optimal investments.

Short-circuit analysis in large-scale distribution systems with high penetration of distributed generators

In this paper a short-circuit computation U+0028 SCC U+0029 procedure for large-scale distribution systems with high penetration of distributed generators based on contemporary technologies is proposed. The procedure is suitable for real-time calculations. Modeling of modern distributed generators differs from the modeling of traditional synchronous and induction generators. Hence, SCC procedures found on the presumption of distribution systems with only traditional generators are not suitable in nowadays systems. In the work presented in this paper, for computation of the state of the system with short-circuit, the improved backward U+002F forward sweep U+0028 IBFS U+0029 procedure is used. Computation results show that the IBFS procedure is much more robust than previous SCC procedures, as it takes into account all distribution system elements, including modern distributed generators.