Annual Energy Outlook

Annual Energy Outlook 2008 With Projections to 2030

The notion of secure operation of power systems, with its present semantic, dates back to the last decade. Since then, tremendous research effort has investigated secure operation of the power system. Nevertheless, operators are still faced with security issues, with even larger uncertainty, however, caused by the integration of renewable energy sources (RES). The system's ability to cope with the volatility of RES and load becomes a critical issue in power system operation. This paper surveys the literature on the concepts of power system flexibility, indices of flexibility, and implementation of the concept of flexibility in power system security. The paper proceeds to review the origin of the reserve problem, the meaning of reserve, its technical classification and related economical aspects. The paper highlights the effect of renewables on these aspects, and suggests new research directions.

A Review of Power System Flexibility With High Penetration of Renewables

This paper proposes a robust optimization approach for optimal operation of microgrids. The uncertain output variation of renewable energy sources (RESs) is addressed by collaboratively scheduling of energy storage (ES) and direct load control (DLC) through a two-stage complementary framework: an hour-ahead charging/discharging of ES and a quarter-hour-ahead activation of DLC. The objective is to maximize the total profit of the microgrid considering operation and maintenance costs of ES units, wind turbines and photovoltaics, and transaction with main grid and customer loads. Assuming the power output of RES randomly varies within a bounded uncertainty set, the problem is modeled to a two-stage robust optimization model and solved by a column-and-constraint generation algorithm. Compared with conventional operation methods, the ES and DLC are coordinated in different time-scales, and RES uncertainties are fully addressed during operation decision-making, ensuring the solutions to be optimal and robust for any realization of uncertainty. The proposed methodology is verified on the IEEE 33-bus distribution system through a wide range of different tests.

Robust Operation of Microgrids via Two-Stage Coordinated Energy Storage and Direct Load Control

Optimal battery storage operation for PV systems with tariff incentives

High penetration levels of renewable resources impose increasing uncertainty and variability on power system operations. Traditionally, power systems rely on conventional generators (CGs) to balance the uncertainty in renewable generation. However, as renewable penetration levels increase, CGs may suffer from higher operating costs while receiving lower profits. In contrast, since bulk energy storage can store and shift clean energy and have fast ramping capability, they may become more competitive under high renewable penetration levels. In this paper, a stochastic unit commitment model with energy storage will be presented to evaluate the short-term profitability of CGs and energy storage under different levels of renewable penetrations. The short-term profitability of CGs and energy storage units will be compared to identify the impact of increasing renewable penetration on the attractiveness of bulk energy storage in comparison to CGs.

Economic Assessment of Energy Storage in Systems With High Levels of Renewable Resources

The fast growing expansion of renewable energy increases the complexities in balancing generation and demand in the power system. The energy-shifting and fast-ramping capability of energy storage has led to increasing interests in batteries to facilitate the integration of renewable resources. In this paper, we present a two-step framework to evaluate the potential value of energy storage in power systems with renewable generation. First, we formulate a stochastic unit commitment approach with wind power forecast uncertainty and energy storage. Second, the solution from the stochastic unit commitment is used to derive a flexible schedule for energy storage in economic dispatch where the look-ahead horizon is limited. Analysis is conducted on the IEEE 24-bus system to demonstrate the benefits of battery storage in systems with renewable resources and the effectiveness of the proposed battery operation strategy.

Flexible Operation of Batteries in Power System Scheduling With Renewable Energy

The requirements of a network equivalent to be used in new planning tools are very different from those used in traditional equivalencing procedures. For example, in the classical Ward equivalent, each generator in the external system is broken up into fractions. For newer long-term investment applications that take into account such things as greenhouse gas (GHG) regulations and generator availability, it is computationally impractical to model fractions of generators located at many buses. To overcome this limitation, a modified- Ward equivalencing scheme is proposed in this paper. The proposed scheme is applied to the entire Eastern Interconnection (EI) to obtain several backbone equivalents and these equivalents are tested for accuracy under a range of operating conditions. In a companion paper, the application of an equivalent developed by this procedure is used to perform optimal generation investment planning.

Optimal generation investment planning: Pt. 1: network equivalents

While there are growing interests in using pumped hydro storage to facilitate the integration of renewable resources, the flexibility of storage is not being fully utilized by existing energy and market management systems. Today, one common approach to operate pumped hydro storage is to determine schedules for a future time horizon based on a look-ahead operational planning stage, with limited adjustment in real-time. However, as renewable penetration levels increase, such approaches do not fully utilize the flexibility of pumped hydro storage. In this paper, a policy function approach is proposed to enhance the utilization of pumped hydro storage in real-time operations. The performance of the approach is evaluated and compared with other benchmark approaches using the IEEE RTS 24-bus system. The result shows that the policy function approach is effective in utilizing the flexibility of pumped hydro storage and has minimal added computational complexity to the existing dispatch process.

Enhanced Pumped Hydro Storage Utilization Using Policy Functions

Power system planning and market behavioral analysis using the full model of a large-scale network, such as the entire ERCOT system, are computationally expensive. Reducing the full network into a small equivalent is a practical way to reduce the computational burden. In a companion paper, a modified-Ward equivalencing procedure has been proposed. In this paper, the proposed scheme is applied to the ERCOT system to obtain several backbone equivalents, the accuracy of which are tested under a range of operating conditions. The ERCOT equivalent is used in a system planning tool to perform optimal generation investment studies with promising results observed.

Nan Li

Papers

Economic Assessment of Energy Storage in Systems With High Levels of Renewable Resources

Flexible Operation of Batteries in Power System Scheduling With Renewable Energy

Optimal generation investment planning: Pt. 1: network equivalents

Enhanced Pumped Hydro Storage Utilization Using Policy Functions

Optimal generation investment planning: Pt. 2: Application to the ERCOT system