Risk assessment and reserve requirements for power systems with high wind power penetration

Integration of wind power generation is increasing rapidly worldwide. The variability and uncertainty of wind energy may lead to significant load-generation imbalances resulting in large frequency deviations, and hence increase system operational risks, especially in small and isolated power systems with low inertia and limited capabilities of providing frequency responses.

Traditionally, security assessment is performed based on deterministic criteria. It requires that a power system must be able to withstand an outage of any single system component without violating any system operating limits. This is a worst-case scenario-based criterion – it provides a simple rule in the system design and operation and has satisfied the needs of the power industry for decades. However, it is recognised that the deterministic method may no longer be adequate for modern power systems with market driven dispatch and high penetration of renewable energy and distributed generation. Increasing installation of unobservable distributed and PV generation with unknown operating characteristics increases demand uncertainty and forecasting errors. This together with large-scale integration of wind power generation (WPG) increases system uncertainties due to wind intermittency and creates a number of technical challenges in the power system planning and operation. This raises a need to investigate new security assessment methods, which can cope with the uncertain behaviour of WPG as well as the probabilistic nature of system conditions and events in the operation planning studies.

The security of a power system is defined as the degree of risk in its ability to withstand random contingencies without interruption to customer service. The higher the risk the lower the security, and vice-versa. Although risk cannot be eliminated fully due to unexpected and random faults, it has to be assessed and managed within an acceptable level in power system planning, design and operation activities.

This paper proposes a risk assessment approach to the quantitative evaluation of security of power systems with significant WPG for short-term operation planning, i.e. hour(s)-ahead, in order to facilitate day-to-day system operation. The proposed risk assessment approach is concerned with steady-state voltage and overload evaluations, and frequency response adequacy. Frequency response adequacy is defined as the capability of frequency control resources to prevent frequency deviations exceeding specified limits. The system operational risk is defined as the product of probability and severity of the system failure states in terms of expected load interruption cost (ELIC) taking into account the randomness of contingencies as well as the uncertainty of operating conditions caused by load and WPG forecasting errors.

In the case study presented in the paper, the proposed approach was used to evaluate operational risks of the nine-bus power system with characteristics similar to the Tasmanian power system. The results showed that the integration of WPG significantly affected the system operational risk, especially risks associated with frequency response inadequacy. Impacts of different factors including load and WPG forecasting uncertainties, wind power penetration levels, and operating reserves on the system security were investigated. It also showed that the proposed approach could assist system operators in operation planning such as setting constraints for wind generation curtailments and determining operating reserves.