Guidelines for Reserves Calculation during Power …

1 Guidelines for Reserves Calculation during Power system restoration SHAHRAM JADID, ABOLFAZL SALAMI Department of Electrical Engineering Iran University of Science and Technology Narmak, Tehran IRAN Abstract: - This paper describes a methodology for determination of system reserve requirement during Power system restoration . Types of Reserves are defined and their amount is determined to improve frequency control process. reserve requirements are calculated from characteristic of different Power plants basic principles, taking into account the largest unit dispatch and the frequency sensitivity of loads. It is shown that effectiveness of Reserves is one of the important problems during load pick-up.

2 Aim of this paper is to propose a novel method for investigation of Reserves capability in different Power plants which can be used for evaluating the stability and frequency response of Power plants during load pickup in the early stages of Power system restoration . The proposed scheme for Reserves Calculation provides good performance for load pickup. In this work a case study is shown regarding a number generating systems which shows process of Reserves Calculation . Key-Words: - Power system Reserves , Power system restoration , Power plant, Dynamic modelling, Spinning reserve , Dynamic reserve , Under frequency load 1 Introduction restoration of a Power system following a blackout is a complex, stressful and time-consuming task [1].

3 In recent years, due to economic competition and deregulation, Power systems are operated closer and closer to their limits. At the same time, Power systems have increased in size and complexity. Both factors increase the risk of major Power outages. After a blackout, Power system needs to be restored as quickly and reliably as possible, and consequently, detailed restoration plans are necessary [2, 3]. There are many problems in Power system restoration that must be considered. To restore the whole system successfully, it is required to know the characteristics of all the system , components and network structure in details.

4 After considering various coordination constraints, the modelling of the bulk Power system is divided into generating systems, load pick-up characteristic, system frequency response and transmission network [4]. In the initial phase of restoration , operators are often concerned with the size of load pickup, and the effectiveness of the generation reserve . A certain amount of reserve is required for satisfactory frequency regulation when system is subjected to normal or unexpected load variations. The prime mover models can be used to determine reserve rates of prime movers. The amount of Reserves of a system are chosen so that the loss of one or more generating units does not cause too far a drop in system frequency [5].

5 If a severe under frequency condition occurs, these Reserves should be activated as quickly as possible in attempt to prevent system from collapse [6]. Comprehensive studies have been undertaken in designing more effective and coordinated spinning reserve and under frequency load shedding strategies for the interconnected system [7]. The problem of Reserves during load pickup is considered less in past works, and proposed method do not consider under frequency load and different type of Reserves [8]. This paper investigates different Power plants with their prime mover models including combustion turbine, hydro turbine and steam turbines.

6 This work defines different types of Reserves which are effective during Power system restoration . during the initial phase of restoration , operators are concerned with the amount of Reserves . A simple guideline has been developed for determining the amount of Reserves to meet the loss of largest generator. The proposed method has been applied for the different Power plants, and simulations results have been carried out to obtain characteristics which are needed for Reserves calculations. 2 Power system Reserves during system restoration , each control area should carry enough operating reserve to cover its largest generator contingency in each isolated area.

7 This reserve can either be on-line generation that can produce additional Power within ten minutes or customer load that can be shed manually within ten minutes. Operating reserve is required to enable the control area to restore its area (or subsystems) to a pre-contingency state (both tie lines and frequency) within ten minutes of a contingency. Connecting two or more systems together may result in a lower combined operating reserve requirement. However, caution needs to be taken to ensure that load is not added too fast and the system collapses again. reserve is effort to afford restoration from any abnormal frequency operating conditions.

8 Two categories of Reserves are to be considered in Power system restoration . These Reserves include spinning reserve and dynamic reserve . Spinning reserve is defined as on-line generation that can be loaded within ten minutes or load that can be shed manually in ten minutes for Power system restoration . Dynamic reserve is defined as amount of available reserve in order to preserve the system during a frequency disturbance. The security of an island is greatly increased when there is sufficient dynamic reserve to cover the loss of single largest generation unit in the island. Amount of this reserve must be enough to survive the largest energy contingency.

9 This dynamic reserve can be a combination of load armed to trip by under frequency load shedding relays and governor actions. The goal in the early stages of creating the island is to restore load with connected under frequency load shedding relays but in very early stages of system restoration not to restore under-frequency load and under frequency relay load should be no more than 50% of total dynamic reserve . As more generating units are added to the island the requirement for load connected to under frequency load shedding relays is reduced. The ultimate goal is to rely on generation unit governor response to arrest frequency declines in an island during restoration .

10 Dynamic reserve is automatic, as opposed to spinning reserve which is manual. Dynamic reserve must be calculated for each island. Dynamic reserve from generation is determined by generator load pickup factor for units paralleled to the system . Load pickup factor is maximum load a generator can pickup as a percentage of generator rating without incurring a decline in frequency below safe operating levels. Load pickup factors for different Power plant have been calculated in next section. 3 Reserves in Different Units Frequency can exhibit large excursions due to load pick-up. Frequency decline is due to inadequate system response following loss of generation or picking up a larger load than can be absorbed by the generators online.