Smart Solar PV Inverters with Advanced Grid Support Functionalities. Rajiv K. Varma
Чтение книги онлайн.
Читать онлайн книгу Smart Solar PV Inverters with Advanced Grid Support Functionalities - Rajiv K. Varma страница 17
Название: Smart Solar PV Inverters with Advanced Grid Support Functionalities
Автор: Rajiv K. Varma
Издательство: John Wiley & Sons Limited
Жанр: Физика
isbn: 9781119214212
isbn:
Figure 1.6 Phasor diagrams for network with active power injection from the solar PV system; (a) network with X/R = 3; (b) network with X/R = 1/3.
1.1.3 Frequency Response with Synchronous Machines
A steady‐state system frequency results when synchronous power generation matches the system load and the losses in power system supplying those loads. In case of a system disturbance, such as sudden loss of a major generation unit, the typical variation in system frequency and its subsequent restoration to the pre‐disturbance level is depicted in Figure 1.7 [8]. The frequency restoration is enabled through three sequential stages of frequency control – primary frequency control, secondary frequency control, and tertiary frequency control [8–13].
Assume that the power system is operating at steady state at t = 0− and a large generation loss occurs at t = 0+. The kinetic energy of all the synchronous machines (generators, condensers, motors) is autonomously extracted to supply the load (inertial response), leading to a decline in the speed of generators and consequently the system frequency. The decline in frequency continues till additional power injection from synchronous generators balance out the load. The rate at which the frequency decreases is termed “Rate of Change of Frequency (ROCOF).” The lowest level at which the frequency is eventually arrested is known as “frequency nadir.” The time period from the onset of disturbance to reaching the frequency nadir is known as “arresting period.”
Primary frequency control (also referred as Frequency Containment Reserve [FCR]) is provided by synchronous generator turbine governors by injecting power from the generators during the arresting period and continuing thereafter. This causes the frequency to stabilize at the “settling frequency,” which is higher than the nadir but still lower than the steady‐state frequency before the disturbance. This time period until settling frequency is reached is termed “rebound period.”
Secondary frequency control through Automatic Generation Control (AGC) is then exercised to restore system frequency to its pre‐disturbance scheduled level. The period over which this secondary frequency control is provided is known as “recovery period,” which extends over 5–10 minutes (or more).
Tertiary frequency control is provided subsequently which involves restoration of the synchronous generator and other reserves which provided primary frequency control, to their preset levels, so that they can respond to any future loss‐of‐generation events. The tertiary control involves coordinated changes in the dispatch levels (outputs) of different generators. In this control, some generators are dispatched down to restore their reserve capability while some other generators are dispatched up by a corresponding amount, while maintaining the scheduled system frequency. Deployment of tertiary frequency control is the final stage of frequency restoration in the recovery period.
Figure 1.7 Sequential frequency controls after a sudden loss of generation and their impact on system frequency.
Source: Eto et al. [8]. Reprinted with permission from Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
The focus of this book is on the arresting period, frequency nadir, and initial parts of the recovery period with and without the high penetration of inertia‐less solar PV systems. The key performance indicators involved in frequency response are explained below.
1.1.3.1 Rate of Change of Frequency
The ROCOF is a measure of how quickly the frequency changes following a sudden imbalance between generation and load [9]. ROCOF at any point on the frequency response curve is the tangential line at that point. However, it is usually calculated as the change in frequency over a short period of time, immediately after the sudden loss of generation.
The initial ROCOF after generation loss at t = 0 s is typically calculated as:
(1.5)
A high ROCOF indicates that the system frequency can potentially fall to a level where Under Frequency Load Shedding (UFLS) may be initiated. ROCOF is used to determine the approximate time required to reach the UFLS threshold. This helps systems planners and operators to provide adequate amount of frequency response within this period to prevent load shedding. It is noted that as soon as the frequency starts to decline, several factors come in to play such as inertial response, load response, and other nonlinear actions which may alter the actual time to reach UFLS [9].
1.1.3.2 Factors Impacting ROCOF
ROCOF is influenced by the following factors [9]:
1 Size of the contingency, i.e. amount of the lost generation or magnitude of power over a major tie line (e.g. HVDC line) from neighboring interconnections or load (for overfrequency events).
2 Total system inertia contributed by synchronous machines (synchronous generators, synchronous condensers, and synchronous motors).
3 Magnitude and the speed of energy injected from generating resources subsequent to contingency.
4 Sensitivity of loads to change in system frequency.
5 Incremental losses in the bulk power system due to modification in power flows after the contingency.
Factors (1), (2), and (3) have a more significant contribution to ROCOF among all the above factors.
1.1.3.3 System Inertia
Synchronous inertia is defined as “the ability of a power system to oppose changes in system frequency due to resistance provided by rotating masses” [14]. Inertia of a synchronous machine is defined as the ratio of stored kinetic energy to the MVA rating of the machine and is expressed as MW‐second/MVA, i.e. in seconds. The inertia constant effectively indicates the time duration over which the kinetic energy stored in the rotating mass will allow the production of rated output of the synchronous machine.
The system inertia, however, represents the aggregation of kinetic energy stored in the rotating masses of all the synchronous machines (generators, condensers, and motors) in the system. This system inertia provides the Synchronous Inertial Response (SIR) to arrest the system frequency as soon as the frequency starts to decline following the loss of a major generation [13].
The system inertia dictates the initial ROCOF as below [9]:
СКАЧАТЬ