Standalone System

The RES is connected differently to the electric power system (EPS) depending upon the overall system structure. A standalone power system is defined as an off-grid system, which operates independently without a grid support. Similarly, a standalone RES is defined in this thesis as a single source which is operated individually without any support systems, such as storage or back up source to counteract the stochastic nature or weather dependency. Such a standalone RES cannot be operated effectively and efficiently in an islanded (off-grid) system; thus, it is always operated in a grid connected mode to maximize its optimal use and credentials, as shown in Figure 2.8.

Wind Source PE I Interfacing Loads PCC Grid DC link AC link PE III Interfacing

Figure 2.8: A typical RES in a grid connected mode

The RES is connected to a grid at the point of common coupling (PCC) through a series of power electronic interfacing units as shown in Figure 2.8. The fluctuating wind speed has impact on the output power which also varies irrespective of load demands. The mismatch power between the RES and loads are compensated through a grid. In other words, the deficit and abundance power supply from RES is supplied and consumed by a grid to balance the power flow in order to maintain the stability and reliability of the system, which is not possible for a standalone RES in an islanded mode.

Control

The RES is normally operated under power or current control mode, where the active and reactive power are controlled based on the reference voltage and frequency (say 170 V rms line to line and 60 Hz) at the PCC of the grid. The power electronic (PE) I interfacing unit or DC-DC converter is controlled under maximum power point tracking (MPPT) scheme whereas PE III interfacing unit or inverter is used to regulate the DC link voltage in order to generate the power flow to the grid [126].

2.4.2 Hybrid Systems

A hybrid system, in this thesis, is defined as a combination of different kinds of energy sources and energy storage systems. For example, if a wind source is combined with a storage element or a complementary source e.g. PV or a dispatchable source, such as diesel generator can all be referred as a hybrid system. The hybrid system has a great potential to provide higher quality and more reliable power to customers. This hybrid

system can be operated either on a grid connected mode or in an islanded (off-grid) mode. Hybrid system can be of different combinations utilizing mainly RESs. The two most popular RESs are wind and PV. The combination between these two sources can also form one of the suited hybrid systems because wind and PV can complement each other meteorologically. Similarly, any RESs: standalone, wind, or PV can combine with dispatchable DG such as a diesel generator or energy storage such as a battery to form another type of hybrid system. Different hybrid systems can be formed with the combination of different dispatchable and non-dispatchable DGs and energy storage.

In this thesis, standalone wind is combined with battery energy storage to form a hybrid system which operates in an islanded mode. The energy storage such as battery storage has a capability of supplying and consuming power to balance the power flow between RESs and loads. Battery storage can also be used as short term energy storage to supply fast transient and ripple power. Therefore, this hybrid system can be used in many applications in rural or urban areas as an islanded and grid connected system. Thus hybrid energy systems are becoming popular in the EPS world. A typical hybrid wind and battery energy storage structure is shown in Figure 2.9.

Wind Source PE I Interfacing Loads PCC Grid Battery Storage PE II Interfacing Loads DC link AC link PE III Interfacing

Figure 2.9: A typical RES based hybrid system

The wind energy source is connected to the point of common coupling (PCC) through a series of power electronic interfacing units or converters. Similarly, battery energy storage is also connected to DC link through a PE II interfacing unit to match the voltage level with a separate controller. However, battery energy storage can be connected to a DC link without a PE II interfacing unit by matching the voltage level and combined control strategy. This hybrid system can be operated either in a grid connected mode or in

an islanded mode as shown in Figure 2.9. The purpose of battery energy storage is to counteract the effect of the stochastic nature of wind energy source.

Control

In a grid connected mode, the PE I interfacing unit of wind energy source is controlled to extract the maximum power under different wind speed whereas the PE III interfacing unit is operated under power or current control mode similar as in the case of standalone system. In this control mode, the voltage and frequency at the PCC of the grid are considered as given references, whereas the active and reactive powers are controlled in a stable and reliable manner [57]. The battery energy storage is inactively floating or charging depending upon the control strategy or can actively participate for short term transient conditions or to regulate the DC link voltage to form a dispatchable wind energy source [127].

In an islanded mode, the PE I interfacing unit of wind energy source is operated under maximum power point tracking (MPPT) scheme, whereas PE III interfacing unit is operated under voltage control mode. In this control mode, the amplitude and the frequency of AC link voltage are regulated at the given reference values (say 170 V rms line to line and 60 Hz) irrespective of wind speed fluctuations. In other words, the power flow is the output of the AC link voltage regulation. The PE II interfacing unit of battery energy storage is used to regulate the DC link voltage [53].