Enhanced Frequency Regulation Using Multilevel Energy Storage in Remote Area Power Supply Systems Abstract: Frequency support from renewable power generators is critical requirement to ensure the frequency stability of remote area power supply (RAPS) systems with high penetration of renewable power generation. However, traditional control strategies and the stochastic nature of wind resource constrain wind energy conversion system (WECS) such as permanent magnet synchronous generator (PMSG) from participating in frequency regulation. This work proposes to integrate hybrid energy storage including ultra capacitors (UCs) and lead-acid batteries (LABs) into a PMSG to provide frequency support. The UCs deal with fast changing frequency by emulating conventional inertial response, whereas the LABs mimic automatic governor response (i.e., primary frequency response). The mechanical power reserved in wind turbine using suboptimal maximum power point tracking (SOPPT) strategy is utilized to restore system frequency (i.e., secondary frequency response). Moreover, supplementary control strategies are proposed to enable the UCs and LABs to assist primary frequency response and secondary frequency response respectively. Simulation study and experimental test are carried out to validate the effectiveness of frequency response provided by the multilevel energy storage. The multilevel energy storage solution can effectively regulate RAPS system frequency while avoiding abrupt and frequent charging/discharging of the LABs and significant mechanical/electromagnetic stress on the WECS. Existing system: An existing approach is to combine LAB with UC, which alleviates the drawbacks associated with the single energy storage. Hence, the hybrid energy storage may radically improve frequency regulation in a self-reliant RAPS system. In, sizing methods are proposed to determine the optimal capacity for battery and UC to cope with frequency deviation. A centralized hybrid storage and converter are implemented at the point of common coupling (PCC) in to smooth the generation from wind farm. Contrarily, a grid connected PMSG with integrated hybrid energy storage is studied in. Majority of the studies are focused on numerical study of the
balance between generation and load demand, which provide good guidelines for the optimization of system component sizing. The contribution of hybrid energy storage for improving RAPS system dynamic performance remains to be explored. Authors in implemented centralized hybrid energy storage to emulate a synchronous generator, but the coordination with generation resources are not considered. Proposed system: In, sizing methods are proposed to determine the optimal capacity for battery and UC to cope with frequency deviation. A centralized hybrid storage and converter are implemented at the point of common coupling (PCC) in to smooth the generation from wind farm. Contrarily, a grid connected PMSG with integrated hybrid energy storage is studied. Majority of the studies are focused on numerical study of the balance between generation and load demand, which provide good guidelines for the optimization of system component sizing. The contribution of hybrid energy storage for improving RAPS system dynamic performance remains to be explored. Authors in implemented centralized hybrid energy storage to emulate a synchronous generator, but the coordination with generation resources are not considered. Advantages: The solution also relieves WECS from abrupt mechanical/electromagnetic stress when the WECS is engaged in frequency regulation. Hence, these advantages can be beneficial to the extension of service life and reduction in lifetime cost for WECS and LAB. Furthermore, the benefit of the multilevel energy storage solution is not restricted to LAB. The contribution is also applicable to general battery technique for which the number of charging/discharging cycles presents as a problem. Disadvantages: One major problem with renewable power generation is that it is not dispatch able as in the case of conventional energy resources such as thermal and hydro.
Therefore, it becomes onerous for conventional generators to meet load side requirements in RAPS systems due to the increasing penetration level of renewable energy resources. The adoption of power electronics based renewable power generators such as wind energy conversion systems (WECSs) aggravates the frequency stability problem since these generators are not naturally involved in frequency regulation. The capability of frequency regulation using doubly-fed induction generator (DFIG) based WECS has been widely investigated due to the large market share of DFIG in renewable rich power networks. Modules: Wind energy conversion systems: The low inertia characteristic of small scale RAPS system results in system’s high sensitivity to the mismatch between generation and load demand. Large frequency excursions are common in such systems. The adoption of power electronics based renewable power generators such as wind energy conversion systems (WECSs) aggravates the frequency stability problem since these generators are not naturally involved in frequency regulation. The capability of frequency regulation using doubly-fed induction generator (DFIG) based WECS has been widely investigated due to the large market share of DFIG in renewable rich power networks. The WECS is being developed to very high capacity and achieving popularity in remote locations like offshore where wind resource is abundant but the harsh environment demands a greater degree of reliability on WECS. Permanent magnet synchronous generator (PMSG) based WECS with full-scale converter can be a good option to meet this requirement. PMSG is suitable for wind power generation due to its high torque-to-volume ratio, elimination of the requirement for excitation windings and its capability of direct drive variable speed operation without a gearbox. PMSG is gaining attention on its contribution to frequency regulation. In, a frequency controller based on virtual inertia and droop loop is introduced to PMSG. Ultra capacitor:
Energy storage can be a promising technique to relieve WECS from the stress caused by frequency regulation. Particularly, ultra capacitor (UC) is a decent shortterm energy storage device due to its high round-trip efficiency, high number of charging/discharging cycles, and fast charging/discharging rates. These advantages enable UC to support instantaneous load spikes. UC is applied in for dynamic frequency support under frequency disturbance in an isolated power system. However, due to the limited energy density of UC, the capability of dynamic .frequency support is limited to primary frequency response that lasts less than 30 s Batteries have high energy density and suitable for long term frequency regulation. Particularly, lead-acid batteries (LABs) are still widespread in RAPS systems due to their low initial costs, in spite of the availability of advanced battery technology such as lithium based batteries. Nevertheless, the drawback of faster degradation rate demands sophisticated approaches to reduce lifetime cost. An existing approach is to combine LAB with UC, which alleviates the drawbacks associated with the single energy storage. Multilevel Energy Storage Based Frequency Regulation Strategy: Conventionally, upon the occurring of a frequency disturbance in power system, synchronous generators release their stored kinetic energy inherently to provide inertial response and limit the rate of change of frequency (ROCOF). Primary frequency response reserve is activated in a few seconds when the frequency excursions exceed the noncritical frequency deviation band. This process is generally achieved by increasing/decreasing mechanical power output from the prime mover using a droop-based governor. Secondary frequency response restores the active power balance between generation and load demand thus recovers the system frequency to the nominal value. Secondary frequency response lasts up to 15 mins until tertiary frequency control is activated to free up the secondary frequency response reserve. In this paper, the hybrid energy storage and power reserve available in the de-loaded PMSG provide frequency response in different time scale. The control mechanism is sketched in Fig. 2. Three levels of energy storage (i.e., UC, LAB, and mechanical power reserve) and three types of frequency response (i.e. inertial response, primary frequency response, and secondary frequency response) are presented. The tertiary frequency control is usually implemented manually and not considered in this paper.
Secondary Frequency Response: Secondary frequency response usually implements centralized automatic generation control (AGC) to modify the active power set points of interconnected generators and restore frequency to nominal value. The integration of LAB enables PMSG to participate in secondary frequency response. Apart from the energy in LAB, mechanical power reserve is established in the wind turbine by operating the turbine away from its typical maximum power point intentionally. The strategy to deload the PMSG is referred as SOPPT by the authors [3]. The mechanical power reserve can also be utilized as charging energy source for the UC and LAB. A constant amount of mechanical power (e.g., 0.01 p.u. in this work) is reserved by manipulating the wind turbine to follow the SOPPT curve. As explored in [22], WECS is capable of participating in AGC using set point control. With two secondary frequency response reservoirs (i.e. LAB and mechanical power reserve), a coordination strategy is required to accomplish power sharing between the wind turbine and LAB.