Energy storage systems control methods in micro grids Class-30, Subject: Distribution Generation and Smart Grid
- 1. Energy Storage Systems Control Methods in Micro-grids Course: Distribution Generation and Smart Grid Prof. (Dr.) Pravat Kumar Rout Department of EEE, ITER, Siksha ‘O’Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India
- 2. Architecture of AC Microgrid
- 3. Architecture of DC Microgrid
- 4. Architecture of AC/DC Microgrid
- 7. ESS Connection Usually, the ESSs generate DC voltage, and so, power electronics interfaces are needed to connect them to the AC power system and AC MG . In addition, some energy consumers may use DC electrical power. Power electronics interfaces, made of semiconductor switches, provide more controllability for ESSs . For example, a DC/DC convertor should usually be used for altering the DC voltage level of a BESS, a DC/AC convertor which is called inverter, is used to connect a SC to the MG, a AC/DC converter which is called rectifier, is used for DC loads and AC/DC/AC (or back-to-back) converters are used for the FESS.
- 8. Description of different ESSs
- 9. MG Control strategies Voltage in DC microgrid and the frequency and voltage in AC microgrid need to be controlled. ESSs as fast and reliable power supplies play an effective and important role. In a MG, some controllers are near loads, and some others are near DGs location or microsources, which are called the Load Controller (LC) and Microsource Controller (MC) respectively. A controller that is usually located at PCC and sends the control signals throughout MG is called the MG Central Controller (MGCC). MG architecture and controllers locations.
- 10. Centralized control strategies In the centralized control strategy, the MGCC has a unique role. It sends and receives all the control signals of the MG. These signals are transferred to LCs and MCs to control the voltage in DC MG (or voltage and frequency in AC MG) and optimize power flow in feeders, etc. The centralized controller is relied on communication architectures. This reduces the system reliability. In this control method, usually all the DGs have one owner which wants to optimize operational and economic criteria for all of them .
- 11. Decentralized and Distributed Control Strategies In contrast to the centralized strategy, the decentralized one does not rely on MGCC and communication architecture. In this method, LCs and MCs play an important role in MG stable operation. Another control method is the distributed control strategy. In distributed control strategy, there are controllers that are geographically distributed and functionally integrated. Indeed, in this strategy, some controllers are the interface between MGCCs and the local controllers . Distributed control strategy has a concept between two-mentioned strategies. In this strategy, MG components exchange control signals.
- 12. Control methods of MG
- 13. Hierarchical control strategy The hierarchical control strategy, which is very close to the distributed control strategy. The hierarchical control strategy has three control levels, which are in different time domains known as: 1-primary, 2-secondary and 3-tertiary control. The first control level is activated immediately after a change in MG parameters such as frequency, voltage or load changes and tries to keep the frequency and voltage in stable ranges. The operation time of this level is about several seconds. The Primary control is the fastest level in the hierarchical control system . After performance of this level, a steady-state error may exist for voltage and frequency. Moreover, other parameters such as active and reactive power may be influenced by this error. To overcome the mentioned problems, the secondary control is activated. It tries to reduce errors between desired and real parameters. Finally, the tertiary control, which is the slowest control level is activated. Usually, the main goal of this level is the economical or market issues.
- 17. Droop Control Method The conventional droop control method is a method that can be applied to ESSs of a MG using centralized and decentralized control strategies. This method is a mimic of synchronous generators, when frequency and voltage drop proportionally with generated active and reactive power, respectively. To determine the reference frequency and voltage, local voltages and currents are measured and processed and as a result, it does not require communication infrastructure, so it has also been called the wireless method. E and φ are the voltage amplitude and angle of the ESS. Z and θ are the amplitude and angle of the line impedance. Equivalent circuit of ESS connected to a bus
- 18. Continue... The active and reactive power (P + j Q) can be calculated as follows:
- 19. Continue... Assuming that the output impedance of an inverter is inductive and the phase difference between E and V is very small, (3) and (4) can be written as follows . Based on the mentioned terms, droop equations can be expressed as follows:
- 20. Continue... This equations is based on inductive lines impedance, but in many cases the line is resistive or resistive-inductive. Similar to inductive case, for an MG with resistive line, the droop equations can be achieved as follows :
- 21. Continue... For a DC MG, the new droop equation is defined as follows : ✓The droop method is suitable for microsources control and it stabilizes the MG under load changes. ✓It can be used in centralized and decentralized control strategies. ✓Since, it relies on the local measurement in each DGs, it can be applied for several independent ESSs in the MG.
- 22. Control methods of ESSs In islanded mode of MG, the ESSs are planned to be charged to a distinct level of SoC determined by local or central controllers. An active and reactive power set point have been defined and using two Proportional–Integral (PI) controllers, the ESSs inject or absorb power. This control strategy is called the PQ control strategy. This control strategy is called the PQ control strategy. Usually, some DGs with a slow response such as FC, might be controlled by this control strategy as shown in figure. P-Q CONTROL STRATEGY
- 23. Figure shows this control strategy for inverter-based ESS, which is controlled in the d-q frame. Control of inverter-based ESS. P-Q control strategy
- 24. Reference currents achievement in grid-side converter controller As can be seen, using id, iq, vd and vq, active and reactive powers are calculated. Then their difference from ordered value passes though PI controller and the current reference values are calculated. P-Q control strategy
- 25. In islanded mode, some of the ESSs must participate in voltage and frequency control of MG. These ESSs usually act as controllable voltage sources . This control strategy is called the V/f control strategy. The reference set points of the voltage and frequency are received from higher control levels. The ESSs, after islanding or load switching, immediately compensate the lack or excess power in primary control and PQ controlled DGs might corporate in secondary control. A summary of the control strategies applied to ESSs in the MG has been shown in Figure below. (a) ESS control strategies and (b) primary and secondary frequency control V/f control strategy.
- 26. Three main general configurations of ESSs in a MG 1-distributed 2-aggregated 3-hybrid
- 27. Distributed Energy Storage System In distributed configuration, several ESSs are dispersed in MG
- 28. Aggregated Energy Storage System All the ESSs are supposed to be in one location to facilitate ESSs modeling.
- 29. Hybrid Energy Storage System •None of ESS can provide all the characteristics needed for a power system or MG. •Due to this reason, combined application of ESSs with different types is a common solution called Hybrid ESS (HESS).
- 30. Control technique based on ESSs State of Charge (SoC) One of the most important aspects in controlling ESSs, especially in distributed and hybrid configurations, is SoC control. When two or more ESSs operate in a power system or MG, they should be simultaneously charged and discharged. This causes an increase in the average lifetime of ESSs and improving the response of voltage control. So it is desirable to equalize the SoC of ESSs all the time with proper control. The most important issue in the MG control is its stability. Therefore, in the system operation optimization process, the MG stability should be considered as an essential condition.
- 31. Continue... The research works in ESS SoC control can be classified in researches in DC and AC MGs. In DC MG, the voltage and power control are very important. However, in AC MG, the stability is more complex where the voltage, frequency, active and reactive powers should properly be controlled.
- 32. Discussion and future works The MG operation in off-grid mode is challenging. In this situation, reliable energy producers such as ESSs play an important role. Considering the required power, energy density and economic issues, the MG operators should select the best choice from different ESSs. In the islanded mode, three main configurations of ESSs are used; aggregated, distributed and hybrid. In aggregated configuration, all the ESS units have been installed in one location or only a large ESS has been used in the MG, while in the distributed one, they have been dispersed in the MG area.
- 33. Continue ... The ESSs can be controlled locally or decentralized or they can be controlled by the MGCC (centralized). Two mentioned strategies might be executed in the hierarchical structure. This means that similar to power system operation, the primary, secondary and tertiary control levels should be applied to the ESSs. The droop control method has been applied to the ESSs that provide good response in stability aspects of the MG. It is concluded from research works that the distributed and hybrid applications are more preferable compared to the aggregated one. Moreover the hybrid application of the ESS is useful for the MG with high and low frequency changing the load power profile.
- 34. Continue... One of the important issues in ESSs control, operation and maintenance, is SoC balancing among different ESS units. The lifetime of the ESS is influenced by charge/discharge times. Since the stability of the MG is the most important issue, after that, the maintenance issues are important. In the future, by developing of electrical vehicles such as plug-in hybrid electric vehicles (PHEVs) in the microgrids, they can participate in voltage and frequency of MG. on the other side by reducing renewable energy costs in the future they can compete by common energy producers but they need ESSs to provide acceptable reliability. Some ESSs such as Lithium batteries are developing to charge and discharge thousands times which, this facilitate its application in the future.
- 35. Conclusion In the aggregated ESS, all the ESS units are located in one location and in the distributed configuration, several units are placed in different locations. There are three main control strategies for ESSs control. In the grid connected MG, the ESSs are usually controlled by the PQ control strategy, which causes a distinct level of SoCs to adjust for ESSs. In the islanded MG with aggregated ESS, the V/f control strategy might be applied. The droop control strategy has been used for cooperation of different ESSs. Some ESSs, such as BESS have high energy density and other ones such as FESS and SMES have high power density. In the practice, these characteristics are important. The SoC of ESSs should be controlled to ensure the suitable operation of the MG.
- 36. Future prospective Todays, BESSs with high density of power and energy are available, which can be used for different applications in MGs. Effective and efficient control of these MGs is one of the most important aspects of their operation. This can be obtained by proper combination and allocation of EESs in MG, and also selection of effective control strategies for ESSs and DGs. Considering the current ESSs technology progresses, it is expected that more efficient control strategies will be designed for ESSs and the islanded operation of the MG will be facilitated.
- 37. References Arani, A. K., Gharehpetian, G. B., & Abedi, M. (2019). Review on energy storage systems control methods in microgrids. International journal of electrical power & energy systems, 107, 745-757. Etxeberria, A., Vechiu, I., Camblong, H., & Vinassa, J. M. (2010, October). Hybrid energy storage systems for renewable energy sources integration in microgrids: A review. In 2010 Conference Proceedings IPEC (pp. 532-537). IEEE.