Due to the considerable investment and rapid development of photovoltaic (PV) and wind generation technologies, the average levelized cost of electricity (LCOE) of these renewable energy resources (RESs) has fallen within the cost range of fossil fuel-based traditional power plants [1]. Thus, these environmentally friendly technologies are the main energy sources to meet the ever-increasing worldwide energy demand. Despite the promising achievements, the intermittent nature of large-scale RESs is still the main challenge to connecting them to transmission or distribution networks. To cope with this problem, grid-scale battery energy storage systems (BESS) are suggested that have the capability of storing and dispatching energy in the shortest period required to compensate for abrupt RESs fluctuations. Such features can significantly improve the power supply stability and reliability. In addition, energy storage systems can participate in various operation services like frequency regulation, peak shaving, power loss reduction, load management, and quality improvement [2]. Hence, interest in coupled PV and BESS is rising considerably among utilities and power system operators. On the other hand, the high transfer efficiency, and low costs of voltage source converter-based (VSC) DC networks have made them attractive solutions for RESs integration compared with their conventional AC transmission counterparts. VSC-HVDC networks provide asynchronous interconnection capability without sub-synchronous resonance issues, cable skin effect, and reactive charging current. These networks can also resist AC fault propagation and provide better active and reactive power controllability over long distances [3][4][5]. VSC-HVDC networks are connected to the AC grids through modular multilevel converters (MMC), including a large number of submodules. These converters can produce almost ideal sine wave voltage, considerably decreasing voltage and current harmonics and minimizing the need for filters. Researchers have investigated the application of multilevel cascaded Hbridge and Full-bridge inverters, MMCs, and neutral-point clamped inverters (NPT) during normal or abnormal situations [6][7][8][9][10]. Thus, some novel MMC configurations have been proposed to improve the MTDC capacity, reliability, and stability. The new MMC-based topology introduced in [11] enhances the performance of MTDC systems while faults occur in DC lines. In [12], the authors proposed an adaptive integrated and coordinated control approach that improves the MMC-MTDC system's response and stability.
Hence, a proper connection of BESS to hybrid AC/DC transmission networks can help power system operators benefit from both technologies while integrating RESs. Research efforts have been investigating the connection of BESS to VSC-HVDC systems, resulting in various configurations. The methods of integrating BESS in hybrid AC/DC networks can be divided into two fundamental categories: 1-separate RESs and BESS units integrated through their DC-DC converters [1][2][3]12], 2-embedded BESS into MMCs in HVDC systems [14][15][16][17][18][19]. Dominant configurations of the first category were described in detail in [2], including 1-loosely DC coupled, 2tightly DC coupled, and 3-AC-coupled. This paper investigates the performance of co-located but independent PV and BESS systems integrated into MTDC links that connects two parts of an AC transmission network through back-to-back (B2B) converters. PV can be connected to the MTDC lines through DC-DC converters, while BESS is connected to the MTDC network by a bidirectional DC-DC converter. The performance of the proposed configuration is evaluated through a transient AC fault analysis.
This section discusses the proposed hybrid AC and MTDC transmission system topology and implemented control systems.
The proposed AC/MTDC transmission configuration allows the system operators and utilities to integrate largescale PV plants and battery energy storage systems (BESS) into the MTDC networks, which enhances the system's availability and reliability. This new topology provides a bidirectional power flow control while maximum energy can be harvested from solar farms. Fig. 1. shows the introduced topology of the hybrid AC transmission system with an MTDC network. MMC-based B2B converters provide the MTDC system with connecting PV plants and BESS. B2B converters are connected to the AC transmission lines through step-up and step-down transformers. In this study, it is assumed that PV plants and the BESS system are co-located units having independent control strategies. A staircase output voltage (with N+1 levels) can be obtained via proper modulation of MMC submodules. The arm current direction determines the capacitors' charging and discharging states. Switching SMs allows the capacitor voltage to be delivered to the terminals, and the capacitors are bypassed -and terminals have zero voltages-when SMs switches are off. However, these voltage changes at high frequencies result in unwanted voltage changes within the MMCs and voltage unbalance between the three phases that cause a circulating current within the legs. This unwanted current leads to drawbacks like increased losses and disturbances. An inner control approach is often applied to cope with this problem.
This section introduces the control system implemented for the control of converters, inverters, maximum power point tracking (MPPT) technique, and the BESS control strategies. In this study, typical control methods for the B2B controls include 1-an outer control technique for regulating active and reactive power, 2-an inner current technique for reducing the circulating current, and 3-a voltage balancing approach for the capacitor voltage. In addition, a PWM (with phase-shifted) technique is used for controlling to control the power and each leg of MMCs. For PV plants, the MPPT method is employed to control the output of PV modules. Fig. 4. illustrates the implemented control system. I. represents the case study network parameters. Fig. 5. shows the hybrid AC transmission network with the MTDC network simulation implemented in the SIMULINK environment. A transient fault is applied to one of the buses of the lower AC network while PV is producing its maximum available power and BESS is charging at %70 of its nominal capacity. Considering BESS for the same fault scenario, there is a slight voltage increase after the fault, but the system is completely stable (lower figure). Fig. 7 shows the B2B inverter voltages and currents (upper AC network) during and after the fault, with and without BESS. It is evident that BESS can considerably improve the inverter outputs (voltages and currents) after the fault.
Before the fault happens, BESS is in the charging state (starting from 70%). However, during the fault, the BESS control system switches to discharging state to keep the MTDC voltage within standard limits. Fig. 8. represents the voltage, current, power, and SOC of BESS during and after the fault. Due to the appropriate performance of BESS and MMCs control systems, voltages and currents of the upper AC network and MTDC are not much affected during the fault. Fig. 9. represents the MTDC line voltage and current (flowing from the converter side to the inverter side) without and with BESS, respectively. PV plant outputs (including generated power, grid voltage, and line current) are also illustrated in Fig. 10.
This article presented a new topology for hybrid AC and MTDC networks while integrating PV plants and BESS. An MMC-based B2B converter is used to provide the MTDC connection, and DC-DC boost and bidirectional converters facilitate connections of PVs and BESS to the DC grid. Outer and inner controllers were applied to regulate the active and reactive power of B2B converters and decrease the circulating current, respectively. The unbalance issue of converters was also dealt with by a capacitor voltage balancing control strategy. MPPT technique helps harvest the maximum accessible energy from the PV farm. Simulation results verify the effectiveness of the suggested topology during a transient AC fault.
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