Decentralized Primary Control of MTDC Networks With Energy Storage and Distributed Generation

Abstract

Multi-terminal dc (MTDC) networks are drawing a lot of interest lately in applications related to distributed generation, especially in those that also integrate energy storage (ES). A few approaches for controlling the operation of such systems have been proposed in the literature; however the existing structures can be significantly enhanced. This paper proposes an improved primary control layer, based on custom droop characteristics obtained by combining concepts of droop and dcbus signaling control. This approach is designed to be generic and takes into account the various operating states of the network. Five operating bands, similar to the operating states of the ac-grids, as well as various droop characteristics for different elements connected to the dc-network, are defined. For the energy storage, the state of charge (SoC) is taken into account at the primary control level and included in the droop characteristic, creating a two variables droop surface. The proposed control strategy is validated through simulation and experimental results obtained from a case study that involves a micro dc network composed of a PV generator, a lead-acid battery, and a connection point to the ac grid.

A Superconducting Magnetic Energy Storage-Emulator/Battery Supported Dynamic Voltage Restorer

“…Two cases of sag duration have been considered; a sag lasting two seconds and a sag lasting two minutes. The energy required by the DVR inverter during the two second sag was found to be 638J from (20)- (26), whereas the energy required to support the two minute sag was found to be 38251J based on IGBT module loss parameters given in [37]. The design procedure described in [38] was then used to estimate the superconducting material requirement assuming a working temperature of 50K.…”

“…The objective of the top-level energy storage control strategy was to control the charge/discharge of each energy storage device. A current vs. voltage active droop characteristic was chosen as this strategy has been shown to provide good stability and active power sharing [26]. The converter reference currents are based solely on the level of the DC bus voltage which is advantageous as high-bandwidth communication between the three different converters is not necessary.…”

“…For a given long-term under-voltage, the required discharge power provided by the DVR can be found from (20). The DVR energy can then be determined using the procedure shown in (21) - (26).…”

“…The short-term energy storage system capacity was estimated to support a sag to 35% of nominal voltage lasting approximately 100ms using (26). The battery bank was sized based on available lead acid batteries.…”

Design considerations for primary control in multi-terminal VSC-HVDC grids

“…Such an example is shown here in Fig. 1a, based on our previous work presented in [31]. Here, a bidirectional VSC (i.e., a VSC that can operate both as a load and as a generator) reacts to a decrease in the voltage V dc by increasing the generated current with i dc .…”