Distributed Primary and Secondary Power Sharing in a Droop-Controlled LVDC Microgrid With Merged AC and DC Characteristics

Abstract

In an ac microgrid, a common frequency exists for coordinating active power sharing among droop-controlled sources. A common frequency is absent in a dc microgrid, leaving only the dc source voltages for coordinating active power sharing. That causes sharing error and poorer voltage regulation in dc microgrids, which in most cases, are solved by a secondary control layer reinforced by an extensive communication network. To avoid such an infrastructure and its accompanied complications, this paper proposes an alternative droop scheme for low-voltage dc (LVDC) microgrid with both primary power sharing and secondary voltage regulation merged. The main idea is to introduce a non-zero unifying frequency and a second power term to each dc source by modulating its converter with both a dc and a small ac signal. Two droop expressions can then be written for the proposed scheme, instead of the single expression found in the conventional droop scheme. The first expression is for regulating the ac frequency and active power generated, while the second is for relating the dc voltage to the second power term. The outcomes are better active power sharing and average voltage regulation in the dc microgrid, coordinated by the common injected ac frequency. These expectations have been validated by results obtained from simulations.

Proportional Load Sharing and Stability of DC Microgrid with Distributed Architecture Using SM Controller

“…To address the aforementioned challenges, different control schemes for DC microgrids are reported in [7,[10][11][12][13][14][15][16]. Commonly, these control schemes can be categorized as centralized and decentralized control [10][11][12][13][14][15][16][17][18][19][20][21].…”

“…Commonly, these control schemes can be categorized as centralized and decentralized control [10][11][12][13][14][15][16][17][18][19][20][21]. In centralized control, the controller collects system data using a high bandwidth communication link, schedules the tasks based on the collected information, and directs control decisions [12,[16][17][18].…”

On Secondary Control Approaches for Voltage Regulation in DC Microgrids

“…To increase the accuracy of the load sharing, large droop gains should be employed at the primary level. Larger droop gains cause higher voltage drop in the case of dynamically stable operation [7]- [11]. The secondary control approach has been carried out to compensate the voltage drop due to the droop method.…”

“…In centralized schemes, the voltage of localized loads connected to a common bus or the voltage at the coupling point into the utility grid should be regulated [9], [10]. On the other hand, in decentralized methods, the average voltage of generator busses (busses with voltage source converter), is controlled [6], [7], [10]- [12], [16] In the centralized secondary approach, it is considered the loads are localized at a common buss and the secondary controller regulates the voltage of the common bus at a reference value. A central control unit measures the voltage of common buss and send the set point voltage to the voltage controlled converters [8]- [10].…”

“…Furthermore, in order to improve the system resiliency and reliability, a consensus protocol based secondary approaches are presented in [11], [17], where only the voltage of generation busses are communicating among the converters. Furthermore, a frequency droop based approach is also presented in [7], [16] in which the average voltage of generation busses can be properly regulated without utilizing any communication system. Furthermore, dc voltage in the dc MG is a local variable and voltage variation due to the feeder resistances at different points of MG is necessary in order to control the current flow.…”