3.2 Types and Commonalities of DG & PCS Systems
To be able to properly design a power conversion sytem (PCS) for distributed energy resource (DER) interconnection, the type of distributed energy resource must be identified and taken into account. There are two basic types of distributed energy resources, direct current (DC) and alternating current (AC) voltage producing sources. For either type to be connected to the utility system their raw outputs need to be processed.
The follwing figure, Figure 1, shows possible DC and AC distributed energy resource based PCS. Though direct voltage conversion from a DC or AC source to an AC grid, ready voltage can be obtained (top topologies in Figure (a) & (b)), these PCS topologies are not used due to hardware protection issues [10-15].
In the remaining topologies of Figure 1, the first section of the PCS is dedicated to the processing of raw DER energy into DC energy. This conversion cannot be easily standardized due to the vast differences between DER technologies. For example: micro-turbines and wind-turbines can both produce variable frequency AC voltages, but the range of frequencies for the micro-turbines are generally higher than those of the wind-turbines. In addition to this, the voltage levels between the two types also vary. Micro-turbines have a constant supply of fuel to generate electricity as opposed to wind-turbines which are dependent on the weather conditions. Hence, wind-turbines require energy storage to help manage power flow to the utility. Distributed energy resource (DER) technologies often differ in produced range of frequencies, supply source, and energy storage requirements. The processing portion is the site for accounting for these differences. Due to this the processing portion of a PCS system cannot be standardized for all DERs. However, the grid interfacing inverter has the potential for standardization. It will be analyzed to explore this potential for standardization.
Figure 1. (a) DC DER based PCS; (b) AC DER based PCS
Table 1. summarizes various DER types, possible PCS topological configurations, and the functions that the PCS need to perform to convert the raw DER energy into viable grid-ready energy. From this it can be seen that the grid-interfacing inverters have a common feature for all types of DER systems; all DER systems require grid inverters. These inverters produce three phase AC grid voltage (60Hz) from the DER generated power. This commonality between all DER PCS types makes the inverter a ideal site for standardizing the grid interconnection.
Table 1. Examples of specific DERs and the needed PCS functions for interconnections.
Additional figures depicting the relationship of area and local electric power systems (EPS) to each other and the utility are presented in Figures 2(a) and 2(b). Figure 2(b) shows detailed PCS configuration and interconnection with area/local EPS and utility. These diagrams also further emphasize the potential for standardization at the grid interface.
Figure 2. (a) Top, Area EPSs of a Utility System showing DG interconnection. (b) Bottom, Black diagram of DER, PCS, Area EPS, and the grid interconnection.
The grid interfacing inverter is the portion of the PCS with the most constraints and protection demands placed upon it to be able to connect with a utility system. This makes the standardization of inverter advantageous for the control design. A voltage source inverter (VSI) was selected to be used in the PCS for this study. The following sections will detail the rationale behind this selection.