Distributed generators (DGs) are small generating units that are connected to the distribution network at voltages below 69 kV. DG units usually have capacities of 10MW or less, and are based on different energy sources, such as wind, solar and diesel. The distribution network is generally a radial system and designed for one direction of load flow, i.e. from the electric grid to the load. The unidirectional flow assumption is no longer valid when DG is interconnected at the customer or load side since the flow of power can now go in either direction: from the load side to the grid or from the grid to the load side. This fundamental change affects how an impact study, generally required to identify and mitigate any changes to reliability of the distribution system, for DG interconnection is conducted. Reference 1 summarizes several typical tasks required in an interconnection impact study.
This Tech Blog discusses several key issues related to the steady state impact of DG.
Variable output. Most DGs do not have an energy storage component. DG output varies according to the time of the day and the season. For example, DGs using photovoltaic (PV) cells have a power output that is nearly constant on a clear day, but may change dramatically if a weather front comes through, as shown in Fig. 1. The interconnection analysis thus requires consideration of generation and load profiles under different conditions in order to effectively evaluate the potential impact of the DG. These potential impacts may include excessive tap changing and capacitor switching operation. The simulation can be performed using a 3-phase power flow program, such as OpenDSS (an open source program available at http://sourceforge.net/projects/electricdss/).
Voltage impact. To analyze the voltage impact of a DG interconnection, some extreme scenarios need to be studied. For example, the voltages seen by customers should be within the acceptable range with the DG full on and also when fully off. DG-caused under-voltage occurs when the feeder to which the DG is to be connected to is equipped with Line Drop Compensator (LDC). With the DG in service, current from the main substation is reduced. This reduction may result in the LDC’s regulator to set the tap at a low position leading to low voltage at the feeder’s end. As shown in Fig. 2, with a DG connected close to the main substation, voltage falls below 0.95 pu on some section of the feeder when the DG is in service. On the other extreme, the presence of DG with reverse load flow might counteract the normal voltage drop, and cause over-voltage. Typically, the LDC on the substation is set for the forward direction only and has the reverse direction disabled when DG is connected to the network. Besides the location and size of the DG, the voltage issue arises when the feeder is relatively long.
Thermal Loading. With DG full on and the worst case contingency occurs, feeders and transformers should be within their contingency thermal ratings. The contingency scenario is typically one feeder out, and some loads are switched to the feeder under the study. The system is studied with different load scenarios, taking into account of load growth, operating conditions and seasons, and contingencies.
Electrical Losses. If the DG does not provide any reactive power to the distribution system, the DG’s impact on the power loss is similar to that of a capacitor; the difference lies in the fact that for DG, the power loss may be reduced by reducing the active flow on the feeder instead of the reactive flow for a capacitor. In cases where a large size generator is connected to a remote bus, an increase in loss may occur. A rule of thumb is to locate the DG as close as possible to the “big” load and to size the DG in such a way that the DG would not export or deliver power to neighboring feeders or the grid to avoid the increase in power loss. If the loads are about evenly distributed along the feeder, the 2/3 rule of thumb, which is applicable for a capacitor placement, can also be applied to the DG placement in order to minimize losses. If the distribution network has a major single phase load, a three phase power flow is required to properly study the impact on power losses.
Power factor. Currently DGs are not permitted to regulate voltage on the distribution system (IEEE Standard 1547) so DG units cannot be treated as conventional generators with reactive power capability. DG interconnections are typically required to maintain unity power factor at the interconnection point which may require additional reactive compensation with the DG interconnection.
Conclusion
Several steady state issues are discussed in this Tech Blog some of which are unique to DG interconnecting at low voltage feeders. In order to tackle the issues, different approaches and tools are required besides the ones used in the traditional radial distribution network.
References:
- K. Dartawan, R. Austria, Distributed Generation: Things You Don’t Want to Miss, October 12, 2010
- Y. Baghzouz, Some General Rules for Distributed Generation-Feeder Interaction, PES General Meeting, 2006, IEEE.
- IEEE Std P1547 – Standard for Distributed Resources Interconnected with Electric Power systems, IEEE Press, January, 2003