Distributed energy resources (DERs), which include renewable energy power sources, such as solar photovoltaic (PV) arrays and wind turbines, are often connected directly to distribution systems that are part of an area electric power system (EPS). A large fraction of renewable energy resources will be installed in utility and customer owned distribution systems as individual States try to attain their Renewable Portfolio Standards (RPS). At 50% or greater total renewable generation, it will be difficult to control frequency and voltage to within acceptable standards without active feedback controls. Additionally, some cities such as San Diego are planning on having 100 percent renewable energy within its city limits by 2035. Many of the renewable power sources will come from small residential rooftop solar systems operating in low voltage distribution systems. There will be many independent participants installing renewable DERs without knowledge of the impact on the grid of the distributed resources and without adequate control of local frequency and voltage. The renewable energy resources (Solar PV, fuel cell, battery, wind) have little or no inertia since they are connected to a power electronics device that converts DC power into AC power. Diesel generators, on the other hand, have inertia and will contribute this to the adjacent connected grid load. Controllable loads can also be considered DERs since their power consumption can be regulated thus providing an additional means of control and providing inertia from the load side. These forms of DERs have higher inertia loads than typical renewable generation; however, they both should be used in a coordinated control system to regulate the frequency and voltage of the local EPS.
Low inertia systems are difficult to control compared to systems with high inertia from rotating energy sources. The IEEE 1547.4 standards clearly point out the sensitivity of DERs to instability and voltage stability issues in the presence of low inertia generation sources. Lack of control of the power characteristics DER power injection can cause large variations in frequency or voltage exceeding standards that can cause the feeder or substation to disconnect from the area EPS. In a typical distribution feeder, one or more DERs may supply up to 10 MW of power. With adequate control and coordination, one or more DERs in combination with multiple feeders can form the basis of a microgrid.
U.S. Pat. No. 8,457,912 describes a method of creating a smooth angle from the discontinuous angle measured from the PMU. The method includes detecting the change in direction of the angle and compensating for the discontinuous wrap at plus or minus 180°. This method is required in order to compute a smooth angle and a smooth angle difference that are used in the control system.
U.S. Pat. No. 8,498,752 describes a method of decoupling real and reactive power from changes in voltage and angle. It also teaches that the control system can be reversed so that the voltage and angle can be controlled to a constant value by simultaneously changing the real and reactive power. The controller uses the basic principle that the response to real and reactive power injection causes a simultaneous change to voltage and angle by fundamental physics known as Ohm's law.
The controller also assumes that the network impedance is constant and is a known value. The nonlinear systems are linearized around and operating point resulting in a linear set of equations that are used in the coupled controller. The patent teaches how the system can be linearized around an operating point and then any linear control System Technology can be used to configure the controller.
U.S. patent application Ser. No. 14/956,684 teaches how multiple decoupled controllers can be configured in cascade mode to form a hierarchical control system. It provides an explicit example of how the Smith predictor controller could be used in the control system design. Additionally, the controller technology recommended is based on commonly used proportional plus integral plus derivative control. The patent also teaches that the controls can be reversed so that the input and output variables at any one level can be reversed to form a set of hierarchical controllers that can be configured in cascade mode to perform a number of control system functions in Electric Power Networks.