Today’s electrical grid in the United States is a complex array of generators, transmission lines, substations and loads. The grid has been evolving since its inception. Renewable generation is growing rapidly on the grid. This article will review some of the basis properties of the electrical gris as it relates to renewable generation.
Supply & Demand
The electrical grid is generally split into two systems: the Bulk Power System (BPS) and the Distribution System (DS). The BPS is designed to generate and deliver electrical energy to the DS loads. Generally, the BPS operated at voltages >69 kV and the DS operated at voltages <69 kV. The power flow within the electrical grid is balanced, meaning the generation matches the load. Theoretically, when a load is added, like switching on a light bulb, a generator somewhere on the grid has to increase its output to match the added load. If the system becomes unbalanced, grid frequency varies away from 60 Hz. If generation > load, the grid frequency rises. If load > generation, the grid frequency falls below 60 Hz. This is the effect of active power (watts) loads and generation on the grid. Reactive power (volt-amp reactive – Var) on the grid effects grid voltage. VArs can be understood as both useless and necessary. It is the generated or absorbed by electrical generators to provide voltage support to the grid.
For further reading
NERC Reliability Guideline
Enter Renewable Generation
With the growth of renewable generation: wind and solar photovoltaic (PV), the generation resources are being installed on both the BPS and DS. Some are in close proximity to human population, like rooftop solar arrays. The renewable generation fluctuates as a function of its “prime mover”. As the wind blows and the sun shines, the ability of the renewable generator fluctuates. Sometimes these fluctuations are in sync with the load demand and sometimes they are not. For that reason, other traditional generation ramps up & down to both meet the demand of the load and the fluctuations in renewable generation. This inverter-based resource (IBR) generation is required to interact with the grid differently on the BPS compared to the DS.
Grid Interaction of IBRs
There are standards that govern the IBR on both the DS and the BPS. For the BPS, IEEE2800 is the standard by which IBR generators must comply. IEEE1547 is the standard by which DS generators must comply. These standards provide requirements for generator behavior during a fault, called ride through. They also prescribe certain aspects of the controls for maintaining voltage and frequency at the point of interconnection as well as voltage and frequency values where generator tripping should occur.
The electrical grid generation is quickly becoming much more distributed with the growth of the renewable energy sector. Wind turbines and solar PV sites are now connected to both the BPS and DS successfully augmenting the energy mix on the U.S. grid. Battery Energy Storage Systems (BESS) are also in use providing a method whereby the energy produced by the wind and sun can be stored and released into the grid at night or when the wind is calm.