Distributed renewable energy sources, such as solar photovoltaic (PV) panels or wind turbines, present an exciting opportunity for the electric utility industry to provide consumers with clean electricity, rather than electricity produced by burning fossil fuels. Distributed renewables must be integrated into the electric power grid to address climate change. However, when grid-connected inverters are used with renewable energy sources, issues can arise with the quality of electricity.
Electric grid operators are typically notified of a proposed renewable energy installation late in the game, often after a site has been selected and equipment vendors have signed on. Renewable energy project managers expect that electric utilities can easily provide a connection to a nearby transmission or distribution line.
New renewable energy sources pose a challenge in that they provide variable power to the grid depending on the time of day, presence of wind, etc. Electric utilities are concerned about the quality of electric service to nearby customers, which is not always consistent when renewable energy sources are in play. This is further complicated by fact that new renewable energy sources may not be built in locations that are ideal for connecting to the grid. Issues with quality of voltage, frequency, in-phase power, quadrature power, short circuit current, phase balance, and harmonic filter are all of concern to electric utilities.
These issues can be addressed with the implementation of dynamic renewable energy inverters. However, before we discuss the solutions, let’s take a closer look at the challenges that new renewable energy sources pose to electric utilities.
Renewables Create Voltage Inconsistencies
Electric utilities establish voltage control in substations and at locations along distribution lines that are equipped with voltage regulators. Traditionally, power flowed from a substation to customers along distribution lines. The main variable that utilities typically accounted for was seasonal load. Peak loading occurred during hot summer months; near peak loads occurred during cold winter months; and lighter loads occurred in spring and fall.
When renewable energy sources are connected to distribution lines, voltage variations differ greatly from the seasonal variations that utilities currently anticipate. Peak load occurs when renewable energy sources are offline, such as at night or during still days. Seasonal loads will differ from the traditional variations because, for example, solar panels will provide more power in the summer even as consumers use more electricity to run air conditioners.
Frequency Control is Vital
Traditionally, large remote generating stations were equipped with speed controls that increased or decreased output power to maintain power flow at 60 Hertz. If an instantaneous 1000 MW change in power production occurred in a metropolitan area, speed controls managed output power so that only a 0.05 Hz change in frequency would occur, which mitigated any potentially damaging impacts.
With the introduction of renewable energy sources, challenges in frequency control can be expected. For example, rapid changes in power production will occur at sunrise and sunset. Additionally, if a substation or distribution line is disconnected from the power grid and renewable energy sources continue to provide energy, frequency control may be lost. Renewable energy sources need the capability to increase power output if frequency drops below 59.95 Hz, to decrease power output if frequency rises above 60.05 HZ, and to shut down immediately if frequency rises above 63 Hz or decays below 57 Hz.
Not All Power will be In-Phase
Kilowatts are in-phase power, that is, power is produced remotely and flows into buildings to be used by consumers. Since the 1920s, electric utilities have focused on in-phase power as their revenue parameter. However, with the introduction of roof top solar panels, power can flow into a residence during the night and out of a residence during the day. Variable power flow like this affects both voltage and frequency controls.
Quadrature Power Cannot be Disregarded
Electric utilities refer to quadrature power as vars or imaginary power. This misnomer has created the false idea that quadrature power is irrelevant. Motors, transformers, and distribution lines require quadrature power to set up electromagnet fields.
Renewable energy sources need to provide both in-phase power and quadrature power.
However, renewables currently do not provide both because electric utilities focus on in-phase power costs, and often disregard quadrature power. Plus, quadrature power must be consumed when voltage is above 126 volts and produced when voltage is less than 120 volts.
Short Circuit Current Differences
Renewable energy sources need to disconnect from the power grid when short circuits occur. Ideally, renewable energy sources would contribute fault current so that the fault isolation device closest to the fault location would open. With traditional energy sources, short circuit current is 5 to 10 multiples of load current. With renewable energy sources, fault current will not be greater than load current. This means that it becomes very difficult to differentiate between load current and short circuit current.
Potential for Damage to Grid Components
When power flows from remote, centralized generating stations to consumers, phase balance is controlled by monitoring how many customers are connected to each phase. With renewable energy sources, this is complicated by the fact that renewable energy can be concentrated on a single phase, while traditional, centralized generating stations must produce balanced, three phase energy. The concern is that three phase motors overheat when operated with unbalanced phase voltage.
Additionally, switching operations that result in instantaneous changes in voltage or current waveforms create harmonics. The concern is that if harmonics are filtered off the power system by motors, transformers, and capacitors, these components will overheat and fail unless they are designed to accept harmonics.
Solution: Dynamic Renewable Energy Inverters
Each of the challenges discussed above presents an opportunity for electric utilities to implement new ideas and technologies to better accommodate renewable energy sources. In our next post, we’ll discuss potential solutions that can allow for a smooth integration of distributed renewable energy sources into the existing electric power grid.
To learn more about Prescient’s innovative ideas for the next generation power grid, browse our next generation blog collection. Or contact us to discuss the opportunities and advantages of dynamic renewable energy inverters at greater lengths.
Want to learn more about Prescient’s recommendations to integrate renewable energy sources into the existing power grid? Check out these other posts:
Preparing the Grid for Renewables: Electric Powerways
Preparing the Grid for Renewables: Electric Serviceways
Before Renewable Energy Expands, FIDVR Must Be Resolved
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