This article provides solutions to issues addressed in our prior article Renewable Energy Poses Challenges to Electric Power Quality.
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. To accommodate renewable energy sources, electric utilities need to change the way they design and operate their energy distribution systems.
The first step utilities should take is to require that renewable energy providers become accountable for their impact on electric energy grid infrastructure, voltage control, and frequency control. Next, utilities should upgrade distribution lines to electric serviceways with dynamic voltage and power factor control. Upgrading to electric serviceways will not require a total overhaul of distribution lines; instead, new components will be added to the existing lines to modify them.
In this article, we’ll focus on addressing voltage inconsistencies introduced by renewable energy sources. Stay tuned for a future article that dives into frequency inconsistencies created by renewable energy sources.
With the addition of specialized inverters and inductors, which will be added to the grid as part of the upgrade to electric serviceways, voltage inconsistencies associated with renewable energy sources can be stabilized. Let’s take a closer look at how that happens.
Electric Serviceways Facilitate Renewable Energy Transfer
To implement a vital change in electric power distribution systems, distribution lines should be converted to electric serviceways that optimize the use of renewable energy provided by solar PVs and other distributed renewable energy sources. This change will facilitate the transfer of excess renewable energy while providing constant voltage at all consumer locations.
Electric serviceways will have dynamic voltage and power factor control. This will require the installation of serviceway modules, shown in Figure 1, composed of inductors and capacitors that are switched using time-biased, predictive, microprocessor-based controllers that respond to changes in energy flow across the serviceway. Serviceway modules will respond within 25 milliseconds to control voltage within +/-0.5% of the setpoint by injecting vars during low voltage conditions and absorbing vars during high voltage conditions.
Specialized Inverters Minimize Voltage Inconsistencies
When renewable energy sources are connected to distribution lines, day/night voltage variations are more significant than seasonal load variations. To address new load variations, renewable energy inverters should be equipped with inductors that are energized whenever renewable energy sources are producing energy.
Voltage drops along electric serviceways increase when load increases, and decrease when load decreases. Inductors will counteract the natural voltage rise that occurs as renewable energy sources produce energy. This is low cost and easy to implement within existing distribution lines.
The Grid Needs More Than Kilowatts
Kilowatts are in-phase power, that is, the energy purchased by consumers. Voltage drop is calculated using complex numbers. Voltage drop (Vd) equals current (I) multiplied by impedance (Z). Current has an in-phase component (watts) and a quadrature component (vars) which can be expressed as “I” at an angle, in degrees, such as 10 amps @100 lagging where the in-phase component is a function of the cosine of the angle and the quadrature components is a function of the sine of the angle. The magnitude and angle of the current changes continuously - sometimes quickly, sometimes slowly.
Impedance of electric serviceways (and distribution lines) has resistive and quadrature components that are functions of conductor material, conductor length, conductor cross sectional area, and conductor spacing. Impedance is a fixed value that is calculated when serviceways are constructed. Typically, the quadrature component of impedance of serviceways (and distribution lines) is 5 to 10 times greater than the resistive component.
Current has in-phase and quadrature components that are functions of the energy source and the connected load. Renewable energy sources need to accommodate voltage control as well as energy supply. This means that electric utilities need to consider holistic solutions.
Vars Must be Considered
Electric utilities refer to quadrature power as vars or imaginary power. This misnomer has created the false idea that quadrature power is irrelevant. Table 1 illustrates the voltage drop at the end of one mile of three phase, 12.47 KV electric serviceway that was built using 500 MCM copper conductors installed in a below grade duct bank. Voltage drops and rises are cumulative, meaning that voltage two miles from a substation would be expected to vary more than voltage one mile from a substation.
The values in Table 1 are simplified representations of complex numbers that were developed using impedance and current values.
Inductors, when energized, increase lagging current and increase voltage drop. Renewable energy sources reduce current and reduce voltage drop. Combining inductors and renewable energy sources can stabilize voltage along electric serviceways.
Acoustic Monitoring Can Detect Low Energy Faults
Widespread use of renewable energy sources will have a huge impact on protective relaying. Traditionally, electric utilities use time and instantaneous overcurrent relays, impedance relays and differential relays to detect and isolate short circuits. Charles Fortescue, Charles Wagner, and Ruth Evans introduced symmetrical components as the basis for short circuit analysis more than 100 years ago at a time when slide rules, as seen in Figure 2, were used to complete complex calculations with three digit accuracy.
With today’s technology, acoustic monitoring of components with fiber-based detectors should be used to supplement voltage and current monitoring. Acoustic monitoring can detect low energy faults below the threshold of overcurrent relays, can verify relay accuracy for high energy faults, and can reduce the misoperation rate of protective relaying schemes to six sigma values. Protective relaying practices need to change as renewable energy sources are installed.
Phase Balancing Tech Can Prevent 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 produce balanced, three phase energy. The concern is that three phase motors overheat when operated with unbalanced phase voltage.
To prevent damage to grid components, phase balancing technology should be installed. Additionally, rewiring to distribute single phase renewable energy sources across all three phases will minimize damage. Serviceway modules equipped with Wye-grounded – Delta transformers can serve as phase balancers as well as voltage control units.
More Solutions Are Needed
The solutions discussed in this article address voltage inconsistencies associated with distributed renewable energy sources. Frequency inconsistencies from renewables also require creative solutions. We’ll address these solutions and more in a future article, so stay tuned!
To take a closer look at the issues that these solutions are addressing, check out our article Renewable Energy Poses Challenges to Electric Power Quality. To learn more about Prescient’s innovative ideas for the next generation power grid, browse our next generation blog collection, or contact us.