Renewable Energy Requires Next Generation Voltage Control
Updated: Apr 20, 2021
The electric power grid is transitioning from antiquated to innovative. One of these big innovations is the increased usage of renewable resources like solar and wind. But is the grid ready for these changes? Not yet, but it can be with the right updates. This post explores the changes to the grid that will be vital for the accelerated integration of renewable energy sources.
As a quick note, this post is going to be in-depth and technical, so I wouldn’t recommend multi-tasking while reading!
Levelized Voltage is Essential
In order to move renewable energy across the power grid, levelized voltage is essential since the primary parameters that influence power transfer are sending end voltage and receiving end voltage. Traditionally, sending end voltage was tightly controlled and receiving end voltage vary was allowed to vary. The flow of power was from a remote power generating station or a major substation. When renewable energy sources are added, the flow of power can be across the street, down the block or to the other side of town.
Let’s take a look at levelized voltage control using enhanced inverters that convert solar energy to AC power.
Traditional vs. Next Generation
Traditionally, power system voltage has been controlled using:
Generator voltage regulators (at power plants)
Transformers equipped with load tap changers (at substations)
Step-voltage regulators (at substations and on distribution lines)
Capacitors (at substations and on distribution lines)
Static var compensators (at substations)
While effective, load tap changers, step-voltage regulators, and switched capacitors are slow to react to voltage variations. Typical reaction time is measured in seconds. With next generation renewable energy sources, reaction time is measured in milliseconds. Rapid reaction time could be 16 milliseconds, quick reaction time could be 100 milliseconds, and sustained reaction time could be 500 milliseconds.
The difference between traditional voltage control methods and next generation voltage control is illustrated in Figure 1 for a typical 10-mile radial distribution line. Renewable energy next generation voltage controls should incorporate solid state switching devices and microprocessor-based control algorithms that maintain steady state voltage within 0.5% of nominal, and transient voltage within 2.5% of nominal.
Existing load tap changers, step voltage regulators, and capacitor on-off switches are mechanical devices with operating times of 50 milliseconds and service life of 10,000 – 100,000 operations. Existing voltage control devices maintain steady state voltage within 5% of nominal and transient voltage within 20% of nominal. Existing voltage control devices are limited to 20 actuations per day (based on service life).
Next generation voltage control technology will utilize next generation renewable energy power supplies and voltage control modules provided by electric utilities.
Updates to Microprocessor-based Controls
Incorporating predictive voltage algorithms in microprocessor-based controls will facilitate distribution line voltage adjustments in 16 milliseconds. Predictive algorithms should include:
Scheduled voltage setpoints that respond whether or not renewable energy sources are producing power.
Transient voltage setpoints that respond to large motor starts as well as voltage recovery after a fault is cleared.
Response time parameters defined as rapid, quick, or sustained.
Next generation renewable energy sources should be categorized as:
Kilowatt (KW) Supply – kilowatts to the power grid
KW+C Supply – kilowatts + capacitor adds vars to the power grid.
KW+L Supply – kilowatts + inductor removes vars from the power grid.
Kilovolt-ampere (KVA) Supply – kilowatts to the power grid and vars to / from the power grid.
Additionally, renewable energy suppliers should receive maximum credit for KVA power supplies and less credit for others. Let’s take a look at that more.
Credits to Renewable Energy Suppliers and Consumers
In areas where customers are paying $0.10 per KWH, renewable energy providers with KVA capability should be paid $0.06 for every KWH supplied to the power grid. Renewable energy providers with KW+C or KW+L capability should be paid $0.05 for every KWH supplied to the power grid. Renewable energy providers with KW capability should be paid $0.04 for every KWH supplied to the power grid. Fee schedules recognize contributions to power system voltage stability.
KVA supplies should be designed with a 70% capacitive power factor (KW+C) and a 90% inductive power factor (KW+L). Residences that install 5 KVA roof top solar panels should reduce their electric bills by $1300 per year. Residences that install 5 KVA+C roof top solar panels should reduce their electric bills by $1200 per year. Residences that install 3.6 KW roof top solar panels should reduce their electric bills by $1100 per year.
The reimbursement schedule for residences should be 125% of the average cost of energy as electric utility costs incurred due to losses will be reduced. Using New York Independent System Operator, Inc. (NYISO) as a benchmark, the marginal cost of energy was $0.01727 per KWH at 1:17 P on December 31, 2020. During a calendar year, the marginal cost of energy can be expected to vary from $0.001 per KWH to $0.40 per KWH. Next generation rate schedules and financing options will be presented in future posts.
Rapid Reaction Times in the Next Generation
During the transition from traditional voltage control to next generation voltage control, electric utilities will replace step-voltage regulators and capacitors with voltage control modulators that react to voltage excursions in 16 milliseconds. Voltage control modulators should be available in 3600 KVA, 2400 KVA, and 1200 KVA pad mounted units that will be connected to existing distribution lines. Each modulator should be small enough to fit in an average parking space.
In the near future, perhaps as early as 2030, digital voltage controllers that replicate analog voltage controllers will be replaced with predictive voltage controllers that utilize real time data to rapidly stabilize system voltage. Next generation power systems will provide every customer with 100% of nominal voltage at the meter base at all times, day and night, summer and winter, spring and fall.
Our next post will discuss levelized voltage control using voltage control modules that are installed and maintained by electric utilities, so be sure to check back in next week.
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