Get Smart – Advanced Inverters

Intermediate

Inside this Article

Hawaii’s closed-system infrastructure (and excellent solar resource) means that variations in energy resources and demands need to be carefully balanced.
The renewable generation capacity in Germany is high enough that cities experience times when peak production exceeds total national demand. That’s impressive, but necessitates advanced system control.
SMA’s Sunny Boy line of inverters was among the first to receive UL1741 Supplement A certification for California’s Rule 21.
Smart microinverters from Enphase Energy can provide estimates of energy lost to curtailment from export limiting and can apply voltage rise compensation, which avoids excess reactive power production (due to volt-var) or active power curtailment (due to volt-watt). Soon, these inverters will also be capable of reporting energy loss estimates for commanded curtailment, volt-var, volt-watt, and frequency-watt.

Distributed energy resources (DERs), such as grid-tied PV systems, offer tangible benefits to customers by reducing their utility bills, and to the utility by reducing use of grid energy at customer locations—but there are consequences when many systems export power into the grid. And in areas with lots of grid-tied systems, new grid-supporting products called smart inverters are becoming a necessity for protecting the grid and ensuring good power quality for all customers.

The Effects of DERs

Germany is one of the first countries that experienced having significant amounts of distributed PV and its resultant problems. Originally, grid-tied PV inverters were set to cease power production and trip offline under fairly narrow power-quality tolerances, which posed the threat of tripping many inverters offline and causing a large blackout. Since it was such a large portion of the nation’s energy mix, a countrywide DER “retrofit”—new inverter settings and firmware updates—had to be applied. This allowed the systems to “ride through” grid disturbances, rather than tripping offline. But the retrofit process was complicated, expensive, and time-consuming.

As California’s DERs have become more prevalent, and to avoid a problem similar to Germany’s, its investor-owned utilities (IOUs) and stakeholders began developing requirements for “smart” inverters through the Smart Inverter Working Group. This group convened to determine how Rule 21—California’s technical requirements for interconnecting DERs to the grid—would be implemented. Phase 1 of the working group added the requirements noted in the “Rule 14 & Rule 21 Inverter Default Settings” table. Since September 2017, any new grid-connected inverters in California must be smart inverters.

Hawaii has nearly 50,000 PV-powered residences (about one in three private rooftops) on Oahu alone. In late 2014, Hawaiian Electric began implementing ride-through. Like Germany, the response of inverters to grid disturbances was a major concern. Hawaii has even more of a challenge, however, since generation and load must be balanced on each island’s independent electrical system. Starting in March 2018, Hawaiian Electric’s Rule 14 (the technical requirements for interconnecting generators to the grid) requires smart inverters. While similar to California’s 21, in addition to ride-through requirements, it also has frequency-watt (for frequency support) and volt-watt (for voltage support) function requirements.

Smart Inverter Functions

Smart inverters have been developed to not only ride through during brief fluctuations in power, but to also assist in raising or lowering grid voltage and help regulate frequency. While there are multiple grid-support functions available in each smart inverter, it is up to each state and/or utility to determine which functions will be required to be activated. Below is a breakdown of the available functions. Only one reactive power function (constant power factor or volt-var) may be turned on at a time. All others can run simultaneously.

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Comments (1)

Solarfire's picture

Dear Mr. Lydic,

I totally enjoyed reading your article since I experience curtailment issues on a daily basis, and I find this a rather obscure area to explain to others unless they experience it themselves, and you have done an outstanding job in providing some insight into this subject.

Some homeowners with roof photovoltaic systems fail to understand that curtailment significantly increases their cost per watt, increases their payback period, and reduces their return on investment (ROI).

I have been wondering if using a residential battery system could somehow decrease the curtailment issue by decreasing the percentage of waste. This waste is, of course, due to not obtaining credit for produced energy.

Somehow the inverter would need to know that a larger percentage of produced energy is ‘consumed,’ and, therefore, it should be accounted as produced energy rather than curtailed by the inverter.

This would allow the utility’s smart meter to, also, account for this produced energy even if it does not flow out into the grid since it has been stored in the battery bank.

In my case, I have two smart meters (LOL), one is for consumption and one is for production, and somehow together they can tell whether consumption was from self generated energy or consumption was from grid energy.

I would be grateful to you for any insight you could provide in this issue.

Best regards,

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