In study after study, we (Pterra) are encountering this seemingly mounting issue of ride-through capability in solar photovoltaic (“PV”) inverters. For now, the matter is isolated to frequency ride-through in small grids such as those that may be found in the Hawaiian islands. However, there is potential for this to be critical in even larger systems as the number of inverter-based PV arrays interconnecting to existing distribution and transmission systems increases.

The crux of the present issue relates to the frequency ride-through settings and capability for commercial PV inverters. The industry standard that addresses frequency settings for solar PV is IEEE standard 1547, Interconnecting Distributed Resources With Electric Power Systems, which specifies that (for 60 Hz systems):

• For inverters < 30 kW, trip at frequency < 59.3 Hz clearing in 0.16 sec
• For inverters > 30 kW, trip at frequencies 57-59.8 Hz clearing in 0.16 to 300 sec (adjustable setpoint), or at frequency < 57 Hz clearing in 016 sec

Certain commercial PV inverters are equipped with extended frequency ride-through capability that will not trip until frequency drops below 55 Hz.

Renewable energy resources such as solar and wind, produce power in a manner that generally does not contribute to frequency control of interconnected power systems. For wind turbines, the reason for this is that the generators used to convert wind to electric energy have small inertias that dissipate rotational energy more readily than conventional steam turbines. Also, wind turbines are operated such as to generate optimal power from the available wind, and hence do not have much spinning reserve. For inverter-based solar generation, the solid-state controls have no rotating component at all. (Solar thermal power is usually produced with synchronous generators and thus contribute to frequency control as most thermal-type power plants are able to do.)

However, both wind turbines and solar inverters have the important characteristic of fast, programmable controls. The question then comes up: Is it possible for these power sources to participate in frequency control response of interconnections? This is an intriguing question that merits some further investigation.

A wave of new solar photovoltaic (“PV”) installations for power generation is hitting many distribution circuits around the country. These installations are typically in the range of 10-2000 kW and comprise of a set of solar PV arrays or trays and inverter modules. The inverters are needed to change the direct-current produced by the arrays to the alternating current standard used by the distribution circuits. The smaller installations connect single-phase, while the larger sizes are three-phase. Interconnection voltage at the point of common coupling between the PV installation and the distribution circuit varies from 120 volt up to 34.5 kilovolt (“kV”).

The concept of integrating these new PV installations with existing distribution circuits is similar to that of interconnecting larger generators in the transmission grid; i.e., the new installation should “do no harm” to the existing system. There are three aspects to this concept as follows. (1) If the existing circuit meets specified standards or criteria of performance, the circuit should still meet the same standard or criteria when the new PV is installed. (2) If the new PV introduces a violation of standard or criteria, mitigation measures need to be included as part of the the new PV’s installation to resolve the violation. (3) If the existing circuit already violates a standard or criteria, the new PV either should not make the violation worse, or limit its impact such that the violation is not worse or even reduced or eliminated.

In various Blogs on this site, we have talked about unique technical issues associated with integrating wind farms into existing grids. This Blog now addresses the matter of transient overvoltages, or TOV, specifically with respect to potential risks to customers and any required mitigation associated with wind farms installations.

Unlike base load power plants such as nuclear and some coal plants which operate near full capacity for days at a time, solar photovoltaic (PV) and wind farms are variable resources whose output is dependent on the minute-by-minute change in weather conditions. For solar PV arrays, clouds and atmospheric interference are the sources of variability. While for wind power installation, gusts and weather patterns are the main culprits. This difference in operating characteristic for variable resources requires a novel approach to determining the impact of transmission capacity on the size of the plant.

Pterra conducts training in power technology subjects, not as a primary line, but in response to a perceived need. Occasionally, work in analytical consulting leads to knowledge and skills that clients and associates desire to acquire. And we are more than happy to oblige, if only to break the stream of days spent talking to computers (instead of people). Plus there is something strangely attractive in speaking to minds that are just exploring this lifetime field, electric power. We hope that most will stay on and help the industry. And we hope that some new insight will consolidate our own understanding of how electrons move. This is not to say that these courses are aimed for Gen X’ers alone. But a noticeable percentage who attend do come from that demo.

Yesterday was the 7th anniversary of the founding of Pterra, LLC.   The original team of 5 who started this journey remain, with some worthy additions.   All have grown somewhat older, hopefully wiser, and after all the contingencies encountered through the years, more resilient and united as ever.

Our core competencies remain the same: power engineering analysis, new technologies, modeling and simulation.   But service applications have grown, from the initial focus on transmission planning and interconnection of new generation, Pterra now offers distributed generation studies, solar photovoltaic and wind power modeling, applications training, assessment for high voltage direct current transmission, expert witness, among others.

No seven-year itch here.   Just some wistful reminiscing and cautionary tales for the next 70 years.   Overall, one can say that it is possible to follow the dream, to have a workplace adopted to family, health, faith, other life situations.   Or, to use an electric power analogy: to be like a lightning arrester, withstanding the normal and continuous challenges and allow all other extraordinary surges to flow.

If only for this one new feature, the trip to attend the meeting (held April 28-29 in sunny Orlando, Florida) was worth it. The new feature is …

PSLF now allows “continuous” tap solutions for phase angle regulators, or PARs. Why does this matter? It matters a lot to those who work in the U.S. Eastern Interconnection (EIC) where most utilities use the competing software package, PSS/E.

Distributed generation (DG) has become a viable option and is gaining wider acceptance to utilities, customers, and independent power producers. While DG offers many advantages, the interconnecting utility typically requires a system impact study for interconnecting DG to the existing electric grid to ensure it would not adversely impact the operation, reliability and safety of the grid. By its nature, DG would interconnect to lower voltage systems generally classified as “distribution”. The studies can range from relatively quick feasibility assessments to comprehensive studies involving extensive equipment and power system modeling, measurements, and detailed simulations. Specific topics for such studies include: islanding, steady state power flow, voltage regulation, short-circuit, protective relaying, power quality (flicker and harmonic), power factor, system stability, grounding, and ground fault overvoltage.

Distributed generators (DGs) are small generating units that are connected to the distribution network at voltages below 69 kV. DG units usually have capacities of 10MW or less, and are based on different energy sources, such as wind, solar and diesel. The distribution network is generally a radial system and designed for one direction of load flow, i.e. from the electric grid to the load. The unidirectional flow assumption is no longer valid when DG is interconnected at the customer or load side since the flow of power can now go in either direction: from the load side to the grid or from the grid to the load side. This fundamental change affects how an impact study, generally required to identify and mitigate any changes to reliability of the distribution system, for DG interconnection is conducted. Reference 1 summarizes several typical tasks required in an interconnection impact study.