Pterra was engaged by the New York State (NYS) Department of Public Service (DPS) to provide some insight into technical issues associated with battery energy storage systems (BESS) interconnecting into distribution feeders. This work was part of ongoing support Pterra is providing to the NYSDPS on NYS Standard Interconnection Requirements (SIR) procedures.

By Ketut Dartawan

(This topic was presented at the PSCAD Users group Meeting held in Atlanta, GA on Sept. 20-21, 2018. For the full presentation, please see this link.)

Many interconnection challenges exist when connecting photovoltaic (PV) resources to the electrical distribution grid. Various challenges on the distribution feeders are covered in some technical papers; however, one of the urgent topics – as recently mentioned by utilities and recognized by inverter manufacturers as well as the developers – is the potential for ground fault overvoltage (GFO) on sub-transmission systems feeding distribution feeders via a delta-wye transformer (see Figure 1).

by  Ketut Dartawan,  Amin M. Najafabadi

Pterra is presenting a paper on the above subject at the IEEE General Meeting 2017- Chicago 16~20 July.  Abstract of the paper follows:

IEEE updated its recommended practice and requirement for harmonic control in electric power system after more than two decades. The most updated version of the standard (IEEE Std. 519-2014) revised the 1992 version and its static harmonic voltage and current limits. Unlike the 1992 and the older versions of the standard, the 2014 version introduces a newer approach which considers the stochastic nature of harmonic distortions.  Furthermore, it recommends limits based on the number of times distortions may occur. For example, for the harmonic current distortion, it recommends three limits: daily 99th percentile, weekly 99th percentile, and weekly 95th percentile values. Applying the IEEE Std. 519-2014 for planning studies and for harmonic assessment of proposed projects can be very challenging because presently there is no known commercial tool which fully considers the stochastic simulations and limits required in the standard. This paper demonstrates the approach used by the authors in applying IEEE Std. 519-2014 to a harmonic study recently performed for a 180 MW solar farm.

Index Terms- harmonic analysis, harmonic filters, solar power generation, statistical analysis, time series analysis

In recent work performed by Pterra, the issue of ground fault overvoltage (GFOV) was raised in relation to integration of distributed generation (DG).  In particular, can inverter-based photovoltaic systems, connected in distribution feeders, induce GFOV on the high -side of the substation transformer?  And if so, under what conditions could this occur? Pterra was engaged to conduct a research study by NYSERDA (the New York State Energy Research and Development Authority) to answer these very questions.

The seemingly innocuous flickering of lamps could be a new technical battleground for the further growth and spread of photovoltaic (“PV”) electric power. On one side of the impending conflict is the flicker standard, a venerable reference that could very well trace its roots back to the advent of the electric age. On the other side are the new darlings of the power industry — environment-friendly, renewable solar power. The one thing about solar power is that in bulk amounts, its units need to be connected to existing electrical systems, and a side effect of this integration is the production of flicker. The more PV devices connected to the same electrical circuit, the more flicker is produced and the closer the level of flicker is to the allowable limit defined by the flicker standard.

(This Blog is a continuation of an ongoing series on integrating inverter-based solar photovoltaic generation with existing electric distribution circuits. Link to Part 1)

Solar PV (shorthand for photovoltaic) generation is growing in support and implementation in part because of a supportive regulatory environment. Among the more common types of interconnection terms are NEM and FIT.

Harmonics is a very specialized and not widely understood topic in the electric power field which can become a major issue when inverter-based photovoltaic (“PV”) generators, (popularly referred to as solar power), are added to existing distribution circuits. This Blog provides a quick overview of the phenomena, potential negative impacts, causal conditions, and mitigating measures associated with harmonics. The bulk of the material presented here is based on an oral presentation at the SOLAR 2012 Conference of the World Renewable Energy Forum (WREF 2012) held last May 13-17, 2012, at the Colorado Convention Center in Denver.

“Harmonics Issues that Limit Solar Photovoltaic Generation on Distribution Circuits,” by K. Dartawan, L. Hui – Pterra, M. Suehiro – Maui Electric Co., presented at SOLAR 2012, World Renewable Energy Forum (WREF 2012), May 13-17, 2012, at the Colorado Convention Center in Denver.

One more item to add to the list of technical items to consider when interconnecting solar and wind farms is grounding. The grounding issue often appears when (a) integrating solar power inverters or wind turbines to existing distribution circuits, or (b) designing collector systems for solar and windpower farms.

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.