NYSERDA R&D Neutral-Grounding for Inverter-Connected DER Tom Key and Devin Van Zandt, EPRI June 26, 2019 w w w . e p r i . c o 2019 Electric Power Research Institute, Inc. All rights reserved. m Research Project Abstract Challenge: Grounding practices for rotating machine DER aimed to control ground fault overvoltage in utility distribution are not well defined for ungrounded inverter-connected PV. Practices are evolving and requirements appear to be inconsistent across systems Developers point out that they are uncertain what requirements must be met. NYSERDA Study Objective: Define evaluation approach and methods to determine effective grounding (and/or supplemental grounding) configurations with inverter-based DER. 2 ww w.epri.co
m 2019 Electric Power Research Institute, Inc. All rights reserved. IEEE Guide for Neutral Grounding C62.92.1 Introduction and definitions (vintage 1954) C62.92.2 Synchronous generator systems C62.92.3 Generator auxiliary systems C62.92.4 Distribution systems (neutral grounding, 1972) C62.92.5 Transmission and sub-transmission systems C62.92.6 Systems supplied by current-regulated sources (2017) provides definitions and considerations related to system grounding where the dominant sources of system energization are currentregulated or power-regulated power conversion devices. EPRI-NYDERDA analytical work to address practical applications of C62.92.6 3 Screening of 3 or 4-wire DER connections w.r.t. primary transformer configuration
Supplemental grounding transformers, sizing, effectiveness, other options Models used in short circuit protection tools (CAPE, ASPEN, CYME, Synergi) ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved. Background/Motivation 1. Traditional power system grounding (based on IEEE C62.92.1-5) addressed synchronous generators (low-impedance voltage sources) 2. In 2017 IEEE C62.92.6 was added to address grounding of systems supplied by inverter DER (current-regulated sources). 3. 4 With few exceptions, grid-connected three-phase inverter DERs are controlled to act as positive-sequence current sources
This behavior is fundamentally different than machines, indicating need to review and update effective (neutral) grounding practices. PV plant grounding requirements are not harmonized among different utilities and between jurisdictions of the same utility ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved. DER System Grounding in the Unintentional Islanding Scenarios 1. 2. Inverter DER are expected to have insignificant effect on system grounding when the feeder breaker is closed The main concern is ground fault overvoltage (GFO) in case DER supports an unintentional island (feeder breaker open) with a ground fault GFO is the first order concern in this scenario 5
ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved. Conventional Notion of an Effectively Grounded Power System Effective grounding is defined as a Coefficient of Grounding (CoG) CoG 0.8 pu limits GFO to 0.8 x Vl-l or 0.8 x of Vl-g effective grounding is a system condition defined by voltages CoG VL-G(fault)/VL-L(no fault) X0/X1 3, R0/X1 1 are an approximation (works for synchronous generators) Conventional situation:
6 Relatively low impedance of rotating generator sources dominates system Loads and other shunt impedances are relatively unimportant to fault and GFOV calculations Thus, a voltage source with a low X0/X1 will yield effective grounding when energizing any practical system ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved. DER Connection Screening Practice: Used in FERCSGIP, NY SIR, CA Rule 21 and most states 7 Aim to screen out DER connections prone to ground-fault overvoltage (GFO) and to limit allowed connection types based on concerns for ground fault and open-phase This table is commonly used (but may not be appropriate for inverter DER)
Failing the screen typically leads to alternatives such as adding a grounding transformer ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved. Project plans over then next 18 months 8 TASK 1. Assess Current Utility Practice To Inform Objectives Determine Current Utility Practice (guided interviews) Clarify Grounding Objectives and Specific Requirements TASK 2. Develop Relevant Scenarios and Models to be Analyzed (includes Laboratory and HIL Test, and Modeling Objectives)
TASK 3. Coordinated and Concurrent Analysis, Simulation, and Testing TASK 4. Develop Guidelines for Inverter Effective Grounding TASK 5. Deliver Final Report, Outreach and Education (ITWG workshop later in 2019 and planning tutorial near the end of the project) ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved. Q1 What are your common transformer connections for 3-Phase DER? 9 ww w.epri.co m
2019 Electric Power Research Institute, Inc. All rights reserved. Typical Grounding Practices for Synchronous Generator DER Feeder Grounded wye generator, GSU must be Yg-yg Feeder Feeder Yg- GSU, no separate GT needed, neutral resistor (options) 10 Grounding transformer on LV, GSU must be Yg-yg Feeder Grounding transformer on MV, GSU any configuration ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved.
Q2 When specifying a local ground source for DER, is your requirement the same or different for inverters and rotating machines of the same rating? 11 ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved. Q3 When do you require a DER to provide additional ground source (either by a grounding transformer or a grounded-wye/delta transformer)? 12 ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved. Tech Brief on DER Open
Phase Combiner Panel Qty: 4 Inverters 36 kW Qty: 27 Ceases to energize Load Center #4 Cu Qty: 27 Avg: 50' DGIT 1 MVA 12.47 kV-277/480V 5.75% Z Phase C blown
Riser pole 1/0 Al Qty: 1 5000' 600 MCM Cu Qty: 4 Avg: 130' POI Fuses R 336 AAC Overhead 25' To 4 more 1 MVA PV sites w/ identical construction X0=0.14 Transformer type
Open phase voltage Yg/Yg 5-leg Yg/ 3-leg 5-leg With GT and inverter capacitance Primary/ Secondary 1.2/1.18 pu ~1 pu ~1 pu Without GT, with inverter capacitance Primary/
Secondary ~1 pu ~1 pu Without GT and all capacitance Primary/ Secondary Ferroresonance 0.6 pu 1 pu 1 pu 13 ww w.epri.co m /Yg Ferroresonance
Ferroresonance < 0.5 pu 2019 Electric Power Research Institute, Inc. All rights reserved. Reference: Distributed Energy Resources Field Experience: Open Phase (3002015949) Q4 Are you seeing more incidents and concerns for open phase because of DER Plants? 14 ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved. Q5 Are open phase commissioning tests conducted before DER installation? 15
ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved. Questions/Comments? 16 ww w.epri.co m 2019 Electric Power Research Institute, Inc. All rights reserved.
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