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Tom Key, EPRI, [email protected]
February 6, 2020
Neutral-Grounding for Inverter-Connected DERFor NY ITWG
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NYSERDA Effective Grounding Project
TASK 1. Assess Current Utility Practice To Inform Objectives
– Determine Current Utility Practice (Guided interviews/Survey)
– Clarify Grounding Objectives and Specific Requirements, Dec 4,2019
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 by July 19
TASK 5. Deliver Final Report, Outreach and Education (proposed ITWG workshop in 2020)
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Q3 – Do you require additional ground sources for DER (either by a grounding transformer or a grounded-wye/delta transformer)?
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Element Pickup Range Time Delay
Undervoltage (27) 0.5 pu 1 secUndervoltage (27) 0.7 pu 10 secUndervoltage (27) 0.88 pu 20 secOvervoltage (59) 1.1 pu 12 secOvervoltage (59) 1.2 pu 0.16 secUnderfrequency (81u) 56.5 Hz 0.16 secUnderfrequency (81u) 58.8 Hz 299 secOverfrequency (81o) 61.2 Hz 299 secOverfrequency (81o) 62.5 Hz 0.16 sec
Element Pickup Time Delay(cumulative)
TOV 2 pu 0.0016 sec
TOV 1.7 pu 0.003 sec
TOV 1.4 pu 0.016 sec
TOV 1.3 pu 0.166 sec
Inverter overvoltage (TOV) trip settings
ABC
120kV 2500MVA
ABC
abc YgYg
12.47 kV / 120 kV47 MVA
abc
ABC
Feeder Section25 miles
A B C
a b cYg
Yg480V / 12.47 kV 1 MVA
ABC
abc
Breaker
ABC ABCSLG fault
abc
ABC
Feeder Section11 miles
abc
ABC
Feeder Section31 miles
A B CLoad1
3PH INV
Preliminary Simulation Study – Overview
Trip settings in IEEE 1547-2018, Category III
1 MVA
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Overvoltage Scenarios and Grounding 1. Load Rejection Overvoltage (LRO) in balanced 3-phase system
– Yg/Yg DER transformer (effective grounding does not matter)– Main breaker opens (no ground fault)– Load varies from 0 pu -1 pu (Δ or Y connected load)
2. Combined LRO and ground fault overvoltage (GFO)– Yg/Yg and Δ/Yg DER transformers (with and w/o supplemental grounding)– Permanent ground fault and main breaker opens– Load varies from 0 pu -1 pu (ungrounded load, spot check with 30% grounded)
3. GFO with varying amount of grounded load– Yg/Yg and Δ/Yg DER transformer (no supplemental grounding)
– Permanent ground fault occurs and main breaker opens– Load to generation ratio = 1, grounded load % varies from 0% –100%
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7.4 Limitation of Overvoltage Contribution (1547-2018)
7.4.1 Limitation of overvoltage over one fundamental frequency period For Ground Fault Overvoltage (GFO) - on any portion of the Area EPS that is designed to
operate effectively grounded to exceed 138% of its nominal line-to-ground fundamental frequency voltage for a duration exceeding one fundamental frequency period.
7.4.2 Limitation of cumulative instantaneous overvoltage
For Load Rejection Overvoltage (LRO)
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PV System LRO Events two different inverter simulations.Pass 1547.1 LROV Test Fail 1547.1 LROV Test
0
1
2
3
VRM
S-Se
c (p
u)
A
B
C
-2
-1
0
1
2
Vsec
(pu)
3.97 3.98 3.99 4 4.01 4.02 4.03 4.04 4.05
time (s)
0
0.2
0.4
0.6
0.8
1
Trip
sig
nal
OUFP
OUVP
TOVP
SW opens
Inv trips
18 ms
1.38 pu@ 4.1 s
0
1
2
3
VRM
S-Se
c (p
u)
-2
-1
0
1
2
Vsec
(pu)
4 4.05 4.1 4.15 4.2
0
0.2
0.4
0.6
0.8
1
Trip
sig
nal
1.38 pu@ 4.1 s
SW opens
Inv trips
164 ms
• With transient overvoltage protection (TOVP)• Inverter is able to trip within the required time
• TOVP disabled• Inverter fails to trip within the required time
time (s)
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Scenario 1 – LRO
3.9 3.95 4 4.05 4.1 4.15 4.2 4.25 4.3
t/s
-2
-1
0
1
2
Vinv
(pu)
A
B
C
X 4.02
Y 1.729
3.9 3.95 4 4.05 4.1 4.15 4.2 4.25 4.3
t/s
-2
-1
0
1
2
Vinv
(pu)
X 4.02
Y 1.7268
Certified inverter
Uncertified inverter
25.7 ms by TOVP
167 ms by OVP
2.00
1.73 1.69 1.66 1.62 1.591.52
1.341.21
1.101.02
0.9
1.1
1.3
1.5
1.7
1.9
2.1
0 0.2 0.4 0.6 0.8 1
Peak
pha
se v
olta
ge (p
u)
load/Gen ratio
Uncertified
Certified
166 167 167 168 172 174 175 182
267 267 267
19.6 25.7 28.3 29.6 33.6 3953.2
182
267 267 268
0
50
100
150
200
250
300
0 0.2 0.4 0.6 0.8 1Tr
ip ti
me
(ms)
load/Gen ratio
Uncertified
Certified
TOVP
TOVP - Transient overvoltage protectionOVP - Overvoltage protectionOFP - Overfrequency protection
OVP
OFP
• Peak LRO decreases with higher load/gen ratio• Uncertified inverter causes longer trip time, but similar LRO
G3BK1
Variable load, 0 – 1 pu
Test condition: No grounding transformer (GT), delta connected load, No ground fault
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G3BK1
Variable load, 0 – 1 pu
Scenario 2 – LRO + GFOYg/∆ for grounding
Test condition: w/wo grounding transformer (GT), delta connected load, permanent ground fault
3.37
2.95 2.892.79 2.77
2.562.38
2.22 2.171.99
1.85
2.72
2.29
2.001.90
1.631.49
1
1.5
2
2.5
3
3.5
0 0.2 0.4 0.6 0.8 1
Peak
pha
se v
olta
ge (p
u)
load/Gen ratio
Uncertified inverter
No GT With GT
3.27
2.692.50
2.392.26
2.132.01
1.88 1.86 1.84 1.76
2.52
2.25
2.01
1.7031.518
1.407
1
1.5
2
2.5
3
3.5
0 0.2 0.4 0.6 0.8 1
Peak
pha
se v
olta
ge (p
u)
load/Gen ratio
Certified inverter
No GT With GT
• Uncertified inverter suffers higher overvoltage in this case due to the longer trip time • Grounding transformer helps to mitigate the overvoltage
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G3BK1
1pu load
Scenario 3 – GFO
• Overvoltage caused by ground fault (GF) tends to reduce with higher portion of ground load (Yg connected)
• Grounding transformer (GT) helps with overvoltage mitigation when grounded load is light, but this effects drops with increasing grounded load. Specially, GT slightly increase GFO when 60% loads are grounded
• Further investigation is needed to study the different overvoltage change trend in uncertified inverter, when 40%-80% grounded load and no GT exist
Test condition: w/wo grounding transformer (GT), load/generation ratio = 1, delta ang Yg connected load, permanent ground fault
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2
0% 20% 40% 60% 80% 100%
Peak
pha
se v
olta
ge (p
u)
Percentage of grounded load
Uncertified inverter, no GT Certified inverter, no GT
Uncertified inverter, with GT Certified inverter, with GT
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Comparison between Scenario 1 and 2Scenario 1 (S1): No grounding transformer (GT), delta connected load, No ground fault
Scenario 2 (S2) : w/wo grounding transformer (GT), delta connected load, permanent ground fault
3.27
2.692.50
2.392.26
2.132.01
1.88 1.86 1.84 1.76
2.52
2.252.01
1.701.52 1.41
2.00
1.73 1.69 1.66 1.62 1.59 1.521.34
1.211.10 1.02
0
0.5
1
1.5
2
2.5
3
3.5
0 0.2 0.4 0.6 0.8 1
Peak
pha
se v
olta
ge (p
u)
load/Gen ratio
Certified inverter
S2- No GT S2 - With GT S1 - No GT
• Overvoltage caused by ground fault and load rejection cannot be simply superposed
• Even with grounding transformer, the ground fault overvoltage (GFO) is higher than the load rejection overvoltage (LRO)
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Test Plan Objective
– Characterize the inverters’ dynamic response to voltage changes– Estimate equivalent negative sequence impedance for theoretical
analysis– Identify impact of volt-var function on inverter response
Test scenarios (with and without volt-var) Response to step change of positive sequence grid voltage Response to negative sequence voltage variation
– Magnitude variation– Phase angle variation
Test approach – Lab test of commercial inverters– Controller hardware-in-the-loop (CHIL) with different negative
sequence current control schemes– Will be performed in parallel
Expected outcomes– Validated inverter simulation model with accurate dynamic response– Characteristics of inverter negative sequence impedance when
different control schemes are implemented
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Future Work
Perform lab and CHIL tests per designed test plan Simulation model refinement
– Arrester– Capacitor bank– …. Simulation of other scenarios
– Yg/∆ transformer for grounding– Varying generation before island forms– Inverter with grid-support functions– ….
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Together…Shaping the Future of Electricity