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Confidential & Proprietary | Copyright © 2019 Slide 1
A l i D a n e s h p o o y , P h D , P E
Over‐voltages In Inverter‐based Systemsi‐PCGRID 2019 Workshop
Confidential & Proprietary | Copyright © 2019 Slide 2
Contents
Inverter vs. Conventional Voltage stresses Inverter TOV Grounding system Analysis methods Mitigation
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Introduction
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Inverter vs. Rotating Machine
Rotating machines (conventional): Synchronous and Induction (Types 1&2)– Rotating Inertia: Synchronous generator demonstrates continuous
frequency response.• Synchronous voltage cannot jump• Inverter is based on PLL and software generated, limited to dc cap voltage
– Magnetic Inertia: Large short circuit current.• Large units are cooled using liquid or gas• Synchronous: 8-10 times rated current• Induction: 5-6 times the rated current
Note: The above list neither is exclusive nor complete.
Confidential & Proprietary | Copyright © 2019 Slide 5
Inverter vs. Rotating machine
Inverter: – Current limited switching devices– GTO, IGBT: Semiconductor wafer is thin and expensive– Low thermal inertia– Losses: I2R, Switching losses– Maximum current is the design criteria– Short circuit current < 1.4 times the rated current (large units 1.1)
– Six-pulse bridge• DC AC
Note: The above list neither is exclusive nor complete.
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Inverter vs. Rotating Machine
Dominant sources of energization of power systems have been synchronous generators.– Represented in power system analysis by a constant ac voltage source
in series with a reactance. Inverter-based generators, however, generally behave like constant
ac current (or power) sources. – Current source characteristic has impact on the overvoltages caused by
ground faults,
Assessment of system grounding, as defined in IEEE Std. C62.92, must properly consider behavior if inverter-coupled power sources dominate in a system.
Confidential & Proprietary | Copyright © 2019 Slide 7
Inverter Voltage Performance
Voltage ride-through (PRC-024-2)
Above 120% for0.16 s, ~ 10 cycles
Inverter does not generate TOV above 120% for morethan 10 cycles. – Canyon 2 Fire (NERC)– 400 MW OV tripping
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Overvoltages
Over-voltage protection is typically provided using Surge Arresters. – Surge arresters demonstrate low resistance for voltages above their
MCOV and high resistance for voltages below their MCOV.– Limits transient overvoltages –not intended to limit temporary
overvoltages. – Temporary overvoltages are caused by faults, load rejection, line
energizing, resonance conditions, ferro-resonance, or by some combination of these factors.
– Note: inverters are harmonic sources, which can contribute to resonance conditions.
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Arrester Power Frequency vs. Time
𝑉𝑉
Source: ABB
TOV ratio versus arrester rated voltage drops with time.
Arrester cannot handle 133% TOV longer than 1 second.
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Ground Fault
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Ground Fault Overvoltage When a feeder is disconnected, the voltage of feeder collapse due
to lack sources.
Feeders with DER– island detection can be
up to 2 seconds.
Transmission-connectedcan have no load connected.
Source: IEEE
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Ground Fault Overvoltage
Study methodologies Symmetrical component
– +/-/0 components Phasor study
– Ac system analysis Time domain
– Inverter control scheme• Proprietary models
– Many manufacturers provide compiled code in PSCAD Source: EPRI
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Ground Fault Overvoltage
Inverters control scheme identifies and arrest conditions outside pre-determined thresholds. – Transformer can impact inverter fault identification.
Symmetrical Components: Inverter (current source mode) – Positive sequence: constant current source.
• Phase inductor can be ignored. – Negative sequence: Dependent on control philosophy
• Can range from phase inductor to infinity. • May actively cancel negative sequence currents
– Zero sequence:• w/o neutral connection: Open circuit (more common) • w/ neutral connection: Dependent on control philosophy, can range from
phase inductor to infinity.– Not valid under saturated condition (voltage source at limited output)
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Ground Fault Overvoltage
Sym. Components: Load– +/-/0: Shunts– Depending on the
load type: Commercial, Residential, Motor, Service transformer
I1: pre-fault value Z2: Variable, worst case ZGT: Typically open circuit
ZL1=ZL2= V2/Sload
ZL0: Critical & Load dependent
I1 ZL1V1
ZL2V2
ZL0V0
Z2
ZGT
Positive sequence
Negative sequence
Zero sequence
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Ground Fault Overvoltage
How good the system is grounded? Effectively grounded:
– Grounded through a connection of sufficiently low impedance (inherent, intentionally added or both) that a ground fault that may occur cannot build up voltages in excess of limits established for apparatus, circuits, or systems so grounded.
– Coefficient of Grounding is the ratio of the highest line-ground voltage during a fault to the line-line voltage with fault removed (location).
– COG does not exceed 80%; i.e. 0.8ꞏ√3 = 1.386. • ratio of the zero-sequence reactance to the positive-sequence reactance
(X0/X1) is positive and < 3, and • ratio of the zero-sequence resistance to the positive-sequence reactance
(R0/X1) is positive and < 1.
Source: IEEE
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Non-fault Situations
High Inverter Output to Load ratios– For an isolated DER and load an overvoltage can result if the source
power output exceeds the load demand. – Load Rejection Overvoltage (LROV)– Constant current (source) x Constant impedance (load)
• Grounding is irrelevant– Compounded with ground fault may result in TOV, where the sequence
components become unsuitable, and time domain analysis including emtp-type studies will be needed.
Source: IEEE
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Impact of supplemental ground Pre-fault
Post-fault– α: ratio of pre-fault
generationto connected load
Ground Fault Overvoltage
ZL
V0
V2
αZL1
αZL0 Load solidly groundedZGT
αZL20→∞ Z2
V1
V
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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.60
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Z2 = InifinityZ2 = 0.02+j0.3Z2 = 0.1+j1
Generation/Load ratio (alpha)
Max
. Pha
se v
olta
ge (p
u)
Maximum phase voltage vs. pre-fault current
Excess generation Can be compounded
with load rejectionovervoltage (LROV)– Inverter controls
only the LROV
Load pf = 1.0 GT = Infinite
Ground Fault Overvoltage
1.13
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Non‐effective
Effective
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 11
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
pf = 1pf = 0.9 lagpf = 0.9 lead
Ratio of Grounded load
Max
. Pha
se v
olta
ge (p
u)
Maximum phase voltage vs. Ratio of grounded load
Load = Generation Impact of load pf on TOV
– Over-compensation– Ungrounded load
– Z2 = 0.1+j1– GT = Infinite
Ground Fault Overvoltage
1.39 = 0.8 x √3
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Impact of inverter negative sequence
Sensitive for large X2/R2
Load = Generation Pf = 1.0 GT = Infinite
Ground Fault Overvoltage
0.01 0.1 1 10 1000.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
X2/R2 = 10X2/R2 = 1X2/R2 = 0.1
Inverter X2 (pu)
Max
. Pha
se v
olta
ge (p
u)
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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.60
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
w/o GT, Load pf=1.0w/GT, Load pf=1.0w/GT, Load pf = 0.9 lagw/GT, Load pf=0.9 leadw/o GT, Load pf=0.9 lead
Generation/Load ratio (alpha)
Max
. Pha
se v
olta
ge (p
u)
Impact of supplemental ground
XGT = 0.6 pu– X/R = 4
Z2 large– Active cancelation
TOV increases with GT
Depends on pf
Ground Fault Overvoltage
1.1
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Impact of supplemental grounding Grounding may
increase TOV
Load = Generation Pf = 1
Ground Fault Overvoltage
0.1 1 10 1000.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Z2 = 0.02+j0.3Z2 = 0.1+j1.0Z2 = Infinite
Inverter X0 (pu)
Max
. Pha
se v
olta
ge (p
u)
GT impedance (pu)
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Additional Cases
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Other Situations
Constant Power Regulation– Fault normally causes a decrease in the voltage.– If the source regulates power the current increases to maintain
reference power level.– Thus it can increase the un-faulted phase voltages. – Practically constant power control is implemented as an outer loop,
hierarchical, and is sufficiently slow. • Thus the source will trip.
– In case of faster scheme, feed-forward, it can cause voltage rise proportional to the allowable current; i.e. 110-120%
Source: IEEE
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Load Unbalanced– The simple sequence network is not valid for substantial phase
unbalance under islanded mode. • Detailed network or time-domain analysis
Zero-sequence Isolation– COG =100%
• Full neutral shift– Load < 125% Inverter P
• Mitigation needed
Other Situations
ZT0
• V=√3I1ZL• V2 = 0• V1=‐V0
ZLV1
ZLV2
V0
Z2
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Single-phase inverters may not be aware of other phases– 120° separation not guaranteed
Load balanced and grounded star connected. – No inter-phase coupling– No overvoltages during ground fault
Banks of single-phase sources w/o coordinated control
ZY
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Single-phase inverters may not be aware of other phases– 120° separation not guaranteed
Load balanced, but not grounded – Post-fault the current sources of un-faulted phases will be in-phase – Un-faulted phase voltage IsourceꞏZY = IsourceꞏZΔ/3– If pre-fault output exceeds ⅓ of the load, sources reach their output
voltage limits
Banks of single-phase sources w/o coordinated control
ZY=ZΔ/3 ZΔ
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Single-phase inverters may not be aware of other phases– 120° separation not guaranteed
Load balanced, but not grounded– Post-fault the current sources of un-faulted phases will be in-phase. – Un-faulted phase voltage IsourceꞏZY = IsourceꞏZΔ/3– If pre-fault output exceeds ⅓ of the load, sources may reach their output
voltage limits
Banks of single-phase sources w/o coordinated control
ZY ZY
ZY ↓I=0
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Mitigation measures
Effective grounding– COG is the measure
Adequate load relative to Inverter Output– Supplemental ground may be needed
Coordinated transfer trip– Inverter disconnected before feeder opens
Fast Inverter overvoltage tripping– Inverter may not see TOV on primary side
Fast islanding detection Sacrificial arrester
– Difficult
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Conclusion
Ground fault on inverter-based systems can cause TOV Symmetrical component analysis can provide practical insight Sequence impedance of the Inverter and load can impact the
resultant TOV Time domain simulation, including inverter control scheme, provides
better resolution Inductive supplemental grounding can increase TOV Islanding can compound TOV due to mismatch between DER power
to connected load, LROV, worsens under reverse power flow Better modelling should be used for planning studies
Confidential & Proprietary | Copyright © 2019 Slide 31
References
1. IEEE Std. C62.92, “Guide for Application of Neutral Grounding in Electrical Utility Systems”, Parts I & VI
2. IEEE Std. 1313.1, “Standard for Insulation Coordination”3. UL-1742, “Standard for Inverters, Converters, Controllers and Interconnection System
Equipment for Use With Distributed Energy Resources” 4. IEEE Std. 1547, “Standard for Interconnecting Distributed Resources with Electric Power
Systems” 5. NERC, “Recommended Practices for Modeling Momentary Cessation”
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