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Directional Element Design for
Protecting Circuits with
Capacitive Fault and Load
Currents
Mike Benitez, P.E., EPSII, Joe Xavier, ABB Inc.
Karl Smith, P.E., David Minshall, ABB Inc.
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▪ ETAP – Irvine CA
▪ Mohammad Zadeh, Ph.D., SMIEEE, PE
▪ Sajal Jain, PE
▪ Norwegian University of Science and Technology (NTNU)
▪ Anniken Liland Fredriksen
▪ Professor Hans Kristian Hoidalen
Acknowledgements
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▪ When do capacitive loads and faults occur? Why do they
cause directional elements to mis-operate?
▪ History of mitigation techniques and their disadvantages
▪ Measurement conventions – VAR flow in wind farm
collector circuits. Isolated and compensated networks
▪ Introduce flexible directional element design features
▪ Importance of modeling and simulations…
▪ Colorado Highlands wind farm example – Retracted
operating area to prevent mis-operation for capacitive load.
▪ Refsdal distribution system example –Extended Operating
area to prevent mis-operations for capacitive faults.
▪ Future work and conclusion
Overview
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When do Capacitive Loads and Faults Occur ? Why traditional directional Elements Mis-Operate?
▪ Capacitive Loads – Occurs when Type 3 & 4 wind turbine
generators consume VARS to bring down terminal voltage
at the utility. This will cause traditional directional elements
to mis-operate for the following reasons:
▪ Load condition appears as a fault. Generating point
enters forward operating area of directional element
▪ Over-current elements set very sensitive (≈ 120% of
maximum auxiliary load).
▪ Capacitive Faults – Occur in isolated networks or in a
compensated network (reverse direction) connected in
parallel to an isolated network.
▪ Two settings groups required to prevent mis-operation.
Not always practical.
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Mitigation Techniques – Historical Disadvantages
▪ Load Encroachment – An impedance based solution to
block trip in forward directional operating area outside
expected fault region. Various methods documented
▪ Numerous settings and logic required to adapt to wind
farm collector circuit applications. Not built-in to
directional element
▪ Pick-up (≈ 120% of max aux load) must be set above
10% of CT secondary in relay. Positive sequence only
▪ Skewing MTA – Lowering MTA to rotate forward operate
area away from capacitive region.
▪ Z1ANG angle setting limited 5º lagging. Does not
provide fault coverage for highly capacitive loads
especially if measurement error is introduced
▪ Prevents blocking in first quadrant (upper right) of
phasor plot, and therefore does not cover the entire
range of possible load generating angles
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Mitigation Techniques – Historical (Continued) Disadvantages
▪ Reverse Power Element Supervision – Allows trip only
when power flowing into wind turbine generator. Blocks trip
when power is flowing out of wind turbine generator.
Drawbacks similar to skewing the MTA.
▪ Additional settings and logic required. Not built-in to
directional element
▪ Does not block in first quadrant (upper right) of phasor
plot, and therefore does not cover the entire range of
possible load generating angles
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Capacitive Load – Wind Turbine Collector Circuits VAR Conventions – Phasor and Impedance Diagrams
Phasor Diagram Impedance Diagram To
Utility
Collector Bus
Relay
VT
Forward
Power Flow
I1
MTA
VARS IN (Collector Bus)
VARS IN (WTG)
V1Watts OUT
(Collector Bus)Watts OUT
(WTG)
Capcitive Load Direction
Watts OUT (Collector Bus)
Watts OUT (WTG)
VARS IN (WTG)
VARS IN (Collector Bus)
R
XZ1
MTA
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Isolated Network - Capacitive Ground Fault The “Healthy Feeds the Faulty” Principle
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Compensated Network – Resistive Ground Fault Inductive Current Cancels Capacitive Current
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Over-compensated Network Changing Reference from Vo to -Vo
I0
REV REGION
FWD REGION V0
(Over-compensated)
V0 Reference - V0 Reference
FWD REGION
REV REGION - V0
(Over-compensated)
I0
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New Flexible Directional Element Design - Features
▪ Modes of Operation – Set operate criterion to desired
application
▪ Phase angle – Operate sector defined by Min/Max
forward and reverse angle settings relative to Relay
Characteristic Angle
▪ I0Sin – Uses reactive component of operate current in
isolated networks
▪ I0Cos – Uses active component of operate current in
compensated networks
▪ Relay Characteristic Angle – Adjust direction element
operation according to method of single point grounding.
▪ Used in ‘Phase angle’ mode of operation
▪ 360º setting range
▪ Retract or Extend Operating Area - Simply accomplished
using Min/Max forward and reverse angle settings
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Modes of Operation –Phase Angle Criterion
‘Phase angle’
‘I0Sin’
‘I0Cos’
-V0
ⱷ RCA = - 90°
ⱷ
I0Sinⱷ
Non-operating area
Correction angle
I0
Forward operate
area
Reverse operate
area
Min operating current
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IOP
IOP
- VPOL
Design Flexibility – Relay Characteristic AngleAdjust According to Method of Neutral Point Grounding
ⱷ RCA = -90°
(ⱷ RCA = 0°)
Compensated neutral
RLL V0
Isolated neutral
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Setting Relay Characteristic AngleAdjust According to Method of Neutral Point Grounding
V0
Isolated neutral
(ⱷ RCA = 0°)
Compensated neutral
RLL
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Design Flexibility – Retract Forward Operating Area
X
IOPMin forward angle
VPOL
(ⱷ RCA =°)
X
Max forward angle
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Retracted Operating Area Phase Angle Characteristics
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Design Flexibility – Extend Operating Area
IOP
Min forward angle
- VPOL
(ⱷ RCA = 0°)
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Extended Operating Area Phase Angle Characteristics
-V0
I0 Max forward angle = 80°
Min forward angle = 170°
Min reverse angle
Relay characteristic
angle ( RCA) = 0°
Min operate current
Reverse operate
area
(Over- compensated)
Forward operate
area
(Reverse fault – Compensated/Isolated)
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Example – Colorado Highland Wind Farm
(David Martinez/Journal-Advocate)
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Event Report – Comtrade FilesFrequent Mis-operations
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Simplified One Line Diagram –Oscillography Report
X
231°
VA
IA
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Simplified One Line Diagram Wind Farm Collector Circuit and Relay
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Relay 67/50P-1 Directional Settings Traditional Operate Area vs. Retracted Operate Area
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Test Result – Directional Element ModuleTraditional Operate Area vs. Retracted Operate Area
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Modeling in Protection Design SoftwareOne line Diagram
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Modeling - Impedance Trajectory PlotEffect of Changing Utility Voltage (0.95 -1.05 pu) for Active Power Levels of 0, 20 & 80%
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Modeling in Protection Design SoftwareRelay Setting Interface for Retracted Operating Area
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Simulation Results – Load Flow Study (0 Active Power)Most Secure Operating Area – Blocks all generating Points Outside Expected Fault Region (0 - 85º lagging)
Phasor Angle CharacteristicsImpedance Plot
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Determining if WTG Load will Enter and Pick-up in the Expected Fault Region
▪ False trips are not inevitable. It all comes down to the
following two questions:
▪ What is the minimum active power (low wind) of the
WTG generation unit while producing maximum
capacitive current
▪ What is the maximum auxiliary load consumed by
the wind generation unit (modeled at 10% of rated
MVA )
Sandia National Laboratories
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Options if Load Pickups in Expected Fault Region
▪ The following actions can be taken to prevent mis-
operations when load flow studies indicate generation point
will pick-up in the expected fault region:
▪ Increase overcurrent pick-up (if setting based on
120% of auxiliary load which is most sensitive) until
simulation shows no operation for load in expected
fault region. Then check if relay can see faults at most
remote lateral or low side of WTG step-up
transformer. If pick-up above 120% of total rated WTG
load then over-current protection can be non-
directional
▪ Re-evaluate minimum active power operating limit
▪ This condition is based on theoretical results from the
load flow study. The likelihood of operating at zero or low
active power is unknown and requires further investigation
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Example – Refsdal Distribution System, Norway
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Refsdal Distribution System Description and Operation
▪ Owned by energy producer Statkraft
▪ 66/22 kV Auto-transformer is rated 6 MVA
▪ Normally operates with an isolated transformer neutral.
▪ In case of an emergency the system is connected in
parallel to a compensated system known as ‘Hove’
owned by a local distributor Sognekraft
▪ Coil set to 5% overcompensated
▪ Two parallel resistors Rp1 and Rp2.
▪ When Vo exceeds 10% of phase voltage:
▪ Rp1 is disconnected after a 1.5 s delay then…
▪ Rp2 is connected 2 s later.
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One Line Diagram
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Modeling in Protection Design Software One line Diagram
(Reverse)
X
X
(Forward)
Relay
Parallel Resistors (Rp1 & Rp2)
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Directional Relaying Philosophies Existing Solution
▪ Using only one mode of operation for directionality is not
possible since the method of neutral point grounding
changes
▪ When the system is operated isolated, faults in both
directions are capacitive
▪ If connected to the compensated system, then faults in
the forward direction are resistive (with inductive
component due to over-compensation), and faults in the
reverse direction are capacitive.
▪ Existing solution uses two setting groups, one for the
isolated system (I0Sin) and one for the compensated one
(I0Cos).
▪ Not always practical since the grounding facility may be
located several kilometers from the substation
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Directional Relaying Philosophies Proposed Solution
▪ Idea proposed in a master’s thesis by Anniken Liland
Fredriksen from the Norwegian University of Science and
Technology (NTNU) to apply the same settings for both
isolated and compensated systems
▪ Thesis Titled ‘Earth fault protection in isolated and
compensated power distribution systems’.
▪ Solution was to extend the forward operating set for a
compensated system (RCA = 0°) to allow fault detection
in an isolated system
▪ System data and recommended relay settings in thesis
modeled in protection design software similar to ATPDraw, a
program developed by professor Hans Kristian Hoidalen of
NTNU
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Fault Study – Modeling Considerations
▪ The impact of configuration change in an emergency
condition as well as parallel resistor selection on directional
protection are studied. The following was taken into
account:
▪ The shunt capacitance of the lines and cables were
assumed transposed
▪ Capacitive discharge (due to unbalance) was not taken
into account
▪ For conductive discharge the recommended value of
15% of capacitive discharge from the thesis was used
▪ Zero fault resistance (ZF = 0)
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Proposed Solution Recommended Settings and Phase Angle Characteristics
-V0
I0 Max forward angle = 80°
Min forward angle = 170°
Min reverse angle
Relay characteristic
angle ( RCA) = 0°
Min operate current
Reverse operate
area
(Over- compensated)
Forward operate
area
(Reverse fault – Compensated/Isolated)
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Simulation Results – Fault StudyExtended Operating Area
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Conclusion and Future Work
▪ Two applications addressed where capacitive loads and
faults are problematic for traditional directional elements
▪ Overcoming these challenges required a new way of
thinking with respect to how directional elements are
designed and the systems they are protecting are modeled.
▪ From design standpoint, extending and retracting the
operating area using the flexible Minimum/Maximum
forward and reverse angle settings, and adjusting the RCA
to accommodate the method of neutral point grounding are
the key factors.
▪ From Modeling Standpoint, Advancements in protection
design software now allow for seamless integration of
relays, using custom relay setting interfaces, and
components, such as wind turbine models, to run the
necessary simulations to ensure directional security.
▪ Likelihood of zero or low power conditions require further
investigation