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.

▪ 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

▪ 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

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.

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

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

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

Isolated Network - Capacitive Ground Fault The “Healthy Feeds the Faulty” Principle

Compensated Network – Resistive Ground Fault Inductive Current Cancels Capacitive Current

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

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

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

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

Setting Relay Characteristic AngleAdjust According to Method of Neutral Point Grounding

V0

Isolated neutral

(ⱷ RCA = 0°)

Compensated neutral

RLL

Design Flexibility – Retract Forward Operating Area

X

IOPMin forward angle

VPOL

(ⱷ RCA =°)

X

Max forward angle

Retracted Operating Area Phase Angle Characteristics

Design Flexibility – Extend Operating Area

IOP

Min forward angle

- VPOL

(ⱷ RCA = 0°)

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)

Example – Colorado Highland Wind Farm

(David Martinez/Journal-Advocate)

Event Report – Comtrade FilesFrequent Mis-operations

Simplified One Line Diagram –Oscillography Report

X

231°

VA

IA

Simplified One Line Diagram Wind Farm Collector Circuit and Relay

Relay 67/50P-1 Directional Settings Traditional Operate Area vs. Retracted Operate Area

Test Result – Directional Element ModuleTraditional Operate Area vs. Retracted Operate Area

Modeling in Protection Design SoftwareOne line Diagram

Modeling - Impedance Trajectory PlotEffect of Changing Utility Voltage (0.95 -1.05 pu) for Active Power Levels of 0, 20 & 80%

Modeling in Protection Design SoftwareRelay Setting Interface for Retracted Operating Area

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

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

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

Example – Refsdal Distribution System, Norway

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.

One Line Diagram

Modeling in Protection Design Software One line Diagram

(Reverse)

X

X

(Forward)

Relay

Parallel Resistors (Rp1 & Rp2)

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

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

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)

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)

Simulation Results – Fault StudyExtended Operating Area

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