DOCUMENT FOR SOTF PROTECTION FINAL

1. 2012
G EKANANDHAN
ABB
8/19/2012
SWITCH ONTO FAULT PROTECTION

2. SWITCH-ON-TO-FAULT PROTCTION


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TABLE OF CONTENTS

1.Purpose

2. Introduction

3.SOTF scheme Application

4.Present logic in REL670 realay
4.1.SOTF Enable logic
4.2.internal dead line detection logic
4.2.1.ULLevel mode logic
4.2.2.UILvl&imp mode logic
4.2.3.impedance mode logic
4.2.4.Merits of this method
4.2.5.Limitation of this method
4.3.Using External trigger method
4.3.1.ULLevel mode logic
4.3.2.UILvl&imp mode logic
4.3.3.impedance mode logic
4.3.4.Merits of this method
4.3.5.Limitation of this method

5.Model of SOTF Application Configuration

6.Different kind of External sotf initiation logic

7.Conclusion

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1.PURPOSE

“When determining the switch on to fault element settings, each utility
Shall verify that the schemes will not operate for emergency loading
conditions.”

This document is intended to provide transmission protection system owners with guidance for
the
review of existing switch-on-to-fault schemes to ensure that those schemes do not operate for
non-switch-on-to-fault conditions or under heavily stressed system conditions. This document
also provides recommended practices for application of new switch-on-to-fault schemes.
2.INTRODUCTION

Switch-on-to-fault (SOTF) schemes are protection functions intended to trip a transmission line
breaker when closed on to a faulted line. Dedicated SOTF schemes are available in various
designs, but since the fault-detecting elements tend to be more sensitive than conventional,
impedance-based line protection functions, they are generally designed to be “armed” only for a
brief period following breaker closure.
3.SOTF SCHEME APPLICATIONS

SOTF schemes are applied for one or more of two reasons:

1. When an impedance-based protection scheme uses line-side voltage transformers, SOTF
logic is required to detect a close-in, three-phase fault to protect against a line breaker
being closed into such a fault. A phase impedance relay whose steady-state tripping
characteristic passes through the origin on an R-X diagram will generally not operate
if there is zero voltage applied to the relay before closing into a zero-voltage fault. This
condition most often occurs when a breaker is closed into a set of three-phase Earth
switch which operations/maintenance personnel failed to remove prior to re-energizing
the line.
2. When this occurs in the absence of SOTF protection, the line protection will not operate
to trip the breaker, nor will breaker failure protection be initiated, possibly resulting in
time-delayed tripping at numerous remote terminals. Unit instability and dropping of
massive blocks of load can also occur.

Note: SOTF current fault detector pickup settings must be low enough to allow positive
detection of close-in three-phase faults under what is considered to be the “worst case”
(highest) impedance to the source bus.

3. When an impedance protection scheme uses line-side voltage transformers, SOTF current
fault detectors may operate significantly faster than impedance units when a breaker is
closed into a fault anywhere on the line. The dynamic characteristics of typical
impedance units are such that their speed of operation is impaired if polarizing voltages
are not available prior to the fault.

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By considering the above three point SOTF protection should be implemented on all the line
protection
And let us see such a protection how to implement in numerical relay and its method

4.PRESENT LOGIC IN OUR REL670 RELAY:
The switch-onto-fault function (SFV) can be activated externally by Breaker Closed Input or
internally (automatically) by using UI Level Based Logic see page no: 6. The activation from
the DLD function is released if the internal signal DeadLine from the UILevel function is
activated at the same time as the input ZACC is not activated during at least for a duration tDLD
and the setting parameter AutoInit is set to On.
The operation of SOTF has three different modes for defending the criteria for trip:
i.Impedance
ii.UILevel
iii.Both
i.Impedance
1.When Mode is set to Impedance, the operation criteria is based on binary coded information
from impedance zone measurement. A non directional output signal should be used from an
overreaching zone (STCND output on the ZMQ block). The selection of Impedance mode gives
increased security.

2. zones will trip without waiting for their usual time delays. Thus tripping can even occur for
close-up three phase short circuits where line connected VTs are used, setting “impedance”
allows instantaneous tripping to occur for all faults within the trip characteristic

ii.UILevel
When mode is set to ULLevel the tripping criteria is based on the setting of IPh< and UPh<. The
choice of UILevel gives faster and more sensitive operation of the function, which is important
to reduce the stress that might scours when energizing onto a fault.

iii. UILvl&Imp
when the mode is set to UILvl&Imp then trip is initiated based on impedance measured criteria
or UILevel detection


Above mentioned all three mode of SOTF operation is initiated by two method which is
explained below:

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4.1 SOTF Enable logic is activated in 2 cases

Let us see the two different type of SOTF initiation method as well logics

Case1: If no external closing command (manual or by remote communication via control
system) is present then the internal levels detectors(based on current and voltage) have detected a
three pole open for more than 60 s (Settable in relay parameter tDLD) as soon as all poles are
closed, then SOTF is enabled for tSOTF(approx 500 ms) and then reset,

Case 2: When an external closing command (manual or by remote communication via control
system) is present: the SOTF logic is activated immediately. As soon as all the poles are closed
(after the
external closing order ); SOTF is enable for tSOTF(approx 500 ms) and then is reset.


Note: Why SOTF function is triggered by non directional zone element:

When line-side potential transformers are used, the use of non-directional distance zones secures
switch-on-to-fault tripping for close-in three-phase short circuits. Use of non-directional distance
zones also gives fast fault clearance when energizing a bus from the line with a short-circuit
fault on the bus.
4.2 Internal dead line detection principle:

Principle: if all three phase currents and voltages are below the setting IPh< and UPh< for
more than 60 s then the internal “Internal DeadLine level detector” output is activated(SOTF
will be enable).Logic below
4.2.1.Mode set to ULLevel: Then relay setting and operation logic will be like below

Relay setting mut be like below:

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Logic operation:

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4.2.2.Mode set to UILvl&Imp: Then relay setting and operation logic will be like below

Operation logic:


For this method relay setting should be like below:

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4.2.3. Mode is set to impedance
In this mode during CB energizing if the impedance element detect the fault on line then ZMQ
block output ZMQSTND(ZACC) get high immediately output of Autoinit on no longer persist
even though the drop off timer will enable the SOTF for 500ms and it reset. the moment both
ZACC and Drop OFF timer output goes to 1 then SOTF function will a trip command see the
logic below:

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For this MODE relay setting must be like below:

4.2.4.Merits:
1. Eventhough Goose communication fail between bay control unit and the protection relay
this method will work
2. Multiple breaker arrangement scheme(like 1 ½ breaker) need multiple external input to
trigger the SOTF
3. Safe and secure for three phase close in fault where line voltage input is zero such a
condition phase impedance relay whose steady-state tripping characteristic passes
through the origin on an R-X diagram will generally not operate if there is zero voltage
applied to the relay before closing into a zero-voltage fault.


4.2.5.Limitation :
1. Automatic activation based on dead line detection can only be used when the potential
transformer is situated on the line side of a circuit breaker
2. When an impedance protection scheme uses line-side voltage transformers, SOTF
current fault detectors may operate significantly faster than impedance units when a
breaker is closed into a fault anywhere on the line. The dynamic characteristics of typical
impedance units are such that their speed of operation is impaired if polarizing voltages
are not available prior to the fault.

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4.3. Using External Trigger:

1. 1.When an external closing command by manual TNC swich close command or by close
command from remote SCADA System is present then The SOTF logic is activated
immediately. As soon as all the poles are closed SOTF is enable for tDLD
delay(recommended time 500msec) and then is reset.

Les us the three mode of operation and its logic


4.3.1.Mode is set to Impedance: logic operation will be like this

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For this mode relay setting should be like this:

4.3.2.Mode is set to UILevel : Then the relay setting and operation of SOTF logic will be
like this
relay setting:

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Operation logic:


4.3.3.Mode is set to UILvl&Imp :then relay setting and operation logic will be like below:
Relay setting:

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Operation logic:


4.3.4.Merits
1. It will Enable the SOTF only during actual CB closing otherwise not at all
2. This method is applicable for both line VT as well as bus VT
4.3.5.Limitation
1.It will not work during communication failure between BCU and protection relay(If the
External manual close command as well SCADA command taken through the GOOSE)

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5.Model of SOTF Application configuration :



Why the above logic maloperated during the recent grid failure?
1. 1.In the recent grid grid failure logic was right but the only problem is with the tDLD
timer setting it was 200ms nor
2. tDLD The time delay for activating the SOTF function by the internal dead line detection
is by default set to 0.2 seconds. This is not suitable in most applications. The delay shall
not be set too short to avoid unwanted activations during transients in the system
3. To rectify that issue only tDLD timer setting need to be increased , setting should incresd
to 60s(none of the transient condition will not persist up to one minute) by increasing
the time delay we can avoid the maloperation SOTF during transient fault in the system
4. If powergrid not ready to accept this setting I have made some logic for different
scheme below
5. They need to provide all input which is mentioned on below logic

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6.Different kind of External SOTF initiation logic for different breaker arrangement
scheme:

1.One and half breaker scheme
2.Double bus scheme
3.Double bus transfer scheme


6.1.One and half breaker scheme:

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Note: ** is only applicable where the substation has the SCADA System

6.2.Double bus scheme:

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6.3.Double bus transfer scheme:








7.CONCLUSION
SOTF schemes are an important element of transmission system protection. However, unless
they are carefully applied, they may compromise the ability of the transmission system to
tolerate heavy loading. This paper has presented the relationship between the intended purpose of
SOFT schemes and introduced methods by which SOFT schemes can be made more tolerant of
heavy load conditions.