Switch On To Fault (Sotf); Applications; Description Of Operation - Siemens 7SG164 Ohmega 400 Series Manual

7SG164 Ohmega 400 Series Impedance Functions

1.8 Switch On To Fault (SOTF)

1.8.1 Applications

The Zone 1 instantaneous elements of the OHMEGA distance protection are directional and rely upon
polarisation from either the faulted phase and/or a healthy phase. When closing on to a bolted fault
where all three-phase voltages are extremely low, the Zone 1 instantaneous elements may not
operate. Time delayed operation would occur from either the zone 3 offset element or the high set
overcurrent. This is not acceptable and special precautions are necessary to ensure high-speed
clearance for this condition.
The Switch-On-To-Fault feature ensures that for a short period of time after a CB is closed, the offset
Zone 3 elements and the overcurrent elements are allowed to trip at high speed.

1.8.2 Description of Operation

The mode of Switch-on-to-fault logic required can be selected as either AC SOTF or DC SOTF. AC
SOTF utilises three-phase pole dead logic, based on measured current and voltage (i.e. AC
quantities), to determine the circuit breaker status. The DC SOTF uses an auxiliary contact (i.e. a DC
quantity) on the CB closing handle or circuit breaker to determine when the CB is being closed.
The SOTF output is automatically configured to operate the three-phase trip output. The SOTF output
can be mapped to one of the LED's and to any of the output contacts to give an alarm.
1.8.2.1. AC SOTF
The AC SOTF logic monitors the line current and voltage, and so it can only be used where the
instrument transformers are placed on the line side of the circuit breaker. When the relay detects that
the voltage and current are dead (i.e. voltage below 20% of the nominal, current below the SOTF O/C
Operate Level which can be set from 0.3 – 4.00xIn) on all three phases, this will start the
ACSOTFTIMER. This timer has a settable delay on pickup (the AC SOTF Pickup Delay settable from
0-60000ms) which is used to ensure that the circuit breaker has been switched out for maintenance.
This delay is set by default to 10,000ms, so the breaker must have been open for at least ten seconds
before the SOTF logic is initiated. Once the logic has been initiated the relay can cause a SOTF trip in
one of two ways.
Firstly, after this time delay, if the voltage on one or more phases increases above 20% of the nominal
voltage, the ACSOTFTIMER will reset after a fixed time delay of 200ms. This gives a fixed 200ms
window of operation, during which the relay will allow the Zone 3 element to trip instantaneously.
Secondly, if the relay detects the current increasing above the SOTF O/C Operate Level without the
voltage increasing above 20% the relay will allow the relay to trip on SOTF. The SOTFFLSTIMER puts
an inhibit on the tripping for 25ms to ensure that there is no race condition between the voltage and
current reaching the "live" levels. Also, when the voltage on all three phases rises above 20% the
SOTF Overcurrent tripping criteria is removed instantaneously. This prevents the relay from tripping
for high line charging currents. This overcurrent criterion is essential when a uni-directional Zone 3 is
being used (i.e. No offset), because in this case the zone 3 element will suffer from the same
difficulties as Zones 1 and 2.
1.8.2.2. DC SOTF
The DC SOTF logic works in much the same way as the AC SOTF, but has been specifically designed
for situations where the VTs are mounted on the busbar side of the line circuit breaker. This means
that the voltage input to the relay cannot be used to supervise the position of the breaker. The
difference is the way in which the logic is initiated.
Two options are available depending on the particular Ohmega model, to suit the source of the
initiating signal. Manual Close DC SOTF uses a contact on the CB closing switch. DC AUX SOTF
uses an auxiliary contact on the circuit breaker.
A Status Input defined as 'DC SOTF Manual Close' is connected to the Manual Close handle of the
circuit breaker. This Status Input is triggered on the rising edge of the Manual Close signal, and for a
settable time, 400 ms default, after this signal the relay will allow instantaneous tripping of the Zone 3
©2010 Siemens Protection Devices Limited Chapter 2 Page 19 of 21