Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-75
AP
Switchgear Maintenance - Excessive operation resulting from short reclaim times can
mean shorter maintenance intervals
The Reclaim Time setting is generally set greater than the tZ2 distance zone delay
4.2 Current transformer supervision
MiCOM
P54x has two methods of detecting CT problems: Differential CTS and Standard
CTS as detailed in the Operation Section. Both methods could be applied simultaneously
and that is a significant advantage because all protection elements that would tend to
operate upon CT failure will be successfully prevented from operating. The Differential CTS
method should be used whenever differential protection is used, whilst the Standard CTS
stabilizes all local ground protections. Therefore, to be on a safe side and cover all
eventualities the recommended setting is ‘I diff + Standard CTS’ mode. Note that the ‘CTS
Reset mode’ and ‘CTS Time delay’ are common for both techniques.
4.2.1 Standard CTS
The re
sidual voltage setting, CTS Vn< Inhibit and the residual current setting, CTS n> set,
should be set to avoid unwanted operation during healthy system conditions. For example
CTS Vn< Inhibit should be set to at least 120% of the maximum steady state residual
voltage. The CTS n> set will typically be set below minimum load current. The time-
delayed alarm, CTS Time Delay, is generally set to 5 seconds.
Where the magnitude of residual voltage during a ground/earth fault is unpredictable, the
element can be disabled to prevent protection elements being blocked during fault
conditions.
Standard CTS must not be used to inhibit the operation of Current differential protection as
this is a local supervision and therefore it will not be fast enough to inhibit the operation of
the differential protection at the remote end.
4.2.2 Differential CTS
Phase Is1
CTS setting must be set above the phase current of the maximum load transfer
expected, normally at 1.2 In. This setting defines the minimum pick-up level of the current
differential protection once the current transformer supervision CTS is detected.
CTS i1> setting, once exceeded, indicates that the circuit is loaded. A default setting of 0.1
In is considered suitable for most applications, but could be lowered in case of oversized
CTs.
CTS i2/i1> This setting should be in excess of the worst unbalanced load expected in the
circuit under normal operation. It is recommended to read out the values of i2 and i1 in the
MEASUREMENT 1 column and set the ratio above 5% of the actual ratio.
CTS i2/i1>> We strongly recommend to keep the default setting (40% In). If the ratio i2/i1
exceeds the value of this setting at only one end, the CT failure is declared.
Note: The minimum generated i2/i1 ratio will be 50% (case of one CT
secondary phase lead being lost), and therefore setting of 40% is
considered appropriate to guarantee sufficient operating speed.
Any of the previous methods will always block protection elements operating from derived
quantities: Broken Conductor, Earth Fault and Neg Seq O/C. Other protections can be
selectively blocked by customizing the PSL, gating DDB 928: CTS Block (originated by either
method) or DDB 929 CTS Block Diff with the protection function logic.
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