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Detection of Inrush Current and Overexcitation Condition (contd..) |
| (1) |
Harmonic Restraint (contd..) |
| 3. |
 [For restraining inrush current] |
| |
(Where  is the magnitude of second harmonic) |
| 4. |
 [For restraining overexcitation]
|
| |
(Where  is the magnitude of fifth harmonic) |
| |
Typically,  and  are the percentage harmonic restraint and would depend upon type of transformer and steel. For numerical relays, the design of anti aliasing filters also affects the choice of above parameters. During any transient (including the fault condition), harmonics develop much more rapidly than the fundamental and hence, typically numerical relays are restrained for about a cycle indirectly by these transients. Typical setting for used in practice are 10, 20 or 30%. |
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In an integrated substation protection system, it is reasonable to assume availability of bus voltage measurement for transformer protection without extra cost of VT. Then, voltage measurement can be used to restrain the operation of differential protection scheme on inrush or overexcitation. Traditionally, this has been referred in literature as “tripping suppressor” as it suppresses tripping function. If the voltage signal is high, the relay is restrained if |
or 
or 
