Second Order Harmonic Inrush Restraint (SOHIR)

SOHIR offers an alternative to the traditional Zero-Detect method of Inrush Restraint explained in Inrush Restraint Operation.

High inrush current is often due to magnetizing current when a transformer is energized.

Magnetizing currents are distinguished by their high second order harmonic content.

SOHIR provides rapid harmonic detection of inrush current based on harmonic content.

Making SOHIR Available

SOHIR is an optional feature and can be made available through WSOS at:

Display > Configuration > Feature Selection > Protection or on the operator interface.

SOHIR Configuration

When SOHIR has been made available, an additional field, Inrush Restraint Type, is added to the Inrush Restraint settings. The setting options for this field are:

Zero Detect
SOHIR

Figure 149
Inrush Restraint Type setting

If SOHIR is selected as the Inrush Restraint Type, two more settings are added:

Inrush Restraint Threshold
Inrush Restraint Algorithm

Figure 150
Inrush Restraint SOHIR settings

SOHIR can be configured using the parameters described in the table below:

Table 44 SOHIR Settings Descriptions

Setting	Default	Description
Inrush Restraint Time	0.10 s	The time Inrush Restraint will remain active once triggered.
Inrush Restraint Multiplier	X4	Setting current multiplier while Inrush Restraint is active
Inrush Restraint Threshold	20%	The 2^nd harmonic component of current on the line (expressed as a percentage of the fundamental component) required for SOHIR to activate.
Inrush Restraint Algorithm	AND	Configured for either AND or OR. In the case of AND logic, all phases must have harmonics above the threshold setting, in order for SOHIR to become active. In the case of OR logic, only the phase with the highest current needs to have harmonics above the threshold setting.

SOHIR can also be configured on the operator interface.

SOHIR is configured per protection group.

SOHIR Operation

SOHIR monitors pickups by either Phase, Earth or NPS protection elements.

Once a pickup is detected, SOHIR restrains all overcurrent protection trips from the Phase, Earth or NPS elements (except those using Definite Time curves) for 30 ms.
During this 30 ms time period, SOHIR calculates the harmonics on the faulted phases as a percentage of the fundamental component.
If one or more phases has a second harmonic component above the Inrush Restraint Threshold setting, a decision is made on whether to become active based on the Inrush Restraint Algorithm.
If SOHIR does become active it will restrain all overcurrent trips from the Phase, Earth and NPS element for the configured Inrush Restraint Time.

SOHIR will be deactivated if either:

Harmonics measured during the 30 ms period are below the threshold.
The Inrush Restraint timer has expired.

Once SOHIR has been deactivated it cannot reactivate until all relevant protection elements have reset.