Siemens ISGS SG8158-00 User Manual download pdf (Page 44)

Control & Communications

38 Siemens Energy & Automation, Inc.

Figure 6.4 Voltage Balance Threshold

Monitoring is done when the maximum voltage is larger than

the voltage balance pickup value. The voltage is considered

balanced and will not cause an alarm if the voltage min/max

ratio is larger than the voltage balance factor. The voltage is

unbalanced and will cause an alarm if the min/max ratio is

smaller than the voltage balance factor.

Failure of this check will cause an event Voltage Balance

Error. This event can activate an output contact.

The voltage balance pickup value (3402) can be set from 40

to 120 V in steps of 1 V. If one of the three phase voltages is

above the preset threshold, the function checks for balance.

The voltage balance factor indicates the amount of unbal-

ance tolerated before the function generates an alarm

(3404). It ranges from 0.58 to 0.95 and can be set in steps of

0.01.

Current Summation

The Current Summation function can be enabled or disabled

(3411). When enabled, it monitors the instantaneous sam-

ples of the A/D converter using the currents flowing into all

four relay CTs regardless of whether there are four primary

CTs connected or not. The calculation is therefore valid for

systems with both residual connections or explicit neutral/

ground/zero sequence CTs.

Figure 6.5 Current Sum Threshold

Failure of this check will cause an event Current Summation

Error. This event can activate an output contact.

The pickup value (3412) for the current summation check

depends on the secondary phase CT rating and the value is

in secondary amperes. The value for 5 A CTs ranges from

0.5 A to 5.0 A; and the value for 1 A CTs ranges from 0.1 to

1.0 A. Both values can be set in steps of 0.1 A. If one of the

three phase currents is above the preset threshold, the mon-

itoring function is activated.

The current summation factor indicates allowable compen-

sation for differences between primary CTs (3414). It ranges

from 0.1 to 0.95 and can be set in steps of 0.01. This factor

is important under high fault currents or when CTs are oper-

ated closely to their rated current.

Current Balance

The Current Balance function can be enabled or disabled

(3421). When enabled, the function monitors the phase cur-

rents to see if they are approximately balanced (of equal

magnitude). Balance is defined as the ratio of minimum to

maximum current, where the maximum current is the largest

and the minimum current the smallest of the three phase cur-

rents.

Current balance monitoring is done when the maximum cur-

rent is larger than the current balance pickup value. The cur-

rent is considered balanced and will not cause an alarm if the

current min/max ratio is larger than the current balance fac-

tor. The current is unbalanced and will cause an alarm if the

min/max ratio is smaller than the current balance factor.

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