ABB ret650, Technical Manual

Page: 229 / 670

Table 151: OEXPVPH Non group settings (basic)
Name Values (Range) Unit Step Default Description
GlobalBaseSel 1 - 6 - 1 1 Selection of one of the Global Base Value
groups
VoltConn Pos Seq
UL1
UL2
UL3
UL1L2
UL2L3
UL3L1
- - Pos Seq Selection of measured voltage
Only PosSeq or UL1L2 should be selected for the VoltConn setting.
9.4.6 Monitored data
Table 152: OEXPVPH Monitored data
Name Type Values (Range) Unit Description
TMTOTRIP REAL - s Calculated time to trip for
overexcitation, in sec
VPERHZ REAL - V/Hz Voltage to frequency ratio
in per-unit
THERMSTA REAL - % Overexcitation thermal
status in % of trip level
9.4.7 Operation principle
The importance of Overexcitation protection (OEXPVPH) function is growing as the
power transformers as well as other power system elements today operate near their
designated limits most of the time.
Modern design transformers are more sensitive to overexcitation than earlier types.
This is a result of the more efficient designs and designs which rely on the
improvement in the uniformity of the excitation level of modern systems. If an
emergency that causes overexcitation does occur, transformers may be damaged
unless corrective action is taken. Transformer manufacturers recommend an
overexcitation protection as a part of the transformer protection system.
Overexcitation results from excessive applied voltage, possibly in combination with
below-normal frequency. Such condition may occur when a transformer unit is
loaded, but are more likely to arise when the transformer is unloaded, or when a loss
of load occurs. Transformers directly connected to generators are in particular danger
to experience overexcitation condition. It follows from the fundamental transformer
equation, see equation
76 , that peak flux density Bmax is directly proportional to the
induced voltage E, inversely proportional to frequency f, and turns n.
1MRK 504 135-UEN A
Section 9
Voltage protection
223
Technical manual