[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Primary Current (Amps)
60
40
20
0
-20
TABLE 3.8.1 Harmonic Content of the Current Signal Shown in Figure 3.8.3
Frequency Component Magnitude (primary amps) Percentage of Fundamental
Fundamental 22.5 100.0
Third 11.1 49.2
Fifth 4.9 21.7
Seventh 1.8 8.1
-40
-60
0.5 1 1.5 2 2.5
Cycles
FIGURE 3.8.3 Exciting current of an overexcited transformer; overvoltage of 150% applied to a 5-KVA, 230/115-
V single-phase transformer.
• Second harmonics are added at 120.
• Third harmonics are added at 180 (they cancel out), etc.
Sonnemann et al. initially claimed that the second-harmonic content of the inrush current was never
less than 16 to 17% of the fundamental [22]. However, transformer energization with reduced voltages
can generate inrush currents with second-harmonic content less than 10%, as will be discussed later in
this section.
3.8.3.2 Transformer Overexcitation
The magnetic flux inside the transformer core is directly proportional to the applied voltage and inversely
proportional to the system frequency. Overvoltage and/or underfrequency conditions can produce flux
levels that saturate the transformer core. These abnormal operating conditions can exist in any part of
the power system, so any transformer can be exposed to overexcitation.
Transformer overexcitation causes transformer heating and increases exciting current, noise, and
vibration. A severely overexcited transformer should be disconnected to avoid transformer damage.
Because it is difficult, with differential protection, to control the amount of overexcitation that a transformer
can tolerate, transformer differential protection tripping for an overexcitation condition is not
desirable. Instead, use separate transformer overexcitation protection, and the differential element should
not trip for these conditions. One alternative is a V/Hz relay that responds to the voltage/frequency ratio.
Overexcitation of a power transformer is a typical case of ac saturation of the core that produces odd
harmonics in the exciting current. Figure 3.8.3 shows the exciting current recorded during a real test of
a 5-kVA, 230/115-V, single-phase laboratory transformer [24]. The current corresponds to an overvoltage
condition of 150% at nominal frequency. For comparison purposes, the peak value of the transformer
nominal current is 61.5 A, and the peak value of the exciting current is 57.3 A.
Table 3.8.1 shows the most significant harmonics of the current signal depicted in Figure 3.8.3.
Harmonics are expressed as a percentage of the fundamental component. The third harmonic is the most
suitable for detecting overexcitation conditions, but either the delta connection of the CTs or the deltaconnection
compensation of the differential relay filters out this harmonic. The fifth harmonic, however,
is still a reliable quantity for detecting overexcitation conditions.
3
FIGURE 3.8.4 Harmonic content of transformer exciting current as a function of the applied voltage. (From Cooper
Power Systems, Electric Power System Harmonics: Design Guide, Cooper Power Systems, Bulletin 87011, Franksville,
WI, 1990. With permission.)
Einval and Linders [13] were the first to propose using the fifth harmonic to restrain the transformer
differential relay. They recommended setting this restraint function at 35% of fifth harmonic with respect
to the fundamental. Figure 3.8.4 [25] shows the harmonic content of the excitation current of a power
transformer as a function of the applied voltage. As the voltage increases, saturation and the exciting
current increase. The odd harmonics, expressed as a percentage of the fundamental, first increase and
then begin to decrease at overvoltages on the order of 115 to 120%. Setting the differential-relay fifthharmonic
restraint to 35% ensures security for overvoltage conditions less than 140%. For greater
overvoltages, which could quickly destroy the transformer in a few seconds, it is desirable to have the
differential-relay fast tripping added to that of the transformer-overexcitation relay.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC