GE Multilin DTP-B, Инструкция по эксплуатации

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3. OPERATING PRINCIPLES
3-2 DTP-B Digital Transformer Protection GEK-106243E
This permits the harmonic restraint principle used in this relay to distinguish between faults and transformer
inrush. The ability to select the magnitude of percent harmonic restraint permits the adaptation of this protection to
any type of power transformer.
The last term, S, represents the sensitivity of the relay. This is the differential current at which the relay will trip in
the absence of through current and harmonics. An operation occurs If the combination of the three factors, one
operate and two restraints, is a positive value greater than the preset sensitivity.
3.2 MEASUREMENT ALGORITHMS
The DTP includes a series of measurement functions in addition to the protection and control functions. The
differential current and through current in each winding of each phase are measured and displayed.
The DTP is a digital microprocessor-based relay and performs all signal processing via software. It performs
simultaneous sampling (necessary to correctly measure the phase of the signals) of all the signals at a rate of 16
samples per cycle using an analog to digital converter of high precision and resolution. This provides
unprecedented benefits which permit among other things:
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High resolution for the protection settings.
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A very precise measurement in a wide dynamic range.
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Phase shift compensation within the relay.
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A zero-sequence filter to avoid undesired trips in certain situations.
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High calibration stability which avoids the need for periodic recalibrations.
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Self-checking.
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Possibility to include oscillography.
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Possibility of hourly clock set.
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Expanded setting ranges for taps, instantaneous levels, sensitivity, harmonics and percentage restraint such
this relay can be applied to any type of transformer.
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Low burden.
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Permits the inclusion of non-linear algorithms which avoid undesired trips in some situations, especially during
inrush with or without load.
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Makes possible a level of integration superior to that of analog models.
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The division of protection and communications functions provides an additional level of redundancy.
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Fault reports.
3.2.1 DIFFERENTIAL CURRENT
The differential current is defined as the difference of the restraint currents. For a two winding transformer, as
illustrated in Figure 7, the differential current is I1 - I2. From here on, all currents will be expressed as a multiple of
relay tap. The fundamental component (50 or 60 Hz depending on the setting) of the differential current is
obtained via a Discrete Fourier Transformation (DFT). The DFT provides excellent frequency filtration, making the
differential current measurement immune to noise, DC components, and wave distortion. This gives the digital
DTP relay a clear advantage over other analog and hybrid (analog measurement and microprocessor support for
other functions) models.
This is illustrated in Figure 8 which is the relay block diagram. The differential current passes through a digital filter
which obtains the fundamental component (performed by the DFT). Note that the instantaneous trip function uses
the fundamental component of the differential current. This avoids undesired trips in certain situations and
provides a high level of reliability and selectivity if the instantaneous trip level is properly adjusted.