EFFECT OF POOR POWER QUALITY ON TRANSFORMERS BASIC INFORMATION
What Are The Effects Of Poor Power Quality To Transformers?
Presence of harmonic current increases the core losses, copper losses, and stray-flux losses in a transformer. These losses consist of ‘no load losses’ and ‘load losses’. No load loss is affected mainly by voltage harmonics, although the increase of this loss with harmonics is small. It consists of two components: hysteresis loss (due to non-linearity of the transformers) and eddy current loss (varies in proportion to the square of frequency).
The load losses of a transformer vary with the square of load current and increase sharply at high harmonic frequencies. They consist of three components:
• Resistive losses in the winding conductors and leads
• Eddy current losses in the winding conductors
• Eddy current losses in the tanks and structural steelwork
Eddy current losses are of large concern when harmonic current is present in the network. These losses increase approximately with the square of frequency. Total eddy current losses are normally about 10% of the losses at full load. Equation (1) gives total load losses (PT) of a transformer when harmonics are present in the network [Hulshorst & Groeman, 2002].
Where,
PCU = total copper loss
PWE = eddy current losses at 50Hz (full load)
PCE1 = additional eddy current losses at 50Hz (full load)
PSE1 = stray losses in construction parts at 50Hz (full load)
In = rms current (per unit) at harmonic ‘n’
IL = total rms value of the load current (per unit)
I1 = fundamental component of load current (per unit) at 50Hz frequency
n = harmonic number
Other concern is the presence of ‘triple-n’ harmonics. In a network, mainly the LV nonlinear loads produce harmonics. With a MV/LV transformer of Δ/Y configuration, ‘triple-n’ currents circulate in the closed delta winding. Only the ‘non triple-n’ harmonics pass to the upstream network.
When supplying non-linear loads, transformers are vulnerable to overheating. To minimize the risk of premature failure of transformers, they can either be de-rated or use as ‘K-rated’ transformer which are designed to operate with low losses at harmonic frequencies. Increased loading can cause overstressing of transformer and the chance of its premature failure.
This effect is usually expressed in terms of ‘loss of lifetime’. The hot-spot temperature is used for evaluation of a relative value for the rate of thermal ageing as shown in Fig. 4.
It is taken as unity for a hot-spot temperature of 98oC with the assumption of an ambient temperature of 20oC and hot-spot temperature rise of 78oC. Equation (2) shows the calculation of relative ageing rate (V) as a function of hot-spot temperature θh [Najdenkoski et al., 2007].
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