Aaaaaaaaa

Electrical Power Quality and Utilisation, Journal Vol. XI, No. 1, 2005

MEASURING ACCURACY O. ELECTRIC ENERGY
IN HV POWER NETWORKS
Wies³aw JA£MU¯NY

Technical University of Lodz, Poland

Summary: The article presents the effect of secondary circuits of inductive current and voltage
transformers on measuring accuracy in HV power networks. The basic parameters of instrument transformerswere defined, such as errors or rated output. The impedance effect of the
leads connecting secondary terminals with measuring instruments and the impact of burden
power on the energy measuring error were analyzed. The calculation samples illustrate the
procedure when assessing energy (power) measuring error. The algorithm may be applied, with
positive results, by the operating personnel ofmeasuring circuits with instrument transformers.

Keywords:
Electric energy
measuring
Instrument transformers
Measuring accuracy

1. INTRODUCTION

2. THE BASIC METROLOGICAL DE.INITIONS

Instrument transformers are the electrical apparatus indispensable for measuring energy or electric power both in
HV power networks and when testing e.g. power transformers. Their metrologicalcharacteristics, currently specified
by the relatively high accuracy class (of e.g. 0,1 or higher),
are often unsatisfactory, because in real operating measurement conditions, errors caused by the instrument transformers exceed significantly the limit values set by the relevant standards [1, 2]. This fact, often unnoticed by its users
is only investigated, when the measurement of energy (power)performed on the transformers of the same rating in
the same place but of a different type, show significant
differences. Apart from the errors caused by the instrument
transformers, power or energy measurement errors are heavily dependent on the frequently ignored factors such as:
the leads connecting their secondary terminals (S1-S2 for
current transformers, and – the a-n or a-b for voltagetransformers) with measuring devices (the current circuits of
watt-hour meters or measuring instruments), as well as the
power factor of loads of the power network or the tested
objects (i.e. the transformers). .urthermore it cannot be underestimated that the burden power factor of instrument
transformers is different from the rated output (most often
0,8) and the burden power value is lower than25% of their
rated output.
The following article reviews inductive measuring current
and voltage transformers. It investigates the influence of the
power factor of network total load whose energy or power
are measured, the resistance of the leads in secondary circuits of instrument transformers and the effect of the actual
burden of the instrument transformers on the change of necessaryaccuracy of energy or power measurement based on
the catalogue data or the error characteristics provided by
the producer. The overall objective of the analysis is to systematize the error assessment criteria of the introduced instrument transformers which cause errors in energy or power
measuring circuits, both in laboratory research and in the
network operation, with a special regard to hightension systems (110 kV and higher).

In order to clarify further arguments, the most important
definitions, useful from the users’ perspective, were given
and systematised, pertinent to metrological properties of instrument transformers. Moreover the errors’ limit values ascribed to the specific accuracy class of current and voltage
transformers were presented.
2.1. Current transformer (CT).rom the secondary side a current transformer may be
treated as a current source. Its metrological characteristics
are marked by the current error Di and the phase displacement (angle error) di corresponding to the specific effective
value expressed as the percentage of the primary current Ip
and the burden power S.
The current (ratio) error defines the relative difference,
expressed in...