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A3-113
Session 2004
© CIGRÉ
METROLOGICAL PROPERTIES OF HIGH-VOLTAGE INSTRUMENT
TRANSFORMERS AFTER MANY YEARS’ SERVICE
E. Anderson*
J. Karolak
J. Wróblewski
Institute of Power Engineering
A. Hyrczak
A. Ratajczak
R. Zając
Polish Power Grid Company
(Poland)
1. Introduction
The many years’ exploitation of current, voltage and combined transformers in Polish
110 - 750 kV electric power system caused that some of them demonstrate negative
metrological features, rendering impossible their use in measuring energy and causing
difficulties in the correct operation of protections. The latter is caused by the deterioration of
their measuring accuracy, far below the range of the ascribed rated accuracy class. Many
years’ studies of metrological properties of instrument transformers allowed to evaluate the
quote of transformers which do not meet the measuring accuracy requirements of standard
specifications [1,2,3,4] as well as the operational instructions being obligatory in Poland. In
consequence, both theoretical and experimental studies, aiming at the modernization of these
transformers which would allow to continue their application, have been undertaken. The
suggested solution of modernization of instrument transformers would allow to abstain from
the necessity of replacement of large number of operated transformers by new ones and, in
consequence, shall decidedly reduce the investment outlays necessary to restore the wanted
measuring properties of transformers showing a low measuring accuracy unacceptable for
measurements used in accounts.
2. General characteristics of instrument transformers installed in the Polish electric
power system
About seven thousand current, voltage and recently combined voltage - current
instrument transformers are installed in the Polish 110, 220, 400 and 750 kV electric power
systems. The 110 - 400 kV current, voltage and combined transformers, covering 99.7% of all
the installed instrument transformers, constitute a decided majority. The number of operated
instrument transformers rated at 750 kV does not exceed twenty of both current and voltage
ones. The quantities of instrument transformers installed in the systems according to their
types and rated voltages are presented in Fig. 1.
*
edward.anderson@ien.com.pl
1600
1400
1200
pcs.
1000
800
600
400
200
VT
CT
0
110 kV
220 kV
C-VT
400 kV
750 kV
Fig. 1. Number of instrument transformers installed in Polish grids (VT – voltage transformers,
CT – current transformers, C-VT – combined transformers)
Instrument transformers installed in the Polish power systems operate from several to
several tens of years (maximum 40 years) and are intended for protection and energy
measurement serving for accounts between particular users. The structures of the age of
current, voltage and combined transformers are given in Figs. 2a, 2b and 3.
a)
b)
100%
100%
80%
80%
60%
60%
40%
40%
20%
20%
0%
0%
750 kV
> 25 years
400 kV
11 - 25 years
220 kV
110 kV
6 - 10 years
< 5 years
750 kV
> 25 years
400 kV
11 - 25 years
220 kV
110 kV
6 - 10 years
< 5 years
Fig. 2. Age structure of instrument transformers: a) - voltage, b) – current
The majority of current and voltage transformers are oil insulated. The quote of gas
insulated (SF6) transformers is within 0.3% to 9% depending on their rated voltages and
types. In the group of combined transformers the quotas of SF6 gas insulated ones amount to
about 35% at rated voltage of 220 kV and about 15% at rated voltage of 110 kV. All
remaining combined transformers are oil insulated.
All the current transformers belong to inductive type group. Voltage transformers 110
- 400 kV are of inductive or capacitive type. The quotas of particular designs within the
whole population of voltage transformers, according to their rated voltages, are presented in
Fig. 4.
2
100%
100%
80%
80%
60%
60%
40%
40%
20%
20%
0%
0%
220 kV
6 - 10 years
110 kV
< 5 years
Fig. 3. Age structure of combined transformers
400 kV
Inductive
220 kV
110 kV
Capacitive
Fig. 4. Partition of 110 - 400 kV voltage
transformers in relation to their design
The 110 - 400 kV current transformers are designed as multi-range ones (generally 2
to 3 primary current ranges) and provided with several secondary windings (from three to six)
of rated currents 1 A or 5 A and rated output of particular windings up to 90 VA (older
designs) or up to 30 VA (newer designs).
The 110 – 400 kV voltage transformers provided with several (from two to four)
secondary windings of rated output 100 – 200 VA (older designs) and up to 25 VA (newer
designs). The accuracy class of measuring windings in majority of transformers is 0.5. A
small group of transformers has a measuring accuracy class of 0.2.
3. Determination of metrological properties of instrument transformers
During the last decade, measuring proceedings have been undertaken in Poland, to
determine metrological properties of current, voltage and combined transformers installed in
national power grids. Measurements of installed transformers were performed onsite using a
specialized mobile laboratory allowing for testing current transformers rated up to 10000 A
and voltage transformers up to 400 kV. This laboratory has a certificate of the state central
office supervising the quality of measurements. During the said period about three and half
thousand of transformers of all types were tested. The latter tests showed that a certain part of
them does not meet specifications as far as the measuring accuracy specified by standards
[1,2,3,4] and those of national regulations for operation of instrument transformers conform
with the latter in the range of metrological properties. Basing upon the tests carried out in the
years 1997-2001 covering 1542 instrument transformers their metrological properties were
assessed. The results of the classification of applicability of transformers for measuring
purposes depending on their types are presented in Fig. 5. Fig. 6 shows the quotas of
disqualified transformers of different types in reference to their age.
A deterioration of the class of measuring accuracy in reference to the rated class
ascribed by manufacturer is to be seen in all types of the tested transformers. The most
frequent cause of disqualification was an excessive rise of the amplitude error, though in
some cases both the amplitude error and the angular error were evidently above the
admissible value. Combined transformers operated during a rather short period. A half of all
the tested combined transformers was operated less than 5 years. Excessive measuring errors
were stated only in the current part of the transformers.
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80,0
100%
80%
60,0
60%
40,0
40%
20,0
20%
0,0
0%
CT
VT
C-VT
> 25
11 - 25
6 - 10
<5
Years
Positive result
Negative result
C-VT
CT
VT ind.
VT cap.
Fig. 5. Classification of applicability of instrument Fig. 6. Quotas of disqualified instrument
transformers for measuring purposes
transformers of different types in reference to
the time of operation
The tested current and voltage transformers belonged to different age groups, about
70% of whole population being constituted by transformers operating below 10 years.
Within the total population of tested transformers the most important quota of disqualified
ones concerned voltage transformers (about 20%) in particular capacitive type (about 50%).
A deterioration of metrological properties with time of their operation is to be observed. For
instance, in the group of capacitive voltage transformers the percentage of those having a
lower accuracy class ranged from about 24% for transformers with a time of operation below
5 years to about 70% for the oldest ones. In some cases it was stated that in their present state
these transformers can no more be used for energy measuring. Their amplitude errors in
extreme cases reach the level more then ten percent. Moreover, the cooperation of these
transformers with protection equipment leaves much be desired.
The results of many years’ diagnostic tests of instrument transformers, concerning also
their metrological properties show, that from several to over ten percent of the latter,
depending on their types, should be replaced by new ones allowing them to fulfil properly the
measuring functions. This brings about the necessity to install from several tens to several
hundreds of new transformers of each type group and is bound with important investment
outlays. Therefore, the activities aiming at a continued operation of instrument transformers
showing measuring errors not exceeding several percent but whose technical state otherwise
qualifies them as applicable in electric power systems, have been undertaken. A
modernization procedure allowing to restore of measuring properties of instrument
transformers is suggested.
4. Suggested modernization of instrument transformers
Secondary circuits of electric power equipment are subject to continuous
electronization. Measuring systems, remote control system and automatics are today as a rule
of electronic, mostly digital type. The last appliance which resisted electronization for a long
time are instrument transformers. Though studies on unconventional instrument transformers
both current and voltage ones are under way for decades, these transformers are not applied
on a wide scale as yet. This is caused both by technological difficulties in obtaining novel
designs economically justified and a lacking demand for them bound without doubt with a
lack of relevant standards. Today unconventional instrument transformers are covered by
international standards [5,6]. They acquired the name of “electronic” transformers understood
as transformers either comprising electronic systems or intended for electronic receivers of
data bound with currents and voltages of an electric power systems. Conditions have been set
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for manufacture and operation of novel measuring transformers and relevant receiving
equipment. Also the possibility arose to give suitably modernized measuring transformers the
status of electronic transformers. In the authors’ opinion, advantage of this possibility should
be taken as, in many cases, this will be a cheaper and simpler procedure on the way to
implant new standards and to end the inevitable process of electronization of secondary
circuits.
The wide electronization of secondary circuits in electric power systems brought about
changes of specifications concerning high-voltage measuring instrument transformers. The
change of standards was accompanied by the necessity to suitably adapt the parameters of
electronic transformers to electronic receivers of measured quantities. This concerns, in the
first place, the values of rated secondary voltages and currents (100 V, 5 A) and of rated
outputs (tens or even hundreds volt-ampere), exceeding needs at present time. Also dynamic
qualities of classic instrument transformers render impossible or at least difficult a further
progress in the speed of operation of protection equipment, keeping at the same time its
selectivity and reliability.
The applicability of instrument transformers for energy measurements used for
accounts is required. This means the application of transformers having a high measuring
accuracy in steady state. The requirement to use transformers of class 0.2 sometimes even 0.1
becomes common. The dynamic qualities of these transformers, in this case are not essential
as transient states appear occasionally and their duration is very short so they do not influence
energy accounts. Transformers with a small and practically invariable secondary load and
with good magnetic materials are very well adapted as primary sensor. Thus, the adaptation of
the existing measuring magnetic cores provided in instrument transformers is possible.
Taking into account the number of voltage transformers installed in Polish grids, the
quotas of transformers disqualified as measuring device and their economical value as well as
world trends in electronization of secondary circuits, theoretical and experimental studies
have been undertaken in the years 2001-2002. The latter aim at the modernization of the
installed transformers so as to give them the status of electronic ones meeting standard
specifications [5]. Some activities has been developed to modernize inductive and capacitive
voltage transformers by providing passive low voltage electronic system at instrument
transformer and active system directly in protection equipment (Fig. 7). This solution has
been submitted to the Polish patent office.
Modernized voltage transformers are provided with additional electronic outputs while
their up-to-date (classic) outputs are kept unchanged and may be yet applicable. Dynamic
errors of latter outputs remain unchanged what has no meaning for account measurements. On
the electronic outputs the following dynamic errors due to the electromagnetic unit of
instrument transformers are eliminated:
- errors in transient states caused by a sudden change of the amplitude or phase of
primary voltage. The application of the said solution assures a correct operation of
the remote control equipment, particularly in auto-reclosing, renders possible a
proper measurement of the quantity and quality of energy.
- errors due to ferroresonance oscillations arising easy in classic voltage transformers
of capacitive type due to sudden changes of primary voltage or temporary faults in
secondary circuits,
- and errors in transfering of high harmonics.
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HV
Induktive or capasitive
voltage transformer
Energy
meters
Protections
To other
equipment
Passive correction system
Existing cables
3-phase active
correction system
From remaining phases
Fig. 7. Installation scheme of a voltage transformer after modernization
Moreover, bestowment of „electronic state” to a transformer accordingly [5] and the
application of a passive correction system optimising the preliminary load of transformer and
becoming an integral element of its design allows to ascribe it its original or even a higher
accuracy class (this concerns also classical outputs). It ensues from possibility to ascribe to
the transformer rated output much smaller that the initial ones, e.g. only 5 VA instead of 100
VA. Of course, it concerns the transformers having measuring errors not to high (max. 1 2%) and a technological state qualifying them for further operation
The cost of modernization of one substation field provided with three voltage
transformers shall be lower than the purchase, transportation and mounting of a single new
one.
5. Conclusions
1. Many years’ diagnostic tests, including measurements of metrological properties,
performed on instrument transformers installed in electric power grids allow for the
assessment of percentage of transformers not meeting the specifications concerning
their measuring accuracy.
2. The suggested modernization of voltage transformers allows to achieve:
- the correction of measuring errors,
- a complete elimination of dynamic errors caused by electromagnetic unit of the
transformer,
- faithful transfering of high frequency voltage transient.
Moreover, the latter modernization will accelerate the operation of remote protection
equipment and improve the work of fault recorders and localizers.
3. The replacement of voltage transformers of an insignificantly reduced measuring
accuracy can not be necessary if the remaining technological features leave no doubts.
Metrological properties are not decisive at considering the necessity of replacement
as they may be corrected by modernization.
4. The implementation of modern solutions in the range of protection automation, of
operation means and of account measurement is possible by modernization of existing
instrument transformers instead of purchasing new electronic transformers. Taking
into account the fact that in the future the application of electronic transformers shall
be imperative, the suggested modernisation may bring important economies.
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6. References
[1] IEC 60044 – 1: Instrument transformers - Part 1: Current transformers
[2] IEC 60044 – 2: Instrument transformers - Part 2: Inductive voltage transformers
[3] IEC 60044 – 3: Instrument transformers - Part 1: Combined transformers
[4] IEC 60044 – 5: Instrument transformers - Part 1: Capacitor voltage transformers
[5] IEC 60044 – 7: Instrument transformers - Part 7: Electronic voltage transformers
[6] IEC 60044 – 8: Instrument transformers - Part 8: Electronic current transformers
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