Frequency Response of Voltage Transformers
and Power Quality
in Medium Voltage Power Networks
(Measurement Accuracy )
A. LIPSKY, N. MITEVA, E. LOKSHIN
Department of Electrical and Electronic Engineering
Ariel University
Israel, Eilat , 2014
Today, the development of electric power networks and
systems is determined by the concept of “smart grids”.
…
…
Report contents
1. How to measure voltage and its parameters in the
high-voltage power networks.
2. In which modes VTs have to operate?
3. When one needs to know amplitude frequency
responce of VT from practical point of view?
4. Frequency response of inductive voltage transformer.
5.Practical aspects of the problem.
6. Conclusion.
An example of metering in power network
S ~
●
161 kV
●
●
T1
3.15 - 36
●
●
kV
T2
●
●
Cab
●
●
●
U, I, P, Q, EPQI …
measurements
●
Load
●I”
T3
0.4 kV
U, I, P, Q, EPQI …
measurements
We need voltage sensors in HV PN . The most widespread
of them are induction voltage transformers (VT).
To answer the question of the accuracy of measurement
by inductive voltage transformer, it is necessary to
answer the question,
In what modes of power supply network VTs have to
operate?
(Standard IEC 60044-2 )
Voltage mode of power supply network is determined by:
- The regimes and processes
themselves in the networks.
(dips, swells, different types of
transients, voltage harmonics, … ).
U(t)
B.B.
VT
- The modes of load operations
connected to the network
where VT is installed,
Effect of load
Voltage variations in the network may be static or dynamic.
Load operation mode may be static or dynamic.
The static mode. The maximum frequency in the spectrum
of envelope voltage graph of static load does not exceed
0.001 Hz,
Power, p.u.
1.0
0.8
0.6
0.4
Time, h
Voltage, V
The dinamic mode
1
3
Time, sec
5
7
To obtain accurate results of measurements, frequency
responses (!) of voltage transformer must comply with the
spectrum of voltage distortions in its network.
Spectra of voltage distortion in medium
voltage power networks
Effect of energy consumers on voltage modes
in power supply network.
Current spectral density , %
Dynamic mode. The maximum frequency in the spectrum of
envelope voltage graph of dynamic load (arc furnace) does
not exceed 25 Hz.
100
80
60
40
20
0
5
10
15
20
Frequency, Hz
Active current spectrum of current main harmonic of
electric arc furnace.
Based on the the theory of amplitude modulation
voltage spectrum in this case is in the range 25-75 Hz.
At the same time equipment connected to the power
network can to have linear or nonlinear volt-ampere
characteristics. In the latter case this equipment is the
sources of voltage harmonics ultimately.
U
Z linear
Z nonlinear
Loads
Electric arc furnaces
n
Current harmonic spectrum of the electric arc furnace
The harmonic frequency spectrum of bridge rectifiers
Harmonic order:
n  k 6a   1
a  1 for
6pulse rectifier, a  2 for the 12-pulse rectifier, etc.; k
= 1, 2, 3 ... is a series of natural numbers.
a is indicator of rectifier rippling.
Amplitude of the current harmonic is defined as
In 1/ n
For example, for a 24-pulse rectifier harmonic 49 (2450
Hz) of the current has a value not exceeding 2%. It can
be ignored.
Thus, frequency spectrum of the harmonics for
rectifiers does not exceed 2500 Hz.
There are some problems with the
spectrum of the inverters.
Graphs of the instantaneous values ​of voltages in the 6 kV
power supply network of 12-pulse rectifiers of 4 MW
frequency converters (the results are based on
measurements using inductive VT).
Voltage, V
http://www.news.elteh.ru/arh/2005/32/22.php
Time, sec
The voltage harmonic spectrum of 6 kV power
network of the above frequency converter
http://www.news.elteh.ru/arh/2005/32/22.php
Voltage harmonics, %
n < 40
n < 40
n < 40
(10 kHz)
Harmonics number, n
According to EN 50160 the total harmonic distortion
factor THD, calculated using the following expression:
40
THD 
U
2
n
n2
Australian Standard AS 4777.1 – 2005 recommends
measure harmonics up to n = 50 (2500 Hz).
Thus, the power network voltage of different types of
industrial installations are usually contain harmonics
with frequencies which not exceeding 2500 Hz.
Although exceptions may be there.
Effect of power supply network
Regime changes and transients in the power networks
themselves are characterized by changes in the
effective and the instantaneous values ​of voltage. These
changes are characterized by different types of electric
power quality indies in the power supply networks.
These distortions represent dips, swells, and
different types of transients. Short interruptions in the
power supply are considered separately.
Classification of dips
according to residual voltage and duration
Residual
voltage
U, %
Duration t
ms
90 > u ≥ 80
10 ≤ t ≤
200
CELL A1
80 > u ≥ 70
CELL B1
70 > u ≥ 40
CELL C1
40 > u ≥ 5
5>u
CELL D1
CELL X1
200 < t ≤
500
CELL A2
500 < t ≤
1000
CELL A3
1 000 < t ≤
5000
CELL A4
5 000 < t ≤
60000
CELL A5
CELL B2
CELL B3
CELL B4
CELL B5
CELL C2
CELL C3
CELL C4
CELL C5
CELL D2
CELL D3
CELL D4
CELL D5
CELL X2
CELL X3
CELL X4
CELL X5
As it can be seen from the table, the minimum width
of a voltage dip is 10 ms.
The ideal dip (a) and its spectral density (b):
The equation of the spectral density:
S ( )  A pulse
  pulse

 sin
2

  pulse


2





90% of the pulse energy is in the frequency range of
0  2  /  pulse
. Therefore, we can assume that the maximum frequency,
which must be considered, is  m ax   1  2 /  pulse
f m ax  1 /  pulse
or
.
When the pulse duration is 10 ms, the maximum frequency of
voltage envelope is 100 Hz. Because it is carried out the
amplitude modulation of 50 Hz voltage, the spectrum of the
signal contains frequencies of 150 Hz. Taking some reserve,
we can say that the spectrum of the voltage distortion,
characterized by dip and swell, does not exceed 300 - 400 Hz.
Transients
EN 50160 – 2010
5.3.3 overvoltages
Transient overvoltages in MV supply systems are caused by
switching or, directly or by induction, by lightning. Switching
overvoltages generally are lower in amplitude than lightning
overvoltages, but they can have a shorter rise time and/or
longer duration.
Transients.
They fall into two subcategories:
- oscillatory,
- impulsive.
Overvoltages in power networks
Voltage, P.U.
Lightning impulse
(the duration of the order of 100
microseconds)
Commutation overvoltage
(f = 0.1 – 1 MHz)
0
20
Time, ms
40
Oscillatory transients associated with switching
of capacitor banks of 10 kV
The oscillation frequency of the transient is 920 Hz ≈ 0.001
MHz
iB C (t )
it ( t )
Current and voltage during
50 ms
Current and voltage during
25
10 ms
There are another problems, such as fault location in
the overhead and cable lines. There are methods in
which one have to analyze spectra with the
components in units of GHz.
Thus, the voltage distortions with frequencies from tens
Hz up to few GHz, exist in the MV power networks.
Note that disturbances at frequencies of 9 kHz - 30 MHz
are traditionally studied by specialists in communication
and
CISPR
(International
Committee
on
Radio
Interference). It is assumed that the range up to 2÷9
kHz must be supervised by personnel of electrical
systems.
Therefore, it is not clear whether
we should deal with HF problem.
When one needs to know amplitude frequency
responce of VT from practical point of view?
1. Verification of compliance
(network operators, customers, regulators)
– Quality of produced electrucal energy (EN 50160)
– Comparison of compatibility levels and planning levels
IEC 61000-2-x, IEC 61000-3-6
– Troubleshooting in case of malfunctions
(network operators, manufacturers).
- Assessing the impact of voltage distortion on
the electrical equipment.
- Network resonances.
- Harmonic filter effectiveness.
2. Researches:
- Cancellation effects of harmonics.
- Transfer of harmonics between voltage levels.
- Propagation of high frequency harmonics.
The intermediate conclusion:
1. From a practical point of view, frequency spectrum of
voltage distortion and most transients in mediumvoltage power network does not exceed 2500 Hz.
2. Thus, for a voltage transformer used in medium
voltage power networks, from a practical point of view,
the frequency range up to 2500 Hz is important .
Voltage transformers, classification
The equivalent circuit of the voltage transformer for
static mode for 50 Hzr L
r
L
A
1
S1
r0
50 Hz
'
S2
'
2
a
L0
'
zL
x
X
There are no serious problems in static mode for 50 Hz
The problems of voltage transformer are under presence
of harmonics.
The presence of parasitic capacities:
- The capacities between coils,
- The capacities between windings,
- The capacities between windings and body of transformer
32
Scheme the parasitic capacitance location in the induction
voltage transformer for bandwidth up to 2-2.5 kHz
C3
a
A
C1
C2B
C1 B
R load
C2
C4
X
x
Equivalent circuit taking into consideration capacities between coils
and between windings
a
A
C1
C1 B
X
C3
C2B
R load
C2
x
Equivalent circuit taking into consideration ground connection
33
The results of experimental investigations of
frequency responces
The calculation of the frequency responses of
inductive voltage transformers is rather
complicated and problematic. It requires
knowledge of the values ​of all the parasitic
capacitances, which can only be found through
experimentation. Therefore, various experimental
investigations are carried out by different firms
and universities.
The errors of the VT transformation ratio for different
voltage harmonics.
Experimental investigations of the frequency
responses of voltage transformers
for 22 kV and 33 kV power networks
Functional diagram of the setup
There is the problem of accurate measurement of
high voltages without using voltage transformers.
There are different types of voltage dividers (R, RC.
C), which should be correct for different
frequencies. For medium voltages it is possible to
use the standard voltage dividers, for example,
probe.
U, kV
MOUNTING
Type
22
Indoor
Outdoor
indoor
outdoor
UCJ-24, 24 kV
VJL-24, 24 kV
VCN - 36, 36 kV
VJN - 36, 36 kV
33
Results of investigations:
- Only for frequencies above 50 Hz,
- 50 Hz, plus a higher frequency.
- Secondary burden: from nominal load up to idling
- (Fluke 1750 – 2MΩ, 5 mW → 300 W), cosφ = 0.8-1.0
Experimental results
outdoor
indoor
UCJ-22
VJL-22
VJN-33
UCN-33
1- unloaded mode; 2- loaded mode
(basic + high harmonics)
Intermediate conclusions
The carried out experimental studies of the
amplitude frequency response of VTs with cast
resin insulation for 22 and 33 kV power network
showed that the error of VT transformation ratios
depend mainly on the frequency range of the
higher harmonics or disturbances in power
networks and the burden of the VT secondary
winding.
In
general,
the
errors
of
the
transformation ratios for each harmonic do not
exceed ±10% in the range up to 2500 Hz.
Practical aspects of the problem:
How can we evaluate the real
measurement error of the voltage
harmonics in the existing medium voltage power
networks using serial VT with cast resin?
Standard IEC 61000-4-7 gives permissions ONLY for
measuring instruments. For both classes of
harmonics measurements permission level is +/-
5%.
Corresponding Russian standard GOST 13109 gives
+/- 5% as the measurement error of voltage
harmonics. When exists uncertainty of
VT
characteristics allowed error is 1.5 times greater.
What happens in practice?
Taking into account the VT uncertainty, reaching up to
±10%, in relation to the European standard we can talk
about the maximum error in the measurement of voltage
harmonics in ±(10% + 5%) = ±15%.
In relation to the Russian standard ±[10% + (instrument
error)] = 15% > (5% • 1.5 = 7.5)%
Really, measuring instruments ​of the EPQ values
measurement and voltage harmonics in particular, have a
smaller error, for example ±(0.2-0.3)% (like for Fluke).
Therefore, we can say that in the medium voltage power
networks
using
VT
with
cast
resin
insulation,
measurement error of voltage harmonics practically does
not exceed ±10.5% ≈ ±11%.
Conclusion
1. The performed experimental studies of VT AFR
with cast resin insulation for 22 kV and 33 kV power
networks revealed that in the bandwidth up to 2.5
kHz the relative error of the transformation ratio for
voltage harmonic in comparison with the VT
transformation ratio on the industrial frequency
does not exceed ±10%.
2. The measurement errors of the voltage harmonics in
22, 33 kV power networks practically does not exceed
±(10-11) % when using VT cast resin insulation.
3. An increase the accuracy of measurement
results up to standard demand values ​can be
reached by introducing a correction factor in the
calculation program, for example, levels of higher
harmonics, of electric power quality monitoring
devices.
4. There is the problem of creation of the voltage
sensor with suitable frequency characteristics in the
range up to 1-2 GHz
47