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Файл:The Electromagnetic Interference in the Electrical Power Supply System. The long-term variance of the voltage specifications. Study guide
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As the analysis showed, the peculiarity of this current spectrum is the
high level of currents of the 5
th
and 7th high harmonics. The high level of
high harmonics of the current consumed from the network is an important
drawback of the VFDs studied in the work.
Also, HH in frequency-controlled converters were calculated by the other way. During the work, oscillograms of the currents consumed by the
ACS150 and ACS550 VFDs were obtained at a maximum speed of HELL
of 695 rpm and a moment of resistance developed by the DC motor, M = 20
N × m (figures 3.17 and 3.18, respectively).
Fig. 3.17. Oscillogram of the mains current consumed by the ACS150
VFD at 695 rpm and M = 20 N × m
Fig. 3.18. Oscillogram of the mains current consumed
by the ACS550 VFD at 695 rpm and M = 20 N × m
41
Figures 3.10 and 3.11 show that the level of the high harmonics of the
current consumed by the VFD from the network is rather high. There is a
high level of the fifth and seventh harmonics, and the harmonics multiple of
three аre absent, since a three-phase rectifying scheme for uncontrolled
VFD rectifier with a number of rectification phases was m = 6. The nonsinusoidal current ratio for the ACS150 VFD is 1.41, for the ACS550 VFD -
0.7. Thus we can conclude that the ACS150 is much simpler in design and
cheaper than the ACS550.
3.4. Consumer Damage due to High Harmonics
Impact on control systems and automation. Violations or conversions of
technological processes caused by HH are possible even if very reliable electrical supply schemes with multiple redundancy are used. When using electronic control devices or controlling computers, a “burst” of HH can cause
failures in control devices leading to the shutdown of the main electrical
equipment, which cannot be started without making adjustments to the technological process (chemical fiber plants, non-ferrous metallurgy, etc.) [11].
HH current, penetrating into the power supply network, leads to deterioration of high-frequency communication and automation systems, to the
failure of computer networks and digital information processing and transmission systems, as well as causing false positives of some relay protections; The influence of HH on devices containing semiconductor elements is
particularly significant.
Impact on consumer equipment. Harmonic components cause distortion
of television images, disruptions in the operation of fluorescent lamps and
reduction of the service life of incandescent lamps. Recently, there was noticed a negative effect of high harmonics on various household appliances,
primarily radio receivers and devices with high-quality sound reproduction,
causing various kinds of acoustic interference.
Ballast devices for fluorescent and mercury lamps sometimes contain
capacitors, and under certain conditions resonance may occur, leading to
lamp failure [12].
Interference in telecommunications networks can occur where power
and telecommunication cables are located relatively close. Due to the highfrequency current harmonics flowing in the power cables, telecommunication cables may be interfered with. The magnetic fields of high harmonic
currents of the direct and inverse sequence partially compensate each other,
so harmonics that are multiples of three produce the greatest influence on
42
telecommunications. The higher the order of the harmonics, the greater the
level of interference induced by them in telecommunication cables [7].
Impact on the control, protective, measuring equipment. It should be
particularly noted that even low levels of high harmonics can cause disturbances in the operation of certain types of control, protective and measuring
equipment due to distortions in the shape of the measured voltage and current curve on the secondary windings of measuring transformers. As a result, for example, high harmonics lead to the occurrence of false positives of
protective relays at transformer substations [15].
In low-voltage networks, sometimes unreasonable operation of fuses
and circuit breakers occurs because of the additional heating of the internal
elements of protective devices. This process is due to the flow of nonsinusoidal currents and, therefore, the impact of the surface and proximity
effects. The operation occurred at the load of 80–85% of the set point of the
thermal release of the circuit breaker [7].
Impact on transformers. Harmonics generated by non-linear load create
additional losses in distribution transformers and transformers for the devices of converter equipment [7].
Voltage harmonics in transformers cause an increase in hysteresis losses, losses due to eddy currents in steel, and losses in windings [4, p. 118].
These losses can be the cause of reducing the service life of the insulation
and the failure of transformers due to overheating. Losses due to hysteresis
are proportional to the frequency, and the losses due to eddy currents its
square. In sinusoidal regimes, eddy current losses are small and average 5%
of the nominal transformer short circuit losses [9]. However, in the case of
the flow of high harmonic currents, the additional losses increase sharply
and can reach 30–50% of the short circuit losses.
Transformer losses increase with an increase in transformer power and
an increase in the cross section of the winding conductors. In this case, a
substantial increase in losses is observed when the transformer operates at a
random non-linear load in the presence of a neutral wire loaded with currents of zero-sequence harmonics. As a result, when the transformer applies
voltage to symmetric single-phase rectifiers, the losses are greater than
when the same transformer operates on the Larionov circuit. This is due in
the first case to the presence of current harmonics that are multiples of three,
which increase the losses in the transformer windings [16, 17].
The increase in losses in the windings is the most important in the case
of a converter transformer, as the presence of a filter, usually connected to
the AC side, does not reduce the current harmonics in the transformer.
43
Therefore, it is required to install a larger power transformer. In addition,
there is a local overheating of the tank. An important component of the effect of harmonics on high-power transformers is the circulation of three
times the zero-sequence current in delta-connected windings. This can lead
to their overload [7].
Some design features in the implementation of networks low power (LP)
should be noted as they have a significant impact on the magnitude of current and voltage high harmonics. According to the standard design practice,
in distribution LP networks the most widespread are distribution transformers with the star-star winding connections with zero scheme. The main disadvantage of such transformers is the relatively large zero-sequence resistance
, which is determined by the design features of the transformer,
the connection circuit of its windings, the degree of saturation of the cores
of the magnetic circuit, etc. It should be noted that a number of works are
devoted to the problems of determining the active
resistance of a transformer. As for this purpose different technics were
, inductive
and full
used, the results obtained significantly differ from each other. Based on the
analysis conducted by A.K. Shidlovsky and A.F. Zharkin, we can conclude
that for the types of TM, TMA, TCMA transformers, the value
is 8 - 18
times greater than the resistance of the direct sequence. This feature of
transformers with a star-star winding connection with zero makes them very
sensitive to non-sinusoidal phase currents [7].
The negative impact of reduced electrical energy quality is often not detected explicitly, for example, as in the case of SH, which leads to a reduction in the service life of electrical equipment (EE) due to accelerated aging
of the insulation. In this case, there is actually a decrease in the functional
reliability of the EE.
Effect on capacitor banks. Under conditions of non-sinusoidal current, the
operating conditions of the capacitor banks, which are designed to power factor
compensation of the load. Capacitive resistance of capacitors with increasing
frequency of the input voltage decreases. Thus, they change the normal path of
the current flow of high harmonics from a non-linear consumer to the power
source, closing a part of this current through itself. Since the resistance of the
network elements has an inductive character, then when using installations of
reactive power compensation and the presence of nonlinear power consumers,
there is a probability of resonant modes occurrence (both in current and voltage) on individual elements of the power-supply system (PSS) [16].
Effect on rotating machines. When synchronous and asynchronous motors operate in non-sinusoidal voltage conditions, additional power losses
44
arise due to temporary HH current both in the stator and rotor windings.
Additional losses in the stator and rotor steel also appear, however, these
losses are small and can be neglected. The main part of the additional losses
from HH in synchronous machines accounted for the soothing and stator
windings; losses in the rotor winding, as a rule, are smaller. In high voltage
induction motors, the losses in the stator and the rotor are approximately the
same [11].
Moreover, the presence of high harmonics in the stator current curve
leads to the appearance of nonsynchronous magnetic fields in the gap, moving relative to the rotor. In this case, high harmonics of the 5
ders create fields of negative sequence, rotating relative to the rotor in the
opposite direction, and components of the 7
th
and 13
th
th
and 11th or-
orders create direct
sequence fields. However, since their rotation frequency is higher than the
rotor rotation frequency with a magnitude of the order of the harmonic, the
fields of both sequences induce increased frequency currents in the rotor
circuits, which flow in the upper layers of the massive rotor parts and, closing at its ends, cause local overheating [12].
In synchronous machines, as shown by calculations and operating experience, the main share of losses falls on the rotor winding, which can be either laminated or massive. The laminated rotor has a greater resistance, so
even in the case of unacceptable voltage distortions (K
= 10–15%), the
NSR
additional losses due to time harmonics in synchronous motors with stator
and rotor laminated do not exceed a few percents of nominal losses; this is
significantly less than the allowable value of the additional losses, amounting to 0.25…4 % of the rated active power of motors with a power above
1000 kW. Therefore, overheating of salient pole type synchronous motors
with laminated poles at industrial enterprises was not observed [19].
Losses from HH in synchronous motors with massive poles turn out to
be much greater. The operation of such electric motors with non-sinusoidal
voltage may cause unacceptable overheating and damage to the excitation
winding. Equally, this also applies to synchronous compensators having
massive poles. Synchronous compensators installed in networks of enterprises with a high level of HH must have laminated poles.
Based on the obtained dependencies, we can conclude that a significant
decrease in the reliability of asynchronous motor occurs when voltage deviation exceeds 10%. To prevent overheating and premature failure of asynchronous motor with a significant level of high harmonics, asymmetry and
voltage deviation, it is necessary to reduce their load.
45
Impact on isolation. When the voltage is not sinusoidal, accelerated aging of the insulation of electrical machines, transformers, capacitors and
cables is observed. Under operating temperatures, chemical reactions take
place in insulating materials, leading to a gradual change in their insulating
and mechanical properties. As the temperature increases, these processes are
accelerating, shortening the equipment service life .The essence of electrical
aging is the occurrence of partial discharges, which apply only to part of the
insulating gap, for example, partial discharges in gas inclusions. Partial discharges are associated with energy dissipation, which results in electrical,
mechanical and chemical effects on the surrounding dielectric. As a result,
local insulation defects develop, which leads to a reduction in its service
life.
The aging of the insulation of conductors and cables is due to the flow
of non-sinusoidal current, leading to increased heating of the outer surface
of the cable cores due to the surface and proximity effects. Studies have
shown that with the distortion coefficient of the sinusoidal voltage curve
within 6 - 8.5% (the fifth and seventh harmonics prevailed) leakage currents
increased: after 2.5 years of cable operation, an average by 36%, and after
3.5 years –by 43% [9].
The distortion of the voltage curve shape significantly affects the occurrence and flow of ionization processes in the insulation of electrical machines and transformers. In the presence of gas inclusions in isolation, ionization occurs, the essence of which consists in the formation of space
charges and their subsequent neutralization. Neutralization of charges is
associated with energy dissipation, which results in electrical, mechanical
and chemical effects on the surrounding dielectric; as a result, local defects
in the insulation develop, which leads to an increase in dielectric losses and,
ultimately, to a shortened service life.
Examination of the state of the cables insulation in the 10 kV networks
of the old part of one of the metallurgical plants, without valve inverter, and
the new one, the shops of which are widely equipped with thyristor units,
showed that in the second case the insulation condition is much worse; this
manifested itself in an increased accident rate, which is increasing over the
years [11].
The impact on power lines. Сurrent harmonics in the lines lead to additional losses of electricity and voltage. In the case of cable lines, voltage
harmonics increase the effect on the dielectric in proportion to the increase
in the maximum amplitude value. This, in turn, increases the number of cable damage and the cost of repairs. For ultrahigh-voltage lines, voltage har-
46
monics for the same reason (increase in amplitude) can cause an increase in
K
corona losses [10].
The effect of high harmonics pulse-phase operation systems (PPOS)
of
converters can lead to so called harmonic instability, which consists in the
appearance of a high voltage of an even harmonic or a multiple of three on
the tires of a multi-pulse high-voltage valve inverter (VI). In this case, in the
inverter mode of the VI, switching failures are possible; PPOS work may
also be unsustainable. Harmonic instability may occur when the VI is connected to the power system, the short-circuit power of which is commensurate with the power of the EP, if there are other HH sources (for example,
power transformers), and asymmetry of the PPOS control pulses.
In general, the damage from non-sinusoidality is as follows:
there is a negative impact on the insulation of electrical networks and
electrical receivers;
there are additional losses in electric cars;
deterioration, and sometimes the operation of capacitor batteries is
impossible;
the operation of induction electricity meters is violated;
deterioration of the work of communications, automation, relay pro-
tection and other equipment;
complicated issues related to neutral modes in electrical networks.
3.5. Rationing of High Harmonics
Permissible levels of high voltage harmonics in electrical networks are
normalized by STANDARD 32144–2013.
The parameters characterizing the non-sinusoidal voltage are [5]:
– values of the coefficients of the harmonic components of the
K
()Un
voltage up to the 40th order, averaged over a time interval of 10 minutes
during the week. The values of the coefficients should not exceed the values
set in Tables 3.11–3.14 for 95 % of the time interval of one week. They also
should not exceed the values set in tables 3.11–3.14, increased by 1.5 times
during 100 % of the time interval of one week.
– is the value of the total harmonic voltage component. The values
U
of these coefficients for different voltages are given in table. 3.11.
Voltage measurements of the harmonic voltage components should be
carried out in time intervals of 10 periods without intervals between intervals, followed by averaging in a time interval of 10 minutes.
47
Interharmonic voltage components caused by frequency converters and
other control equipment are under review.
T a b l e 3.11
The values of odd harmonic components of voltage, not multiple
of three K
U(n)
, %
The ordering
of the n harmonical
component
5 6 4 3 1,5
7 5 3 2,5 1
11 3,5 2 2 1
13 3,0 2 1,5 0,7
17 2,0 1,5 1 0,5
19 1,5 1 1 0,4
23 1,5 1 1 0,4
25 1,5 1 1 0,4
˃25 1,5 1 1 0,4
The values of odd harmonic components of voltage, multiple
The ordering
of the n harmonical
component
Network voltage, kV
0,38 6-25 35 110-220
T a b l e 3.12
of three K
U(n)
, %
Network voltage, kV
0,38 6-25 35 110-220
3 5 3 3 1,5
9 1,5 1 1 0,4
15 0,3 0,3 0,3 0,2
21 0,2 0,2 0,2 0,2
˃21 0,2 0,2 0,2 0,2
48
The values of the coefficients of the even harmonic components
, %
U(n)
Network voltage, kV
The ordering
of the n harmonical
component
of the voltage K
0,38 6-25 35 110-220
2 2 1,5 1 0,5
4 1 0,7 0,5 0,3
6 0,5 0,3 0,3 0,2
8 0,5 0,3 0,3 0,2
10 0,5 0,3 0,3 0,2
12 0,2 0,2 0,2 0,2
˃ 12 0,2 0,2 0,2 0,2
Values of the total harmonic components of the voltage K
Table 3.13
T a b l e 3.14
, %
U
Period
Network voltage, kV
0,38 6-25 35 110-220
95 % of the times
during one week
100 % of the times
during one week
8,0 5,0 4,0 2,0
12 8,0 6,0 3,0
If we analyze the foreign experience, we can note the following. The
process of standardization in the field of interharmonics (IG) is in the process of accumulating knowledge.
3.6. The Ways of Voltage High Harmonics Decreasing
There are two options for reducing high voltage harmonics. The first is
associated with a decrease in the value of I
equivalent resistance of X
En
or X
SEn
.
An example of the second type of the ways to reduce high voltage harmonics is the installation of filter-compensating devices (FCD) (power fil-
49
, the second is a decrease in the
n
ters), representing a very small, ideally zero, resistance for the specifically
chosen ν
th
harmonic. FCD are connected in parallel non-linear load. FCD is
a series-connected reactor and a battery of capacitors operating in voltage
resonance mode (HF ν = 0,). If it is required to suppress several high harmonics, several FCDs are installed. Fig. 3.19 shows the frequency response
of the FCD.
+Х
Фn
inductance
Х
Фn
1
ν
0
ω
0Ф
Х
ф1
–X
capacitance
Фn
n
ω
n
Fig. 3.19. Frequency response of the νth harmonic FCD
For the first harmonic, FCD is an equivalent capacitance having an
equivalent resistance HF1, that is, it is a source of reactive power and can
be used instead of a capacitor bank when the level of harmonics in the network due to a capacitor bank is unacceptably high, as in the calculation
example.
Based on the use of fully controlled high-frequency thyristors, so-called
active filters are developed that consume (due to a special PWM control
circuit) from the network a non-sinusoidal current containing the same current harmonics as the nonlinear load, but only in antiphase to them. In this
case, the total current of the nonlinear load and the active filter does not
contain high harmonics and the voltage on the receiving tires becomes sinusoidal.
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