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Файл:The Electromagnetic Interference in the Electrical Power Supply System. Study guide
.pdf
The causes of voltage dips
occurance
Load change by switching
Short circuits
Starting large engines in an
enterprise or in a neighboring
enterprise in the same distribution
network
Connecting significant loads to
Faults on adjacent electrically
connected sections of the circuit
Reclosing devices operation
consumers
Fig. 1.3. Causes of voltage dips
The main distinction of an active voltage regulator (AVR) from an uninterruptible power supply (UPS) is that the missing energy in the AVR is taken
from the power supply system, and in the UPS from batteries. This explains
the fact that AVRs are cheaper in cost than UPSs, since the cost of purchasing
batteries is not required. The AVR compensates for any voltage dips at halfcycle speed. In the UPS, great attention should be paid to the condition of the
batteries. As there is no energy storage in the AVR, there is no need to inspect it often. But in case of a complete disconnection of energy from consumers, only the UPS can support the power.
We can conclude that voltage dips in electrical networks pose a danger to
consumers operation. The enterprises must be equipped with the protection
against voltage dips. It depends on the nature of production and its technological process.
Voltage dips usually occur due to malfunctions in electrical networks or
in electrical installations of consumers, as well as when connecting a powerful load. The voltage dip is usually associated with the occurrence and termination of a short circuit or a sharp current increase in a network or electrical
installation connected to an electrical network. In accordance with the requirements of the standard GOST 32144-2013, the voltage dip is considered
as an electromagnetic interference, the intensity of which is determined by
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both voltage and duration. The duration of the voltage dip can be up to 1 min.
In three-phase power supply systems, the moment when the voltage drops at
least in one of the phases falls below the threshold value of the beginning of
the voltage dip is taken as the beginning of the voltage dip. The moment when
the voltage in all phases rises above the threshold value of the termination is
taken to be the end of the voltage dip.
Overvoltage. Overvoltage is usually caused by switching and disconnecting power loads. Overvoltage can occur between phase conductors or between phase and protective conductors. Depending on the grounding device,
short circuits to earth can also lead to overvoltage between phase and neutral
conductors. In accordance with the requirements of this standard, overvoltage
is considered as electromagnetic interference, the intensity of which is determined by both voltage and duration. Overvoltage duration can be up to1min.
1.2. Definition and assessment of voltage dips
and overvoltage
Both phenomena – voltage dips and overvoltage – are unpredictable and
largely random. The frequency of their occurrence depends on the type of a
power supply system, an observation point, a season.
Fig. 1.4. Voltage dip
The voltage dip (Fig. 1.4) is characterized by the duration of the voltage
dip, for which the following norm is established – the maximum allowable
12
value of the duration of the voltage dip in electric networks with voltage up
s
s
to 20 kV inclusively is 30 s. The duration of the automatically eliminated
voltage dip at any point when connecting to electric networks is determined
by the time delay of the relay protection and automation.
A voltage dip is a sudden and significant decrease in voltage (less than
90%
lasting from several periods to several tens of seconds with the
)U
nom
subsequent restoration of voltage.
The causes of voltage dips are triggering of protective equipment and automation when the lightning overvoltage, short-circuit currents are disconnected, as well as during false protection trips or as a result of erroneous actions of operational personnel.
1.3. Calculation of voltage interruptions
Standard 32144-2013 does not standardize the voltage drop, it limits its
duration to 30 s. To tell the truth, the phenomena lasting more than 30 s practically do not happen – the voltage of an automatically eliminated voltage dip
at any point of connection to electric networks is determined by the time delay
of relay protection and automation. A characteristic of a voltage dip is its
duration
equal to
t
n
where
and ft – the initial and final time points of the voltage dip.
t
The voltage dip is also characterized by a depth
tt t , (1.1)
fs
U – the difference
p
between the nominal voltage value and the minimum effective voltage value,
expressed in units of voltage or in percent of its nominal value. The voltage
dip is calculated by the expressions:
UU U
UU
n
nom min
U
U
nom minn
nom
, (1.2)
100%
. (1.3)
Voltage dips and interruptions are classified in accordance with tables 1.1
and 1.2. The numbers placed in the table cells reflect the number of relevant
events.
13
Classification of voltage dips by residual voltage and duration
Table 1.1
Reference
voltage
about
stress U,%
90>u≥85
85>u≥70
70>u≥40
40>u≥10
10>u≥5
0,01<Δtп≤0,2 0,2<Δtп≤0,5 0,5<Δtп≤1 1<Δtп≤5 5<Δtп≤20 20<Δtп≤60
Voltage dip (interruption) duration, Δt
s
p,
Table 1.2
Classification of short-term voltage interruptions by duration
Reference
voltage
about stress
U,%
5>u≥0
0,01<Δtп≤0
,2
Voltage dip (interruption) duration, Δt
0,2<Δt
≤0,5 0,5<Δtп≤1 1<Δtп≤5 5<Δtп≤20 20<Δtп≤60
п
s
p,
Voltage dips and interruptions are measured in accordance with Standard
30804.4.30 based on rms voltage values updated for each half-cycle. The parameters of voltage dips, interruptions, which are the objects of consideration
in this standard, are the residual voltage and duration. In low voltage electrical
networks, four-wire three-phase systems, phase voltages are taken into account; in three-wire three-phase systems, linear voltages are taken into account; in the case of a single-phase connection, the supply voltage (phase or
linear according to the consumer connection) is taken into account. The
threshold value of the onset of the voltage dip is taken equal to 90% of the
reference voltage. The threshold value of the beginning of the voltage interruption is taken to be 5% of the reference voltage.
14
Table 1.3
Characteristics of voltage dips
Voltage dip rate, %
Voltage dip
depth, %
0-10 23.7 27.17 17.34 68.21
10-25 22.54 4.05 0 26.59
25-35 2.31 0 0 2.31
35-60 2.89 0 0 2.89
60-100 0 0 0 0
Number of
total voltage
dips,%
Three
phase
5.44 31.22 17.34 100
Two
phase
One
phase
Total number of voltage
dips, %
Note : While measuring in multiphase systems, it is recommended to determine and record the number of phases affected by each event. For electrical
networks of three-phase systems, multi-phase data reduction should be used,
which consists of determining an equivalent event, characterized by one duration and one residual voltage.
15
2. TEMPORARY OVERVOLTAGE
Temporary overvoltage is the excess of the amplitude values of the voltage by more than 10% of the nominal equal
2 U . Moreover, the duration
nom
of such an excess in order to exclude the influence of switching pulses should
be more than 40 ms.
Temporary overvoltage is characterized by a coefficient of temporary
overvoltage:
U
max
K
перU
a
, (1.4)
2
U
nom
where
U
ing ones
is the maximum amplitude voltage from a number of exceed-
maxa
1,1 2U .
nom
An auxiliary parameter of the temporary overvoltage is their duration
.
t
temU
The values of the temporary overvoltage coefficient, depending on its duration, usually do not exceed the values specified in Table 1.4.
Table 1.4
Values of the temporary overvoltage coefficient
Duration of temporary overload
Coefficient of temporary overvoltage
U, r.u
K
tem
ttemU
, s
up to 1 up to 20 up to 60
1,47 1,31 1,15
On average, up to 30 temporary overvoltages may occur in networks of
0.38 kV with a grounded neutral. If the neutral conductor breaks in these networks temporary overvoltage between the phase and ground can occur up to
the phase-to-phase voltage.
16
Temporary overvoltage is an increase in the electrical network above 1.1
for a duration of more than 10 ms, which occurs in power supply systems
during switching or short circuits (Fig. 1.4).
Overvoltage is measured in accordance with Standard 30804.4.30, subsection 5.4 based on measurements of rms voltage values updated for each
half-period. The threshold value of the beginning of the overvoltage is taken
to be 110% of the reference voltage. On average, about 30 overvoltages a year
can occur at the point of the attachment. When the neutral conductor breaks
in three-phase electric networks with voltages up to 1 kV, working with a
solidly grounded neutral, temporary overvoltage occurs between phase and
ground.
Fig. 1.4. Temporary overvoltage
The level of such overvoltages with significant asymmetry of phase loads
can reach the values of linear voltage, and the duration can be several hours.
In low voltage systems, under certain circumstances, a malfunction that occurs electrically above the transformer can give rise to a temporary overvoltage on the low voltage side for the time during which the current caused by
the malfunction flows. Such overvoltage in the general case do not exceed
1.5 kV. For medium voltage systems, the expected magnitude of such overvoltage depends on the type of grounding in the system. In systems with a
rigidly grounded neutral or with a neutral ground through resistance, the overvoltage usually does not exceed 1.7 Us. In the systems with insulated neutral
17
or with neutral ground through the reactor, the overvoltage usually does not
exceed 2.0 Us. The type of grounding is indicated by the network operator.
The duration of a temporary overvoltage is the time interval between the
initial moment of occurrence of a temporary overvoltage and the moment of
its disappearance:
The temporary overvoltage coefficient is also not standardized.
The values of the coefficient of a temporary overvoltage at the points of
connection of a general electric network, depending on the duration of temporary overvoltage, do not exceed the values given in table 2.4.
When the neutral conductor breaks in three-phase electric networks with
voltages up to 1 kV, operating with a solid grounded neutral, temporary overvoltage occurs between the phase and ground. The level of such an overvoltage with significant asymmetry of phase loads can reach the values of interphase voltage, and the duration can reach several hours.
Temporary overvoltages can cause disruptions in the operation of various
electrical devices in power supply systems. So, for example, an overvoltage
of 1.5 kV for a duration of 3 seconds can cause the lamp to burn out in electric
lighting systems. Overvoltages from high-voltage networks in the 380/220 V
network can cause a failure or malfunction of various electrical devices.
ttt
temU ktem utem
. (2.5)
2.1. Overvoltage during the operation
of circuit breakers
The switching process is accompanied by the occurrence and arc blowout
in the switch. In vacuum and air circuit breakers, widely used in enterprises,
the arc is not stable at low currents. The cessation of discharge in a vacuum
leads to a cutoff of the current until it naturally passes through zero. In this
case, overvoltage may occur due to current cutoff, multi-rate repeated ignitions and three-phase simultaneous shutdown. This switching overvoltage
leads to a breakdown of the insulation of an electrical device and can cause
malfunctions in the central technical
When large currents are switched off (> 100 A), the arc blowout is extinguished during the natural transition of the current through zero. In this case,
dangerous overvoltages do not arise, and the recovering voltage does not exceed the double amplitude of the mains operating voltage.
18
When you turn off the small current (<25 A), current cuts often occur an early transition of the current through zero - at any point on the sine wave.
Such a case takes place, for example, when the no load operation of the transformer (or shunt reactor) is turned off: the idle current of the transformer is
within units or tens of amperes, the capacity of the transformer is very small,
and its dissipation inductance is quite large:
UI
k
L
C
relative unit .
45
L
C
10 100
Ohm,
Similarly, processes occur when the BC is turned off.
There given below the characteristics of circuit breakers widely used in
enterprise power supply systems:
Connect .............................................................. Break-time, s
time, s
Oil tank circuit-breaker ...................................... 0,3–0,5 0,08
Low oil circuit-breaker....................................... 0,3–0,4 0,12–0,17
Air circuit-breaker .............................................. 0,1–0,3 0,04–0,12
Gas-insulated circuit-breaker ............................. 0,08–0,3 0,03–0,075
Vacuum circuit-breaker...................................... 0,08–0,3 0,05–0,075
From the above data it is seen that the switching time of air, gas and vacuum circuit breakers is significantly less than oil ones. Due to this , switching
is accompanied by the appearance of voltage and current pulses having a
steep, almost plumb front.
The occurrence of a switching overvoltage in enterprise networks is associated with operational and emergency switching of individual network elements.
Switching off of inductive loads that is unloaded transformers, reactors,
electric motors leads to the overvoltage on them, as well as on the switch and
cable line between them. The nature of these overvoltages lies in the transition of the (Li2 / 2) electromagnetic energy stored in the inductors into the
(CU2 / 2) electrical energy. Moreover, the overvoltage usually does not ex-
ceed 2.0–2.5
U.
ph
19
2.2. Overvoltage when switching electric motors
M
s
Z
s
In enterprises, a significant part of the load is made up of high-voltage
motors.
The shutdown of the electric motors is accompanied by the overvoltage
caused by the disconnection of the starting current (similar to the shutdown
of the magnetizing currents of transformers and reactors).
When the engine is running, magnetic energy is stored in its scattering
inductance:
L
2
Wi , (2.6)
s
av
2
where
is the scattering inductance of the engine;
L
– the cutoff current.
i
av
The transition of magnetic energy into electrical energy (and vice versa)
is accompanied by overvoltages in the form of high-frequency oscillations.
Overvoltages are determined by the expression:
where
UZi
п sav
is the wave resistance of scattering;
L
;
LC , (2.7)
cse
– equivalent capacitance of
С
е
the cable-motor system.
Maximum overvoltages are generated on the engines of comparatively
low power, in which the wave resistance of the scattering is of greater importance.
2.3. Overvoltage during switching
filter-compensating device (FCD)
To reduce the coefficient of non-sinusoidality in the power supply networks of enterprises are widely used resonant filters of higher harmonics.
During the switching of these FCDs, overvoltage may occur.
The equivalent circuit for the calculation of transients is shown in Fig. 2.5.
Valve converters are switched according to 6 or 12-phase schemes (real
or conditional). Short-circuit inductance
and high-voltage motors, if available at the substation.
considers both the power system
L
k
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