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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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The first group is power consumers, replaced by constant resistance.
Their power, consumed from the network depends on the square of the voltage at the terminals.
The second one is power consumers equipped with an automatic regulator necessary to control the power consumed. For example, an automatic
temperature controller to heat the parts in an electric furnace. In this case,
when the voltage changes, the current consumption from the network is
changed by the regulator as to ensure the required heating level of the parts.
In this case, the allowed change in voltage in the electrical network is determined by the characteristics of a controller.
The third group is power consumers designed for a wide range of power
supply voltages. For example, power supply units of some electronic devices (TVs, computers, etc.) ensure the normal operation of these devices with
the network voltage in the range of 100-260 V. For such power consumers,
the current consumed from the circuit significantly depends on the voltage
in the network.
11
2. VOLTAGE FLUCTUATIONS
2.1. Main Definitions and Standards
Voltage fluctuations are rapid changes that cause the blinking of the
brightness of electric lighting lamps or some other negative manifestations
for other consumers.
The cause of voltage fluctuations is
cause rapid changes in voltage losses (Figure 2.1):
()It peak (shock) loads, which
() () ()
Ut I tR I tX
а Ep E
. (2.1)
Fig. 2.1. The voltage fluctuations
A sample list of technological equipment with a peak (shock) load,
causing voltage fluctuations, is given in Table 2.1. From the above list, the
highest frequency of voltage fluctuations is caused by the operation of piston compressors.
Figure 2.1 shows the graph of the RMS voltage envelope – a step time
function formed by root-mean-square voltage values that are discretely determined at each half-period of the fundamental frequency, that is, at consecutive 10 ms time intervals.
12
Table 2.1
The technological equipment which causes voltage fluctuations
Voltage
fluctua-
tions
frequen-
1
cy, F,
с
20 Piston - type compressors – +
2,0 The electric-welding units – +
0,7 Arc furnaces + +
0,3 Bloomings, slab mills + +
0,03 Continuous hot-rolling mill + +
Technological equipment
with a peak(sock) load
Fast-speed
synchro-
nous com-
pensating
devices
Fast-speed
static compen-
sating devices
0,02
0,0 Cold - rolling mills + +
Mechanical handling facilities
(cranes)
If
time interval between the end of one change and the beginning of
u
the next, occurring in the same direction, is less than 30 ms, then these
changes are considered as one change. Therefore, Fig. 2.1 shows two volt-
age fluctuations: the first demonstrates the change in
value, the second one demonstrates the change in
UU
UU
voltage value.
2ii
voltage
1ii
Voltage fluctuations are characterized by the following values (parameters):
– the excursion of a voltage change;
– the frequency of a voltage change;
– the time interval between voltage fluctuations (between the initial
moments of time following one after another voltage changes);
– a flicker indicator.
13
The first voltage fluctuation excursion in Figure 2.1 is
P
U
1
t
UU
1
ii
U
н
100%
, (2.2)
the second one is
UU
12
ii
1
U
U
t
100%
. (2.3)
н
Voltage fluctuations are characterized by the following parameters: the
excursion of a voltage change, the frequency of a voltage change, the time
intervals between voltage fluctuations (between the initial moments of the
time following one after another voltage changes) and the flicker indicator.
Frequency of voltage changes (voltage fluctuation frequency):
m
F
. (2.4)
T
where m – is the number of voltage changes (fluctuations) during time T;
T – is the observation time interval taken as10 minutes.
The time interval between voltage changes:
ttt
,1 1ii i i
. (2.5)
Here
t
is the beginning of the 1st voltage fluctuation;
i
t
– the begin-
1i
ning of the 2nd voltage fluctuation.
Flicker indicator
T
1
PgUdt
where T is the
short-term flicker indicator, at T = 2 hours, a long-term flicker indicator
ST
U
measurement time interval. At T = 10 min there is a
t
Tftf
T
0
22
f
14
,
%, (2.6)
is
L
P
f
g
F
curve;
;
T
is the f th harmonic expansion in a Fourier series of the
U
tf
– is the derating factor, taking into account the degree of the f
U
t
th
harmonic impact on a human visual apparatus [5].
In the outdated National Standard [3], the maximum permissible values
of the range of
U
voltage changes are depending of the frequency given
доп
of F voltage fluctuations in the form of two curves in Fig. 2.3. In this case,
the voltage fluctuations should have the form of a meander (Fig. 2.2). If the
voltage fluctuations have a form different from the meander, then the derated/range of the voltage change is calculated:
where
is a derating factor of voltage fluctuations of arbitrary shape to
пр
UFU
tпр пр t
the fluctuations in the form of a meander,
changes of an arbitrary shape.
а
b
Fig. 2.2. Voltage fluctuations of arbitrary shape (a);
the form of meandra (b)
, (2.7)
U
is the range of voltage
t
15
Fig. 2.3. Maximum allowed values of the
depending of the frequency of voltage changes repetitions per
minute for voltage fluctuations having the form of a meander
U
range of voltage changes
t
The first curve in Fig. 2.3 defines
UfF
points of their connection to the power supply network. If
()
д
for consumers at the
UU
tпр д
,
then such voltage fluctuations are considered as permissible ones.
The second curve is for consumers using incandescent lamps in the
rooms that require high level eyestrain. The list of premises with the
works requiring significant eyestrain is specified in the regulatory documents.
The maximum allowed value of
should not exceed
10 %
.
UU
in t
in 380 V voltage circuits
The rates of a flicker indicator are set for voltage fluctuations of any
shape. Flicker indicators should be measured in electrical networks with a
special device called a flickermeter, which carries out the computational
procedure (2.1).
The maximum permissible values for short-term and long-term flicker
indicators are 1.38 and 1.0, respectively. If a consumer uses incandescent
lamps in the rooms where high level eyestrain is required, then the permis-
sible short-term flicker indicator is
is
P
lf
0,74
.
P
sf
and long-term flicker indicator
1, 0
16
2.2. The Ways of Voltage Fluctuations Decrease
I
E
Z
E
I
I
I
in the Power Supply Systems
In accordance with (2.1), there are two possible ways to reduce the
range of
with the help of high-speed compensating devices.
due to the amplification of the supply network or
help of a longitudinal compensation.
U
voltage changes.
t
The first one is to reduce
The second way is to decrease
reactive component of the peak current
п
RjX
EE
Voltage loss on equivalent network resistance:
without a battery compensating device (BCD)
equivalent resistance
X
decrease with the
UIR IX
па E пр E
; (2.8)
when using BCD
where
II jI
PPа Pр
UIR IX
па E пр E
is the value of the peak current and its active and
reactive components of a power consumer;
; (2.9)
– the value of BCD current,
K
having a capacitive character.
()
t
BCD consumers
regulated at the rate of the process in accordance with the change in
capacitive current from the network, which is
K
()
It
Pр
current, which reduces the second term in expression (2.2), that is, reduces
U
the
range of the voltage change. There are two types of BCD: high-
t
speed synchronous compensators (BSC) and high-speed static compensating
devices (BSSC) [8].
()
t
BSSC has
greater speed of regulation, lower losses and are char-
K
acterized by better ease of operation, and therefore, at present, they are primarily used. Table 2.1 shows the approximate frequency ranges for which
the use of BSC and BSSC is possible.
There are two types of longitudinal compensation: group and individual.
In the ideal case of full compensation is considered when
XX
BE
the
loss of voltage in the network from the center of the power supply to the
U
receiving buses is significantly less than
shown in Figure 2.1, due
U
17
P
Z
P
P
to the fact that
()0
IX X
Cp E B
, where
III
CP
is equal to the sum of
the currents of peak and normal loads.
When considering the scheme of individual compensation, we will set
RjX
the peak load with the
PP
equivalent resistance. A battery of
capacitors is connected in series with the peak load and compensates for its
X
inductive resistance. With
XX
ideal compensation, the peak cur-
B
rent of the welding machine has only an inductive component, which reduces the voltage loss in the network at the moment of peak current flow.
The U voltage (directly on the welding machine) increases:
where
UU UUU IX
bbPB
U
is the voltage drop across the capacitor bank.
B
,
, (2.10)
This circumstance necessitates the redesign of welding machines to a
higher nominal voltage (
UU
) [9], which requires additional costs. In
b
addition to reducing voltage fluctuations by 4–5 times, the use of individual
compensation for welding machines makes it possible to reduce losses by
4–8 times, increases their
to unity and reduces by 2–3 times the volt-
cos
age unbalance in the network [9].
18
3. VOLTAGE NON-SINUSOIDALITY
Z
E
Z
3.1. The Main Definitions and Standards
The cause of non-sinusoidal voltage in electrical networks is the socalled non-linear load. Non-linear load is the load,
of which depends on the current or the voltage magnitude. Therefore, at a
sinusoidal voltage of the power supply, nonlinear loads consume nonsinusoidal current (Fig. 3.1). Non-sinusoidal current, flowing
a
non-sinusoidal voltage loss and, accordingly,
U
voltage. For this reason, the power supply voltage of other (linear) consumers is non-sinusoidal, which is a disturbance for these consumers, causing a
damage. A linear consumer has to consume non-sinusoidal current when the
voltage is non-sinusoidal.
equivalent resistance
н
causes
U
non-sinusoidal
b
Fig. 3.1. Distortion of voltage waveform
by odd harmonics
19
The main sources of high voltage harmonics (non-linear loads) in elec-
trical networks of power supply systems are:
valve converters, the unit capacity of which in metallurgical enter-
prises reaches 6–8 MW;
electric arc furnaces with unit capacity from 2 to 25 mW;
sets for electric arc-welding of metals;
installations with electric discharge lighting lamps.
In Figure 3.2 (a) line 1 is a normal linear load, for which the relationship
between the instantaneous values of the current and the voltage applied is
linear. Curve 2 is characteristic of high-power rectifiers with high inductance in the rectified current circuit. Curve 3 – for electric receivers that use
an electric arc: electric arc furnaces, electric arc welding sets, etc.
Fig. 3.2. (b) shows the curves of the corresponding instantaneous values
of the currents with a sinusoidal voltage of the electric power supply:
() sin
ut U t
m
. (3.1)
The linear load current replicates a voltage curve in shape and is sinusoidal:
const
where
wave interval.
Z
н
i(t)
it I t
1
is load resistance and does not change in time in a half-
3
Z
m
н
u(t), i(t)
, (3.2)
()
ut
() sin( )
u(t)
1
2
i3(t)
(t)
i
1
(t)
i
2
u(t)
а б
π
Fig. 3.2. Non-linear load currents:
a – dependence of instantaneous values of currents on voltage; b – power
supply voltage and load current curves in a half-wavelength interval
20
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