Добавил:
ivanov666
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:The Electromagnetic Interference in the Electrical Power Supply System. The long-term variance of the voltage specifications. Study guide
.pdf
4. VOLTAGE UNBALANCE
4.1. The Main Definition and Norms
The three-phase system of voltages at the outputs of the generators of
power stations is always symmetrical, since this is determined by their design. Asymmetry of the three-phase voltage system may occur in the nodes
of the electrical network due to asymmetrical loads, causing voltage losses
that are not the same in phases.
Asymmetry of stresses in accordance with GOST 32144-2013 is characterized by the following indicators:
reverse voltage unbalance factor
U
2(1)
KU
2
U
zero sequence voltage unbalance factor
KU
0
U
100 %
1(1)
3
U
0(1)
1(1)
; (4.1)
100 %
, (4.2)
where
quence of the main harmonic;
age of the zero sequence of the main harmonic;
is the effective value of the phase voltage of the reverse se-
U
2(1)
is the effective value of the phase volt-
U
0(1)
is the effective value
U
1(1)
of the interphase voltage of the direct sequence of the main harmonic.
Allowed to take
UU
1(1) nom
when the voltage distortion factor of the
voltage curve is no more than 5 %.
51
To determine the stress asymmetry coefficients by the reverse sequence,
K
N
K
N
K is calculated for the i-th observation using the formula
2
U
i
U
2(1)
i
nom
100
, (4.3)
KU
2
U
i
where
U
is the nominal value of phase-to-phase voltage, V;
nom
is the
U
2(1)i
effective value of the interfacial voltage of the inverse sequence of the fundamental frequency in the i-th observation.
Next, the value of the
voltage unbalance coefficient is calculated
2U
by reverse order in percent when averaging N ≥ 9 observations over a
3-second time interval
The calculation of the
N
1
i
K
2
U
stress asymmetry coefficients using the zero
0U
2
K
2
U
i
. (4.4)
sequence is performed similarly in accordance with the following expressions:
where
N
3
U
0(1)
i
nom
100
;
K
0
KU
0
Ui
is the effective value of the phase voltage of zero sequence of
U
0(1)i
1
i
U
2
K
0
U
i
, (4.5)
the fundamental frequency in the i-th observation.
Normally permissible and maximum permissible values of the coefficient of asymmetry of voltages in the reverse order at the points of common
connection to electric networks are 2.0 % and 4.0 %, respectively.
Normally permissible and maximum permissible values of the voltage
unbalance factor in the zero sequence at the points of common connection to
four-wire electric networks with a nominal voltage of 0.38 kV are 2.0 % and
4.0 %, respectively.
52
When determining
p
1) determine
2) calculate
U
2(1)i
U
2(1)i
K
2Ui
and
and
U
0(1)i
, it is allowed:
K
0Ui
by the method of symmetric components;
by the approximate formula
,
where
UUU
2(1) max(1) min(1)
UU
,
max(1) min(1)
ii
0.62( – )
iii
– the largest and smallest effective values of the
three interphase voltages of the fundamental frequency in the i-th observation, V;
3) calculate
with the symmetry of the phase to phase voltages us-
U
0(1)i
ing the approximate formula
,
where is
UUU
UU
max (1) min (1)
,
hi phi
0. )62( –
0(1) max (1) mi n (1)
iphiphi
– the largest and the smallest of the three
effective values of the phase voltages of the fundamental frequency in the ith observation, V.
The quality of electrical energy according to the voltage unbalance coefficient in the reverse sequence at the common connection point is considered to comply the requirements of this standard, if the greatest of all the
values of the voltage unbalance coefficients measured in 24 hours does not
exceed the maximum permissible value, and the voltage unbalance coefficient value in reverse sequence corresponding to a 95 % probability over a
set period of time does not exceed the normal tolerance forward value.
A similar conclusion holds for the voltage unbalance coefficient in the zero
sequence.
Additionally, it is allowed to determine compliance with the standards
for the total duration of the output time of the measured values of these indicators for normal and maximum permissible values.
At the same time, the quality of electrical energy according to the indicated coefficients is considered to comply the requirements of this standard,
if the total duration of their normal time is not more than 5 % of the established time period, i.e. 1 h 12 min, and beyond the maximum permissible
values – 0 % of this time period.
Under the voltage asymmetry we understand the inequality of phase or
linear voltages in amplitude and shear angles between them (the asymmetry
of the three-phase voltage system).
53
Voltage asymmetry occurs only in a three-phase network under the influence of uneven distribution of loads across its phases. As a probable
cause of voltage asymmetry, GOST 32144-2013 indicates a consumer with
an asymmetrical load.
The sources of voltage unbalance are: steel arc-melting furnaces, traction substations of alternating current, electric welding machines, singlephase electric thermal installations and other single-phase, two-phase and
asymmetric three-phase electric power consumers, including household
ones. Thus, the total load of individual enterprises contains 85…90 % of
asymmetric load, and the voltage asymmetry coefficient for the zero sequence
of a single 9-storey residential building can be 20 %, which on
K
0i
tires of a transformer substation (point of general connection) may exceed
the normally allowable %
The effect of voltage unbalance on the operation of electrical equipment: electric power losses in the networks from additional losses in the
neutral wire increase. Single-phase, two-phase consumers and different
phases of three-phase electricity consumers operate at different non-nominal
voltages, which cause the same consequences as if the voltage failed. In addition to the negative effect of asymmetric voltages, electric fields arise in
electric motors, which rotate counter to the rotation of the rotor.
The overall effect of voltage unbalance on electrical machines, including transformers, leads to a significant reduction in their service life. For
example, during long work with an asymmetry factor in the reverse sequence
= 2...4 %, the service life of an electric machine is reduced by
K
2i
10...15 %, and if it operates at nominal load, the service life is halved.
Therefore, GOST 32144-2013 establishes the values of the stress
asymmetry coefficients in the reverse
and zero
K
2i
sequences – the
K
0i
normal allowable 2 % and the maximum allowable 4 %.
The normalized asymmetry index is the ratio of the inverse voltage sequence, equal to the ratio of the negative sequence voltage U
nal linear voltage
.
U
nom
to the nomi-
2
When the quality indicators go beyond the established limits, the consumption and losses of electricity in the power supply systems increase, the
level of reliability of electrical equipment decreases, there are violations of
technological processes and the output of products decreases.
The most common sources of voltage unbalance in three-phase power
supply systems are electricity consumers whose symmetric multi-phase execution is impossible or impractical for technical and economic considerations. Such installations include induction and arc electric furnaces, traction
54
loads of railways, on alternating current, electric welding aggregates, special
single-phase loads, lighting installations.
Asymmetrical voltage regimes in electrical networks also occur in
emergency situations – with a phase failure or asymmetrical short circuits.
Voltage asymmetry is characterized by the presence of a reverse or zero
sequence in a three-phase electrical network, which is significantly smaller
in magnitude with the corresponding voltage components of the direct
(main) sequence. Asymmetry of the three-phase voltage system arises as a
result of imposing a reverse sequence on the system of the direct voltage
sequence of the system, which leads to changes in the absolute values of
phase and phase-to-phase voltages (Figure 4.1).
Fig. 4.1. Vector diagram of the voltage of the direct and inverse
sequence
In addition to the asymmetry
caused by the voltage of the negative
sequence system, asymmetry may arise
from the imposition on the system of a
direct sequence of voltages of a zerosequence system. As a result of the displacement of the neutral of the threephase system, non-symmetry of the
phase voltages occurs while maintaining a symmetric system of phase-tophase voltages (Fig. 4.2).
55
Fig. 4.2. Vector diagram of voltages
of direct and zero sequences
The coefficient of asymmetry of the voltage in the reverse order is
K
p
equal, %:
U
2(1)
1(1)
100
, (4.6)
KU
2
U
where
fundamental frequency of a three-phase voltage system, V;
is the effective value of the negative-sequence voltage of the
U
)2(1
U
is the ef-
)1(1
fective value of the voltage of the direct sequence of the main frequency, V.
It is allowed to calculate
where
U
nom line-to-line
KU
– the nominal value of the interfacial voltage, V.
by expression, %
K
2i
U
U
2
nom line-to-line
2(1)
100
, (4.7)
The coefficient of asymmetry of the voltage in the zero sequence is
equal, %:
3
U
1(1)
0(1)
, (4.8)
100
where
KU
U
0
is the effective value of the zero-sequence voltage of the fun-
U
0(1)
damental frequency of the three-phase voltage system, V.
It is allowed to calculate
by the formula, %,
0U
U
KU
0
U
0(1)
nom
100
, (4.9)
p
where
U
– the nominal value of the phase voltage, V.
nom
Measurement of the voltage unbalance coefficient by zero sequence is
carried out in a four-wire network.
The relative error in determining
ue of the voltage deviation from
U
nom
and numerically equal to the val-
K
2i
.
56
Normally permissible and maximum permissible values of the coeffi-
cient of asymmetry of voltages in the reverse sequence at the point of common connection to electric networks are 2.0 and 4.0 %.
There are two types of voltage unbalance in three-phase networks: longitudinal (phase failure, different phase resistance) and transverse (not the
same phase load, single-phase and two-phase short-circuit).
Phase failure, as well as single-phase and two-phase short-circuit are
emergency modes and should be automatically switched off by means of
relay protection of electrical networks. Transverse asymmetry, due to the
load unbalanced in phases, creates a voltage loss that is unequal in the network phases and causes asymmetry in the voltage on the receiving buses.
The asymmetry of three-phase voltages has a negative impact on the operation of various electrical equipment and networks:
losses in zero working wires in 380/220 V networks increase;
increases the deviation of voltages in individual phases, which reduc-
es the efficiency of some electrical receivers;
in synchronous and asynchronous electric motors, counter-rotating
magnetic fields arise, causing additional losses, additional heating of the
motors, additional vibration and, as a result, reduction of the service life;
disrupts the normal operation of capacitor batteries;
losses in power transformers increase.
4.2. The Ways Voltage Unbalance Decreasing
Measures to reduce voltage unbalance:
1. Uniform distribution of the load across the phases.
2. The use of balancing devices (resistances in the phases of the balancing device are selected in such a way as to compensate for the reversesequence current generated by the load as a source of distortion).
The asymmetry of voltages due to asymmetrical electrical receivers can
be limited to values of
K
2
U
special balancing devices (SU).
When the ratio of short-circuit power in the network node Sc and single-
phase load
, 50 ,SS S
oph c oph
voltages does not exceed the normalized GOST 32144-2013. Electrical receivers causing asymmetry are connected by network nodes, where the
short-circuit power satisfies the above relation.
, both using circuit solutions and using
0.02
the coefficient of the negative sequence of
57
The balance by means of balanced converter (BC) to compensate for the
equivalent current of the negative sequence of an unbalanced load and the
voltage caused by it.
Distinguish individual, group, centralized and combined methods of
balancing. And these are installed directly with asymmetrical power receivers. When group balancing in different points of the network, several balanced converters are installed, each of which balances a certain network
segment with a group of asymmetrical power receivers connected to it.
In the centralized balancing, a single balanced converter is installed in the
distribution network. The combined method of balancing is the combination
of two or three BC.
And it is possible to eliminate the asymmetry directly at the consumer,
but the installed capacity of the power elements of the balanced converter
is used irrationally. In centralized balancing, less installed power is required
for control elements, but current asymmetry is maintained in the network
with unbalanced loads.
Group method symmetrization combines the advantages and disadvantages of individual and centralized methods. The choice of balancing
method is mainly determined by the network parameters and the nature of
the loads.
Measures and procedures are performed uncontrolled or controlled depending on the peculiarities of the load curve. There a large number of is
BC circuits that have both electrical and electromagnetic connections between elements. Consider some of the famous BC.
Existing BCs can be divided into two large groups: with electrical and
electromagnetic connections. In BC with electrical connections, balancing
elements and loads are included either in a triangle or a three-beam star. By
design, BC is divided into one-, two-, and three-element circuits.
Controlled BSs can have both continuous (analog) and step (discrete)
control.
Capacitor banks discrete BC gathered from several groups, one of which
is connected permanently, and the other has temporal connection. Switching
is carried out using thyristor keys.
Currently, smooth control of capacitive elements is used by connecting
in parallel to the capacitors of reactors controlled by thyristors.
Three elements balance of converters are the most versatile and flexible;
Their disadvantages include a rather low utilization factor.
For balancing the system of linear voltages with one-, two- and threephase unbalanced loads, capacitor banks with unequal phase powers are
widely used to compensate for reactive power in the network.
58
The balancing devices with electromagnetic coupling are divided into
two groups: with dividers (autotransformers) and transformers. In circuits
with dividers, the power of BC is usually chosen equal to the power of the
load. By switching the autotransformer tap, it is possible to balance a nonlinear load with a varying power factor.
Transformer type balancing devices are very diverse. As an example,
Figure 4.3 shows the two-phase load from the Scott transformer and vector
diagrams of currents and voltages. The ratios of the turns of the transformers are chosen so that their secondary voltages are equal.
Transformer type balancing devices are individual and unregulated.
Fig. 4.3. Scheme and vector diagram of currents and voltages when
supplying a two-phase load from Scott transformer
The balancing of the voltage system can also be carried out by means of
a wiring system and a system of voltages electromotive force (EMF): between the source and the receiver, additional sources of EMF are included
in the rupture of linear wires, forming a reverse sequence system. As a result of summing the EMF of the main and additional sources, their symmetrical components of the negative sequence are compensated, the voltage at
the receiver becomes symmetrical. A synchronous generator, seriescontrolled transformers, a transformer with phase-by-phase transformation
ratio control can be used as the source of the additional EMF system.
Other BC scheme are also known but are used very rarely.
Filter-symmetrizing devices (FSU) are also used, which are HH filters
collected on the base of a balancing device BC, the so-called asymmetric
filters. The choice of line voltages for which the FSO filtering circuits are
switched on and the power ratio of the capacitor banks included in the filter
phases is made according to the balancing conditions.
59
However, to ensure the normal operation of the FSU battery and effec-
tively reduce non-sinusoidal conditions, certain conditions are imposed in
the network.
0
In practice, full balancing, i.e. when
E
2
nomically impractical; partial balancing is used when
is technically and eco-
U
E
2
U
0, 2
value is
provided by indirect compensation or the use of SU.
One of the most important questions when choosing the parameters of
multifunctional correction devices is the calculation of reactive power.
60
Соседние файлы в предмете [НЕСОРТИРОВАННОЕ]