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Файл:The Electromagnetic Interference in the Electrical Power Supply System. Study guide
.pdf
T-shaped filters (Fig. 3.5). In these filters, the interference current is reduced twice due to two inductancesincluded in the interference current circuit
and due to the “shorting” of the interference current to the filter capacity,
which has a low resistance at the frequency of the interference current.
P-shaped filters (Fig. 3.5) in which the interference pulse is “shorted”
twice by the low resistance of two capacities, thereby reducing the interference voltage two times, and the interference current decreases due to the high
inductance resistance connected in series with the load.
The correct choice of filters depends on a number of features of the power
supply circuit of the DTS and its features.
So with a large internal inductive reactance of the EMF interference and
a small inductive resistance of the equivalent load, it is necessary to use a
L-shaped filter with a capacitance at the input. In this case, the voltage drop
from the interference current at the internal resistance of the EMF of interference is large, and at the input filter capacitance it is small. The interference
current will decrease further due to the large filter inductance resistance.
With a small inductive internal resistance EMF interference, the most effective is a L-shaped filter with inductance at the input. Here, the low resistance of the capacitance connected to the filter output significantly reduces
the voltage of the impulse noise at the filter output.
With a small capacitive internal resistance of the equivalent EMF and a
small capacitive equivalent resistance of the load, the T-shaped filter is the
most effective, providing two stages of reducing the interference current.
With a large internal inductive impedance of the EMF interference and a
large equivalent inductive resistance of the load, the U-shaped filter is the
best, which provides three stages of suppressing the pulsed interference current. The first stage is the shorting of the voltage pulse at the capacitance C1,
the second is the reduction of the interference current due to the large inductance resistance, the third is the reduction of the interference voltage due to
the small capacitance resistance C2.
The interference filters used in practice, as a rule, have a multi-link structure, with the help of which the value of the interference current penetrating
the internal circuits of the DTS decreases to a safe value.
Interference filters are installed in many network elements and in the designs of various elements of the DTS and have a multi-stage structure. This
is the most massive way to reduce the level of impulse noise directly at the
input of power to the DTS, that is, at the lowest level of power supply systems.
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The degree of suppression (attenuation) of interference is usually estimated in decibels (dB)
U
20 lg , dB.
K
en
U
ex
(3.2)
The higher we go up through the power supply systems from the central
heating system to the power supply sources, the greater the level of impulse
noise due to the reasons discussed above, and here the interference filters become insufficient. Therefore, a more effective way to reduce the level of interference is used here, namely the use of protection devices from impulse
noise (SPD).
The most powerful source of impulse noise from power supply systems
is lightning discharges.
Lightning discharges are powerful impulse overvoltages resulting from
direct lightning striking the power supply network, lightning conductor or
impulse from a lightning bolt at a distance of up to 1.5 km, leading to a failure
of electrical equipment or a malfunction of the apparatus . A direct hit is characterized by instantaneous pulse currents of up to 100 kA with a discharge
duration of up to 1 ms.
In the presence of a lightning rod system, a discharge pulse is distributed
between the lightning rod, the power supply network, communication lines
and household communications. The nature of the distribution largely depends on the construction of the building, the laying of lines and communications.
Switching in the power grid causes a series of pulse overvoltage of various capacities, accompanied by a wide range of radio frequency interference.
The nature of the interference is shown in the example below. For example,
when the isolation transformer with a power of 1 kVA 380/220 V is disconnected from the mains, all the energy stored by the transformer is “released”
into the load in the form of a high-voltage pulse with a voltage of up to 2 kV.
The power of transformers in the power grid is much larger, thus the emissions are more powerful. In addition, switching is accompanied by the appearance of an arc, which is a source of radio frequency interference.
The electrostatic charge accumulating during the operation of technological equipment is interesting in the fact that it has little energy, but it is discharged in an unpredictable place.
32
Fig. 3.6. The nature of the distribution of pulsed voltages
The shape and the amplitude of the pulse overvoltage depend not only on
the source of interference, but also on the parameters of the network itself.
There are no two identical cases of pulse overvoltage, but for the production
and testing of protection devices, standardization of a number of characteristics of current, voltage and form of overvoltage for various applications has
been introduced.
Fig. 3.7. SPD Image
To combat surge voltage, special devices have been developed.
33
Current Standard R 51992-2002 “Devices for protection against pulse
p
I
overvoltage in low-voltage power distribution systems” introduces the concept of SPD:
“A pulse overvoltage protection device (SPD) is a device that is designed
to limit transient overvoltages and to remove current pulses. This device contains at least one non-linear element”.
Nonlinear electrical circuit elements are called elements the parameters
of which depend on voltages, currents, magnetic fluxes, or other quantities.
Currently, such elements for creating an SPD are used in the form of separate
components, various types of arresters, zinc oxide varistors and TVS diodes.
For more effective pulse overvoltage protection these carefully coordinated
components can be housed in a single protection module.
For overvoltage protection are used:
Coal discharger.
Gas-filled spark gap.
Varistor.
TVS diode.
The modern classification of protective devices is built in accordance
with the zone lightning protection concept (IEC-1024-1, IEC-1312-1). The
main classes of protective devices are given in IEC 1643-1 (37A / 44 / CDV:
1996-03) “Pulse overvoltage protection devices for low-voltage power distribution systems. Performance requirements and test methods”. Depending on
the installation location and the ability to pass through various pulsed currents, the overvoltage protection devices are divided into the following classes – A, B, C, and D (Fig. 3.8).
Main characteristics of SPD
1. Rated Discharge Current In. The peak value of the current flowing
through the SPD, with a waveform of 8/20, which the SPD can withstand
many times and not fail.
2. The pulse current
I is determined by the peak value of the
imp
I
eak
current and the charge Q. They are used in the classification of SPDs for class
I tests with a wavelength of 10/350 microsecond.
3. Maximum discharge current
for class II tests: Peak value of the
max
current flowing through the 8/20 wave-type surge protector, which the surge
protector can pass once and not fail.
34
Fig. 3.8. Main characteristics of SPD
P
f
4. The maximum continuous operating voltage
The maximum volt-
U
c
age of the effective value of alternating or direct current, which is supplied to
the terminals of the SPD for a long time. It is equal to the rated voltage.
5. Own power consumption
plying the maximum long-term operating voltage
. Power consumed by the SPD when ap-
c
at balanced voltage
()
U
c
and phase angles in the absence of load. The SPD is connected according to
the manufacturer's instructions.
6. Accompanying current.
I (Parameter for surge arresters based surge
arresters). This is the current that flows through the arrester after the end of
the overvoltage pulse and is supported by the current source itself, i.e. electric
power system. In fact, the value of this current tends to the rated short circuit
35
current (at the installation point of the arrester for this particular electrical
I
I
s
installation). Therefore, for installation in the circuit “L-N; L-PE” it is forbidden to use gas-filled (and other) arresters with I
value equal to 100-400 A.
f
As a result of prolonged exposure to the accompanying current, they will be
damaged and may cause a fire! For installation in this circuit, it is necessary
to use arresters with an I
value exceeding the rated short circuit current, i.e.
f
preferably from 2–3 kA and higher!
7. Nominal load current
. Nominal alternating current (effective value)
L
or direct current, which can be supplied to the load protected by the SPD.
8. Level of protection
. This is the maximum value of the voltage drop
U
r
across the SPD when a pulsed discharge current flows through it. The parameter characterizes the ability of the device to limit the overvoltage appearing
on its terminals. It is usually determined by flowing a rated impulse discharge
current
(.)
n
9. Response time. For zinc oxide varistors, its value usually does not exceed 25 ns. For arresters of different designs, the response time can range
from 100 nanoseconds to several microseconds.
SPD selection. The most frequently asked question when applying SPDs
is by what criteria to choose one or another type of SPD, what parameters of
SPDs play the main role, and which are secondary. The following is an example method for selecting an SPD using the example of TN-C, TN-S, TNC-S networks.
1. Estimate the maximum lightning current in a given area. Imax should
be greater than the expected lightning current, which can go along the protected circuit.
2. Determine the rated voltage of the network at the point of inclusion of the
SPD. Maximum continuous operating voltage
must be greater than
U
.
U
n
3. Determine the most probable nature of the waveform of the overvoltage
impulse at the point of inclusion of the SPD. If the object has an external
lightning protection system and / or there is an air input of power supply, in
which a direct lightning strike (DLS) is possible, then a class I SPD must be
placed at the input to the object. If at the facility the probability of PUM is
extremely small, it is allowed to put class II SPDs at the entrance to the building. Class III surge arresters are placed directly in front of single consumers
as an additional protection in the event of their significant removal from the
switchgear or main switchboard, where there are class I and II surge arresters.
36
4. Determine the required protection level of the surge protection device
s
in accordance with the maximum withstand surge voltage assigned to this
section of the protected network. The protection voltage level Up must be less
than the maximum withstand impulse overvoltage allowed for the connected
equipment.
5. Determine between which network conductors the SPD will be installed. The choice of SPD by the type of non-linear element depends on this.
For example, between the phase and ground in the TN-C, TN-S, TN-C-S
power systems, it is preferable to include varistor SPDs. And between the
neutral and ground the SPD on the arresters (gas, coal, etc.). This is explained
by the fact that during the passage of the overvoltage pulse, the arrester opens
and the pulse current and the short circuit current of the network begin to flow
through it if the arrester is connected, for example, between phase and ground
in the TN-C system. One of the properties of arresters is that it cannot close
until the supply voltage is completely removed from it, which is impossible
in this example. After the overvoltage pulse passes through the electrodes of
the open arrester, the rated mains voltage will remain, which will support the
short circuit current through the arrester. SPDs are designed to absorb large
currents for a short time (tens, hundreds of microseconds). A long short-circuit current will cause the arrester to overheat and fail, the arrester will burn,
which can cause a fire. Varistors, unlike surge arresters, when the overvoltage
decreases to the level of the maximum long-term operating voltage
U
(greater than the rated operating voltage of the network at a given location)
are closed, and do not interrupt the normal operation of the network. The inclusion of arresters between neutral and ground in the case of the TN-S system is justified by the fact that they provide galvanic isolation between them
and only work in case of overvoltage. Therefore, if an SPD based on a spark
gap is placed in a place where a short-circuit current may occur, it is necessary
to apply an SPD protection, for example, in the form of a fuse, connected in
series with an SPD. The SPD itself must have a value of the accompanying
current If greater than the rated short circuit current at a given location in the
network. The rating and type of fuse-link should be selected in accordance
with the recommendations of the manufacturer of the SPD.
A distinctive feature of modern varistor SPDs is that, with the ability to
absorb large pulsed currents from both a waveform of 10/350 μs and a waveform of 8/20 μs, they provide protection at level I of electrical safety category
(protection voltage Up less 1.5 kV). That is, when a varistor SPD of class I is
installed at the input of a building, after it you can immediately connect
household electrical equipment or (DTS). Additional SPDs may be required
37
in the case of a significant removal of the consumer from the switchgear,
when an induced overvoltage pulse may appear on long connecting wires or,
if necessary, protection of especially valuable device is necessary.In this case,
a class III SPD is placed directly in front of the protected equipment, and a
class II SPD in a local switchboard (for example, a storey, far removed from
the switchgear or main switchboard).
SPD Examples:
EZ B 75. The EZ B 75 / * series is designed to protect against direct and
indirect lightning overvoltages in lightning protection zones 0a-1 according
to IEC 62305. It consists of three double, connected in parallel, high-quality
varistor blocks.
SPD Market Overview:
Currently, in the domestic market, a number of supplier companies offer
a wide range of surge protection devices (SPD). In most cases, these are firms
that sell products manufactured in Western Europe, or foreign suppliers that
supply a variety of turnkey technological complexes.
correct
Fig. 3.9. SPD installation schemes
incorrect
incorrect
As a result, products of different manufacturers, when installed on the
same object, are combined with each other without any preliminary verification of their mutual compatibility according to the amplitudes of the transmitted pulse currents and the levels of remaining voltages (protection levels).
38
Fig. 3.10. Appearance of EZETEK ™ EZ B 75 / *
The situation is complicated by the fact that most of the types of proposed
SPDs are designed in accordance with the German national standard DIN
VDE 0675. Although this standard at an early stage played a very important
role in the development and solution of the problem of electromagnetic compatibility (EMC) and the theory of surge protection, it, however, not obligatory for the Russian Federation. In Russia, the more modern standard of the
International Electrotechnical Commission (IEC) IEC 61643-1: 1998, which
is published in the form of GOST R 51992-2002 Devices for surge protection
in low-voltage power distribution systems, has been adopted.
SPD production in Russia. It can be noted that in the field of voltages
above 1 kV, domestic manufacturers produce surge suppressors (surge arresters) in a very wide assortment and of good quality. For voltages less than
1 kV, this problem still remains unsolved.
Until recently, there were no domestic SPDs on the market that fully met
the requirements of GOST R 51992-2002. Now the first steps are being taken
to organize the production of class II and III devices. Their quality and availability will be shown by time. In most cases, the produced varistor SPDs have
a primitive design, the basis of which is a disk varistor and two bolts, nuts,
etc., welded to its lateral planes. Such devices are manufactured on the same
equipment as the varistors for high-voltage arrester, and in essence are components of such a high-voltage surge suppressor.
39
There are SPDs designed for installation on a 35 mm DIN rail, but they
and the designs described above do not include a thermal shutdown device
designed to protect the faulty varistor from overheating and, accordingly,
from the likelihood of a fire in the electrical installation.
It is necessary to add that the majority of domestic SPDs for low-voltage
distribution networks produced are only in the third protection class according to Standart R 51992. These devices are capable of passing through the
maximum pulse current Imax without breaking or thermal breakdown of the
varistor crystal (waves 8 / 20 microsecond) with an amplitude value of no
more than 10–15 kA, while the shape of the current pulse during a direct
lightning strike
I is described by a 10/350 μs wave and significantly larger
imp
current amplitudes (100, 150 and 200 kA (10/350 microsecond ) depending
on picked out level of reliability of external lightning protection system,
you are).
Thus, even under the condition that only part of the current caused by a
direct lightning strike, for example, 10–20%, taking into account its spreading
over other metal structures of the object, will fall on the input power supply,
and the amplitude value of the current
I (waves 10/350 microsecond) may
imp
not exceed the values Imax (waves 8/20 microsecond) = 15 kA, and due to
the current pulse duration
I , which is almost an order of magnitude longer,
imp
the thermal energy allocated on the varistor crystal will damage it!
This process is sometimes accompanied by explosive destruction of the
varistor crystal, which can cause serious injuries, damage to the insulation of
conductors in the electrical installation, and also due to intense sparking, lead
to a fire. At the same time, the task of protecting consumers of electricity may
remain unresolved, since part of the current pulse after failure of the SPD will
go unhindered into the protected equipment and will inevitably damage it.
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