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Файл:The Electromagnetic Interference in the Electrical Power Supply System. Study guide
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Fig. 2.5. An equivalent circuit to calculate the overvoltage
in networks with FCD
Analysis of the transient process in the above diagram allows us to conclude that when the first FCD is turned on, the maximum overvoltage values
on the capacitors and reactors do not exceed the doubled amplitude of the
rated mains voltage. The maximum current through the FCD in the transition
process exceeds the amplitude of the rated current of the capacitor several
times more than the harmonic number to which the FCD is tuned.
When the second FCD is turned on, the overvoltage multiplicities at its
inductance and capacitance in the transient mode also practically do not exceed the doubled amplitude of the rated mains voltage, the transient inrush
current multiplicity, in comparison with the rated capacitor current, is slightly
higher than the harmonic number that is configured.
The intermittent processes in the filter compensating device during sharp
fluctuations of the voltage in the network lead to the appearance of quite significant transient currents in the filters. Short-time overload of FCD by current can be 50-75%. The voltage on the capacitors reaches the voltage value
of the power supply system, which is 5-10% higher than the voltage on the
tires. If the voltage margin of the capacitors of the filter compensating device
is small, systematic voltage increases on the capacitors can lead to damage.
In this regard, from the point of view of the reliability of the FCD operation,
voltage fluctuations are preferably limited to ± 3%.
The duration of transient processes in the network with FCD is no more
than 0.1 s. The interval between load shocks, as a rule, exceeds 0.1 s. Therefore, we can assume that any disturbance acts on the power supply system in
the steady state of the system.
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2.4. Ferroresonant overvoltage
The high energy intensity of metallurgical, engineering and other enterprises determines the presence in the power supply systems of these enterprises of a significant number of transformer substations, cable lines, current
conductors and various power receivers. The complexity of the technological
process requires constant changes in the network configuration and its parameters. Under these conditions, cases of false earth faults, failure of transformers and cable lines due to the occurrence of ferro-resonance phenomena are
not uncommon. Ferro-resonance phenomena in networks of 6 – 10 kV power
supply systems are accompanied by insulation overlap at their inputs, insulation breakdowns of transformers, cables and other equipment. Voltage transformers are most often susceptible to damage.
Rather often, neutral displacements accompanying ferro-resonance phenomena in networks with isolated neutrals lead to false alarms to the ground
and to unjustified blackout of consumers.
Ferro-resonance is an oscillatory process caused by the interaction of capacitive resistance with nonlinear inductive resistance.
In power supply systems, these elements are network capacitance and
non-linear transformer inductance. All other network elements, as well as its
active resistance, do not significantly affect the process under consideration.
Most negative consequences of voltage dips affect the installations of automation, telecommunications, relay protection; they are usually called noisesensitive elements (NSE); the operation of the NSE under the influence of
EMF in some cases leads to a shutdown of the technological process; as a
rule, the permissible time for ES breaks for industrial consumers is in the
range from 0.1 up to the 0.3 s.
Influence on electric motors of multiphase short circuits. The effect on
electrical device of a single-phase short circuit most often occurring in the
network 110 kV was examined above. The following can be said about the
effect of multiphase faults - two-phase, three-phase, and two-phase on the
ground. When these types of short-circuit occur, the depth of voltage dips is
much greater and the behavior of high-voltage electric motors largely depends on the setting of protections in the 110 kV network. If the protection of
the 110 kV lines is provided in accordance with the instructions of clause
3.2.108 of the Electrical Installation Code, i.e. all damage accompanied by a
decrease in the voltage on the bus bars of the power source below 0.65U, are
switched off without delay (the duration of the voltage dip is 0.20–0.25 s),
then high-voltage synchronous and asynchronous motors, as a rule, remain in
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operation. This is due to the fact that the protection of the minimum voltage
of high-voltage electric motors is performed with a time delay of at least 0.5 s
and therefore does not have time to trip. If the protection of 110 kV lines does
not have the necessary speed for deep voltage dips, then synchronous motors,
as a rule, go out of synchronism and special measures must be taken to resynchronize them. As for low-voltage asynchronous electric motors, with deep
multiphase voltage dips, regardless of their duration, contactors and starters
have time to fall off and the electric motors are switched off.
The damage from sudden voltage dips occurs not only in factories with
continuous technological processes. So, in some metal processing operations,
in the event of a sudden interruption in the supply of electricity, expensive
metalworking tools (for example, cutters) may break if not provided for their
automatic removal from the workpiece.
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3. VOLTAGE PULSES
3.1. The main definitions and Standards
So, in all electrical networks there may occur overvoltage caused by various reasons. These overvoltage causes two types of negative effects on electrical devices. The first is damage to the insulation, the second is when penetrating through parasitic capacitances into the logical circuits of a digital technical systems or due to electromagnetic interference in conductors, a malfunction, especially a digital technical systems (DTS).
380/220 V networks are heavily “clogged” with voltage pulses that are
formed when switching protective and switching devices (various electromagnetic devices, contactors, relays, etc.). In fig. 3.1 an example is given of
an oscillogram of a voltage pulse at the terminals of a PME type contactor
that occurs when it is turned off. Since the frequency of the voltage forming
this pulse is of the order of 0.5 – 20 MHz, this pulse, propagating through the
elements of the 380/220 V network, quickly attenuates due to the natural capacitance and inductance of these elements.
In this figure you can see that when the coil power is turned off, there is
a switching overvoltage, which has the form of damped oscillations. The
maximum value of the overvoltage reaches hundreds of volts. The specific
form and parameters of the voltage pulse depend on many factors: the time
of current disruption in the coil of the contactor (determines the magnitude of
the discontinuous current), the characteristics of the current cutoff in the coil
of the contactor, the resistance of circuit elements, etc. The maximum value
of pulse overvoltage occurs when the highest currents (short-circuit) are
switched off by devices with current-limiting capabilities, for example, fuses
of the PPR type. In this case, an overvoltage pulse arises due to a short-circuit
current disconnection in the inductance of the network and reaches a value of
up to 2.5–5 kV, which is dangerous for the central heating system.
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Fig. 3.1. Voltage waveform at the terminals of the PME
contactor when it is disconnected
Figure 3.2 (a) shows half-waves of instantaneous voltage values of the
network on which there is a voltage pulse (overvoltage), and Fig. 3.2 (b)
shows idealized impulse and its characteristic indicators. An important characteristic of the equivalent EMF of pulsed interference is the internal resistance of this EMF.
According to Standard 32144-2013, pulsed voltage refers to the induced
change in voltage, after which the voltage returns to its initial value or approaches it within a few microseconds. There are lightning and switching
voltage pulses. The voltage pulse is characterized by the following values:
1) pulse voltage
2) the amplitude of the pulse
3) pulse duration
U ;
imp
t ;
imp
U ;
.imp a
4) the duration of the instantaneous value of the pulse voltage
UU t is taken into account by the duration of the existence
iimpai
() 0.5
.0.5
of more than half of the pulses.
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To determine the duration of a voltage pulse at a level of 0.5 of its ampli-
s
f
tude
voltage curve (Fig. 3.2 (b)). The initial 0.5
t , a voltage pulse with an amplitude
0.5imp
and the final 0.5
t
U is isolated from the
.imp a
t time points
correspond to the intersection of the voltage pulse curve by a horizontal line
drawn at half the pulse amplitude of 0.5
Duration
0.5 – 0.5
tt .
fs
t 0.5 is calculated with the expression 0.5
imp
U .
.imp a
t
imp
a b
Fig. 3.2. Impulse Voltage Parameters
Switching impulse voltages occur more often in electric networks and the
reason for their appearance is a rapid break in the current, for example, in the
inductance of a contactor retractor coil (Fig. 3.3). When the
current
i flows through the inductance L, which can be considered constant,
K switch is on, a
since the time is considered in microseconds, and the half-period of the industrial frequency is 10 milliseconds. Capacitance C is natural capacitance
between wires or specially connected. When the key
the circuit breaks and an emf occurs on the
ference of potential of self-induction:
L inductance electromotive dif-
d
e
K is open, the current in
.
dt
26
2
UC
Li
m
. (3.1)
Fig. 3.3. Switching voltage pulse
This pulse has the approximate form shown in Fig.3.3 and is characterized
by the following parameters:
UU
– pulse amplitude, kV;
mimpа
t
– pulse duration at the level of 0.5 of its amplitude, microsecond;
5,0imp
– the duration of the leading edge, microsecond.
ph
The greater the inductance of the network element or electrical equipment
in which the current is turned off, the greater the amplitude of the voltage
pulse.
The larger the capacitance C is, the smaller this amplitude.
Voltage pulses coming from the electric network to the clamps of power
receivers can cause damage or malfunction. The shorter the duration of the
leading edge of the voltage pulse, the higher its penetration into the internal
circuits of the electrical receivers. The greatest dangerous effects of the
switching voltage pulses is on various electronic equipment, especially for
digital technical systems (DTS), which without special protection against impulse noise from the electric network cannot work for a long time without
failures. The main ways to protect against voltage impulses include: the use
of surge protection devices (SPDs), interference filters, shielding of electrical
wiring and the correct implementation of grounding.
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3.2. Ways of pulsed noise penetration
into logic DTS chain
The penetration paths of impulse noise from the power supply network to
the logical circuits of the DTS are formed by stray capacitors and electromagnetic pickups in the conductors that make up the logical circuits of the DTS.
Let us consider some of these possible ways using the idealized design of
a central heating system made in the form of a single box (Fig. 3.4). Spurious
capacities having a distributed nature, we’ll show in the figure in the form of
concentrated capacities:
– capacitance between the phase wires of the power supply of the
C
1P
DTS and the conductive housing of the DTS;
– capacitance between the conductive housing of the DTS and the
C
2P
elements of the logical circuits of the DTS;
– winding capacity of the network transformer.
C
3P
Fig. 3.4. The ways of penetration of impulse noise into the logical circuits of the
DTS
Power supply of this central heating system is carried out via a singlephase 220 V line, which has a phase (L) and a neutral wire (N). Pulse noise
has two types (Fig. 3.4). The first is symmetric (e1), the second is asymmetric
(e2). Noise is symmetrical, the equivalent circuit of which, in the form of an
equivalent EMF, is connected between the phase and zero of the supply line.
This interference does not pose any danger to the DTS, since it closes to the
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equivalent capacitance available between the wires L and N and cannot get
into the DTS logical circuits.
More complicated is the situation with asymmetric noise that flows along
the circuit formed by the equivalent EMF interference – parallel-connected
power wires – the DTS conductive housing – spurious capacitances existing
between the elements of the logic circuits and the housing parts (Сp – physical
earth – equivalent EMF). In real installations, these paths can be much more
complicated and confusing. In Fig. 3.4.
the paths of impulse noise penetration from the power supply network
into the logical circuits of the DTS are shown by dashed lines. It should be
noted that there are parasitic capacitances between the power supply wires
and the parts of the central heating system, the presence of a natural capacitance between the primary and secondary windings of the network transformer and many other elements.
Zero conductors and ground in the PZT. In relation to the constructive
implementation of the central heating system, two types of land are distinguished. The first is a real earth, for example, in networks with a grounded
neutral, the second is a system of conductive surfaces and electrical connections, having a specific physical design and a specific functional purpose. The
second type of land is always carried out artificially and can be divided into
four main groups:
1) Protective earth is the so-called grounding chains. The potential of this
system of conductors is far from always equal to the potential of the “physical” earth.
2) A conducting system in relation to which the signal voltage is counted.
The potential of this system is taken equal to zero. Such systems are sometimes called "basic".
3) Electrical connections designed to form paths of reverse currents (supply and signal). An example is the “common bus” of the secondary power
supply or / neutral wire (neutral) of the primary. This land is called “returnable”.
4) Shielding systems.
The listed types of lands are rarely performed separately. Usually, the
base and return systems are combined, as well as the shielding and protective.
The combination of lands entails a deterioration in their properties. For
example, the combination of the base and the return worsens its equipotentiality due to the voltage drop on the return conductors from the working currents, which reduces the noise immunity.
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3.3. Ways to protect a DTS from impulse noise
The main measure of protection of a DTS from impulse noise is the use
of noise filters, which can be divided into two types: active and passive. The
most common are passive filters, as the simplest in design and economical.
Passive filters are a set of capacitors, inductances and resistors connected in
a certain order in order to reduce interference currents penetrating the DTS
logic circuits using the paths discussed above. Passive interference filters are
divided into the following types:
Fig. 3.5. Types of interference filters
G-shaped filters. The operation of such filters is as follows. The voltage
pulse causes the flow of the pulse current along the circuit: the equivalent
EMF noise, the internal resistance of the EMF, the capacitance included in
the filter design, ground. The voltage drop from the interference current consists of two parts: the first is the voltage drop in the elements of the supply
network, the second is the voltage drop across the filter capacitance. The
larger the filter’s capacitance or the greater the capacitive internal resistance
of the equivalent emf noise is, the lower the pulse voltage at the filter input.
A voltage drop across the filter capacitance causes a current through the filter
inductance and through the load. At the same time, the filter inductance,
which has a large resistance at the interference frequency, significantly reduces the interference current flowing from the filter output to the load. Thus,
the two times reduced interference current can penetrate the logical circuits
of the DTS.
G-shaped filters can have two options: with a capacity at the input and
with a capacity at the output.
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