Current-limiting reactors are tested according to the conditions of electrodynamic and thermal resistance, must be carried out the following criteria checks:
- electrodynamic resistance: idin * iud, (3.7)
where idin - electrodynamic resistance at (amplitude value) - see tables 5.14, 5.15; for single (not twin) reactors, only idin is given, and for double reactors, the amplitude value idin and the effective value Idin of the electrodynamic resistance current are given;
taking into account current limitation, is calculated by formulas (2.40) - (2.43);
- thermal resistance:
Iter 2 ter * B, (3.8)
where Iter - thermal resistance at - see table. 5.14, 5.15;
B - thermal current pulse, taking into account the current limitation, is calculated by the formula B = Ip0 * 2(toff + Tae), (3.9)
where toff is the time of switching off by the backup protection; toff = 4 s;
Tae - equivalent time constant of attenuation of the aperiodic component of the short circuit current; Tae = 0.1 - 0.23 s.
The test results are presented in table. 3.5 - 3.7. Checking the electrodynamic and thermal resistance for reactors in the circuit of Fig. 2.1 
The indicated reactors of the RBU 10-1000-0.14U3 type are not sectional, but multi-group, because there are no short-circuit current feeding sources in the section behind the reactor, except for electric motors.
The maximum flows through the reactor at point K2. The corresponding currents, taking into account the current limitation, Ips0 = 13.1 kA and iud.s = 36.2 are calculated in Table 2.6. In terms of electrodynamic resistance, the reactors pass with a large margin - Table 3.5. 
In Table 2.8, the thermal impulse is calculated at B = 86.8 kA2 s after the reactor. Strictly speaking, the indicated thermal impulse takes into account the currents of the engines feeding behind the reactor, which do not actually flow through the reactor at point K2. But, as Table 3.5 shows, even taking into account the overestimation of the thermal impulse, thermal stability is provided with a large margin. Calculation for the SR reactor.
The maximum flows through SR-1 at section C1. The corresponding one, taking into account the current limitation, we calculate through the short circuit calculated in clause 3.2.2 Ip0vg1 = 99.9 kA:
x * (b) \u003d 99.9 1.05 5.78 \u003d 0.061; - from equation (2.31)
Ip0 \u003d 0.061 0.167 1.05 + 5.78 \u003d 26.7 kA, - formula (2.31)
where хр1*(b) = 0.167 is the resistance of the SR reactor.
kud \u003d 1 + exp (-0.01 / 0.1) \u003d 1.905 - formula (2.43)
iud \u003d 2 1.905 26.7 \u003d 71.9 kA - formula (2.42)
B \u003d 71.92 (4 + 0.1) \u003d 2923 kA2 s - formula (3.9)
Calculation for reactor P.
The maximum flows through the reactor P at section 2P.
The corresponding make-up from the system Ip0 = 15.2 kA is calculated in clause 3.2.3. The impact factor remains the same:
isp \u003d 2 1.905 15.2 \u003d 41.0 kA - formula (2.42)
B \u003d 15.22 (4 + 0.1) \u003d 947 kA2 s - formula (3.9) Calculation for the reactor Рres.
The maximum flows through the reactor Рres at directly behind the standby reactor. The calculation in this case completely coincides with the calculation for the working reactor P.
Calculation for the RS reactor.
The maximum flows through the RS reactor at 6.3 kV on group assemblies. The corresponding make-up from the system Ip0 = 13.6 kA is calculated in clause 3.2.4.
iud \u003d 2 1.905 13.6 \u003d 36.6 kA - formula (2.42) 
B \u003d 13.62 (4 + 0.1) \u003d 758 kA2 s - formula (3.9) From Table 3.6 it follows that the determining factor is the verification of reactors for electrodynamic stability. According to thermal resistance, they pass with a large margin, tk. during the flow of the thermal resistance current tter = 8 s significantly exceeds toff = 4 s in formula (3.9).
Checking the electrodynamic and thermal resistance for reactors in the circuit of Fig. 3.2
The resistance of a current transformer to mechanical and thermal influences is characterized by an electrodynamic resistance current and a thermal resistance current.
Short-time electrodynamic current I D is equal to the largest amplitude of the short-circuit current for the entire time of its flow, which the current transformer can withstand without damage that prevents its further proper operation.
Current I D characterizes the ability of the current transformer to withstand the mechanical (electrodynamic) effects of the short circuit current.
Electrodynamic resistance can also be characterized by a multiplicity K D, which is the ratio of the electrodynamic resistance current to the amplitude .
The requirements of electrodynamic resistance do not apply to busbar, built-in and detachable current transformers.
thermal current
thermal current I tt is equal to the largest effective value of the short-circuit current for the interval t t, which the current transformer can withstand for the entire period of time without heating the current-carrying parts to temperatures exceeding those allowed for short-circuit currents (see below), and without damage preventing its further operation.
Thermal resistance characterizes the ability of a current transformer to withstand the thermal effects of a short circuit current.
To judge the thermal resistance of a current transformer, it is necessary to know not only the values of the current passing through the transformer, but also its duration, or, in other words, to know the total amount of heat released, which is proportional to the product of the square of the current I tT and its duration t T. This time, in turn, depends on the parameters of the network in which the current transformer is installed, and varies from one to several seconds.
Thermal resistance can be characterized by a multiplicity K T thermal current, which is the ratio of thermal current to the effective value of the rated primary current.
In accordance with GOST 7746-78, the following thermal resistance currents are established for domestic current transformers:
- one second I 1T or two second I 2T(or their multiplicity K 1T and K 2T in relation to the rated primary current) for current transformers for rated voltages of 330 kV and above;
- one second I 1T or three second I 3T(or their multiplicity K 1T and K 3T in relation to the rated primary current) for current transformers for rated voltages up to 220 kV inclusive.
Between the currents of electrodynamic and thermal resistance there should be the following ratios:
for current transformers 330 kV and above
for current transformers for rated voltages up to 220 kV
Temperature conditions
The temperature of current-carrying parts of current transformers at the thermal resistance current should not exceed:
- 200 °C for aluminum live parts;
- 250 °C for current-carrying parts made of copper and its alloys in contact with organic insulation or oil;
- 300 °C for current-carrying parts made of copper and its alloys not in contact with organic insulation or oil.
When determining the indicated temperature values, one should proceed from its initial values corresponding to long work current transformer at rated current.
Values of currents of electrodynamic and thermal resistance of current transformers state standard are not standardized. However, they must comply with the electrodynamic and thermal resistance of other devices. high voltage installed in the same circuit with a current transformer. In table. 1-2 shows the data of dynamic and thermal resistance of domestic current transformers.
Table 1-2. Data of electrodynamic and thermal resistance of some types of domestic current transformers
Note. Electrodynamic and thermal resistance depends on mechanical strength insulating and current-carrying parts, as well as from the cross section of the latter.
Current transformers are designed to reduce the primary current to values that are most convenient for measuring instruments and relay. (5 A, rarely 1 or 2.5 A), as well as for separating control and protection circuits from primary high voltage circuits. Current transformers used in switchgear act simultaneously as a bushing insulator (TPL, TPO). In complete switchgear, support-through (rod) current transformers - TLM are used. TPLC, TNLM, tire - TSHL. in switchgear 35 kV and above - built-in, depending on the type of switchgear and its voltage.
The calculation of current transformers in a substation, in essence, comes down to checking the current transformer supplied complete with the selected cell. So, the brand of the current transformer depends on the type of cell chosen; in addition, current transformers choose:
1) by voltage;
2) by current (primary and secondary)
In this case, it should be borne in mind that the rated secondary current of 1A is used for 500 kV switchgear and powerful 330 kV switchgear, in other cases, a secondary current of 5 A is used. The rated primary current should be as close as possible to the rated current of the installation, since underloading of the primary winding transformer leads to an increase in errors.
The selected current transformer is checked for dynamic and thermal resistance to short-circuit currents. In addition, current transformers are selected according to the accuracy class, which must correspond to the accuracy class of the devices connected to secondary circuit measuring current transformer (ITT) - In order for the current transformer to provide the specified measurement accuracy, the power of the devices connected to it should not be higher than the rated secondary load indicated in the current transformer passport.
The thermal resistance of a current transformer is compared with a thermal impulse B k:
where is the coefficient of dynamic stability.
The load of the secondary circuit of the current transformer can be calculated by the expression:
where - the sum of the resistances of all series-connected windings of devices or relays;
Resistance of connecting wires;
Resistance contact connections( = 0.05 Ohm, with 2 - 3 devices: with more than 3 devices = 0.1 Ohm).
The resistance of devices is determined by the formula:
where is the resistivity of the wire;
l calc - effective length wires;
q- section of wires.
The length of the connecting wires depends on the connection diagram of the current transformer:
| , | (6.37) |
where m- coefficient depending on the switching scheme;
l- length of wires (for substations take l= 5 m).
When turning on the current transformer in one phase m= 2, when the current transformer is connected to an incomplete star, , when connected to a star, m =1.
The minimum cross section of the wires of the secondary circuits of the current transformer should not be less than 2.5 mm 2 (for aluminum) and 1.5 mm 2 (for copper) according to the condition of mechanical strength. If meters are connected to the current transformer, these sections must be increased by one step.
In the switchgear of the LV substation, it is necessary to select (check) the current transformers in the cells the following types: input, sectional, outgoing lines, as well as in the cells of the auxiliary transformer. The rated currents of these cells are determined by expressions (6.21-6.23), and in the TSN cells:
 ,
| (6.38) |
where S ntsn- rated power of TSN.
The calculation results are summarized in table 6.8:
Table 6.8 - Summary table for the selection of current transformers of the switchgear of the LV substation:
| Transformer parameter | Selection (check) condition | Cell types | |||
| input | sectioning | outgoing lines | TSN | ||
| Transformer type | determined by the cell series (according to the directory) | ||||
| Rated voltage |  | ||||
| Rated current | |||||
| primary | |||||
| secondary | BUT | ||||
| Accuracy class | According to the accuracy class of connected devices | or | |||
| Dynamic Stability |  |
||||
| Thermal stability |  |
Example 1
Select current transformer in input cell power transformer at the substation. The rated power of the transformer is 6.3 MVA, the transformation ratio is 110/10.5 kV. The substation has two transformers. The design load of the substation is S max 10.75 MVA. The 10 kV network is not grounded. The surge current on the low voltage side is 27.5 kA. Ammeters and meters of active and reactive power. Type of cells in RU-10 kV - KRU-2-10P.
Maximum rated input cell current (for the most unfavorable operating mode):
BUT.
The nearest standard current transformer built into the input cell (KRU-2-10P) is selected - TPOL-600 / 5-0.5 / R with two secondary windings: for measuring instruments and relay protection. The rated load of such a current transformer of accuracy class 0.5 - S2= 10 VA ( r2\u003d 0.4 Ohm), the multiplicity of electrodynamic stability, k dyn= 81, multiplicity of thermal stability, k T= 3 s. These data are indicated in /3, 10/.
The selected current transformer is checked for electrodynamic stability:
,
as well as thermal stability:
,
C from the calculation (table 4.4); T a\u003d 0.025 s according to table 4.3;
1105,92 > 121,78.
In ungrounded circuits, it is sufficient to have current transformers in two phases, for example, in A and C. The loads on the current transformer from measuring instruments are determined, the data are summarized in table 6.9:
Table 6.9 - Loading of measuring instruments by phases
| Device name | ||||
| BUT | AT | With | ||
| Ammeter | H-377 | 0,1 | ||
| Active energy meter | SAZ-I673 | 2,5 | 2,5 | |
| Reactive Energy Meter | SRC-I676 | 2,5 | 2,5 | |
| Total | 5,1 |
The table shows that phase A is the most loaded, its load is VA or r app= 
0.204 ohm. The resistance of connecting wires made of aluminum with a cross section is determined q\u003d 4 mm 2, long l= 5 m.
Ohm
where \u003d 0.0283 Ohm / m mm 2 for aluminum;
Secondary circuit impedance:
where r cont= 0.05 ohm.
Comparing passport and calculated data on the secondary load of current transformers, we obtain:
Therefore, the selected current transformer passes through all parameters.
When choosing devices and conductors in the line circuit, it is necessary to take into account that
a) busbar branching from busbars and bushings between busbars and disconnectors (if there are separating shelves) should be selected based on a short circuit to the reactor;
b) the choice of bus disconnectors, circuit breakers, current transformers, bushings and busbars installed upstream of the reactor should be carried out according to the values of short circuit tones downstream of the reactor.
The calculated type of short circuit when checking the electrodynamic resistance of devices and rigid tires with their supporting and supporting structures is a three-phase short circuit. Thermal stability should also be checked for a three-phase short circuit. Equipment and conductors used in circuits of generators with a capacity of 60 MW or more, as well as in circuits of generator-transformer blocks of the same power, must be checked for thermal stability, based on an estimated short circuit time of 4 s. Therefore, for the generator circuit, a three-phase and two-phase short circuit should be considered. The breaking capacity of devices in ungrounded or resonantly grounded networks (networks up to 35 kV inclusive) should be checked by the three-phase short circuit current. In effectively grounded networks (networks with a voltage of 110 kV and above), the currents are determined during a three-phase and single-phase short circuit, in order to check the breaking capacity, they do it in a more severe mode, taking into account the conditions for restoring voltage.
Test for electrodynamic resistance.
Surge short-circuit currents can cause damage electrical apparatus and tire structures. To prevent this from happening, each type of device is tested at the factory, setting for it the highest allowable short-circuit current (peak value of the total current) i dyn. In the literature, there is another name for this current - the limiting through short-circuit current i pr.skv.
The test condition for electrodynamic resistance has the form
i beats ≤ i dyn,
where i beats- estimated shock current in the circuit..
Thermal stability test.
Conductors and devices during a short circuit should not heat up above the permissible temperature, established norms for short term heating.
For the thermal stability of the devices, the condition must be met
where B to - the impulse of the quadratic short circuit current, proportional to the amount of thermal energy released during the short circuit;
I ter - rated current thermal resistance of the device;
t ter - nominal time of thermal resistance of the device.
The device can withstand the current I ter during the time t ter.
Impulse quadratic short-circuit current
where i t is the instantaneous value of the short circuit current at the moment t;
tc - time from the beginning of the short circuit to its disconnection;
B kp - thermal impulse of the periodic component of the short circuit current;
B k.a - thermal impulse of the aperiodic component of the short circuit current.
The thermal impulse B to is defined differently depending on the location of the short circuit point in the electrical circuit.
Three main cases can be distinguished:
Remote short circuit
short circuit near generators or synchronous compensators,
short circuit near a group of powerful electric motors:
In the first case, the total thermal impulse of the short circuit
where I p.0 - effective value of the periodic component of the initial short circuit current;
T a is the decay time constant of the aperiodic component of the short circuit current.
Determining the thermal impulse Bk for the other two short-circuit cases is rather difficult. For approximate calculations, you can use the above expression B to.
According to the PUE, the trip time t otk is the sum of the time of action of the main relay protection of this circuit t r.z and the total time of the switch off t o.v;
t otk \u003d t r.z + t o.v
Cables and busbars are selected according to their nominal parameters (current and voltage) and checked for thermal and dynamic short circuit resistance. Since the short circuit process is short-term, it can be assumed that all the heat released in the cable conductor goes to heat it. The heating temperature of the cable is determined by its resistivity, heat capacity, operating temperature. Cable heating temperature in normal operating mode
where t o.sr - temperature environment(soils); t add - allowable temperature in normal mode, taken equal to 60 ° C; I additional - permissible current for the selected section.
The maximum permissible short-term temperature rises during short-circuit for power cables with impregnated paper insulation are accepted: up to 10 kV with copper and aluminum conductors - 200 °C; 20-35 kV with copper conductors - 175 °С.
Checking the cable cross-section for thermal resistance to short-circuit currents is carried out according to the expression
(10.27)
where AT k - thermal impulse; C = A con –BUT early- coefficient corresponding to the difference in heat released in the conductor after the short circuit and before it.
For 6-10 kV cables with paper insulation and copper conductors With= 141, with aluminum conductors With= 85; for cables with PVC or rubber insulation with copper conductors With= 123, with aluminum conductors With= 75.
During a short circuit, transient currents pass through the current-carrying parts, causing complex dynamic forces in busbar structures and electrical installations. The forces acting on rigid busbars and insulators are calculated from the highest instantaneous value of the three-phase short circuit current i y. This determines the maximum force F on the busbar structure without taking into account mechanical vibrations, but taking into account the distance l between busbar insulators and distances between phases a(Fig. 10.2).
Rice. 10.2. Distance between phases ( b,h- tire sizes)
Permissible stresses, MPa: for copper MT - 140, for aluminum AT- 70, for aluminum ATT - 90, for steel - 160.
In multi-lane tires, in addition to the force between the phases, there is a force between the lanes, the calculation in this case becomes more complicated.
Electrodynamic forces in the current-carrying parts of switches, disconnectors and other devices are complex and difficult to calculate, therefore, manufacturers indicate the maximum through-circuit current allowed through the device (peak value) I rated dyne, which should not be less than the shock current found in the calculation I y with a three-phase short circuit.
Service life of electrical equipment depending on operating modes and environmental characteristics
Lecture No. 12-13 Indicators of the quality of electricity and methods for its provision Standards for the quality of electrical energy and their scope in power supply systems
important integral part of the multifaceted problem of electromagnetic compatibility, which is understood as the totality of electric, magnetic and electromagnetic fields that generate electrical objects created by man and affect the dead (physical) and living (biological) nature, technical, informational, social reality, the power quality subsystem of the PQE becomes , which in the electrical network is characterized by indicators of the quality of electricity. The list and standard (permissible) values of the SQE are established by GOST 13109-97 "Quality standards electrical energy in power supply systems”, introduced on January 1, 1999 to replace the existing GOST 13109-87.
The concept of the quality of electrical energy is different from the concept of the quality of other goods. The quality of electricity is manifested through the quality of the operation of electrical receivers. Therefore, if it works unsatisfactorily, and in each specific case, an analysis of the quality of the consumed electricity gives positive results, then the quality of workmanship or operation is to blame. If the SCEs do not meet the requirements of GOST, then claims are made against the supplier - the energy company. In general, SCEs determine the degree of distortion of the voltage of the electrical network as a result of conductive interference (distributed over the elements of the electrical network) introduced by both the power supply organization and consumers.
The decrease in the quality of electricity causes:
Increase in losses in all elements of the electrical network;
Overheating of rotating machines, accelerated aging of insulation, reduction of service life (in some cases failure) of electrical equipment;
Growth in electricity consumption and the required power of electrical equipment;
Disruption of work and false alarms of relay protection and automation devices;
Malfunctions in electronic control systems, computer science and specific equipment;
The probability of occurrence of single-phase short circuits due to the accelerated aging of the insulation of machines and cables, followed by the transition of single-phase faults to multi-phase faults;
The appearance of dangerous levels of induced voltages on the wires and cables of disconnected or under construction high-voltage power lines located near the existing ones;
Interference in television and radio equipment, erroneous operation of X-ray equipment;
Incorrect operation of electricity meters.
Part of the SCE characterizes the interference introduced by the steady state operation of the electrical equipment of the power supply organization and consumers, i.e., caused by the peculiarities of the technological process of production, transmission, distribution of electricity consumption. These include voltage and frequency deviations, distortion of the sinusoidal shape of the voltage waveform, unbalance and voltage fluctuations. For their normalization, allowed values PKE.
The other part characterizes short-term interference that occurs in the electrical network as a result of switching processes, lightning and atmospheric phenomena, the operation of protective equipment and automation, and post-emergency modes. These include dips and voltage pulses, short-term interruptions in power supply. For these SCEs, the permissible numerical values are not established by GOST. However, parameters such as amplitude, duration, frequency, and others must be measured and made up of statistical data sets that characterize a particular electrical network in relation to the probability of short-term interference.
GOST 13109-97 establishes indicators and standards in electrical networks of power supply systems general purpose alternating three-phase and single-phase current with a frequency of 50 Hz at points to which electrical networks owned by various consumers of electrical energy, or receivers of electrical energy (points of general connection) are connected. The standards are used in the design and operation of electrical networks, as well as in establishing the levels of noise immunity of electrical receivers and the levels of conductive electromagnetic interference introduced by these receivers. There are two types of norms: normally permissible and maximum permissible. Compliance assessment is carried out within a billing period of 24 hours.
The quality of electricity is characterized by parameters (frequency and voltage) at the connection nodes of the levels of the power supply system.
Frequency- the system-wide parameter is determined by the active power balance in the system. When there is a shortage of active power in the system, the frequency is reduced to a value at which a new balance of generated and consumed electricity is established. In this case, the decrease in frequency is associated with a decrease in the rotation speed electrical machines and a decrease in their kinetic energy. The kinetic energy thus released is used to maintain the frequency. Therefore, the frequency in the system changes relatively slowly. However, with a shortage of active power (more than 30%), the frequency changes quickly and the effect of an “instantaneous” frequency change occurs - “frequency avalanche”. A change in frequency at a rate of more than 0.2 Hz per second is commonly called frequency fluctuations.
Voltage in the node of the electric power system is determined by the balance of reactive power in the system as a whole and the balance of reactive power in the node of the electrical network. 11 power quality indicators are established:
steady-state voltage deviation δU y;
range of voltage change δU t ;
flicker dose P t ;
coefficient of distortion of the sinusoidal curve of the phase-to-phase (phase) voltage To U ;
coefficient n- th harmonic component of the voltage To U ( n ) ;
voltage unbalance factor in reverse sequence K 2 U ;
coefficient of voltage asymmetry in the zero sequence K 0 U ;
frequency deviations Δf;
the duration of the voltage dip Δt p;
impulse voltage U imp;
coefficient of temporary overvoltage K per U .
Not all SCEs have standards set by the standard. So, the steady voltage deviation (this term means the average deviation for 1 minute, although the process of changing the effective voltage value during this minute can be completely unsteady) is normalized only in networks of 380/220 V, and at points in networks of higher voltage it should be calculated . For voltage dips, only the maximum permissible duration of each (30 s) in networks with voltage up to 20 kV is established and statistical data are presented on the relative dose of dips of different depths in the total number of dips, but statistical data on their number per unit of time (week, month and etc.). By impulse voltage and temporary overvoltage norms are not established, but given reference Information about their possible values in the networks of power supply organizations.
When determining the values of some KE indicators, the following auxiliary parameters of electric energy are used:
Frequency of repetition of voltage changes F δUt ;
Interval between voltage changes Δt i , i +1 ;
Depth of voltage dip δU P ,
Frequency of occurrence of voltage dips F P ;
The pulse duration at the level of 0.5 of its amplitude Δt imp 0.5 ;
Duration of temporary overvoltage Δt per U .
For all SCEs, the numerical values of the norms for which are in the standard, the mechanism of penalties is contractually launched, which is formed for six SCEs out of 11 listed: frequency deviation; voltage deviation; flicker dose; the distortion factor of the sinusoidality of the voltage curve; voltage unbalance factor in reverse sequence; coefficient of voltage asymmetry in the zero sequence.
Responsibility for unacceptable frequency deviations certainly lies with the power supply organization. The power supply organization is responsible for unacceptable voltage deviations if the consumer does not violate the technical conditions for the consumption and generation of reactive power. Responsibility for violation of the norms for the other four (PQI with defined liability) rests with the culprit, determined on the basis of a comparison of the allowable contribution included in the contract with the value of the considered SQI at the electricity metering point with the actual contribution calculated on the basis of measurements. If the allowable contributions are not specified in the contract, the energy supply organization is responsible for low quality, regardless of the culprit of its deterioration.
