Scheme of power supply of a site or workshop. Design of power supply for the shop of mechanical assembly of parts. All places in a given workshop have local lighting

The scheme of the workshop power network up to 1000 V is determined technological process production, the mutual arrangement of shop TS or power input and electrical receivers, their unit installed capacity and placement over the area of ​​the shop. The circuit should be simple, safe and easy to operate, economical, meet the characteristics of the environment, ensure the use of industrial installation methods.

The lines of the workshop network, extending from the workshop TP or input device, form the supply network, and those supplying energy from the busbars or RP directly to the electrical receivers form the distribution network.

Network schemes can be radial, trunk and mixed - with one-way or two-way power.

Radial power supply scheme of the workshop network

With a radial scheme, energy from a separate power supply unit (TP, RP) is supplied to one sufficiently powerful consumer or to a group of power receivers. Radial schemes are performed as single-stage, when the receivers are powered directly from the transformer substation, and as two-stage, when they are connected to an intermediate RP.


Rice. 1. Radial power supply scheme: 1 - switchboard TP, 2 - power RP, 3 - power receiver, 4 - lighting board

Radial circuits are used to power concentrated loads of high power, with uneven placement of receivers in the workshop or in groups in its individual sections, as well as to power receivers in explosive, fire and dusty rooms. In the latter case, the control and protection equipment for electrical receivers installed on the switchgear is taken out of the unfavorable environment.

Radial circuits are performed with cables or wires in pipes or boxes (trays). The advantages of radial circuits are high reliability (an accident on one line does not affect the operation of receivers powered by another line) and ease of automation. Increasing the reliability of radial circuits is achieved by connecting the buses of individual TS or RP with redundant jumpers, on the switching devices of which (automatic machines or contactors) an ATS circuit can be performed - automatic backup power input.

The disadvantages of radial circuits are: low efficiency due to the significant consumption of conductor material, the need for additional space to accommodate power RP. Limited flexibility of the network when moving technological mechanisms associated with a change in the technological process.

The main power supply circuit of the workshop network

With trunk circuits, receivers are connected to any point on the line (trunk). The mains can be connected to the switchboards of the substation or to the power distributors, or directly to the transformer according to the transformer-line block diagram.

Trunk circuits with are used when supplying receivers of one production line or with receivers evenly distributed over the area of ​​the workshop. Such schemes are carried out using busbars, cables and wires.

Rice. Fig. 2. Trunk circuits with unilateral power supply: a - with distribution busbars, b - transformer-main block, c - chain, 1 - TP switchboard, 2 - power distribution switchgear, 3 - power receiver, 4 - main busbar, 5 - distribution busbar

When installing low-power electrical receivers at the workplaces of a technological line, it is advisable to carry out distribution lines with modular wiring. For the backbone of the modular network, insulated wires are used, laid in pipes hidden in the floor, with the installation of junction boxes at a certain distance from each other (module), on which floor distribution columns are mounted with plug connectors. Electrical receivers are connected to the speakers with wires in metal hoses. Modular wiring is used for line loads up to 150 A,

The advantages of trunk circuits are: simplification of substation shields, high network flexibility, which makes it possible to move technological equipment without altering the network, the use of unified elements that allow installation by industrial methods. The main circuit is less reliable than the radial one, since when the voltage on the main line fails, all consumers connected to it lose power. The use of busbars and modular wiring of a constant cross section leads to some overspending of the conductor material.

Mixed power scheme

Depending on the nature of production, the location of electrical receivers and environmental conditions, power networks can be carried out according to a mixed scheme. Some of the electrical receivers are powered from the mains, some - from the power distributors, which, in turn, are powered either from the switchboard of the transformer substation, or from the main or distribution busbars.

Modular wiring can be powered from distribution busbars or from power distributors connected in a radial pattern. This combination allows you to more fully use the advantages of radial and main circuits.

Rice. 3. Two-way power supply schemes: a - main with a distribution busbar, b - radial with a reserving jumper, c - with mutual redundancy of mains

To improve the reliability of power supply to power receivers according to the main circuits, a two-way supply of the main line is used. When laying several mains in large workshops, it is advisable to feed them from separate transformer substations by making jumpers between the mains. Such main power supply schemes with mutual redundancy increase power supply reliability, create convenience for carrying out repair work at substations, provide the ability to turn off unloaded transformers, resulting in reduced power losses.

The choice of a power supply scheme is inextricably linked with the issue of voltage, power, EP category in terms of reliability, EP remoteness.

With regard to ensuring the reliability of power supply, power receivers are divided into the following three categories.

Power receivers of the first category - power receivers, the interruption of power supply of which may entail: danger to people's lives, threat to the security of the state, significant material damage, disruption of a complex technological process, disruption of the functioning of critical elements public utilities, communications and television facilities.

From the composition of power receivers of the first category, a special group of power receivers stands out, the uninterrupted operation of which is necessary for an accident-free shutdown of production in order to prevent a threat to people's lives, explosions and fires.

Electrical receivers of the second category are electrical receivers, the interruption of power supply of which leads to massive undersupply of products, massive downtime of workers, mechanisms and industrial transport, disruption of the normal activities of a significant number of urban and rural residents.

Power receivers of the third category - all other power receivers that do not fall under the definitions of the first and second categories.

Power receivers of the first category in normal modes must be provided with electricity from two independent mutually redundant power sources, and a break in their power supply in the event of a power failure from one of the power sources can only be allowed for the period of automatic power restoration.

For the power supply of a special group of power receivers of the first category, additional power must be provided from a third independent mutually redundant power source.

As a third independent power source for a special group of power receivers and as a second independent power source for other power receivers of the first category, local power plants, power plants of power systems (in particular, generator voltage buses), uninterruptible power units designed for these purposes, batteries and etc.

If it is impossible to ensure the continuity of the technological process by redundant power supply or if redundant power supply is not economically feasible, technological redundancy should be carried out, for example, by installing mutually redundant technological units, special devices for trouble-free shutdown of the technological process, operating in the event of a power failure.

Power supply of power receivers of the first category with a particularly complex continuous technological process that requires a long time to restore the normal mode, in the presence of feasibility studies, it is recommended to carry out from two independent mutually redundant power sources, to which additional requirements are imposed, determined by the characteristics of the technological process.

Power receivers of the second category in normal modes must be provided with electricity from two independent mutually redundant power sources.

For power receivers of the second category, in the event of a power failure from one of the power sources, power supply interruptions are permissible for the time necessary to turn on the backup power by the actions of the duty personnel or the mobile operational team.

For power receivers of the third category, power supply can be carried out from one power source, provided that power supply interruptions necessary to repair or replace a damaged element of the power supply system do not exceed 1 day.

The issue of choosing a power supply scheme, voltage level is decided on the basis of a technical and economic comparison of options.

For industrial power supply, enterprises use electrical networks with a voltage of 6, 10, 35, 110 and 220 kV.

In the supply and distribution networks of medium-sized enterprises, a voltage of 6–10 kV is accepted. Voltage 380/220 V is the main voltage in electrical installations up to I000 V. The introduction of voltage 660 V is cost-effective and is recommended to be used primarily for newly built industrial facilities.

Voltage 42 V (36 and 24) is used in rooms with increased danger and especially dangerous, for stationary local lighting and hand-held portable lamps.

The 12 V voltage is used only under particularly unfavorable conditions with regard to the risk of electric shock, for example, when working in boilers or other metal tanks using hand-held portable lights.

Two main power distribution schemes are used - radial and main, depending on the number and relative position of workshop substations or other power supplies in relation to the point that feeds them.

Both schemes provide the required reliability of power supply to EA of any category.

Radial distribution schemes are used mainly in cases where the loads are dispersed from the power center. Single-stage radial circuits are used to power large concentrated loads (pumping, compressor, converter units, electric furnaces, etc.) directly from the power center, as well as to power workshop substations. Two-stage radial circuits are used to power small workshop substations and HV power receivers in order to unload the main energy centers (Fig. Z.1). At intermediate distribution points, all switching equipment is installed. The use of multi-stage schemes for intrashop power supply should be avoided.

Rice. 3.1. Fragment of a radial power distribution scheme

Distribution points and substations with electrical receivers of categories I and II are usually powered by two radial lines that operate separately, each for its own section, when one of them is disconnected, the load is automatically taken by the other section.

Main power distribution schemes should be used for distributed loads, when there are many consumers and radial schemes are not economically feasible. The main advantages: allow better loading of cables in normal mode, save the number of cabinets at the distribution point, reduce the length of the trunk. The disadvantages of trunk circuits include: the complication of switching circuits, the simultaneous shutdown of the EP of several production sites or workshops powered by this trunk when it is damaged. For power supply of VP of categories I and II, schemes with two or more parallel through mains should be used (Fig. 3.2).

Rice. 3.2. Scheme with double through highways

Power supply of EP in networks with voltage up to 1000 V of II and III categories in terms of power supply reliability is recommended to be carried out from single-transformer packaged transformer substations (KTS).

The choice of two-transformer PTS must be justified. The most expedient and economical for intrashop power supply in networks up to 1 kV are the main circuits of transformer-trunk blocks without switchgears at a substation using complete busbars.

Radial circuits of intrashop supply networks are used when it is impossible to perform trunk circuits due to the conditions of the territorial distribution of electrical loads, as well as environmental conditions.

For the power supply of shop consumers in design practice, radial or main circuits in their pure form are rarely used. The most widespread are the so-called mixed circuits of electrical networks, combining elements of both radial and trunk circuits.

Power supply schemes and all AC and DC electrical installations of an enterprise with a voltage of up to 1 kV and above must meet general requirements to their grounding and protection of people and animals from electric shock both in normal operation of the electrical installation and in case of damage to the insulation.

Electrical installations in relation to electrical safety measures are divided into:

- electrical installations with voltages above 1 kV in networks with a solidly grounded or effectively grounded neutral;

- electrical installations with voltages above 1 kV in networks with isolated or grounded neutral through an arcing reactor or resistor;

- electrical installations with voltage up to 1 kV in networks with dead-earthed neutral;

- electrical installations with voltage up to 1 kV in networks with isolated neutral.

For electrical installations with voltage up to 1 kV, the following designations are accepted: system TN- a system in which the neutral of the power source is solidly grounded, and the open conductive parts of the electrical installation are connected to the solidly grounded neutral of the source by means of zero protective conductors (see Fig. 3.3–3.7).

Rice. 3.3. System TN-C- system TN, in which zero protective

and zero working conductors are combined in one conductor

throughout its length

The first letter is the state of the neutral of the power supply relative to earth:

T– grounded neutral;

I– isolated neutral.

The second letter is the state of open conductive parts relative to ground:

T– exposed conductive parts are grounded, regardless of the relation to the earth of the neutral of the power supply or any point of the supply network;

N– exposed conductive parts are connected to a dead-earthed neutral of the power source.

Subsequent (after N) letters - combination in one conductor or separation of the functions of the zero working and zero protective conductors:

S– zero worker ( N) and zero protective ( PE) conductors are separated;

C- the functions of the zero protective and zero working conductors are combined in one conductor ( PEN-conductor);

N- zero working (neutral) conductor;

PE- protective conductor (grounding conductor, zero protective conductor, protective conductor of the potential equalization system);

PEN- combined zero protective and zero working conductor.

Rice. 3.4. System TN-S- system TN, in which zero protective

and zero working conductors are separated along its entire length

Rice. 3.5. System TN-C-S- system TN, in which the functions of zero

protective and zero working conductors are combined in one

conductor in some part of it, starting from the power source

Rice. 3.6. System TT– a system in which the neutral of the power supply

deafly grounded, and open conductive parts of the electrical installation

earthed with a grounding device, electrically

source independent of the dead-earthed neutral

Rice. 3.7. System IT– a system in which the neutral of the power supply

isolated from earth or earthed through appliances or devices,

with high resistance, and exposed conductive parts

electrical installations are grounded

Zero working (neutral) conductor ( N) - a conductor in electrical installations up to 1 kV, designed to power electrical receivers and connected to a solidly grounded neutral of a generator or transformer in three-phase current networks, with a solidly grounded output of a single-phase current source, with a solidly grounded source point in DC networks.

Combined zero protective and zero working ( PEN) conductor - a conductor in electrical installations with voltage up to 1 kV, combining the functions of zero protective and zero working conductors.

To protect against electric shock in normal operation, the following protective measures against direct contact must be applied individually or in combination:

– basic insulation of current-carrying parts;

- fences and shells;

– installation of barriers;

– placement out of reach;

– use of extra-low (small) voltage.

For additional protection against direct contact in electrical installations with voltages up to 1 kV, if there are requirements of other chapters of the PUE, residual current devices (RCDs) with a rated differential breaking current of not more than 30 mA should be used.

In order to protect against electric shock in the event of insulation failure, the following protective measures against indirect contact must be applied individually or in combination:

– protective grounding;

– automatic power off;

– equalization of potentials;

– equalization of potentials;

– double or reinforced insulation;

– extra-low (small) voltage;

– protective electrical separation of circuits;

- insulating (non-conductive) rooms, zones, sites.

Electrical installations up to 1 kV in residential, public and industrial buildings and outdoor installations should, as a rule, be powered from a source with a solidly grounded neutral using a system TN.

Power supply of electrical installations with voltage up to 1 kV alternating current from an isolated neutral source using the system IT should be carried out, as a rule, if a power interruption is unacceptable at the first short circuit to the ground or to open conductive parts connected to the potential equalization system. In such electrical installations, for protection against indirect contact during the first earth fault, protective grounding must be performed in combination with network insulation monitoring or RCDs with a rated differential breaking current of not more than 30 mA should be used. In case of a double ground fault, automatic power off must be performed in accordance with the PUE.

Power supply of electrical installations with voltage up to 1 kV from a source with a dead-earthed neutral and with grounding of open conductive parts using a grounding conductor not connected to the neutral (system TT), is allowed only in those cases when the electrical safety conditions in the system T N cannot be provided. To protect against indirect contact in such electrical installations, automatic power off with mandatory application RCD.

In this case, the following condition must be met:

R a I a ≤ 50 V,

where I a is the operating current of the protective device;

R a is the total resistance of the grounding conductor and the grounding conductor of the most distant electrical receiver, when using RCD to protect several electrical receivers.

When using the system TN re-grounding is recommended PE- and PEN- conductors at the input to the electrical installations of buildings, as well as in other accessible places. For re-grounding, first of all, natural grounding conductors should be used. The resistance of the re-grounding earth electrode is not standardized.

In electrical installations with a voltage above 1 kV with an isolated neutral, to protect against electric shock, protective grounding of exposed conductive parts must be made.

App. 3 shows the power supply schemes of individual buildings, and in App. 4 - graphic and letter designations in electrical circuits.

INTRODUCTION

Increasing the level of electrification of production and the efficiency of energy use is based on the further development of the energy base, the continuous increase electrical energy. At present, in the presence of powerful power plants, combined into electrical systems with high reliability of power supply, many industrial enterprises continue to build power plants. The need for their construction is due to the large distance from energy systems, the need for thermal energy for industrial needs and heating, the need for backup power for responsible consumers. The design of power supply systems is carried out in a number of design organizations. As a result of the generalization of design experience, the issues of power supply of enterprises received the form of standard solutions. At present, methods have been developed for calculating and designing shop networks, choosing the power of shop transformers, methods for determining shop loads, etc. In this regard, great importance acquire the issues of training highly qualified personnel capable of successfully solving the issues of power supply design and practical tasks.

In this course project, a diagram of a transformer substation will be considered, a description of its operation. The calculation of the choice of the most optimal transformer will also be made.

The purpose of the course project is: the choice and justification of the power supply scheme and installed electrical equipment for the designed facility.

Object of study: mechanical repair shop

Subject of study: stages of calculation and choice of power supply system for a mechanical repair shop.

Hypothesis: when developing the electrical circuit of the mechanical repair shop, an optimal variant was found that ensures reliable uninterrupted operation of electrical equipment, taking into account the safety of its maintenance.

To achieve the goal and test the hypothesis, the following tasks were set:

Make a choice of the number and power of transformers of the supply substation;

Design a single-line diagram of the power supply of the production workshop.

1. MAIN PART

1 Characteristics of the object

The production workshop is engaged in the manufacture of various parts and metal structures necessary for the main production. The workshop includes various metalworking machines, welding and lifting equipment, fans. The power of electrical receivers of the shop is from 5 to 30 kW. Electric receivers operate in long-term (metal-working machines, fans) and in repeated short-term modes (lifting equipment). Shop electrical receivers operate on alternating 3-phase current (metalworking machines, fans, lifting equipment) and single-phase current (lighting). Shop electrical receivers belong to the third category according to the required degree of power supply reliability. Environment in the workshop is normal, so all the equipment in the workshop is made in normal performance. The workshop area is 367m 2

Characteristics of electrical equipment in table. 1.1

Table 1 . 1

No. according to the plan

Name of electrical receivers

Р nom, kW

lathe

lathe

lathe

lathe

lathe

lathe

CNC carousel

Milling machine

Milling machine

Milling machine

Milling machine

Fan

Fan

Crane - beam PV = 40%

Crane - beam PV = 40%

Fan

Fan

Figure 1.1 shows the plan of the designed workshop

Fig.1.1 Plan of the designed workshop

1.2 Description of the power supply scheme

The power supply of the production shop is carried out from a single-transformer substation 6/0.4 kV with a transformer capacity of 160 kVA. In turn, the 6/0.4 kV transformer substation is fed via the AAB 3x10 cable line, laid in the ground, from the upstream two transformer substation 110/6 kV with transformers with a capacity of 2500 kVA each, which is powered from the power system via the A-70 single-circuit overhead line.

On the 6kV side of TP 6/0.4, oil circuit breakers and disconnectors are installed as protective switching equipment.

On the 0.4 kV side, fuses are installed as short-circuit protection devices

3 Power and lighting network design

Switchboards have been installed in the production workshop to receive and distribute electricity.

Electric receivers are powered from the SHR by a wire laid in pipes

Fuses are used as protection devices against short-circuit currents.

The lighting of the workshop was made by 28 RKU lamps with high-pressure mercury lamps with a power of 400W

Lighting networks are carried out with APV-2.5mm² wire laid in a pipe

The working lighting is powered from the OSHV-12 lighting panel, in which circuit breakers are installed as protection devices against short-circuit and overload currents.

2. CALCULATION PART

1 Lighting calculation

The calculation of lighting is carried out according to the method of utilization of the luminous flux. We will show the calculation using the example of section I. As a light source, we will take a 400 W DRL lamp for installation

The number of light sources is determined by the formula:

where E norms - normalized illumination, E norms \u003d 300 lx - coefficient taking into account the decrease in luminous flux during operation, Z \u003d 1.1

K z - coefficient taking into account the uneven distribution of the light flux on the illuminated surface, K z \u003d 1.5 - area of ​​\u200b\u200bthe room, m²

F l - the luminous flux of one lamp, F l \u003d 22000 lm, - the utilization factor of the luminous flux is determined depending on the type of lamp, lamp, reflection coefficients and room indicator i

The indicator of the room is found by the formula:

where i is the indicator of the room

A - the length of the room, m

B - room width, m

H p - the height of the lamp suspension above the working surface, m

For the RKU luminaire at ρ n = 50%; ρ c = 30%; ρ p = 10% and i = 1.34 u = 0.48

where ρ n - reflection coefficient from the ceiling,%

ρ c - coefficient of reflection from walls, %

ρ p - coefficient of reflection from the working surface, %

we determine by formula (1) the number of lamps: =

We find the number of emergency lighting fixtures (25% of the working one):

We install 8 lamps in 2 rows, 4 pieces in a row

For other sections, the calculation is similar, the results are summarized in Table. 2.1.

Table 2.1

name site

Lamp type

Plot area, m²

2 Calculation of electrical loads

The calculation is carried out according to the load node by the method of ordered diagrams according to the following algorithm

a) All receivers of a given load node are divided into characteristic technological groups

b) For each group, find the utilization factor Ki, active power factor cosφ and reactive power according to the formula:

(2.3)

c) We find the installed capacity for each group of power consumers according to the formula:

R set \u003d N (2.4)

where N - number of receivers nom - rated power of receivers, kW

d) For each technological group, the average shift active P cm and the average shift reactive power Q cm are found according to the formulas:

P cm \u003d K and R set (2.5) cm \u003d P cm tgφ (2.6)

e) Based on this load node, the total installed power, the total average shift active power and the total average shift reactive power are found: ΣР set; ΣP cm; ΣQ cm

f) Determine the group utilization factor by the formula:

K i.gr \u003d ΣP cm / ΣQ cm (2.7)

where ΣР cm is the total average shift active power, kW;

ΣQ cm - total average shift reactive power, kvar

g) Determine the load modulus by the formula:

where P nom.max - active rated power of the largest receiver in the group, kW

P nom.min - active rated power of the smallest receiver in the group, kW

h) Determine the effective number of receivers according to the condition:

if m ≤ 3, n ≥ 4, then n e = n; at m> 3, K i.gr< 0,2, эффективное число приёмников определяют в следующем порядке:

) the largest power receiver of the considered node is selected

) electrical receivers are selected, the power of each of which is equal to or more than half of the largest electrical receiver in terms of power

) count their number n′ and their total rated power Р′nom

) determine the total rated power of all working electrical receivers of the considered node P nom ∑ and their number n

) find n′ * and P′ nom* :

′ * = n′ / n(2.9)

Р′ nom* = Р′ nom / Р nom∑ (2.10)

) by n′ * and P′ nom* determine n′ e* according to the schedule

) find n e:

n e = n′ e* n (2.11)

i) Determine, depending on the group utilization factor and the effective number of power receivers, the maximum coefficient K m according to graphical dependencies or

j) Calculated active power is determined by the formula:

P m \u003d K m ΣP cm (2.12)

k) Calculated reactive power is determined by the formula:

if n e ≤ 10, then Q m = L m ΣQ cm (2.13)

if n e > 10, then Q m = ΣQ cm (2.14)

where L m - coefficient of maximum reactive power, L m = 1.1

m) Determine the total design load S m by the formula:

m) Determine the rated current I by the formula:

where U - rated voltage of electrical consumers, kV

The active design load of lighting is determined by the formula:

P p.o \u003d K s R set (2.17)

where K c - demand coefficient, K c \u003d 0.8

by formula (2.4):

R set \u003d 28 0.4 \u003d 11.2 kW

P p.o \u003d 0.8 11.2 \u003d 8.96 kW

According to formula (2.3) we find: tgφ = 0.62

according to the formula (2.6) we find the calculated reactive lighting load:

Q p.o \u003d 8.96 0.62 \u003d 5.6 kvar

The full load on the tires 0.38 kV TS is determined by the formula:

p \u003d √ (P m∑ + P p.o)² + (Q m∑ + Q p.o)² (2.18)

where P m∑ - total power load on 0.38 kV TS buses, kW m∑ - total reactive load on 0.38 kV TS buses, kVAr

The calculation results for all load nodes are summarized in Table. 2.2

Table 2.2

Naim. node gr. EP

R set kW

P nom kW

Cosφ tgφ

1) milling machines






2) lathe






3) machine carousel. CNC

0,5 1,73






4) crane-beam PV=40%

0,5 1,73






On tires ШР-1


1) milling machines

0,4 2,35






2) Fans

0,8 1,73






On tires ShR-2


1) lathes

0,4 2,35






2) Fans

0,8 1,73






3) crane-beam PV=40%

0,5 1,73






On tires ШР-3


Lighting











On tires 0.38 TP












2.3 Reactive power compensation

The power of the compensating device is calculated by the formula:

ku \u003d α ΣР calc (tgφ avg.vz -tgφ s) (2.19)

where α is a coefficient that takes into account the possibility of reactive power compensation by natural methods, α = 0.9

ΣR calc - total calculated active load, kW

tgφ c - reactive power factor, which must be achieved after reactive power compensation, according to the task: tgφ c = 0.45.

tgφ avg - weighted average value of the reactive power factor, calculated by the formula:

(2.20)

where ΣQ calc - total calculated reactive load

The total design load on the 0.38 kV buses of a transformer substation, taking into account reactive power compensation, is calculated by the formula:

4 Selection of the number and power of transformers of the supply substation

Since the electrical receivers of the production workshop belong to consumers of the 3rd category according to the required degree of reliability of power supply, 1 transformer can be installed at the substation

In accordance with the load, we plan 2 options for the power of transformers:

var - 1 X 160 kVA

var - 2 X 63 kVA

Let's show the calculation on the example of option 2

We check the transformers in the normal mode. We find

load factor of transformers:

(2.22)

where S load - total load power, kVA - number of installed transformers nom.tr - rated power of one transformer, kVA

We check the operation of transformers in emergency mode. Oil transformers allow overload by 40% in emergency mode 6 hours a day for 5 days

When one transformer is disconnected, the second, taking into account, will allow overload:

4 63 = 88.2 kVA

The power deficit will be:

1 - 88.2 = 26.9 kVA

but because electrical receivers are consumers of the 3rd category in terms of reliability of power supply, then some of them can be turned off for the duration of the accident

We check the operation of transformers according to an economically feasible mode

We determine the cost of energy losses by the formula:

C n \u003d C o N T m [(ΔR x.x + K i.p I x.x) + K s 2 (ΔR k.z + K ip U k ] (2.23)

where C o - the cost of one kWh, for the current 2013, C o \u003d 0.81 tons / kWh

T m - the number of use of the maximum load, h

K i.p - Coefficient of loss change, K i.p = 0.03 kW / kvar

ΔР x.x - no-load power losses, ΔР x.x = 0.24kW x.x - no-load current, I x.x = 2.8%

ΔР short circuit - short circuit power losses, ΔР short circuit = 1.28kW to - short circuit voltage, U to = 4.5%

We determine the capital costs by the formula:

K = N C tr (2.24)

where C tr is the cost of the transformer, C tr = 31 tons

We find depreciation costs C a:

C a \u003d K a K (2.25)

where K a - coefficient taking into account deductions for depreciation and operation, for transformers K a \u003d 0.12

Find the total annual costs:

С ∑ = С n + С a (2.26)

For the first variant, the results are summarized in Table. 2.3

Table 2.3

Name of parameters

Option 1 - 1 x 160 kVA

Option 2 - 2 x 63 kVA

ΔR x.x kW

ΔR short-circuit kW

C o, tn/kW∙h

Since C ∑II > C ∑I and K II > K I, then we choose option I - 1 X 160 kVA, as more economical

5 Selection of the location of the supply substation

The location of the SR is determined by the cartograms of the loads, depending on the power of the electrical receivers powered from it.

Distribution cabinets and workshop transformer substation should be installed in the center of electrical loads (CEN). CEN coordinates are determined by the formula:

X tsen = (2.27)

Y ceng =(2.28)

where Xi - coordinate of the i -th electrical receiver along the abscissa axis, m; - coordinate of the i -th electrical receiver along the ordinate axis, m;

Rnom.i - rated power of the i -th electrical receiver, kW.

We will show the calculation using the example of ШР - 1:

X ceng = = 26.1m ceng == 8.1m

For the rest of the calculation, similar results are summarized in Table 2.4

Table 2.4

Estimated coordinates

Installation coordinates


2.6 Calculation of the network 0.38 kV

workshop power supply lighting transformer

Choice of protection devices

The choice of the conductor section for a separate electrical receiver will be shown by an example lathe No. 13. The cross section of the supply conductor is selected according to the allowable heating:

add ≥ I р (2.29)

where I add - permissible current of the conductor, determined by the cross section

current-carrying core, its material, number of cores, type of insulation and laying conditions, A

The rated current is determined by the formula:

p = (2.30) p =

this current corresponds to the APV wire - 2.5 mm² with I add \u003d 19A

We check the selected section for permissible voltage losses:

∆U add ≥∆U р (2.31)

where ∆U add - allowable voltage losses, ∆U add = 5%

∆U р - calculated voltage losses, %

∆U p % = (2.32)

where L is the length of the conductor, km o is the active resistance of 1 km of the conductor, r o \u003d 3.12 Ohm / km,

x o - reactance of 1 km of conductor, x o \u003d 3.12 Ohm / km,

because ∆U p< ∆U доп, то сечение 2,5 мм² соответствует допустимым потерям напряжения. В качестве аппарата защиты выбираем предохранитель по следующим условиям:

U nom.pr > U nom (2.33) nom.pr > I p (2.34) square sun > I peak / α(2.35)

where U nom.pr - rated voltage of the fuse, V nom.pr - rated current of the fuse, A pl.vs - rated current of the fuse-link, A peak - peak current, A

α - coefficient taking into account the start conditions, α = 2.5

peak \u003d K p ∙ I p (2.36)

where K p - the multiplicity of the starting current in relation to the normal mode current

K p \u003d 5 peak \u003d 19 5 \u003d 95A nom.pr > 380V rated > 19A Sq.Sun > 95/2.5 = 38A

We select the fuse PN - 2, I nom \u003d 100A I pl.vs \u003d 40A

We check the selected wire for compliance with the selected fuse according to the condition:

add ≥ K s ∙ I s (2.37)

where K z - the multiplicity of the permissible current of the conductor in relation to the current of operation of the protection device, K z \u003d 1

I c - protection operation current, A

because 19< 1 ∙ 40, то провод не соответствует аппарату защиты поэтому выбираем провод АПВ - 10мм 2 , I доп = 47А

We will show the calculation for a group of electrical consumers using the example of ШР-1

In accordance with formula (2.30) I p = 67.82A. According to the condition (2.29), we select the APV wire - 25mm 2; I add \u003d 80A

By formula (2.32) we find:

∆U p % = 0.2%

Wire APV-25mm 2 corresponds to the permissible voltage losses,

because ∆U p = 0.2% ≤ ∆U add = 5%

We install a fuse as a protection device.

Finding the peak current:

peak \u003d I p - K and ∙ I nb + I start. nb (2.38)

where I nb - rated current of the largest motor powered by ShR-1 start.nb - starting current of the largest motor powered by ShR-1

According to formula (2.30) we find I nb \u003d 91A, according to formula (2.36) I start.nb \u003d 455A peak \u003d 67.82 - 0.13 91 + 455 \u003d 511A

According to the conditions (2.33), (2.34), (2.35) we select the fuse PN-2 nom.pr = 250A, I pl.vs = 250A

Checking the selectivity fuse

The single-line diagram of ShR-1 is given in fig. 2.1

Fig. 2.1 Single-line diagram of ShR-1

The fuse at the input is not selective, so we choose the fuse PN-2 I nom.pr = 400A, I pl.vs = 350A

We check the selected wire for compliance with the selected fuse according to the condition (2.37), since 67.82 ≤ 1 ∙ 350, then the wire does not correspond to the protection device, so we select the cable SB 3 185 + 1 95 s I add = 340A

Taking into account the permissible overload, the cable corresponds to the selected fuse.

For other electrical receivers and distribution cabinets, the calculation is similar, the results are summarized in Table. 2.5

Table 2.5

conductor

fuse

Number of cores




2.7 Calculation of a network with a voltage above 1 kV

We determine the economically feasible section according to the formula:

F eq = (2.39)

where j ek - economic current density, j ek \u003d 1.7 A / mm 2

In accordance with formula (2.30): p = A ek = 9m

We choose the nearest standard section - 10 mm²

We choose the cable AAB-3x10 mm 2

We check the selected cable for thermal resistance to short-circuit currents

The thermally stable cross section to short-circuit currents is determined by the formula

m.y. = (2.40)

where I ∞ is the steady-state value of the periodic component of the short-circuit current ∞ = 2850A (see section 2.8)

C - coefficient taking into account the difference in heat released by the conductor before and after a short circuit, C \u003d 95

t pr - fictitious time at which the steady short circuit current releases the same amount of heat as the actual short circuit current. for real time

at tg = 0.15s, t pr = 0.2s, at β '' = 2 t.y = 2850 = 13

AAB cable 3 x 10 is thermally resistant to short-circuit currents

We finally choose the AAB 3 x 10 cable

2.8 Calculation of short-circuit currents

The calculation is carried out in relative units under basic conditions. In accordance with the assignment and design results, we draw up a design scheme and an equivalent circuit. The design scheme is given in Fig. 2.2, the equivalent circuit in Fig. 2.3

Rice. 2.2 Calculation scheme Fig.2.3 Equivalent scheme

We assume that the base power Sb = 100MVA, the base voltage Ub = 6.3kV

The overhead line resistance is found by the formula:

X vl * b \u003d (2.41)

where U nom.sr is the average rated voltage of the step, kV

X vl * b \u003d 0.4 35 100 / 115² \u003d 0.11 Ohm

The resistance of the transformer is found by the formula:

tr.b =* (2.42) tr.b =* = 4.2Ω

We determine the reactance of the cable line according to the formula (2.41):

X cl * b \u003d = 0.28 ohm

We find the active resistance of the cable line according to the formula

(2.43) cl*b = = 7,97

Using the signs of parallel and series connection of resistances, we find the active and inductive resulting resistances:

X res * b \u003d 0.11 + 2.1 + 0.28 \u003d 2.49 res * b \u003d 7.97

because \u003d res * b = 8,35

We determine the short-circuit current by the formula:

where I b - base current, kA

According to the formula (2.14) we find the base current:

I b \u003d \u003d 9.16 kA

I k.z. = = 1.1 kA

Determine the shock current:

y \u003d (2.45) y \u003d 2.55 ∙ 1.1 \u003d 2.81 kA

We find the short circuit power:

k.z. = (2.46) k.z. = = 11.98 MVA

9 Selection of substation equipment

The choice of disconnectors is made according to the following conditions:

nom.r > U nom. (2.47) nom.r > I calc. (2.48) a. ≥ i y. (2.49)

I t² ∙ t > I to 2 ∙ t pr (2.50)

where U nom.r - rated voltage of the disconnector

I nom.r - rated current of the disconnector a - amplitude value of the preliminary through short-circuit current t - limiting current of thermal resistance - time during which the disconnector withstands the limiting current of thermal resistance

The rated data of the disconnector is found by

The switch is selected according to the following conditions:

nom.v = U nom. (2.51) nom.v > I p (2.52) a. ≥ i y (2.53) t ² ∙ t > I to 2 ∙ t pr (2.54) otk > I to (2.55) out ≥ S to (2.56)

where U nom.v - rated voltage of the circuit breaker, kV nom.v - rated current of the circuit breaker, A off - rated breaking current of the circuit breaker, kA off - breaking power of the circuit breaker, MVA

open = ∙ I open ∙ U nom.v (2.57)

We find the nominal data of the oil circuit breaker. The selection results are presented in table. 2.6

Table 2.6


3. SAFETY AND HEALTH

1 Organizational and technical measures for safe work with electrical installations up to 1 kV

For the safe performance of work, the following organizational measures must be taken:

appointment of persons responsible for the safe conduct of work;

issuance of orders and orders;

issuance of permits for the preparation of jobs and admission;

preparation of the workplace and admission;

supervision during the performance of work;

transfer to another workplace;

registration of breaks in work and its completion.

All work, both with and without stress relief, near or on live parts must be carried out according to a work permit or by order, since ensuring their safe performance requires special preparation of the workplace and the implementation of certain measures. The exception is short-term and small-scale work performed by on-duty or operational-repair personnel in the order of current operation. Their duration should not exceed 1 hour.

One employee can prepare the workplace and admit it.

The order is a task drawn up on a special form for the safe performance of work, which determines the content of the work, the place, time of its beginning and end, the necessary security measures, the composition of the team and persons responsible for the safety of the work. The order can be issued for up to 15 days.

The order is a task for the safe performance of work, which determines the content of the work, places, time, security measures for the persons who are entrusted with its implementation. The order can be oral and written, it has a one-time character. Works lasting up to 1 hour are allowed to be carried out by order of the maintenance personnel under the supervision of a person on duty or a person from among the operational and maintenance personnel, as well as by the duty or operational maintenance personnel themselves. At the same time, the senior person performing the work or supervising should have skill group IV in electrical installations with voltages above 1000 V. If the duration of these works is more than 1 hour or they require the participation of more than three people, then they are issued with an order.

Issuing the outfit, the order establishes the possibility of safe performance of work. He is responsible for the sufficiency and correctness of the safety measures indicated in the work order, for the qualitative and quantitative composition of the team and the appointment of responsible persons, as well as for the compliance of the work performed by the electrical safety groups listed in the work order. The right to issue orders and orders is granted to employees from the administrative and technical staff of the enterprise and its structural divisions having group V.

The work manager is responsible for the implementation of all the safety measures specified in the work order and their sufficiency, completeness and quality of the team briefing conducted by the admission and the work foreman, as well as the organization of safe work. Engineering and technical workers with group V should be appointed as work supervisors.

The person who gives permission for the preparation of workplaces and for admission is responsible for the sufficiency of the measures for disconnecting and grounding the equipment provided for the work and the possibility of their implementation, as well as for coordinating the time and place of work of the admitted teams. Employees from duty personnel with group IV are entitled to give permission for the preparation of workplaces and for admission in accordance with job descriptions, as well as employees from the administrative and technical staff authorized to do so by the instructions for the enterprise.

The person preparing the workplace is responsible for the correct and accurate implementation of the measures for preparing the workplace specified in the order, as well as those required by the working conditions (installation of locks, posters, fences).

The duty officer or employees from the operational and repair personnel who are admitted to operational switching in this electrical installation have the right to prepare jobs.

The admitting person is responsible for the correctness and sufficiency of the security measures taken and their compliance with the measures indicated in the order, the nature and place of work, for the correct admission to work, as well as for the completeness and quality of the briefing he conducts. The admitting person must be appointed from the duty or operational-repair personnel. In electrical installations above 1000V, the admitting one must have group IV. The manufacturer of work performed side by side in electrical installations above 1000V must have group IV. An observer must be appointed to supervise teams of workers who do not have the right to work independently in electrical installations. Employees with group III can be appointed as observers.

Each member of the team is obliged to comply with the safety regulations for the operation of electrical installations and instructions received during admission to work and during work, as well as the requirements of local labor protection instructions.

CONCLUSION

When designing a mechanical repair shop, the following results were obtained:

1. A variant of the power supply scheme was selected, a scheme of the distribution network of power supply was developed

2. In accordance with the power and lighting loads, taking into account economic indicators for the power supply of the production workshop, it is necessary to install one transformer with a capacity of 160 kVA at the 6 / 0.4 kV supply substation

Power networks 0.38 kV, it is advisable to carry out the cable of the AAB brand, laid along cable structures, and the APV wire, laid in pipes in the floor

Fuses must be selected as the protection device

5. Organizational and technical measures for labor protection during work in electrical installations up to 1 kV are given.

The design results are given in the table:

Name of electrical equipment

Brand Type

unit of measurement

Quantity

Three-pole disconnector

Oil switch

VMM-10-320-10tz

160kv*A oil transformer

Fuse

also I nom \u003d 600A I pl.vs \u003d 500A

also I nom \u003d 250A I square.vs \u003d 200A

also I nom \u003d 250A I square.vs \u003d 120A

also I nom \u003d 100A I pl.vs \u003d 80A

also I nom \u003d 100A I square.vs \u003d 50A

also I nom \u003d 100A I pl.vs \u003d 40A

also I nom \u003d 100A I pl.vs \u003d 30A

Cable for voltage 6Kv Cross section 3/10mAPV

Postnikov N.P., Rubashov G.M. Power supply industrial enterprises. L.: Stroyizdat, 1980.

Lipkin B.Yu. Power supply of industrial enterprises and installations. - M .: graduate School, 1981.

Kryuchkov I.P., Kuvshinsky N.N., Neklepaev B.N. Electric part of stations and substations. - M.: Energy, 1978.

6. Handbook of power supply and equipment / Ed. Fedorova A.A., Barsukova A.N. M., Electrical equipment, 1978.

7. Rules for the installation of electrical installations / Ministry of Energy of the USSR. - M .: Energy, 1980.

Khromchenko G. E. Designing cable networks and wiring - M .: Higher school, 1973.

9. E.F. Tsapenko. Devices for protection against single-phase earth fault. - M.: Energoatomizdat 1985 - 296 p.

10. Shidlovsky A.K., Kuznetsov V.G. Improving the quality of energy in electrical networks. - Kyiv: Naukova Dumka, 1985 - 354 p.

Zhelezko Yu.S. Selection of measures to reduce electricity losses in electrical networks. Guide for practical calculations. - M.: Energoatomizdat, 1989 - 176 p.

Electrical networks are used to transmit and distribute electrical energy to shop consumers of industrial enterprises. Energy consumers are connected through intrashop substations and distribution devices using protective and starting devices.

Electric networks of industrial enterprises are carried out internal (workshop) and external. External voltage networks up to 1 kV are of very limited distribution, since at modern industrial enterprises the power supply of shop loads is produced from intra-shop or attached transformer substations.

The choice of electrical networks radial power circuits are characterized by the fact that from the power source, for example, from a transformer substation, lines depart directly to power powerful electrical receivers or separate distribution points, from which smaller electrical receivers are fed by independent lines.

Radial circuits provide high reliability of power supply to individual consumers, since accidents are localized by turning off the automatic switch of the damaged line and do not affect other lines.

All consumers can lose power only in case of damage on the PTS busbars, which is unlikely. As a result of a fairly reliable design of cabinets of these PTS.

Main power circuits are widely used not only to power many electrical receivers of one technological unit, but also to compare a large number of small receivers that are not connected by a single technological process.

Trunk circuits allow you to abandon the use of a bulky and expensive switchgear or shield. In this case, it is possible to use the transformer-trunk block scheme, where bus ducts (bus ducts) manufactured by the industry are used as the supply line. Trunk circuits made by busbars provide high reliability, flexibility and versatility of workshop networks, which allows technologists to move equipment inside the workshop without significant installation of electrical networks.

Due to the uniform distribution of consumers within the mechanical repair shop, as well as low cost and ease of use, the main power supply scheme is selected.

The location of the main equipment is shown in the diagram (Fig. 1).

The choice of a power supply scheme is inextricably linked with the issue of voltage, power, EP category in terms of reliability, EP remoteness.

With regard to ensuring the reliability of power supply, power receivers are divided into the following three categories.

Electrical receivers first category- electrical receivers, the interruption of the power supply of which may lead to a danger to human life, a threat to the security of the state, a significant material damage, disruption of a complex technological process, disruption of the functioning of especially important elements of public utilities, communication and television facilities.

From the composition of power receivers of the first category, a special group of power receivers stands out, the uninterrupted operation of which is necessary for an accident-free shutdown of production in order to prevent a threat to people's lives, explosions and fires.

Electrical receivers second category- electrical receivers, the interruption of power supply of which leads to massive undersupply of products, massive downtime of workers, mechanisms and industrial transport, disruption of the normal activities of a significant number of urban and rural residents.

Electrical receivers third category- all other electrical receivers that do not fall under the definitions of the first and second categories.

Power receivers of the first category in normal modes must be provided with electricity from two independent mutually redundant power sources, and a break in their power supply in the event of a power failure from one of the power sources can only be allowed for the period of automatic power restoration.

For the power supply of a special group of power receivers of the first category, it should be provided extra food from a third independent mutually redundant power supply.

As a third independent power source for a special group of power receivers and as a second independent power source for other power receivers of the first category, local power plants, power plants of power systems (in particular, generator voltage buses), uninterruptible power units designed for these purposes, batteries and etc.

If it is impossible to ensure the continuity of the technological process by redundant power supply or if redundant power supply is not economically feasible, technological redundancy should be carried out, for example, by installing mutually redundant technological units, special devices for trouble-free shutdown of the technological process, operating in the event of a power failure.


Power supply of power receivers of the first category with a particularly complex continuous technological process that requires a long time to restore the normal mode, in the presence of feasibility studies, it is recommended to carry out from two independent mutually redundant power sources, to which additional requirements are imposed, determined by the characteristics of the technological process.

Power receivers of the second category in normal modes must be provided with electricity from two independent mutually redundant power sources.

For power receivers of the second category, in the event of a power failure from one of the power sources, power supply interruptions are permissible for the time necessary to turn on the backup power by the actions of the duty personnel or the mobile operational team.

For power receivers of the third category, power supply can be carried out from one power source, provided that power supply interruptions necessary to repair or replace a damaged element of the power supply system do not exceed one day.

The issue of choosing a power supply scheme, voltage level is decided on the basis of a technical and economic comparison of options.

To power industrial enterprises, electric networks with a voltage of 6, 10, 20, 35, 110 and 220 kV are used.

In the supply and distribution networks of medium-sized enterprises, a voltage of 6–10 kV is accepted. Voltage 380/220 V is the main voltage in electrical installations up to 1000 V. The introduction of voltage 660 V is cost-effective and is recommended to be used primarily for newly built industrial facilities.

Voltage 42 V (36 and 24) is used in rooms with increased danger and especially dangerous, for stationary local lighting and hand-held portable lamps.

The 12 V voltage is used only under particularly unfavorable conditions with regard to the risk of electric shock, for example when working in boilers or other metal tanks using hand-held portable lights.

Two main power distribution schemes are used - radial and main, depending on the number and relative position of workshop substations or other power supplies in relation to the point that feeds them.

Both schemes provide the required reliability of power supply to EA of any category.

Radial distribution schemes are used mainly in cases where the loads are dispersed from the power center. Single-stage radial circuits are used to power large concentrated loads (pumping, compressor, converter units, electric furnaces, etc.) directly from the power center, as well as to power workshop substations. Two-stage radial circuits are used to power small workshop substations and HV power receivers in order to unload the main energy centers(Fig. H.1). At intermediate distribution points, all switching equipment is installed. The use of multi-stage schemes for intrashop power supply should be avoided.

Distribution points and substations with electrical receivers of categories I and II are usually powered by two radial lines that operate separately, each for its own section, when one of them is disconnected, the load is automatically taken by the other section.

Rice. 3.1. Fragment of a radial power distribution scheme

Main power distribution schemes should be used for distributed loads, when there are many consumers and radial schemes are not economically feasible. The main advantages: allow better loading of cables in normal mode, save the number of cabinets at the distribution point, reduce the length of the trunk. The disadvantages of backbone circuits include the complication of switching circuits, the simultaneous shutdown of the electric power supply of several production sites or workshops that are powered by this line when it is damaged. For power supply of VP of categories I and II, schemes with two or more parallel through mains should be used (Fig. 3.2).

Power supply of EP in networks with voltage up to 1000 V of II and III categories in terms of power supply reliability is recommended to be carried out from single-transformer packaged transformer substations (KTS).

The choice of two-transformer PTS must be justified. The most expedient and economical for intrashop power supply in networks up to 1 kV are the main circuits of transformer-trunk blocks without switchgears at a substation using complete busbars.

Radial circuits of intrashop supply networks are used when it is impossible to perform trunk circuits due to the conditions of the territorial distribution of electrical loads, as well as environmental conditions.

For the power supply of shop consumers in design practice, radial or main circuits in their pure form are rarely used. The most widespread are the so-called mixed circuits of electrical networks, combining elements of both radial and trunk circuits.

Rice. 3.2. Scheme with double through highways

Power supply circuits and all electrical installations of alternating and direct current of an enterprise with a voltage of up to 1 kV and above must meet the general requirements for their grounding and protection of people and animals from electric shock both in normal operation of the electrical installation and in case of damage to the insulation.

Electrical installations in relation to electrical safety measures are divided:

- for electrical installations with voltages above 1 kV in networks with a solidly grounded or effectively grounded neutral;

- electrical installations with voltages above 1 kV in networks with isolated or grounded neutral through an arcing reactor or resistor;

- electrical installations with voltage up to 1 kV in networks with dead-earthed neutral;

- electrical installations with voltage up to 1 kV in networks with isolated neutral.

For electrical installations with voltage up to 1 kV, the following designations are accepted. System TN- a system in which the neutral of the power source is solidly grounded, and the open conductive parts of the electrical installation are connected to the solidly grounded neutral of the source by means of zero protective conductors (Fig. 3.3–3.7).

Rice. 3.3. System TN-C- system TN, in which zero protective

and zero working conductors are combined in one conductor

throughout its length

The first letter is the neutral state of the power supply relative to

T– grounded neutral;

I– isolated neutral.

The second letter is the state of open conductive parts relative to ground:

T– exposed conductive parts are grounded, regardless of the relation to the earth of the neutral of the power supply or any point of the supply network;

N– exposed conductive parts are connected to a dead-earthed neutral of the power source.

Subsequent (after N) letters - combination in one conductor or separation of the functions of the zero working and zero protective conductors:

S– zero worker ( N) and zero protective ( PE) conductors are separated;

C- the functions of the zero protective and zero working conductors are combined in one conductor ( PEN-conductor);

N- zero working (neutral) conductor;

PE- protective conductor (grounding conductor, zero protective conductor, protective conductor of the potential equalization system);

PEN- combined zero protective and zero working conductor.

Zero working (neutral) conductor ( N) - a conductor in electrical installations up to 1 kV, designed to power electrical receivers and connected to a solidly grounded neutral of a generator or transformer in three-phase current networks, with a solidly grounded output of a single-phase current source, with a solidly grounded source point in DC networks.

Combined zero protective and zero working ( PEN) conductor - a conductor in electrical installations with voltage up to 1 kV, combining the functions of a zero protective and zero working conductor.

To protect against electric shock in normal operation, the following protective measures against direct contact must be applied individually or in combination:

– basic insulation of current-carrying parts;

- fences and shells;

– installation of barriers;

– placement out of reach;

– use of extra-low (small) voltage.

Rice. 3.4. System TN-S- system TN, in which zero protective

and zero working conductors are separated along its entire length

Rice. 3.5. System TN-C-S- system TN, in which the functions of zero

protective and zero working conductors are combined in one

conductor in some part of it, starting from the power source

Rice. 3.6. System TT– a system in which the neutral of the power supply

deafly grounded, and open conductive parts of the electrical installation

earthed with a grounding device, electrically

source independent of the dead-earthed neutral

Rice. 3.7. System IT– a system in which the neutral of the power supply

isolated from earth or earthed through appliances or devices,

with high resistance, and exposed conductive parts

electrical installations are grounded

For additional protection against direct contact in electrical installations with voltage up to 1 kV, if there are requirements of other chapters of the PUE, apply residual current devices(RCD) with a rated breaking differential current of not more than 30 mA.

In order to protect against electric shock in the event of insulation failure, the following protective measures against indirect contact must be applied individually or in combination:

– protective grounding;

– automatic power off;

– equalization of potentials;

– equalization of potentials;

– double or reinforced insulation;

– extra-low (small) voltage;

– protective electrical separation of circuits;

- insulating (non-conductive) rooms, zones, sites.

Electrical installations up to 1 kV in residential, public and industrial buildings and outdoor installations should, as a rule, be powered from a source with a solidly grounded neutral using a system TN.

Power supply of electrical installations with voltage up to 1 kV AC from a source with isolated neutral using the system IT should be carried out, as a rule, if a power interruption is unacceptable at the first short circuit to the ground or to open conductive parts connected to the potential equalization system. In such electrical installations, for protection against indirect contact during the first earth fault, protective grounding must be performed in combination with network insulation monitoring or RCDs with a rated differential breaking current of not more than 30 mA should be used. In case of a double ground fault, automatic power off must be performed in accordance with the PUE.

Power supply of electrical installations with voltage up to 1 kV from a source with a dead-earthed neutral and with grounding of open conductive parts using a grounding conductor not connected to the neutral (system TT), is allowed only in those cases when the electrical safety conditions in the system T N cannot be provided. For protection against indirect contact in such electrical installations, automatic power off must be performed with the mandatory use of RCDs.

In this case, the condition

Ra I a≤ 50V,

where I a is the operating current of the protective device;

R a is the total resistance of the grounding conductor and the grounding conductor of the most remote electrical receiver when using RCD to protect several electrical receivers.

When using the system TN re-grounding is recommended PE- and PEN- conductors at the input to the electrical installations of buildings, as well as in other accessible places. For re-grounding, natural grounding should be used first. The resistance of the re-grounding earth electrode is not standardized.

In electrical installations with a voltage above 1 kV with an isolated neutral, to protect against electric shock, protective grounding of exposed conductive parts must be made.

App. 3 shows the power supply schemes of individual buildings, and in App. 4 - graphic and letter designations in electrical circuits.

 

It might be useful to read: