Characteristics - Installed power loads

Characteristics

Installed power loads

The examination of actual values of apparent-power required by each load enables the establishment of :

• A declared power demand which determines the contract for the supply of energy
• The rating of the MV/LV transformer, where applicable ( allowing for expected increased load )
• Levels of load current at each distribution board.

Induction motors

Current demand

The reted current in suppliend to the motor is given by the following formulae :






where
In : reted current (in amps)
Pn: nominal power (in kW)
U: voltage between phases for 3-phase motors and voltage between the terminals for single-phase motors (in volts). A single-phase motor may be connected phase-to-neutral or phase-to-phase.

 : per-unit efficiency, i.e. output kW / input kW
 : power factor, i.e. kW input / kVA input.

transactions :

• An examination of the actual apparent-power demands of differnt loads : A necessary preliminary step in the design of a LV installation.
• The norminal power in kW (Pn) of a motor indicates its rated equivalent mechanical power output. The apparent power in kVA (Pa) suppliend to the motor is a function of the output, the motor efficiency and the power factor.

         

Subtransient current and protection setting

• Subtransient current peak value can be very high; typical value is about 12 to 15 times the rms rated value In. Sometimens this value can reach 25 times In.
• If unexpected tripping of the overcurrent protection occurs during starting, this means the starting current exceeds exceeds the nominal limits. As a result, some maximum switchgear withstands can be reached, life time can be reduced and even some devices can be destroyed. In order to avoid such a situation, oversizing of the switchgear must be considered.

Compensaion of reactive-power (kvar) spplied to induction motors

It is generally advantageous for technical and financial reasons to reduce the current supplied to induction motors. This can be achieved by using capacitors without affecting the power output of the motors. The application of this principle to the operation of induction motors is generally referred to as "power-factor improvement" or "power-factor correction".

As discussed in chapter L, the apparent power (kVA) supplied to an induction motor can be significantly reduced by the use of shunt-connected capacitors. Reduction of input kVA means a corresponding reduction of input current (since the voltage remanis constant).

Compensation of reactive-power is particularly advised for motors that operate of long periods at reduced power.

As noted above    so that a kVA reduction will increase (i.e. improve) the value of

.

The current supplied to the motor, after power-factor currection, is given by

where  is the power factor before compensation and   is the power factor after compensation, Ia being the orginal current.

Resistive-type heating appliances and incandescent lamps (conventional or halogen)

The current demand of a heating appliance or an incandescent lamp is easily obtained from the nominal power Pn quoted by the manufacturer.

The power Pn (watts) indicated on the tube of a fluorescent lamp does not include the power dissipated in the ballast.

The current is given by :






where U is the voltae between the terminals of the equipment.
For an incandescent lamp, the use of halogen gas allows a more concentrated light source. The light output is increased and the lifetime of the lamp is doubled.

Note : at the instant of switching on, the cold filament gives rise to a very brief but intense peak of current.

electrical installation guide

Methodology Electrical

Rules and statutory rugulations

Range of low-voltage extends form 0 V to 1000 in a.c and from 0 V to 1500 V in d.c . One of the first decision is the selection of type of current between the alternative current which corresponds to the most common type of current through out the world and the direct current. Then designers have to select the most appropriate rated voltage within these ranges of voltages. when connected to a LV public network, the type of current and the rated voltage are already selected and imposed by the utility.

Compliance with national regulations is then the second priority of the designers of electrical installation. Regulations is then thescond priority of the designers of electrical installation. Regulations may be based on national or internarional standards such as the IEC 60364 series. Selection of equipment complying with national or international product standards and appropriate verification of the completed installation is a powerful mean for providing a safe installation with the expected quality. Defining and complying with the verification and testing of the electrical installation at its completion as well as periodic time will guarantee the safety and the quality of this intallation all along its life cycle. Conformity of equipment according to the appropriate product standards used within the installation is also of prime importance for the level of safety and quality.

Environmental conditions will become more and more stringent and will need to be considered at the design stage of the installation. This may iclude national or regional regulations considering the material used in the equipment as well as the dismantling of the installation at its end of life.

Characteristics - Installed power loads 

A review of all applications needing to be supplied with electricity is to be done. Any possible extensions or modifications during the whole life or the electrical installation are to be considered. Such a review aimed to estimate to current flowing in each circuit of the installation and the power supplies needed. The total current or power demand can be calculated from the data relative to the location and power of each load, together with the knowledge of the operating modes (steady state demand, starting conditions, non simultaneous operation, etc.)

Estimation of the maximum power demand may use verious factors depending on the type of application; type of equipment and type of circuits used within the electrical installation. From these data, the power required from the supply source and (where appropriate) the number of sources necessary for an adequate supply to the installation is readily obtained. Local information regarding tariff structures is also required to allow the best choice of connection arrangement to the power-supply network, e.g.at medium voltage or low voltage level.

Connection to the MV public distribution Netwok

Where this connection is made at the Medium Voltage level a consumer-type substation will have to be studied, built and equipped. This substation may be an outdoor or indoor instalallation conforming to relevant standards and regulations (the low-voltage section may be studiend separately if necessary). Metering at medium-voltage or low-voltage is possible in this case.

Connection to the LV utlity distribution network

where the connection is made at the Low Voltage level the installation will be connected to the local power network and will (necessarily) be meteriad according to LV tariffs.

MV and LV architecture selection guide

The whole electrical system including the MV installation and the LV installation is to be studiend as a complete system. The customer expectations and technical parameter will impact the architecture of the system as well as the electrical installation characteristics. Determination of te most suitable architecture of the MV/LV main distribution and LV power distribution level is often the resuit of optimization and compromise. Nautral earthing arrangements are chosen accoriding to local regulations, constraints realated to the power-supply, and to the type of loads.

The distribution equipment (panelboards, switchgears, circuit connections,...) are determined from building plans and from the location and gruping of loads. The type of premises and allocation can influence their immunity to external disturbances.

LV distribution

the system earthing is one protective measure commony used for the protection against electric shocks.. These systems earthings have a major impact on the LV electrical installation architecture and they need to be analysed as early as prossible. Advantages and drawbacks are to be analysed for a correct selection. Another aspect needing to be considered at the earlier stage is the external influences. In large electrical installation, different external influences may be encountered and need to be considered independently. As a result of these external influences proper selection of equipment accrording to their IP or IK codes has to be made.

Protection against electric shocks and Electric fires

protection against electrc shock consists in providing provision for basic protection ( protection against direct contact ) with provision for fault protection ( protection against indirect contact ). Coordinated provisions result in a protective measure. One of the most common protective measures consists in " automatic disconnection of a system earthing. Deep understanding of each standardized system ( TT, TN and IT system ) is necessary for a correct implementation. 

Electrical fires are caused by overloads, short circuits and earth leakage currents, but also by electric arcs in cables and connections. These dangerous electric arcs are not detected by residual current devives nor by circuit breakers or fuses. The arc fault detector technology makes it possible to detect dangerous arcs and thus provide additional protection of installation of installations.

Sizing and protection of Conductors

selection of cross-sectional-areas of cables or isolated conductors for line conductors is certainly one of the most important tasks of the design process of an electrical installation as this greatly influences the selection of evercurrent protective devices, the voltage drop along these conductors and the estimation of the prospective short-circuit currents : the maximum value relates to the overcurrent protection and the minimum value relates to the fault protection by automatic disconnection of supply. This has to be done for each circuit of the installation. Similar task is to be done for the neutral conductor and for the Protective Earth (PE) conductor.

LV switchger : Functions and Selection

Once the short-circuit current are estimmated, Protectives can be selected for the overcurrent protection. Circuit breakers have also other possible functions such as switching and isolation. A complete understanding of the functionalities offered by all switchgear and controlgaer within the installation is necessary. Correct selections of all devices can now be done. A comprehensive understanding of all functionalities offered by the circuit breakers is of prime importanc as this is the device offering the largest variety of functions.

Overvoltage protection

Direct or indirect lightning strokes can damae electrical equipment at a distance of serveral kilometers. Operating voltage surges, transient and industrial frequency over-voltage can also produce the same condequences. All protective measures against overvoltage need to be assesed. One of the most used corresponds to the use of Surge Protective Devices (SPD) ; Their selection; installation and protection within the electrical installation request some particular attention.

Energy efficiency in electrical distribution

Implementation of active energy efficiency measures within the electrical installation can produce high benefits for the user or owner : reduced power consumption, reduced cost of energy, better use of electrical equipment. These measures will most of the time request specific design for the istallation as measuring electricity consumption either per application ( lighting, heating, process...) or per area ( floor, workshop ) present particular interest for reducing the electricity consumption still keeping the same level of service provided to the user.

Reactive energy

The power factor correction within electrical installations is carried out locally, globally or as a combination of both methods . Improving the power factor has a direct impact on the billing of consumend electricity and may also have an impact on the energy efficiency.

Harmonics

Harmonics currents in the network affect the quality of energy and are at the origin of many disturbances as overloads, vibrations, ageing of equipment, trouble of sensitive equipment, of local area networks.

Particular supply sources and loads

Particuler items or equipment are studied ;

• Specific sources such as alternators or inverters
• Specific loads with special characteristics, such as induction mtors, highting circuits or LV/LV transformers
• Specific systems, such as direct-current networks

EMC Guldelines

Some basic rules must be followed in order to ensure Electromagnetic Compatibility. Non observance of these rules may have serious consequences in the operation of the electrical installation : Disturbance of communication systems, nuisance tripping of protection devices, and even destruction of sensitive devices.