Current Transformers (CT) – Types, Characteristic & Applications

Current Transformers (CT) – Construction, Types, Installation, Characteristic & Applications

What is Current Transformer (CT) ?

Current transformers (CT) are used in High Voltage (HV) and Medium Voltage (MV)^[1] installations to give an image of electrical current to protection relays and units and metering equipment and they are designed to provide a current in its secondary proportional to the current flowing in its primary.

CT are connected in series and protection devices and metering equipments are connected to the secondary of the CT in series association, as shown in Figure 1.

Figure 1 – Schematic connection of a current transformer

• You may also read: Transformer, Construction, Working, Types Application & Limitations

Installation and Procedure of Current Transformer

HV CT are commonly installed outdoors, in AIS substations (Air Insulated Substation) – Figure 2 – or indoors, in GIS substations (Gas Insulated Substation) – Figure 3. MV CT are usually installed indoors, in MV Switchgears – Figure 4.

Figure 2 – Current transformer in an AIS substation

Figure 3 – Current transformer in a GIS substation

Figure 4 – Current transformer in MV switchgear

CT secondary circuit must be grounded, and grounded at one point only. If the secondary of CT is left unloaded a risk of explosion exists.

• You may also read: Transformers Fire Protection System – Causes, Types & Requirements

Special precautions must be taken when connecting CT primary (connection points are usually identified by P1 and P2), and secondary (connection points are usually identified by S1 and S2) in order to assure the correct flow of electric current and the proper functioning of the devices, what is explained in Figure 5.

Figure 5 – Connection of a CT

With this connection the directions of primary and secondary currents are:

• P1  è  P2
• S1  è  S2   (Externally)

When testing a CT using an Omicron test equipment it is possible to verify if the CT is correctly connected:

• If the connection is correct the test equipment will display an angle of 0°.
• If the connection is not correct the test equipment will display an angle of 180°.

You may also read: Transformer Phasing: The Dot Notation and Dot Convention

Construction and Types of Current Transformers

Two types of CT are manufactured:

• “On line” (straight-through) CT (Figure 6) – bar primary type and wound primary type.
• “Ring type” (doughnut) CT (Figure 7)

“Ring type” CT is constructed of an iron toroid, which forms the core of the transformer, and is wound with secondary turns. The doughnut fits over the primary conductor, which constitutes one primary turn.

Figure 6 – On line CT

Figure 7 – Ring type CT

Ring type CT are commonly used in cables, busbars and transformers bushings.

Usually HV CT use oil or gas (SF6) as insulation medium and MV CT use synthetic resins.

CT may have one or more cores; typical applications of these cores are:

• Core 1 – metering; energy metering; recording.
• Cores 2 e 3 – protection.

The use of more than one core for protection is justified when the installation have two protections sets – main and backup.

• You may also read: Transformer Efficiency, All day Efficiency & Condition for Maximum Efficiency

Characteristic  and Specification of Current Transformers

Main electric characteristics of CT are:

• Rated voltage (maximum voltage the CT can withstand)
• Rated primary current
• Ratio
• Accuracy class
• Burden power
• Rating factor (RF)
• Magnetizing curve

According to IEC^[2] Standard 61869-2, Clause 5.201, rated primary currents of CT are: 10 – 12,5 – 15 – 20 – 25 – 30 – 40 – 50 – 60 – 75 A and their decimal multiples or fractions.

The ratio of a CT is the relation between the values of the primary and the secondary currents; the usual secondary values are 1 A and 5 A.

Some CT has special primary coils that allow a double ratio, when an increase of the installation is foreseen (example: 200-400/1 A) – see Figure 8.

Figure 8 – Schematic connection of primary CT windings with double ratio

The accuracy class of a CT is the percent allowable error, and is associated to burden power, the apparent power, expressed in VA, that is taken up from the secondary core (secondary load), and for which the accuracy is assured.

According to the above referred IEC Standard, CT most common accuracies and burdens are:

• Energy metering: 2 or 0.5/2.5
• Metering: 5/10 VA
• Protection: PX, 5P10, 10P10, 5 P20 or 10P20/ 15 VA or 30 VA; the first figures (“5” and “10”) are associated to maximum error allowance and the second figures (“10” and “20”) are associated to accuracy limit factor (ALF) that represents the capacity of the cores to reproduce short-circuit currents without being saturated^[3]. “P” stands for protection.

Class PX is the most accurate and is used usually for main protections. This accuracy class was retained by IEC in 1966 in the Amendment nr. 1 to former Standard 60044 to include the accuracy class “X”, defined on the withdrawn BS 3938:1973.

• You may also read: Transformer Losses – Types of Energy Losses in a Transformer

This transformer as a low leakage reactance for which the knowledge of the transformer’s secondary excitation characteristics, secondary winding resistance, secondary burden resistance and turns ratio is sufficient to assess its performance in relation to the protective relay system with which it is to be used.

The specification of a PX accuracy CT is:

• Rated primary current
• Ratio (maximum error: 25%)
• Knee point voltage
• Magnetizing (excitation) current (at specified voltage)
• Secondary resistance (at 75°C)

Common accuracies and burden powers, as well as error limits, according to IEC Standard 61869 are indicated in Table 1.

Table 1 – Common accuracies and burden powers of CT and error limits

RF, which is a characteristic of metering and energy metering cores, represents the amount by which the primary load current may be increased over its nameplate rating without exceeding the allowable temperature rise, that is to say, the transformer’s overload capability. Common value for RF is 1.5.

Conversely, the minimum primary current a CT can accurately measure is “light load,” or 10% of the rated current

Rating factor of a CT is largely dependent upon ambient temperature. Most CT have rating factors for 35º C and 55º C. Common value for RF is 1.5.

• You may also read: EMF Equation of Transformer

Also important to consider in a CT is the magnetizing curve that is similar to the one shown in Figure 9.

Figure 9 – Magnetizing curve of a CT

For this CT to operate satisfactorily at maximum fault currents, it must operate on the linear part of the magnetizing curve, i.e., below the point at which saturation occurs, which is known as the knee point.

The knee point is defined as the point at which a 10% increase in voltage produces a 50% increase in magnetizing current.

The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much tighter but constrained within a very small bandwidth of the current transformer rating, typically 1.2 to 1.5 times rated current. However, the concept of knee point voltage is very pertinent to protection current transformers, since they are necessarily exposed to currents of 20 or 30 times rated current during faults, and is most critical on differential protection that will be discussed later.

The point on the magnetizing curve at which the CT operates is dependent upon the resistance of the CT secondary circuit.

• You may also read: Can we operate a 60HZ Transformer on 50Hz Supply Source and Vice Versa?

Good to Know:

^[1] Being U_n the rated voltage of the network: HV – U_n ≥ 60 kV; MV – 1 kV < U_n ≤ 49.5 kV.

^[2] IEC: International Electrotechnical Comission.

^[3] A magnetic material is said to be saturated when the increase of applied external magnetic field does not increases the magnetization of the material.

About the Author: Manuel Bolotinha

-Licentiate Degree in Electrical Engineering – Energy and Power Systems (1974 – Instituto Superior Técnico/University of Lisbon)
– Master Degree in Electrical and Computers Engineering (2017 – Faculdade de Ciências e Tecnologia/Nova University of Lisbon)
– Senior Consultant in Substations and Power Systems; Professional Instructor

You may also read:

• What is the difference between Power Transformers and Distribution Transformers?
• What will happen if the primary of a transformer is connected to DC supply?
• Under what condition is D.C supply applied safely to the primary of a transformer?

The post Current Transformers (CT) – Types, Characteristic & Applications appeared first on Electrical Technology.

March 12, 2018 at 03:32AM by Department of EEE, ADBU: http://ift.tt/2AyIRVT

Why does generator-current breaking differ from other switching devices in T&D systems

There are few switching situations that differ from the duties standardized for switching devices in transmission and distribution systems. One of these special situations are generator-current breaking. Generator circuit-breakers are... Read more

Credit- Electrical Engineering Portal. Published by Department of EEE, ADBU: tinyurl.com/eee-adbu

Insulating And Dielectric Materials – Types, Properties & Applications

An Introduction to Insulating And Dielectric Materials

Definition of Insulating and Dielectric Materials

An electrical insulating material can be defined as the material that does not allow electric current to pass through it.

For electrical applications a particular category of insulating materials is used to separate electrically the conducting parts of equipment from each other and from earthed and “no live” components of equipments and networks.

Dielectric materials. A dielectric is an electrical insulating material that can be polarized by an applied electric field (notation: E; unit: volt per meter – V/m). When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization, which an example can be seen in Figure 1.

Figure 1 – Dielectric polarization

Due to dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction, what creates an internal electric field which reduces the overall field within the dielectric itself.

• Also read: Insulation Resistance of a Cable | Why Cables are insulated?

Difference between Insulator, Insulating and Dielectric Materials

From the above statements, It has been cleared that all dielectrics are insulators, but all insulators are not dielectrics.

In very simple words,

Insulator or Insulating Material:

are those substances which will not allow the flow of electron through them due to very low free electrons in it and they are having low dielectric constant (Relative permittivity = ε_r). It is same like resistance of a resistor.

Examples: Porcelain insulators used in the transmission & distribution poles and towers, rubber, glass, plastic etc.

Dielectric or Dielectric Materials:

are those substances same as insulators but will allow the flow of electron through them when subjected to an external electric field as they can be polarized. It also can be defined as the ability to store charge (energy) by mean of polarization such in a capacitor. Also, they are having high dielectric constant (Relative permittivity = ε_r).

Examples: A common example of a dielectric is the electrically insulating material between the metallic plates of a capacitor, (such as mica, laminated paper). Other examples: air, ceramic etc.

Good to know:

• All dielectrics are insulators, but all insulators are not dielectrics.
• Everything is a conductor at some point of temperature or electric field due to breakdown as every insulator has its limits to withstand a potential difference across the material

Also read: Submarine Cables – Construction, Characteristics, Cables Laying & Joints

Types of Dielectric Materials

Dielectric materials are largely used in electrical equipments and networks, being the most common used the types indicated in Table 1,

Table 1 – Common dielectric materials

• Also read: Why AC Needs More Insulation Than DC for the Same Working Voltage Level?

Application of Dielectric Materials

A major application for inorganic materials is in high and medium voltage substation equipments and overhead lines as insulators or as bushings on high voltage transformers and switchgears.

Plastic films have been used as films in a variety of applications such as the insulation between foils in capacitors and slot insulation in rotating electrical machines.

Common use for flexible insulating sleevs is the protection of cables and components from the deleterious effects of mechanical and thermal damage, and may find application in electrical machines, transformers, domestic and heating appliances, light fittings, cable connections (joints and terminations) and switchgears.

Resins and varnishes are used by in the impregnation and coating of electrical equipment (dry type transformers, as an example) in order to improve its resistance to working conditions, to enhance its electrical characteristics and to increase its working life.

Elastomers and thermoplastics are commonly used in the insulation of power, control and communications cables.

Nowadays the principal uses of liquid dielectrics, mainly hydrocarbon mineral oils, are as an insulation and cooling medium for transformers, earth reactors and shunt reactors, capacitors and rheostats.

The important properties of dielectric liquids are therefore electric strength, viscosity, chemical stability and flashpoint.

Two gases already in common use for insulation are nitrogen and sulphur hexafluoride (SF6). Nitrogen is used as an insulating medium in some sealed transformers and Gas Insulated Lines (GIL), while SF6 is used in in high and medium voltage switchgears and circuit breakers, because of its insulating properties and its arc-extinguishing capabilities, and also in Gas Insulated Transformers (GIT) as an insulation and cooling medium.

However, due environmental conditions, in medium voltage installations (circuit breakers, contactors and capacitors) vacuum has nowadays a preferential use.

• You may also read: Corona Effect in Transmission Lines & Power System

Properties and Behavior of Dielectric Materials

The most important properties of dielectric materials are:

• Volume resistivity or specific resistance.
• Permittivity (notation: ε; unit: farad per meter – V/m), is defined as the resistance of the dielectric to an electric field in a particular medium.
• Relative permittivity, or dielectric constant (notation: ε_r – adimensional), which is defined as the ratio of the electric flux density produced in the material to that produced in a vacuum by the same electric field strength, or the relation between the permittivity of the dielectric and the permittivity of vacuum (notation: ε₀ – ≈ 85×10⁻¹² F/m): ε/ε₀ .
• Dielectric strength, which is the ability to withstand electric stress without breaking down. It is usually quoted in kV/mm (typical values may range from 5 to 100 kV/mm).
• Dielectric loss, or electrical dissipation factor, which is defined as the ratio of the power loss in a dielectric material to the total power transmitted through it. It is given by the tangent of the loss angle and is commonly known as “tan δ”. In an ideal insulator the current that passes through it is totally capacitive (I_C), but real insulators do not have 100% purity, this meaning that the current through the insulator as also a resistive component (I_R), and we say that insulator has losses that are represented by tan δ, being δ the angle shown in Figure 2.

Figure 2 –

Figure 2 – Angle of losses and currents of an insulator

Also read: Resistor & Types of Resistors | Fixed, Variable, Linear & Non-Linear

Another significant aspect of all dielectric materials is the maximum temperature at which they will perform satisfactorily.

Generally speaking, dielectric materials deteriorate more quickly at higher temperatures and the deterioration can reach a point at which the insulation ceases to perform its required function.

This characteristic is known as ageing, and for each material it has been usual to assign a maximum temperature beyond which it is unwise to operate.

The ageing of a dielectric depends not only on the physical and chemical properties of the material and the thermal stress to which it is exposed, but also on the presence and degree of influence of mechanical, electrical and environmental stresses.

• You may also read: How to Test a Capacitor? 6 Ways to Check a Capacitor.

Dielectric materials may be deteriorated (premature ageing) when subjected to excessive heat and overvoltages and may be contaminated by other materials, such as copper particles, water and gas, causing dielectric failure.

The definition of a useful lifetime will also vary according to the type and usage of equipment; that must be taken into account when choosing the dielectric material for a particular application.

About the Author: Manuel Bolotinha

-Licentiate Degree in Electrical Engineering – Energy and Power Systems (1974 – Instituto Superior Técnico/University of Lisbon)
– Master Degree in Electrical and Computers Engineering (2017 – Faculdade de Ciências e Tecnologia/Nova University of Lisbon)
– Senior Consultant in Substations and Power Systems; Professional Instructor

You may also read

• Advance Voltage Drop Calculator and Voltage Drop Formula
• Three Phase Electrical Wiring Installation in Home
• Transformers Fire Protection System – Causes, Types & Requirements

The post Insulating And Dielectric Materials – Types, Properties & Applications appeared first on Electrical Technology.

March 09, 2018 at 04:46AM by Department of EEE, ADBU: http://ift.tt/2AyIRVT

Thumb Rules-13 ( Quick Reference Earthing -CPWD)

 

Earthing Strip for Sub-Station Equipment
CPWD-TABLE VIII
Type of Installation	Earth Electrode	Earth Strip
Indoor sub-station with HT panel, Transformer capacity up to  1600  KVA, LT panel, D.G Set.	Copper Plate	25 x 5 mm Copper Strip
Indoor sub-station with HT  panel, Transformer  capacity  above  1600  KVA, LT panel, D.G Set	Copper Plate	32 x 5 mm Copper Strip
HT Outdoor sub-station	Copper Plate	25 x 5 mm Copper Strip
LT Indoor sub-station with generator	Copper Plate	25 x 5 mm Copper Strip
LT   switch   room   with Main   LT  D.B	Copper Plate	20 x 3 mm Copper Strip

 

Neutral Earthing of Transformers and Generators
CPWD-TABLE VIII
Type of Installation	Earth Electrode	Earth Strip for Neutral
Transformer  of  capacity  up  to  1600 KVA	Copper Plate	25 x 5 mm Copper strip
Transformer  of  capacity  above  1600  KVA	Copper Plate	32 x 5 mm Copper strip
Generating set of all capacity	Copper Plate	26 x 5 mm Copper strip
Type of Installation	Earth Electrode	Earth Strip for Neutral
Transformer  of  capacity  up  to  1600 KVA	Copper Plate	25 x 5 mm Copper strip

 

Earthing Strip for Bus Trunking and Rising Main
CPWD-TABLE VIII
Type of Installation	Material of Main Conductor	Earth Strip
Bus   trunking   up   to   2500   Amp   capacity	Copper/ Aluminum	2 No 25 x 5 mm Copper Strip
Bus   trunking   above   2500   Amp   capacity	Copper/ Aluminum	2 No 32 x 5 mm Copper Strip
Bus  trunking for  generating set and LT panel	Copper/ Aluminum	2 No 25 x 5 mm Copper Strip
Rising main up to 400 Amp capacity	Copper/ Aluminum	2 No 20 x 5 mm Copper Strip
Rising  main  above  400  Amp  and  up to 800 Amp	Copper/ Aluminum	2 No  20 x 3 mm Copper Strip

 

The Size of Earthing conductors
As per CPWD
Size of phase conductor	Size of Earthing conductor of the same material as phase conductor
Up to 4 sq.mm.	Same size as that of phase conductor
Above 4 sq.mm. up to 16 sq.mm.	Same size as that of phase conductor
Above 16 sq.mm. up to 35 sq.mm.	16 sq.mm.
Above 35 sq.mm.	Half of the phase conductor

 

Materials and Sizes of Earth Electrodes
CPWD-TABLE IX
Type of Electrodes	Material	Size
Pipe	GI medium class	40 mm dia 4.50 m long (without any joint)
Plate	(i) GI	60 cm x 60 cm x 6 mm thick
	(ii) Copper	60 cm x 60 cm x 3 mm thick
Strip	(i) GI	100 sq. mm section
	(ii) Copper	40 sq. mm section
Conductor	(i) Copper	4 mm dia (8 SWG)
Note :  Galvanization of GI items shall conform to Class IV of IS 4736 : 1986.

 

Minimum Sizes of Earthing Conductors for Use Above Ground
CPWD- TABLE X
Material and Shape	Minimum Size
Round copper wire or copper clad steel wire	6 mm diameter
Stranded copper wire	50 sq. mm or (7/3.00 mm dia)
Copper strip	20 mm x 3 mm
Galvanized iron strip	20 mm x 3 mm
Round Aluminum wire	8 mm diameter
Aluminum strip	25 mm x 3 mm

 

Minimum Sizes of Earthing Conductors for Use Below Ground
CPWD- TABLE XI
Material and Shape	Minimum Size
Round copper wire or copper clad steel wire	8 mm diameter
Copper strip	32 mm x 6 mm
Round galvanized iron wire	10 mm x 6 mm
Galvanized iron strip	32 mm x 6 mm

 

Selection of Type of Earthing Electrodes
As per CPWD
Type of electrode	Application
GI pipe	Internal electrical installations like Distribution Board and Meter Boards (in residential quarters), feeder pillars and poles etc.
GI plate	(i) For Fire Fighting pumps and water supply pumps.
	(ii) Lightning conductors.
Copper plate	Neutral earthing of transformers/ generating sets.
Strip/ Conductor	Locations where it is not possible to use other types.

 

Number of Earth Electrodes
As per CPWD
Equipment	No of Earthing
For neutral earthing of each transformer	2 sets
For body earthing of all the transformers,	2 sets
HT/LT Panels and other electrical equipment
in the Sub-station/ power house
For neutral earthing of each generating set	2 sets
For body earthing of all the generating sets,	2 sets
LT panels, other electrical equipment in the generator room

 

Size of protective conductor
As per CPWD
Size of phase conductor	Size of protective conductor of the same material as phase conductor
Up to 16 sq.mm.	Same as Phase Conductor  Size
16 to 35 sq.mm.	16 sq.mm.
35 sq.mm	Half Size of Phase Conductor

 

Earthing Points
As per CPWD
Earthing	Description
Location for Earth Electrodes	Normally  an  earth  electrode  shall  not  be  located  closer  than  1.5  m  from  any  building.
Installation of Pipe	Pipe electrode shall be buried in the ground vertically with its top at not less than 20 cm below the ground level
Installation of Plate	Plate electrode shall be buried in ground with its faces vertical, and its top not less than 3.0 m below the ground level.
The strip or conductor	The strip or conductor electrode shall be buried in trench not less than 0.5 m  deep
More Earthing Electrode	When more than one electrode (plate/pipe) is to be installed, a separation of not less than 2 m shall be maintained between two adjacent electrodes.
Earthing Electrode	If the electrode cannot be laid in a straight length, it may be laid in a zigzag manner with a deviation upto 45 degrees from the axis of the strip. It can also be laid in the form of an arc with curvature more than 1 m or a polygon
Earthing Pit Cover	A cast iron / MS frame with MS cover, 6 mm thick, and having locking arrangement shall be suitably embedded in the masonry enclosure.
Earthing Wire Protection	The  earthing  conductor  from  the  electrode  up  to  the  building  shall  be  protected  from mechanical injury by a medium class, 15 mm dia. GI pipe in the case of wire, and by 40 mm dia, medium class GI pipe in the case of strip. The protection pipe in ground shall be buried at least 30 cm deep (to be increased to 60 cm in case of road crossing and pavements)
No of Earthing Conductor	Two protective conductors shall be provided for a switchboard carrying a 3-phase switch gear there on.
Earthing Electrode	No earth electrode shall have a greater ohmic resistance than 5 ohms as measured by an approved earth testing apparatus. In rocky soil the resistance may be up to 8 ohms.
Earthing Resistance	Each   of   the   earth   stations   should   have   a   resistance   not   exceeding   the product  given  by  10  ohms  multiplied  by  the number  of  earth  electrodes  to be  provided  therein.  The  whole  of  the  lightning  protective  system,  including any  ring  earth, should  have  a  combined  resistance  to  earth  not  exceeding  10 ohms without taking account of any bonding
More Earthing Resistance	If the value obtained for the whole of the lightning protection system exceeds 10 ohms, a reduction can be achieved by extending or adding to the electrodes, or by  interconnecting  the  individual  earth  terminations  of  the  down  conductors  by  a  conductor

 

March 02, 2018 at 10:24AM by Department of EEE, ADBU: http://ift.tt/2AyIRVT

Emergency Generator Set – Construction, Installation, Maintenance & Wiring

An Overview of Emergency Generator Set

(Manuel Bolotinha)

Introduction to Emergency Generator Set

Hospitals, airports, shopping malls and office buildings, just to mention some examples, are very sensitive, in what concerns safety of people and goods, to power outages caused by faults in medium (MV) and low voltage (LV) distribution networks and even in high voltage (HV^[1]) transmission networks.

When a power outage occurs is necessary that communication systems, emergency lighting, smoke exhaust fans, fire fighting water pumping stations, security, lighting in buildings and other critical electrical systems and equipments, designated as essential loads (or critical), continue running.

To solve such problems common solution is the installation of LV diesel emergency generator sets, whose applications, characteristics and installation procedures must be in accordance with IEC^[2] Standard 60034.

Rated Power and Neutral Grounding of Emergency Generator

Rated power of LV diesel emergency generator sets^[3] depends on the operation regime: standby, prime and continuous, as defined in ISO^[4] Standard 8528.

In standby regime the available power supplied by the generator set varies with the value of the load during the lack of normal power supply and average power output is 70% of emergency standby power rating. Typical operation of this regime is 200 hours per year with a maximum of 500 hours per year.

In prime regime the available power supplied by the generator set varies with the value of the load for an unlimited period of time and average power output is 70% of prime power rating. Typical peak demand of 100% of prime-rated ekW^[5] with 10% of overload capability for emergency use for a maximum of 1 hour each 12 hours; overload functioning may not exceed 25 hours per year.

In continuous regime output is available without varying load for an unlimited time. Average power output is 70 – 100% of the continuous power rating. Typical peak demand is 100% of continuous rated ekW for 100% of operating hours.

According to the definitions stated above is easy to understand that the same generator set has different rated powers for each operation regime. Rated power in standby regime is higher than rated power in prime regime, which is higher than rated power in continuous regime.

Rated power of generator sets main be defined in kVA or kW for a cos Φ (Power Factor)^[6] = 0.8. It is also necessary to define the network frequency (50 Hz or 60 Hz).

In Table 1, as an example, are shown the values of rated power (kVA) of some generator sets of the same manufacturer and the same manufacturing series, according to the operation regime (f = 50 Hz).

Table 1 – Examples of emergency generator sets rated power according to operation regime

As the LV network that the generator set wills electrically supplies is isolated and has unbalanced loads, neutral point of the windings of the alternator^[7] is directly earthed since this neutral grounding system improves the behavior of the alternator with those types of loads.

Construction and Components of Emergency Generator

General Considerations

Main components of a generator set are (see Figure 1):

• Engine
• Alternator
• Main assembly/Frame.
• Starting battery and charger.
• Lubrication system.
• Cooling and exhaust systems, including the radiator.
• Fuel system.
• Voltage regulator
• Exhaust piping and silencer.
• Control and monitoring panel.

Figure 1 – Main components of a generator set

Engine

The engine that must be in accordance to ISO Standard 3046 is an internal combustion machine, four stroke cycle and usually diesel fueled (there also models gas fueled). The power of the engine must be suitable for the rated power of the alternator.

Main components of the engine are:

• Electronic speed governor, usually with a variation of ± 0.25
• Acceleration control and run and stop lever
• Standard steel ring to assure a rigid connection to the coupling system of the alternator enclosure
• Flywheel and elastic union for alternator coupling

The engine must start and work at least 8 hours at full load, followed by 1 hour of 10% overload under the specified temperature conditions. The engine must have electrical start-up, which time shall not be higher than 10 s.

Engine cooling may be done by circulation of air or water, in a close circuit with a radiator. To improve engine start-up it must be envisaged an oil, water or combustion air pre-heating system (usually resistors), which must work with the temperature increase of carter grease.

Alternator

The alternator is of synchronous type, single or three-phase, self excited, regulated and ventilated; the ventilation of the alternator shall be performed by a shaft coaxial turbine.

Main characteristics of alternators are:

• Rated voltage: 230 V (single-phase alternators); 400/230 V (three-phase alternators).
• Rated frequency: 50 Hz or 60 Hz.
• Rated power (depending on the operation regime – see Section 2).
• Power factor: usually 0.8.
• Insulating class: usually H.

Insulating class of alternators are established, according to IEC Standard 60085, taking into account the maximum temperature windings can withstand, since temperature is often main factor that contributes for insulating material ageing. In Table 2 are shown the alternators insulating classes.

Table 2 – Insulating of alternators

Other characteristics of alternators that must be considered are:

• Radio interferences suppression.
• Harmonic distortion: ≤ 2 % off-load; ≤ 3.5 % with balanced load.
• Voltage regulation: ±1.5% for off-load and full-load variations, power factor between 8 and 1 and speed variations of ± 4.5 %.
• Capability to recover voltage up to 3% of rated voltage within 3 s, when full-load is suddenly applied with cos Φ = 0.8.
• Capability to withstand short-circuits currents up to 300% of rated current during 5 s before the actuation of internal protection devices.

Other Components and Systems

The assembly frame of the engine and alternator (and even of the daily diesel tank) is usually constructed with standard steel profiles, electrically welded, and anti-vibrate supports shall be installed; these profiles must be calculated in order that the self vertical oscillation should be around 7 Hz.

The starting battery is lead-acid type and connected to a battery charger that will assure the maintenance charge of the battery and is normally installed in the control and monitoring panel.

Exhaust piping must include a silencer (see Figure 2) and a flexible connection from exhaust outlet of the engine to the outgoing pipe. Both silencer and piping must be thermally insulated with glass wool covered with aluminum sheet.

Figure 2 – Exhaust piping of a generator set

Fuel system includes the daily tank (with level indicator and maximum and minimum level switches) and must have such a capacity that assures the generator set woks during a defined time period, at full-load rated power for the established operation regime.

This system may also include an eventual diesel cistern, transfer pumps of daily tank (manual and electrical) and the required piping.

Control and Monitoring Panel

In the control and monitoring panel it shall be Installed the following equipment:

• Metering equipment (ammeters; voltmeters; frequency meter).
• Control and monitoring equipment: start and stop push-buttons; oil manifold; water thermometer.
• Control switch with 4 positions: Automatic/Manual/Test/Out-of-service.
• Start and stop generator set automatic system.
• Circuit breaker for alternator protection.
• Monitoring lamps.
• Emergency shutdown system of the engine.
• Battery charger (see Section 3.4).
• Control and monitoring equipments for all the auxiliary components of the generator set.

Installation Procedures of Emergency Generator Set

Generator sets are usually installed indoors, in a dedicated room, which must be provided with incoming and outlet grilles, calculated to assure the required air flow for equipment cooling.

Outgoing air grille is installed face to the radiator and shall allow an easy dismantling for generator set removing. In situations where is not possible to assure the required air flow for generators set cooling it is necessary to install radiator separated from the engine.

In these situations generator set cooling is done through fresh water circulation in a close circuit, using a centrifugal pump. Temperature is thermostatically controlled and heat dissipation is achieved by radiator, installed at the primary circuit, and a fan. The radiator must have a low noise level and must be mounted on anti-vibrate supports.

Figure 3 gives an example of what was explained above.

Figure 3 – Installation scheme of a Generator set with separated radiator

If the noise level of the generator set is too high according to noise regulations, namely in circumstances where the equipment is installed outdoors, the generator set must be installed inside an enclosure, as shown in Figure 4.

Figure 4 – Generator set inside an enclosure

Maintenance of Emergency Generator Set

As referred in Section 1 generators sets play a critical role in safety of people and goods of certain electrical installations and for that reason is important to establish an accurate maintenance program that must include:

• Run monthly, or twice a month, at off-load conditions for a period of 4 hours (to assure that it will run when necessary).
• Manual running.
• Generator set start in case of absence of network voltage and verification of the functioning of the automatic inverter switch.
• Visual inspection.
• Verification of earthing connection of neutral and all metallic parts.
• Verification of oil level and
• Battery status (visual inspection; voltage measurement of the elements; situation of the electrolyte).

Generator Start-Up. Automatic Transfer System

Generator set start-up shall me automatic, when there is an undervoltage or a negative sequence voltage in the power supply network and it is done through an automatism system installed in the generator set control and monitoring panel. This automatism system will provide also the disconnection and the stop of the generator set when the network voltage returns to normal.

The above referred automatism system will control the automatic transfer system, hereafter referred as automatic inverter switch, and must assure a number of start-up consecutive trials (never less than 3) with a time between trials that is the required time for battery regeneration.

Automatic inverter switch, which preferably shall be installed in the main incoming switchboard, must provide transfer of essential loads to the generator set power supply when a disturbance in the network voltage occurs and provide the re-transfer of those loads to network power supply when the situation backs to normal.

The automatic inverter switch may be built with 2 of the following equipments:

• Circuit breakers.
• On-load switches.
• Contactors.

Those equipments must have a mechanical and electrical interlocking to avoid the parallel association between the network and the generator set.

All switchboards must have disconnecting equipment (circuit breaker; on-load switch; contactor) to split the loads between essential and non essential, to avoid these loads to be powered by the generator set.

Figure 5 shows an example of a schematic diagram of a transfer system.

Figure 5 – Schematic diagram of transfer system

Wiring & Connection of Portable Generator

You can wire and connect a portable generator to the home supply system by three methods here.

^[1] HV: U_s ≥ 60 kV; MV: 1 kV < U_s ≤ 49.5 kV; LV: U_s ≤ 1 kV. U_s is the rated voltage of the network.

^[2] IEC: International Electrotechnical Comission.

^[3] Hereafter we will use the expression generator sets to refer to LV diesel emergency generator sets.

^[4] ISO: International Organization for Standardization.

^[5] ekW: Electrical active power.

^[6] cos Φ: power factor.

^[7] See Section 3.3.

About the Author: Manuel Bolotinha

-Licentiate Degree in Electrical Engineering – Energy and Power Systems (1974 – Instituto Superior Técnico/University of Lisbon)
– Master Degree in Electrical and Computers Engineering (2017 – Faculdade de Ciências e Tecnologia/Nova University of Lisbon)
– Senior Consultant in Substations and Power Systems; Professional Instructor

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March 01, 2018 at 06:52AM by Department of EEE, ADBU: http://ift.tt/2AyIRVT