Current & Instrument Transformers

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Inside the Catalogue ∗ Range

∗ Specifications ∗ Ordering Code

Type MKE11

Current & Instrument

Transformers

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M-Kris Electric is a new firm in electrical sector committed to provide quality and reliable products and solutions to its customers for improving the performance of electrical systems. The current range of M-Kris includes current and instrument transformers and electronic devices (timers, power analysers, phase monitoring relays, earth leakage relays, digital meters etc.).

Introduction

Seemingly obscure and unsung, these are ‘behind the scene’ electrical products which form the crucial link between the electrical system and protection & control devices. The accurate and reliable perform-ance of these instrument transformers over a period of time is the key to the well being of the electrical system as a whole.

Typical applications of instrument transformers are in switchboards for use with measuring instruments like ammeters, energy-meters, multi function power analyzers, and protection devices such as overload relays, earth fault and earth leakage relays, protection relays etc.

M-Kris Instrument Transformers are designed and manufactured in accordance with relevant IEC stan-dards and available in standard configurations as well as custom specifications.

Functions of Instrument Transformers

Principle of Operation

All the current based instrument transformers work on the principle of constancy of ATs (ampere-turns) i.e. higher current in the primary can be transformed to lower current simply by increasing the number of turns in the secondary winding. Thus a suitable number of turns are provided in the secondary circuit to bring the current to manageable level for metering and protection purposes. Though technically speak-ing, ATs remain equal on both the primary and secondary, in reality, some losses happen in magnetizing the core and heating of winding. These losses lead to ratio error and phase error. Quality of core and winding material determines the quantum of error.

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Range - at a glance

Low Voltage Current Transformers

Extensively used in low voltage switchboards, the LVCTs transform the line currents to appropriate levels making them suitable for use with measuring instruments and protection devices. It is clear that accuracy of these current transformers is critical for reliable operation of these measuring instruments and protec-tion devices.

Classification

LVCTs can be classified on the basis of construction, material, function, shape:

• Function: Metering / Protection

• Material: Tape wound / Resin cast / Nylon casing / ABS moulded • Construction: Wound primary / bar

• Shape: Round / Rectangular

Metering CTs are specified with the accuracy class of 3, 1, 0.5 or 0.2. Lower number indicates higher level of accuracy. Metering CTs are required to transform the primary current in the current range from 5% to 120% of its rated primary current, within the tolerance limits specified by its accuracy class. Metering CTs must saturate above 120% of the rated primary current to prevent damage to the measuring instruments connected to the secondary of the CT. VA capacity of the CT specifies the maximum level of VA burden of devices / instruments on the secondary circuit of the CT e.g. it will be inappropriate to use a 2VA CT for a measuring device having VA burden of 4VA.

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User benefits for M-Kris CTs

• Wide range – helps in selecting the right CT for the application • Size of aperture/opening – accept all the sizes of busbars/cables • Minimum space requirement

• VA Capacity – wide range from 1VA to 15VA helps in matching closely the CT and the device • Low losses – lesser heating and hence lesser deterioration of CT leading to its longer life

Selecting the accuracy class for Metering CTs

Application Recommended Accuracy Class

Calibration 0.2

Tariff based metering 0.5

Non-tariff ‘sub’ metering 1.0

General purpose measure-ments

1.0– 3.0

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MKE1106M – Metering CT upto 600/5A

MKE1106S – Metering CT upto 600/5A

MKE1107M – Metering CT upto 750A Class 1

MKE1108S - Metering CT upto 800A

CT Ratio Class 1 Class 3

60/5A to 80/5A VA 1 100/5A VA 1 125/5A VA 1 150/5A VA 1 2.5 200/5A VA 2.5 250/5A VA 2.5 300/5A VA 2.5 5 400/5A to 600/5A VA 5

RATIO Class 1 Class 3

60/5A to 80/5A VA 1 100/5A VA 1 125/5A VA 1 150/5A VA 1 2.5 200/5A VA 2.5 250/5A VA 2.5 300/5A VA 2.5 5 400/5A to 600/5A VA 5

50/5A VA 1.25 60/5A VA 1.25 75/5A VA 1.25 80/5A VA 1.25 100/5A to 750/5A VA 5

RATIO Class 0.5 Class 1 Class 3

150/5A VA 2.5 200/5A VA 2.5 250/5A VA 2.5 300/5A VA 2.5 5 400/5A VA 5 7.5 500/5A to 800/5A VA 5 7.5

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MKE1108M - Metering CT upto 800A

MKE1120M - Metering CT upto 2000A

150/5A VA 2.5 200/5A VA 5 250/5A VA 5 300/5A VA 5 400/5A VA 5 7.5 500/5A to 800/5A VA 7.5

RATIO Class 0.5 Class 1

400/5A VA 10 500/5A VA 10 600/5A VA 10 15 750/5A VA 10 15 800/5A VA 10 15 1000/5A VA 10 15 1200/5A to 1600/5A VA 15

RATIO VA Class 0.5 Class 1

400/5A VA 5 7.5

500/5A VA 5 7.5

600/5A VA 5 7.5

750/5A to

2000/5A VA 10 15

RATIO Class 0.5 Class 1

400/5A VA 5 500/5A, 600/5A VA 5 10 750/5A , 800/5A VA 10 15 1000/5A to 1600/5A VA 15 2000/5A , 2500/5A VA 15 30 3000/5A VA 30 4000/5A VA 30

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Medium Voltage Current Transformers

Available upto 33kV. Double ratio – triple core CTs are also available Specifications:

Core Balance Current CT (CBCT)

Also called Zero Sequence Current CTs, the CBCTs work on the summation principle of current that the vector sum total of currents in three phases at any point should be zero. However, this condition gets violated in case of earth fault and earth leakage when part of the current ‘leaks’ into earth. CBCTs detect this abnormal condition and give an output current in the secondary circuit.

Specifications:

Leakage current: 300mA, 1A, 3A, 5A Construction: Tape wound / Resin Cast

Aperture: As per customer requirement (round/rectangular)

Summation Current Transformers

When the currents in multiple feeders need not be individually metered but their combined value needs to be monitored, a summation current transformer is used. The summation CT consists of two or more primary windings which are connected to the feeders to be summated, and a single secondary winding, which feeds a current proportional to the summated primary current. A typical ratio would be 5+5+5/ 5A, which means that three primary feeders of 5A are to be summated to a single 5A meter. Secondary cur-rents of 1A are also used.

Specifications:

Parameter Value

Voltage 3.3/6.6/11/33 kV

Primary current 10A to 2000A

Secondary Current 1A / 5A Insulation Level 30kV Class of Insulaton F Parameter Value Primary current 1 – 5A Secondary current 1 or 5A Accuracy class 0.2, 0.5 or 1 VA Burden 1 to 30VA

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Phone/Fax: 0091 11 25071122 E-mail: [email protected] Web: www.mkris.com 383, Shriniketan

Plot-1, Sec-7, Dwarka, New Delhi—110075 INDIA

M-Kris Electric

Other products offered by M-Kris Electric:

Voltage Transformers

Also called PTs (potential transformers), these instrument transformers are used for stepping down the voltage to 110V (usually) for compatibility with the measuring and protection devices.

Specifications for VTs/PTs:

Parameter Specified Values

Construction Resin Cast / Oil type

Primary Voltage 3.3/6.6/11 kV

Secondary Voltage 110/√3 V, 110V

Accuracy Class 0.2, 0.5, 1.0 for metering and 3P for protection

Burden Upto 200VA

Overvoltage factor 1.2 continuous, 1.5 for 30 sec for effectively earthed tem and 1.9 for 8 hours for non-effectively earthed sys-tems

Neutral Connected to earth

Connection Y/Y, Y/∆

COMBINED CT-PT UNITS

Also called metering units, these can be offered in dry type/oil immersed – dry type being primarily used for indoor applications and oil type for outdoor applications.

The metering unit may consist of:

• 2 nos. single phase CTs and 1 no. three phase PT • 3 nos. single phase CTs and 1 no. three phase PT • 2/3 nos. single phase CTs and 3 nos. single phase PTs

Note: Please contact us for technical details of Tape Wound and Resin Cast Current Transformers. Custom built CTs, Control Transformers and Interposing CTs also available.