kpb intra s.r.o., Ždánská 477, cz-68501 bučovice, tel.: +420 517 380 388, fax: +420 517 381 433,...

KPB INTRA s.r.o. , Ždánská 477, CZ-68501 Bučovice, tel.: +420 517 380 388, fax: +420 517 381 433, [email protected], www.kpbintra.com

KPB INTRA s.r.o.

How to order (demand) transformers correctly


How to order (demand) a transformer correctly

1. Transformer type selection

a. Measuring voltage or current transformer

b. Indoor or outdoor environment

VTS 12VTS 12 CTS 12CTS 12

CTS 25CTS 25 CTSO 38CTSO 38


c. Types of current transformers


CTS 25CTS 25

•Supporting typeSupporting type

CTB 25CTB 25

•Bushing typeBushing type

CTB 25CTB 25

•Pass typePass type



VTS 25VTS 25

•Supporting typeSupporting type•Earthed measuring phase – earthEarthed measuring phase – earth•E.g. E.g. 22000/22000/√3//100///100/√3/100/3 V/100/3 V

VTD 25VTD 25

•Supporting typeSupporting type•Unearthed measuring between phasesUnearthed measuring between phases

•E.g. E.g. 22000//100 V22000//100 V

c. Types of voltage transformers



VTS 25VTS 25

•Supporting type of Supporting type of •earthed voltage transformerearthed voltage transformer

•with fusewith fuse

VTD 25VTD 25

•Supporting type of Supporting type of •unearthed voltage transformerunearthed voltage transformer

•with fusewith fuse



Functions and types of fuses

A fuse is used to protect adjacent equipment in case of transformer destruction An appropriate type of fuse corresponds to each type of transformer

JT6 600 mAVTS 12 – 1 pcVTD 12 – 2 pcs

JT6 300 mAVTS 25 – 1 pcVTD 25 – 2 pcs

Bussman2 3.15 A VTS 38



2. Technical parameters of measuring transformers

a. Measuring current transformer

2.a.1 Insulation levels

Insulation voltage/Testing voltage/Testing impact voltage e.g. 25/50/125 kV

36/70/170 kV acc. to IEC 60044-1

38.5/80/180 kV acc. to ČSN

40.5/95/200 kV acc. to GOST

Rated frequency 50 (60Hz)



2.a.2 Rated conversion and number of secondary windings

E.g. 100//5 A or 250//1/1 A • If switch-over type is required, the primary current is to be stated in conversion in the form of e.g. 100-200//5 A (one branch) or 50-100-200//5 A (two branches) • Rated conversion consists of rated

a. Primary current b. Secondary current

Note:It is appropriate to separate the primary current from the secondary one with a double slash // in the notation in order to increase clarity.



2.a.2.a Primary current

Selection from series in accordance with standards

10,12.5,15,20,25,30,40,50,60,75 A An atypical current, e.g. 65 A or 630 or 515 A should be consulted with the designer.

2.a.2.b Secondary current

Only 5 A or 1 A (or 2 A) is acceptable according to standards

Other values, e.g. 2.5 A should be consulted with the designer (accuracy may not always be guaranteed as we do not have the necessary measuring equipment)



Number of secondary windings

- Is given by the values after a double slash in conversion Examples 400//5 A One secondary winding 5A

150//5/1 A Two secondary windings, first winding - 5 A , second winding 1 A

700//1/1/1 A Three secondary windings (all 1A)

150-300//5//5 A Two secondary windings (both 5A) with one branch on secondary winding

Number of windings may be 1 , 2 , 3 or exceptionally 4



Single core type

Two-core type

Three-core type

Basic diagrams of winding of measuring current

transformer



Two-core type with onebranch on each core

Three-core type with onebranch on each core

Single core type with one branch

Basic diagrams of winding of measuring current

transformer



The following parameters should be given for individual windings:

I. Rated load (burden) in VA

E.g. 15 VA

II.a) Accuracy class for measuring windings

0.2 ( 0.2S)0.5 (0.5S)

II.b) Accuracy class for protection winding 5P or 10P

5P corresponds to 1 % error at rated current passing10P corresponds to 3 % error at rated current passing



III.a) Instrument security factor FS for measuring windings

It is stated as FS , e.g. FS5 or FS10

It expresses the ratio of resulting exciting current with respect to rated current at which the transformer’s magnetic circuit will be oversaturated

This parameter is important for protection of measuring instruments on the transformer’s secondary side

The real instrument security factor for measuring instruments is always lower than the value stated in the label, e.g. for FS5, the max. factor is 4.999

In the case of passing a primary current higher than a quintuple of the primary current, a current lower than 25 A will appear in the secondary circuit at conversion to 5A (quintuple of 5A)

Or, in the case of conversion to 1 A, a current lower than 5 A will appear in the secondary circuit (quintuple of 1A)



Primary current

[A]

Secondary current

[A]

100 5

200 10

300 15

400 20

500 24.995

600 24.995

Example

0

5

10

15

20

25

30

100 200 300 400 500 600

primární proud [A]

seku

nd

árn

í pro

ud

[A

]



III.b) Instrument security factor for protection windings

Accuracy limit factor:

Ratio of rated primary overcurrent at a given precision and rated primary current

Follows always after accuracy class

E.g. 5P10 or 10P20 (instrument security factor is either 10 or 20)

Real instrument security factor of protection windings is always higher than the stated value.

It must be guaranteed for transformer winding that measuring error will not exceed 10 % in the case of passing overcurrent in the value of a multiple of instrument security factor 10%

It is important for correct operation of protections

There is a risk of protection failure if it has been set incorrectly (higher instrument security factor than that stated on the label (transformer will be oversaturated before protection activation)



Example:

Transformer 50//5 AAccuracy class in combination with instrument security factor 10P10 The transformer will measure with max 10% error if a current of 500 A is passing, i.e. measured current will be at least 450 A

0

10

20

30

40

50

60

70

80

0 100 200 300 400 500 600 700 800

primární proud [A]

se

ku

nd

árn

í p

rou

d [

A]

skutečný proud ε=10%

ideální proud za předpokladu ε=0%

10%

Instrument security factor 10 guarantees correct functioning of protection winding up to tenfold the rated current. However, rated load in VA may not be exceeded as this could affect the instrument security factor.

E.g. If the above-mentioned burden is 10 VA and instrument protection factor 10, the instrument security number will be reduced to 7 upon winding loading with a burden with output of 15 VA

Load (burden) may be increased by too long a supply line (several tens of meters) between the protection and terminal board or by too small a cross section of wires, which is why this fact should be considered when dimensioning rated burden of protection winding.



2.3 Short-circuit currents

2.3.a) Rated short-time thermal current

Ith - rated short-time thermal current: effective value of primary current that the transformer withstands for 1 second with secondary winding short circuit without damage to the transformer.Also times other than 1 second such as 0.5 s, 2 s, 3s may be agreed upon.

2.3.b) Rated dynamic current

Idyn - rated dynamic current: peak value of primary current which the transformer withstands without electric or mechanical damage caused by electrodynamic force in the case of secondary winding short-circuit.



Rated dynamic current (1s) corresponds to a 2.5 multiple of short-term thermal current Ith short-circuit duration 1s. Short-circuit conditions in the mains will be specified by the specialized designer.

If a different short-circuit duration time than one second is stated, it is necessary to state this fact in a request or an order!

Idyn = 2.5 x Ith

These short-circuit currents should be written in a request as follows: Ith/Idyn kA,the value is to stated in kA, e.g. 16/40 kA.



Data which should be included in a request (summary)

Current transformer type

Conversion

Insulation levels

Rated frequency

Parameters of individual windings – burden/accuracy class/instrument security factor

Short-circuit currents Ith/Idyn

Official verification (calibration) – if requested Non-standard modifications E.g. tropical design, PR super-structures, different base boards,

etc.

Such data must be specified for each winding separately as specification should beunambiguous and complete in order to allow offering of precisely the required type of the instrument transformer



Example of incorrectly specified demand

Frequently the customer describes an instrument with the following parameters:

CTS 25 100//5/5 15 VA 0.5 FS5

It is apparent from conversion that it is a two-winding current transformer; however, parameters of the other winding are not specified and we can only estimate that the other winding is the same as the first one. If the same windings are concerned, it is advisable, because of the unambiguous character of the specification, to state the type as follows:

CTS 25 100//5/5 15 VA 0.5 FS515 VA 0.5 FS5

Please always state complete data in order to prevent any misunderstanding!

An example of a correctly specified demand

CTS 25 300//5/5/5 A25/50/125 kV50 Hz25/63 kA 15 VA 0.2 FS5 10 VA 0.5 FS1015 VA 5P10Official verification for the first winding



2. Technical parameters of measuring parameters

b. Measuring voltage transformer

2.b.1 Insulation levels

Insulation voltage/Testing voltage/Testing impact voltage E.g. 25/50/125 kV

36/70/170 kV acc. to IEC 60044-1

38.5/80/180 kV acc. to. ČSN

40.5/95/200 kV acc. to GOST

Rated frequency 50 (60Hz)



2.b.2 Rated conversion and number of secondary windings

E.g. VTS 25 - 10000/√3//100/√3/100/3 V or VTD 25 - 22000//100 V • Instead of √3, it is possible to write V3 (VTS 25 22000/V3//100/V3Note:It is appropriate to separate the primary current from the secondary one by a double slash //in the notation in order to increase clarity.

2.b.2.a Standardized levels of primary voltages

Standard voltage levels in the Czech Republic

2.b.2.b Standardized values of secondary voltages (measuring zone)

Note: In the case that an unearthed transformer is used as a supply of low output of low voltage, it is also possible to select different secondary voltages, e.g. 220 V, 231 V, 380 V, 400 V, 500 V, etc.

Earthed type: 11000/√3 V, 22000/√3 V, 35000/√3 V

Unearthed type: 11000 V, 22000 V, 35000 V

Standard voltage levels in the Czech Republic

Earthed type – measuring and protection winding: 100/√3 V, 110/√3 V

Unearthed type – auxiliary winding: 100/3 V, 110/3 V



Number of secondary windings

- is given by values after the double slash in conversion Examples 22000/√3//100/√3 V one secondary winding 100/√3 V

11000/√3 //100/√3 /100/3 V two secondary windings -the 1st measuring winding 100/√3 V-the 2nd auxiliary winding 100/3 V

35000/√3 //100/√3 /100/√3 /100/3 V three secondary windings-the 1st measuring winding 100/√3 V-the 2nd measuring winding 100/√3 V-the 3rd auxiliary winding 100/3 V

Number of windings may be 1 , 2 , 3 or exceptionally 4



Parameters for individual windings

Each winding should include data as follows:

Burden size in VA Accuracy class 0.2, 0.5, 1, 3P, 6P

Accuracy classes for measuring windings are: 0.2,0.5,1Accuracy classes for protection transformers: 3P and 6PAccuracy class for auxiliary windings: 6P

If a measuring transformer contains more measuring windings,it is necessary to take into account the fact that individual windings

affect each other so that a sum of their output may reach a maximum ofthe output of the transformer with one winding



Example

Transformer VTS 25 output with one measuring secondary winding can reach an output of 30 VA in accuracy class 0.2

1. The same transformer VTS 25 with two measuring windings reaches only max. output1st winding 15 VA in class 0.2 2nd winding 10 VA in class 0.2

2. The same transformer VTS 25 with three measuring windings reaches only max. output 1st winding 5 VA in class 0.2 2nd winding 5 VA in class 0.2 3rd winding 5 VA in class 0.2

Note: For spatial reasons, it is usually impossible to reach the maximum limit for a given accuracy classby the sum of individual windings, e.g.

15 VA + 10 VA < 30 VA

Please consider whether it is necessary to require high outputs, e.g. 50VA in class 0.2 or 30 VA and 30VA in class 0.2

for a two-winding measuring voltage transformer.These requirements increase demands for instrument design and

the transformer is less resistant to the phenomenon called ferro-resonance.



Data which should be included in a demand (summary)

Voltage transformer type

Conversion

Insulation levels

Rated frequency

Parameters of individual windings – burden/accuracy class

Official verification (calibration) if required

Non-standard modificationsE.g. Tropical modification, PR superstructures, different base boards, etc.

Such data must be specified for each winding separately as specification should beunambiguous and complete in order to allow offering of precisely the required type of the instrument transformer.

Correctly specified demand:VTS 25 22000/V3//100/V3/100/3 V25/50/125 kV50 Hz 30 VA 0.550 VA 6POfficial verification not required



Ferro-resonance

The phenomenon called “ferro-resonance” appears in electric networks.

Causes of its occurrence:

Presence of capacitors (cable supply lines, capacitor batteries, etc.)

Effect of inductivity of chokes and the transformer’s own inductivity

Occurrence of oversaturation and impacts at transient phenomena

Oscillation of electric power between instrument capacitances and inductivity

Consequences:

Disturbing insulation of measuring voltage transformers

Progressive instrument destruction

In some cases, even sudden explosion of the instrument

Note: This phenomenon will appear more frequently in future as a consequence of replacement of existing analog voltmeters with electronic ones because electronic voltmeters have a lower burdenand thus also lower damping effects, which means that the existing transformers are less resistant with respect to this phenomenon.



Potential solution of the problem called ferro-resonance

1. Increase of primary winding resistance

• Inconvenient as it increases instrument dimensions• It increases production costs substantially• It increases product price

2. Undersaturation of magnetic circuit 0.4T

• It protects transformer against oversaturation• It is necessary in some cases, e.g. for railway applications• It increases instrument dimensions and product costs• It limits some existing parameters of the instrument, e.g. output

3. Use of resistor connected in open triangle with auxiliary windings MTN (68 Ohm/ 200W (TR305 manufacturer: Tesla Blatná)

• It reduces ferro-resonance effects• Low costs with respect to instrument purchase price

Note: In the case that resistor connection would result in non-activation of parallelprotection or signalling relay, it is possible to reduceconsumption by connecting two resistors in series.

connectiondiagram



Thank you for your attention. Should you have any questions please feel free to contact:

Ing. Robert Knápek - Executive: +420 603 481 128

[email protected]

Ing. Petr Novák – Business Department Manager: +420 604 237 033

[email protected]

Ing. Josef Stejskal – Technical Department Manager: +420 603 862 464

[email protected]

www.kpbintra.com

mailto:[email protected]



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kpb intra s.r.o., Ždánská 477, cz-68501 bučovice, tel.: +420 517 380 388, fax: +420 517 381 433,...

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