Power Quality Technical Faq

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22/07/2009 | Slide 1

Technical FAQ for Transformers Horace Gay, Quality School, 17 August 2010



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Topics

Differences among normal, ACS and multi-tapped trafo

What are the different types of transformers and their

uses?

How does the tap changer work to regulate the HV input to

achieve desired LV output? Purpose of having a neutral ground resistor

How to determine the vector group required for our TFs

What is differential protection?

What are the different types of CT and their applications?



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Types of Transformers

3 phase

distribution

Converter TF Multi Tapping

Uses Residential /

commercial /

industrial use

Variable speed drive

systems are used,

e.g. industrial pumps,

paper, steel

production

Network adaptability

Vector Dyn11 Dd0y11 / Dd0y1 YNyn0 / Dyn11

Operating

Frequency

50 or 60 50 or 60 50 and 60

No. of windings 2 3, axially stacked 3, Single LV, Dual HV



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Tap Changer

How does the tap changer work to regulate the HV input to

achieve desired LV output?

Device that faciliates voltage regulation for the transformer

The regulation of a transformer is defined as the drop in

secondary voltage from no load to full rated loadexpressed as a percent of the full load voltage.

How does tap changing maintains voltage regulation?

To help match the incoming primary voltage to your

equipment needs which is connected to the secondary

side. In other words, it works by adding or subtractingthe number of transformer windings and hence the

turns and voltage ratio.



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Tap Changer - Voltage Regulation Example

A huge 3-phase heating load was connected to the

secondary of a step-down transformer (nominal 433V p-p)

resulting in a 5% voltage drop. Now the voltmeter

measures 411V p-p. How to regulate back to the nominal

voltage?

Given, the transformer (Dyn11) has a voltage regulationcapability on the Primary ± 2 X 2.5%.

411 X 83.6 / 1.73 = 432.98 V p-p

The answer is to tap up from position 3 to position 5.

Tap Vpri Vsec Turns

Ratio

1 23100 : 433 92.4

2 22500 : 433 90.2

3 22000 : 433 88.04 21450 : 433 85.8

5 20900 : 433 83.6



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Vector Groups

What is the purpose of vector groups?

A Vector group is the IEC method of categorizing theprimary and secondary winding configurations of threephase. Within a polyphase system power transformer itindicates the windings configurations and the differencein phase angle between them.

Note in ANSI, vector diagram is given to show thephase relationship between the primary and thesecondary.

Features & Type of connections.

First symbol: HV winding connection Second and subsequent symbols: Intermediate and LV

winding.

Lastly, phase displacement ‘clock number’



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Vector Groups

Popular Connection: Delta on

Primary and Star on Secondary

This will produce a 30° phase shift

between the two windings. As our

Delta Connection (5 taps) is A0B6,

B0C6 and C0A6, which would resultin a phase displacement of 30°

anticlockwise to the 11 o’clock

position.

A B

C

A0, B6

B0, C6C0, A6



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Vector Groups

Common connection for distribution transformers: Dyn11

Delta connected windings eliminate / suppresses third

harmonics from feeding back into the primary network.

Also provides better current balance for the primary source

should there be large amount of unbalanced loads. The neutral point provides a neutral earthing for the

purpose of safety.

Due to large installed base of distribution transformers with

such vector group and for parallel operations of

transformers



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Vector Groups

Other vector groups

Generator step-up – Ynyn0, Ynd1, or Ynd11



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Neutral Ground Resistor

NGR’s limit the fault current when one phase of the system

shorts to ground. In the event that a ground fault condition

exists, the NGR typically limits the current to approx 300A.



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A very reliable method of protecting generators,transformers, buses, andtransmission lines from theeffects of internal faults.

Detection of faults is performed

primarily by CTs. The activation upon detection is

performed by relays.

The sum of all currents enteringand leaving the bus must beequal to zero under normalconditions or if the fault is outsidethe protected zone.

If there is a fault on the bus, therewill be a net flow of current to thebus and differential relay willoperate.

Differential Protection



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Current TransformersGeneral

A current transformer is used to transform a primary

current quantity in terms of its magnitude and phase to a

secondary value such that in normal conditions the

secondary value is substantially proportional to the primary

value.

Protection and Metering CTs



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Current TransformersBasics

IEC 60044

Ratio, Burden & Class

E.g. 3300/1A, Burden 10VA, Class 3



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Current Transformer Basics

Burden = Impedance of pilot wire between CT &

Relay/Instruments + Impedance of the Relay/Instrument



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Current Transformer Basics

Class

Accuracy Class Application

0.1 Precision Testing, Calibration of other CTs

0.2 Precision Metering

0.5 General Tarrif Metering

1.0 Non Revenue measurement incl. power and energy

3.0 General industrial measurements

5.0 Approximate measurements

5P Protection

10P Protection, lesser accuracy

X Protection - Restricted earth fault relay

Measurement

Protection



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Types of CTsProtection CT

Application: Neutral Earth, Differential Protection

Typical ratings: 1600/5 15 VA Class 5P10

1600/5 is the current ratio.

15 VA is the VA output at rated secondary current, Class

5P indicates that this is a protection (P) CT with acomposite error of < 5% at rated accuracy limit primary

current and 10 is the rated accuracy limit factor (RALF) for

the CT, i.e. overcurrent = 10 X rated normal current



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Protection Current Transformers- Selection

Protective System CT Secondary VA Class

PE current for phase &

earth fault

1A

5A

2.5

7.5

10P20 or 5P20

10P20 or 5P20

Unrestricted earth fault1A

5A

2.5

7.5

10P20 or 5P20

Sensitive earth fault 1A or 5A Class PX use relay manufacturers formulae

Distance protection 1A or 5A Class PX use relay manufacturers formulae

Differential protection 1A or 5A Class PX use relay manufacturers formulae

High impedancedifferential

impedance

1A or 5A Class PX use relay manufacturers formulae

High speed feeder

protection1A or 5A Class PX use relay manufacturers formulae

Motor protection 1A or 5A 5 5P10



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Types of CTsMetering / Measuring CT

Application: WTI

Example of rating on 2MVA 22/0.433kV, Dyn11

3300/1A Class 3 Burden 10VA

2666A

1A



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References

Differential Protection, NGR – Practical Power System,

Hewiston, Brown and Balakrishnan

NGR – Resistance Grounding System Basics, Michael

Seal

CT – ABB Protection Application Handbook

CT – Rudolf CT selection guide

CT – ITL selection guide

Neutral Earth – J & P Transformer Book,

The Basics of Delta-Wye Transformers, ecmweb.com

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