Introduction to Instrument Transformers

INSTRUMENT TRANSFORMERS INTRODUCTION Various types of instruments are employed in the power system for various purposes. These include monitoring, measurement, indication, protection etc. These equipment cannot be safely connected directly to the system at such high voltage and current. For example if we are to connect a voltmeter to measure 330kV transmission voltage, we will need a meter with such voltage capacity. This will be very difficult, expensive, big and will be dangerous should there be an insulation breakdown. Now consider an ammeter to be connected to 2000A circuit. The cross-sectional area of the cable will be quite large with the attendant losses and safety concerns. To forestall this, instrument transformers are used to bring down the voltage and current that can easily and safely be used for purposes of metering Instrument transformers are a category of transformers that are used to isolate high current and voltage in power systems to a standardized lower current and voltage to voltage  to enable metering by measuring instruments. They act as a buffer between the high voltage or high current circuits and the measuring equipment used for measuring electricity in power systems. The primary function of Instrument transformers is to step down system voltage and current to standardized levels so that the metering equipment is safeguarded from high current and voltages running in the system. The most common rating used by metering instruments are 1A/5A & 110V/120V as the case maybe. In a modern power system, the primary winding of an instrument transformer is connected to the high voltage or high current circuit while the metering instrument is connected to the secondary circuit. The advantages of instrument transformers are: 1. The large voltage and current of AC Power system can be measured me asured by using small rating measuring instrument i.e. 5 A, 110 – 120 V. 2. By using the instrument transformers, measuring instruments can be standardized. Which results in reduction of cost of measuring instruments. More ever the damaged measuring instruments can be replaced easily with healthy standardized measuring instruments. 3. Instrument transformers provide electrical insulation between high voltage power circuit and measuring instruments. This reduced the electrical insulation requirement for the measuring instruments and protective circuits brings about reduced cost and also protects operators from the hazards of handling high voltage equipment. 4. Several measuring instruments can be connected through a single transformer to power system. 5. Due to low voltage and current level in measuring and protective circuit, there is low power consumption in measuring and protective circuits.

Instrument Transformers are primarily categorized into two types 1. 2.

Current Transformers (CT) Potential or Voltage Transformers (PT/VT)

Current Transformer This is a type of instrument transformer that is designed to produce a proportional amount of current in its secondary terminal compared to the primary. Note that voltage is not no t significant as the primary voltage is very negligible. neg ligible. This is because most common CTs used in our circuit have designated primary side P1 and P2 on the th e terminals that are so close that the potential difference across the two terminals will be very negligible. In electrical power systems, the role of current transformers is to reduce high value currents to proportional low value currents to enable measurement of high value currents using a standard measuring instrument called ammeter or to connect to relays as in protection circuits. The standard secondary current is usually 1 ampere or 5 amperes. The primary winding of a CT consists of one or a sometimes a few more turns in its primary winding, while the secondary will have several turns.

Current Transformer

The failure of protective system to perform its function correctly may be due to incorrect application of these transformers. Hence, current transformer must be regarded as constituting part of the protective system and be carefully matched with the relays to fulfill the requirements of the system. The requirements of a protective current transformer are quite different from that of a metering C.T . The metering C.T is only required to perform its function over the normal range of load current, while the protective C.T is required to give satisfactory protection over a wide range of fault conditions. Theory of Current Transformers A C.T functions with the same basic working principle of electrical power transformer. In an electrical power transformer or other general purpose transformer, the load determines the secondary current. The secondary current itself determines the primary current. However in the case of a CT, the primary current determines the secondary current that merely flows through the load connected to the CT. This load is very negligible compared to the primary current. In a power transformer, if load is disconnected, there will be only magnetizing current flowing in the primary. The primary of the power transformer takes current from the source proportional to the load connected with secondary. But in case of CT, if the load is disconnected, the primary is still the system current which may be far more than the magnetizing current. In fact this is why the secondary terminals must not be left open circuited otherwise there may be explosion. When CT secondary terminals are not to be connected, they must be shorted out. So current through its primary is nothing but the current flows through that power line. The primary current of the CT, hence does not depend upon whether the load or burden is connected to the secondary or not or what is the impedance value of burden. Generally CT has very few turns in primary whereas secondary turns is large in number. NP IP = NS IS and EP NS = ES NP . These are common simple equation you found in C.T related formula, Where:

p and s denote primary and secondary E - Voltage I - Current N - Number of turns.

Also, the primary winding is connected in series with the load and it is the latter which determines the current induced in the secondary winding.

CLASSIFICATION OF CURRENT TRANSFORMERS Current Transformers can be classified in a variety of w ays. The following are the major classifications: 1. Depending on the location of installation  Indoor  Outdoor 2. Depending on the application  Metering  Protection 3. Depending on the location in the circuit  Main C.T.  Auxiliary C.T 4. Depending upon the type of construction  Bar  Ring  Wound  Split core  Linear  Cascade 5. Depending upon the type of insulation  Dry type  Oil impregnated paper  Epoxy  SF6 6. Depending upon the location of the secondary core and winding.  Tank type or dead tank  Inverted type or live tank  Insulator type or cross connected type

Bushing Current Transformers

Ring Type Metering Current Transformer

Outdoor Current Transformer

Indoor type current transformers

Ring type current transformer

Insulated Current Transformer

Interposing Current Transformer

Accuracy Limit Factor (ALF) The accuracy limit current is the highest primary current at which a current transformer still meets the specified requirements as regards total error. The accuracy limit factor is the ratio of the accuracy limit current to the rated primary current. The standardized accuracy limit factors are 5, 10, 15, 20 and 30. For example, as per IEC 5 P 20 means a C.T. for protection having maximum total error of 5% at 20 times the rated current. Marking: Accuracy limit factor is written after the accuracy class. E.g. 10 VA 5P10, 15 VA 10P10, 30 VA 5P20 Instrument Security Factor (ISF) The rated instrument security factor is the smallest primary current at which an instrumentation core exhibits a current error of 10%.

The Instrument Security Factor ISF is the ratio of the rated instrument safety current to rated primary current. The instrument security factor defines the behavior of a metering C.T. core under over-current conditions. The ISF is specified to protect instruments connected to the metering C.T. core from system short circuit currents. The ISF to be chosen should be as low as possible. It is expressed as a number ‘n’  5 or n  10. Knee point voltage (V k) This is the sinusoidal e.m.f of rated frequency applied to the secondary terminals of the C.T., with all other windings being open circuited, which when increased by 10% causes the exciting current to increase by 50% or more. This is illustrated below:

Example V1 = 100 V V2 = 110 V Percentage Increase = 10% Corresponding currents C1 = 0.35A C2 = 0.7A Percentage increase = 50% V is the knee point voltage Vk.

The knee point voltage indicates the voltage above which the C.T. enters into saturation and exciting current increases rapidly with a very little increase in voltage The Vk is also limited by practical design and manufacturing consideration as: Vk = Rated output in VA x ALF Secondary rated current

Rated Primary and Secondary Currents These are the values of the primary and secondary current on which the performance of the current transformer is based. Standard values of primary currents are: 5, 10, 15, 20, 30, 60, 75, 50, 100, 150, 200, 300, 400, 600, 800, 1000, 1500, 2000, 3000, 4000 and above. Standard values of secondary currents as per BS 3938 are 5A, 2A and 1A and as Per IEC, 5A or 1A. However there are cases where occasionally ratings of 0.577A, 0.866A or 2.87A have been used.

The selection of the primary current of a C.T. shall always be adopted as closely as possible to the full load or rated current of the installation by rounding off to the next higher standard. However the C.T must be capable of continuously carrying the maximum expected current in service. It is advisable to consider a permitted overload of 20% of the full load current while deciding the rated current. Another factor to be considered is also the load growth and the increase in capacity of an installation. It is for this reason that multi ratio primary currents are adopted like 800-400 - 200 - 100 A. The selection of the secondary current depends upon the secondary current of the equipment already in service where interchangeability is a consideration.

The following are the advantages and disadvantages of CT’s with 5A and 1A secondary currents. • The number of turns required on the secondary side is less for a 5A C.T. than for a 1A C.T. for a given primary current. • A thicker gauge wire is required for a 5A C.T than for a 1A C.T. • Both the above factors contribute to the cost reduction of a 5A C.T. when compared to a 1A C.T. • Since the number of turns is less for a 5A C.T, the voltage induced on the secondary side during secondary saturation or secondary open circuit is less whe n compared to a 1A C.T. •

•

The lead burden, however, becomes excessive for a 5A C.T since the same is proportional to the product of the square of the current and resistance of the lead wire. The lead burden in a 1A CT. will be very low. • In view of the reduced number of secondary turns in a 5A C.T., it is difficult to provide for turns compensation to design and manufacture low current higher accuracy class CT’s. However in a 1A C.T. it is possible to achieve the desired accuracy class because of the increased number of turns and by providing compensating turns. The internal resistance of a 5A C.T. is comparatively less ( 1 ohm) when compared to that of 1A C.T. (Generally 3 to 12 ohms)

Examples

A case study of Estimation of Burden, Knee Point Voltage, Accuracy Class etc of a Protective Current Transformer Requirement of a C.T. to protect a 15 MVA, 132/33 KV Delta/Star connected transformer. Data available % Impedance of Transformer = 10 Fault level at 132KV side = 1400 MVA

Transformer full load current per phase 6

=

15 x 10 _____ 3

3 x 132 x 10 = 65.61 A Hence select primary current = 100 A i.e. Ip = 100 A Select secondary current Is as 5A. A 5A C.T secondary has a winding resistant of less than 1.0 ohm. A typical value may be chosen as 0.601 ohms. •

Assume (a) Distance from C.T to Relay control panel as 100 metres and C.T. secondary leads of 10 sq mm. (R  = 0.1627 ohms for 100 metres) L

(b) Connected relays are GEC CDG 11 over-current and earth fault relays with VA burden of 1.8 and 4 respectively.

2

2

Relay burden =I R  + 2I R  + VA of (OCR + EFR) S

S

S

L  2

 2

= (5)  0.601 + 2(5)  0.1627 + (1.8 + 4) = 15.0 + 8.135 + 5.8 = 28.935 VA Hence select relay burden or output as 30 VA Select Accuracy class 5 P 20 Knee point voltage

 V

=

VA x ALF__

k

Sec. current =

30 x 20  5 600  5 120 V 

= =

6

Fault current at C.T. installation =

1400 x 10 ___ 3

= Or 6.124 KA

3 x 132 x 10 6123.6 A = Isc

Ith   Isc   [t + 0.05] KA rms for 1 sec Assume operating time of breakers, relays etc = 1 sec I  6.124 [1.05] th

 6.275 KA rms Select I  as 10 KA rms for 1 sec th

I  short time rating = 10 KA rms for 1 sec. th

Idyn

=

2.5 I

th

=2.5 x 10 = 25 KA for 1 sec.

Hence complete specifications for this protection C T will be: Voltage class: 132 KV Primary current: 100 A Highest System Voltage: 145 KV Secondary current: 5A Accuracy class: 5 P 20 V : 120 V. k

I : 10 KA rms for 1 sec. th

Idyn:

25 KA for 1 sec.

VOLTAGE TRANSFORMER This is a type of instrument transformer that is designed to produce a proportional amount of voltage in its secondary terminal compared to the primary. Note that here current is not significant as the primary current is very negligible because the load called the burden is also very small. Remember, V1I1=V2I2 The VTs are used as step down transformers. Note that for voltage step down, the primary winding has more turns than the secondary windings. Hence there more turns in the primary windings of a VT than on the secondary windings. Note also that while the primary voltage of the VT is rated depending on the voltage to be measured but the secondary voltage is 110V. The ratio of the primary rated voltage to the secondary rated voltage is called the transformation ratio. The power rating of the VT is dependent on the maximum burden in VA it can deliver within a specified limit of error. The instruments to be connected are also have VA ratings which when added up must not exceed the rated VA of the VT. This contrasts with the power transformer where allowable temperature rise must not be exceeded. The VT

Errors In Voltage Transformers Just as there is no ideal power transformer, there is no ideal VT. Hence the following errors: 1. Voltage Ratio Error: Ideally, the system voltage Vp should be equal to the primary voltage of the VT. If, the voltage across the secondary terminals of the VT is Vs and, the voltage ratio of the VT is KT then Vp/Vs =KT Vp= VsXKT However, in practice, the Vs measured across the terminals of the secondary windings multiplied by the transformation ratio is less than the system voltage. The shortfall or the difference is the transformation error. This can be stated as a percentage as:

The reasons for this error:  

This is because of voltage drop due to the resistance and inductance in the primary winding. The induced voltage also drops due to the resistance and inductance of the secondary windings.

2. Phase Error or Phase Angle Error in Potential or Voltage Transformer The angle ′β′  at which the secondary voltage differs in phase with system voltage and this is measured in minutes. NOTE The two errors above increase with the burden connected to the VT.

Rated Burden of a VT This is the total VA burden (from all the instruments namely voltmeter, watt meter etc) that could be connected to the secondary winding of the VT without compromising the accuracy of the VT.

The Limiting or the Thermal Rating of a VT This is greatest burden in VA at which the VT operates continuously without overheating the windings.

Difference between C.T. and V.T. Few differences between C.T. and V.T. are listed below – Sl. Current Transformer (C.T.)

Voltage Transformer (V.T.)

1

Connected in series with power circuit.

Connected in Parallel with Power circuit.

2

Secondary is connected to Ammeter.

Secondary is connected to Voltmeter.

3

Secondary works almost in short circuited condition.

Secondary works almost in open circuited condition.

4

Primary current depends on power circuit current.

Primary current depends on secondary burden.

5

Primary current and excitation vary over wide range Primary current and excitation variation with change of power circuit current are restricted to a small range.

6

One terminal of secondary is earthed to avoid the insulation break down.

One terminal of secondary can be earthed for Safety.

7

Secondary is never be open circuited.

Secondary can be used in open circuit condition.

Some Important Things To Note about VTs and CTs 1. Don’t leave unused windings of a CT open circuited. Windings that are not connected to other instruments must be shorted out. On the other hand, windings of VTs that are not in use should be left open circuited 2. Use the right class of both the CT and VT. Note that the class for metering is different for the class for protection. Choose the right for your application. 3. Note the frequency of the CT and VT and ensure they conform with the frequency of the system where they are connected.

CT and VT Connections

CTs and VTs in use in our networks come in single units only. Hence, for three phase supply, three units will be used. The primary winding of each VT or CT is connected to a phase ensuring that the polarity is maintained. The secondary windings are connected accordingly.