Instrument Transformers

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

Power System protection Dr. Mohamad Tawfeeq Instrument Transformers

Instrument Transformers � CT – Current Transformer � Current Scaling � Isolation � VT – Voltage Transformer � Voltage Scaling � Isolation

1.Current Transformers  Current and voltage transformers are responsible for scaling primary system signals.  Typical nominal secondary values are 5 A and 1 A for current transformers and 110 V for voltage transformers.  Conventional magnetic current and voltage transformers are still in wide use in power systems. For voltage levels greater than 230 kV, we substitute capacitive-coupled voltage transformers (CCVT) for VTs.CT saturation and bad transient response of CCVTs create protection problems.  New trends in current and voltage transducers are low- power-output current and voltage transformers (recently included in an IEC Standard) and magneto-optic current and voltage transducers (MOCT and MOVT). MOCTs use the Faraday Effect, and MOVTs can use either the Faraday or the Pockels effect.

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

Principles of Current Transformers

The initial problem was how to connect a low-voltage device to the high-voltage system and have the ability to handle large fault currents (kilo-Amps). How can we make the relay measure the currents flowing in the high-voltage system in order to detect these faults? The solution consists of using a special type of transformer, called a current transformer. The main parts of a current transformer are: • Iron core • Secondary winding • Primary conductor • External insulation Some current transformers do not have a primary conductor. In those cases the primary is the line or bus itself. Sometimes the core and its secondary winding are directly installed in the bushing of the circuit breakers or transformers. These CTs are called “bushing CTs”. Some current transformers may have a primary that consists of several turns. Typically there are no more turns than the natural, which is equivalent to say that the primary number of turns is 1. The total load connected to the CT terminal (g and h in this case) is called “burden”. Ideally, the secondary current of a CT is perfectly proportional to the primary current. It will be shown later that in reality this is sometimes not true.

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Core and Secondary Winding Example

High-Voltage CT Example

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

Note that in all cases there are polarity marks. The following conventions are used to mark the reference for AC currents:

ANSI: Polarity marks

IEC: P1, P2, S1, S2

VDE: K, L, k, l

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

CT Common Connections

These are the two of the most common connections of current transformers in three phase systems.

At the left, the “Y” connection provides the line currents at the secondary. At the right, the “∆” connection provides the difference currents (delta currents) to the secondary loads.

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

Current Transformer Ratio (CTR)

The Current Transformer Ratio, CTR, expressed as a fraction, is the ratio between the magnitudes of the primary and the secondary current for ideal working conditions of the current transformer.

� Denominator is the Secondary Rated Current

� Typically: 1 A or 5 A

� Numerator Is Not Always the Primary Rated Current

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

Current Transformer Equivalent Circuit

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

CT Performance Calculations

The performance of a C.T may be found from two approaches : 1. The formula method. 2. The saturation curve method.

2- The saturation curve and error method.

The relation between Es and Ie is not linear like the relation shown in the Fig.1 due to the saturation of C.T core. Therefore the C.T. has ratio error.

Fig.1 Magnetizing curve

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 The deviation of Is’ from Is is called the C.T error, and can be expressed as a percentage.

I  I ' I C.T  s s x100%  e x100% error Is Is

Where:

Is –Is’ = Ie

Is: Total current in the CT secondary Is’: Current due to the CT load

Example:

Assume that a C.T has rated current ratio of 500/5 A. The impedance of the secondary winding Z2 = 0.242 Ω, and the burden impedance ZB = 0.351 Ω .The core area A = 0.00193 m2. The C.T must operate at máximum primary current of 10 kA.If the frequency is 60 Hz and the core is built from silicon steel: (a) Determine whether or not the C.T will saturate. (b) Determine the C.T error. Solution

Is= 10,000 x (5/500)= 100 A If we neglect the excitation current Ie. ’ Is ≈ Is= 100 A. Es = Is (ZB+Z2) = 100 x (0.351 +0.242) = 59.29 V Es = 4.44 * f * N2 * A * Bm So: 59.29 B  1.15T m 500 4.44x60x x0.001935 5 As we know that the lower limit for the silcion steel saturation is Bm = 1.2T Hence the C.T will not saturate. From the saturation curve the core corrsponding Ie for Es = 59.29 V is Ie = 0.1 A.

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

I ' 0.1 C.T % = e x100%  x100%  0.1% error I s 100

Saturation Curve for Silicon Steel Used in Current Transformer Cores for different CTRs.

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

CT Classes and accuracies

Accuracy Class Class % error Applications

0.1 ± 0.1 0.2 ± 0.2 Metering 0.5 ± 0.5 1.0 ± 1.0 5P ± 1 Protection 10P ± 3

C.T specifications:  A rated burden at rated current or (VA).  An accuracy clases  (ALF) accuracy limit factor an upper limit beyond which the accuracy is not guaranteed.

Example: 1. 15 VA clases 0.5→mettering C.T 2. 15 5p 20 →protection C.T

Rated burden 15VA Accuracy class 5p ALF= 20 (20 times the rated current)

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

Power System protection Dr. Mohamad Tawfeeq Instrument Transformers

2.Voltage Transformers

Voltage transformers are connected across the points at which the voltage to be measured. Types of voltage Transformers: There are three main types of voltage transformers:  Magnetic voltage transformers (ordinary two winding type – used for L.V. and M.V).  Capacitive Voltage Transformers (CVT), used for high and extra high voltages.  Magneto—optic voltage transformers (new ).

2.1 Magnetic Voltage Transformer (VT)

Line

Vs = 110 or 120

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VT Equivalent Circuit Referred to the Secondary Side

2.2 Capacitive Voltage Transformers (CVT) There are two types of CVTs:  Coupling capacitor voltage transformer  Capacitor – bushing voltage transformer These types are shown in figure below:

Figure : Capacitor voltage transformers: (a) coupling-capacitor voltage divider (b) capacitance-bushing voltage divider .

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

CVT equivalent circuit

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VT error : Errors in magnitude can be calculated from :

Error VT= {(n Vs - Vp) / Vp} x 100%.

Table : Voltage transformers error limits Class Primary Voltage Phase error voltage error (±min) (±%) 0.1 0.8 Vn , 1.0 0.1 0.5 0.2 Vn and 1.2 0.2 10.0 0.5 Vn 0.5 20.0 1.0 1.0 40.0

0.1 1.0 40.0 0.2 1.0 40.0 0.5 0.5 Vn 1.0 40.0 1.0 2.0 80.0

0.1 0.2 80.0 0.2 2.0 80.0 0.5 Vn 2.0 80.0 1.0 3.0 120.0

Vn =Nominal voltage

Power System Protection Part – 3 Dr.Prof .Mohammed Tawfeeq

VT connections

(b) delta – wye connection (a) Open delta connection