Instrument Transformers

Contents

Recall the Instrument Transformers 2

Classify the Instrument Transformers 2

List the Advantages of Instrument Transformers 3

Describe the Construction and the Working Principle of Current Transformers 4

Recall the Core Construction of Current Transformers 4

Recall the Effects of Open Circuited Secondary of Current Transformers 5

Recall the Equivalent Circuit of Current Transformers 6

Draw the Phasor Diagram of Current Transformers 6

Equivalent Circuit of Current Transformer: 6

Recall the Errors in Current Transformers 8

Describe the Construction and the Working Principle of Potential Transformers 9

Recall the Equivalent Circuit and draw the Phasor Diagram of Potential Transformers 10

Recall the Errors in Potential Transformers 12

Recall the Ratios of Instrument Transformers 13

Recall the Ratio Correction Factor of Instrument Transformers 13

Describe the Burden of an Instrument Transformer 14

Recall the Wilson’s Compensation Method 15

Recall the Instrument Transformers

Instrument transformers are devices that are used to measure high voltage and high current electrical systems safely and accurately. They are designed to provide a smaller, low-power output that can be easily measured and analysed by instruments or metres.

There are two types of instrument transformers: current transformers and voltage transformers.

Current transformers (CTs) are used to measure high currents in power systems. They are typically installed in series with the line carrying the current to be measured. The CT steps down the current to a lower value that can be safely measured by a current metre or relay. CTs are commonly used in power systems to measure the current flowing through transformers, generators, and transmission lines.

Voltage transformers (VTs), also known as potential transformers (PTs), are used to measure high voltage levels in power systems. They are typically installed in parallel with the circuit to be measured. The VT steps down the voltage to a lower value that can be safely measured by a voltmeter or relay. VTs are commonly used in power systems to measure the voltage across transformers, generators, and transmission lines.

Both CTs and VTs are designed to be highly accurate, with minimal error or distortion. They are typically used in conjunction with protective relays, metres, and other monitoring devices to ensure the safe and efficient operation of power systems.

Classify the Instrument Transformers

Instrument transformers can be classified into two main types:

1. Current transformers (CTs): These transformers are used to measure current. They are connected in series with the power line and produce a proportional current in their secondary winding, which is connected to the instrument or metre. CTs are used to measure large currents that cannot be measured directly by instruments or metres.
2. Voltage transformers (VTs): These transformers are used to measure voltage. They are connected in parallel with the power line and produce a proportional voltage in their secondary winding, which is connected to the instrument or metre. VTs are used to measure high voltages that cannot be measured directly by instruments or metres.

List the Advantages of Instrument Transformers

The advantages of using instrument transformers in electrical power systems are:

1. Safety: Instrument transformers protect personnel and equipment from high voltages and currents. They allow the use of lower-rated instruments and metres, which can be more easily and safely handled.
2. Accuracy: Instrument transformers provide accurate measurements of electrical quantities, even at high voltages and currents. This accuracy is essential for proper operation and maintenance of power systems.
3. Reliability: Instrument transformers are designed and manufactured to high standards of reliability and durability. They are subjected to rigorous testing and inspection to ensure proper performance and long service life.
4. Versatility: Instrument transformers can be used in a variety of applications, including protection, metering, and control. They are essential components of many power system devices, such as relays, metres, and protective relays.
5. Cost-effectiveness: Instrument transformers are cost-effective compared to other measurement technologies. They are also easy to install and maintain, reducing overall costs and downtime.

Describe the Construction and the Working Principle of Current Transformers

Current transformers (CTs) are devices used to measure the current flowing in an electrical power system. The construction of a CT typically includes a primary winding, a magnetic core, and a secondary winding. The primary winding is connected in series with the power line and carries the current to be measured. The magnetic core is a toroidal-shaped iron core that surrounds the primary winding. The secondary winding is wound on the magnetic core and is connected to the instrument or metre.

The working principle of a CT is based on the magnetic induction principle. When an alternating current flows through the primary winding, it creates a magnetic field in the magnetic core. The strength of the magnetic field is proportional to the current flowing in the primary winding. The magnetic field induces a voltage in the secondary winding, which is proportional to the magnetic field strength. The voltage induced in the secondary winding is proportional to the primary current and is typically much lower in magnitude.

Recall the Core Construction of Current Transformers

Current transformers (CTs) are devices used to measure high current levels in power systems. The core construction of current transformers is a key factor in their accuracy and performance.

The core of a CT is typically made of high-permeability magnetic material such as laminated silicon steel or ferrite. The core is typically formed in the shape of a toroid, with a primary winding wrapped around the core and a secondary winding connected to a measuring device.

The primary winding is the conductor carrying the current to be measured, and it is typically made of a few turns of a thick, low-resistance wire. The secondary winding is typically made of many more turns of a fine wire, and it is connected to a measuring device such as an ammeter or protective relay.

The accuracy of a CT is dependent on the quality of the core material and the precision of the winding. The core must be constructed in a way that ensures a uniform magnetic field and minimises losses due to hysteresis and eddy currents. The winding must be precisely positioned and insulated to ensure a high level of coupling between the primary and secondary windings.

Recall the Effects of Open Circuited Secondary of Current Transformers

When the secondary winding of a current transformer (CT) is open-circuited, it means that there is no load connected to the secondary winding. In this condition, the voltage induced in the secondary winding is not consumed by any load, and the voltage across the secondary winding increases, which can damage the insulation of the CT.

The open-circuited secondary of a CT can also cause errors in the measurement of the primary current. Since the secondary current is zero, the CT behaves as an inductor, and the primary current may cause a high voltage across the primary winding, leading to saturation of the core. This can cause a distortion of the primary current waveform and lead to errors in the measurement of the primary current. Therefore, it is essential to ensure that the secondary winding of a CT is always connected to a load.

Recall the Equivalent Circuit of Current Transformers

The equivalent circuit of a current transformer (CT) includes the primary winding, the secondary winding, the magnetising branch, and the core loss branch. The magnetising branch represents the magnetising current required to establish the magnetic field in the core, and the core loss branch represents the core losses. The secondary winding is connected to a load impedance, which represents the instrument or metre connected to the CT. The equivalent circuit of a CT is used to analyse the performance of the CT under different conditions and to determine the errors in the CT.

Draw the Phasor Diagram of Current Transformers

The phasor diagram of a current transformer (CT) shows the phasor representation of the primary current, the secondary current, and the excitation current. The phasor diagram is used to analyse the performance of the CT and to determine the errors in the CT.

In the phasor diagram, the primary current phasor is drawn horizontally, while the secondary current phasor is drawn vertically. The angle between the primary current phasor and the secondary current phasor represents the phase shift between the primary current and the secondary current. The excitation current phasor is also shown, and it represents the magnetising current required to establish the magnetic field in the core

Equivalent Circuit of Current Transformer:

The equivalent circuit and phasor diagram of the current transformer is shown in figure below.

Where

n = turn ratio

r_s = resistance of secondary winding

x_s = reactance of secondary winding

r_e = resistance of burden connected to secondary

x_e = reactance if burden connected to secondary

E_p = Induced emf in CT primary

E_s = Induced voltage in CT secondary

N_p = CT primary winding turns

N_s = CT secondary winding turns

I_p = CT primary winding current

I_s = CT secondary winding current

Ø = Flux in CT core

ɵ = Phase angle of CT

I₀ = Excitation current of CT I_m = Magnetizing current of CT

Recall the Errors in Current Transformers

There are several types of errors that can occur in current transformers (CTs), including:

1. Ratio error: This occurs when the actual turns ratio of the CT is different from the nominal turns ratio. Ratio error can be caused by variations in the number of turns in the primary and secondary windings or by changes in the magnetic properties of the core.
2. Phase angle error: This occurs when the phase angle between the primary current and the secondary current is different from the ideal value of zero degrees. Phase angle error can be caused by the magnetising current and core losses.
3. Burden error: This occurs when the load impedance connected to the CT is different from the rated value. Burden error can cause a change in the secondary current and affect the accuracy of the CT.
4. Saturation error: This occurs when the magnetic core of the CT becomes saturated, leading to a distortion of the primary current waveform and errors in the measurement of the primary current.
5. Frequency error: This occurs when the frequency of the primary current is different from the rated frequency of the CT. Frequency error can cause a change in the magnetic properties of the core and affect the accuracy of the CT.

Describe the Construction and the Working Principle of Potential Transformers

Potential transformers, also known as voltage transformers, are instrument transformers used to step down high voltage levels to a safe and measurable level for the purpose of measurement and protection. They are commonly used in power systems to provide accurate voltage measurements and to isolate low-voltage instruments from high-voltage circuits.

Construction of Potential Transformers:

1. Primary Windings: The primary winding of a potential transformer is connected in parallel with the high-voltage line whose voltage is to be measured. It typically consists of a few turns of thick insulated copper conductor.
2. Secondary Windings: The secondary winding is connected to the low-voltage measuring or protection devices. It is wound with a large number of turns of fine insulated copper conductor. The turns ratio of the primary and secondary windings determines the step-down ratio of the potential transformer.
3. Magnetic Core: The primary and secondary windings are wound around a laminated iron core. The core is made of high-permeability material to enhance the magnetic flux and minimize losses. The laminated construction reduces eddy current losses.

Working Principle of Potential Transformers:

The working principle of a potential transformer is based on the principle of electromagnetic induction. When a high-voltage current flows through the primary winding, it creates a magnetic flux in the core. This magnetic flux induces a proportional voltage in the secondary winding according to the turns ratio.

The secondary voltage of a potential transformer is proportional to the primary voltage and is determined by the turns ratio. The primary voltage is stepped down by the turns ratio, typically ranging from 1000:1 to 5000:1. This enables safe measurement and protection of the system voltage using low-voltage instruments.

Potential transformers are designed to have high accuracy, low voltage regulation, and minimal phase shift between primary and secondary voltages. They are also equipped with protective measures such as fuses, voltage-limiting devices, and grounding connections to ensure safe operation.

Applications of Potential Transformers:

1. Metering: Potential transformers are commonly used in electrical metering applications to provide accurate voltage measurements for billing and monitoring purposes.
2. Protection: Potential transformers are used in protective relays and devices to sense abnormal voltage conditions and trigger protective actions such as circuit breaker tripping.
3. Instrumentation: Potential transformers are used in various industrial and laboratory instruments that require precise voltage measurements, such as oscilloscopes, data loggers, and control systems.

In summary, potential transformers play a crucial role in electrical power systems by stepping down high voltages to safe and measurable levels. They provide accurate voltage measurements and isolation between high-voltage circuits and low-voltage instruments, ensuring reliable operation and protection of electrical systems.

Recall the Equivalent Circuit and draw the Phasor Diagram of Potential Transformers

Equivalent Circuit of Potential Transformers:

The equivalent circuit of a potential transformer consists of an ideal transformer and associated impedance components. The primary side of the potential transformer is connected to the high-voltage line, and the secondary side is connected to the low-voltage measuring or protection devices.

The equivalent circuit includes the following components:

1. Ideal Transformer: The ideal transformer represents the step-down ratio between the primary and secondary voltages. It has a turns ratio Np/Ns, where Np is the number of turns in the primary winding and Ns is the number of turns in the secondary winding.
2. Magnetizing Reactance (Xm): The magnetizing reactance represents the magnetic flux created by the primary winding when energized. It is represented by an inductive reactance Xm.
3. Leakage Reactance (Xl): The leakage reactance represents the leakage flux and associated losses in the transformer windings. It is represented by an inductive reactance Xl.
4. Burden Impedance (Zb): The burden impedance represents the impedance of the connected load on the secondary side, such as measuring instruments or protective relays. It is represented by an impedance Zb.

Phasor Diagram of Potential Transformers:

The phasor diagram of a potential transformer illustrates the relationship between the primary and secondary voltages and currents. It helps visualize the voltage step-down and phase shift between the primary and secondary sides.

In the phasor diagram, the primary voltage Vp is represented by a phasor at the reference angle of 0 degrees. The secondary voltage Vs is represented by a phasor at the angle of phase shift relative to the primary voltage. The angle of phase shift depends on the power factor of the connected load and the impedance components of the potential transformer.

The ideal transformer phasor is drawn at the turns ratio angle, showing the step-down relationship between the primary and secondary voltages. The magnetizing reactance and leakage reactance phasors are added to represent the corresponding components in the equivalent circuit.

The burden impedance phasor represents the load impedance on the secondary side. Its angle is determined by the load power factor.

The phasor diagram helps analyze the voltage regulation, phase angle difference, and the effect of load impedance on the secondary voltage. It is a visual representation that assists in understanding the behavior and performance of the potential transformer under different load conditions.

Note: The specific values and angles in the equivalent circuit and phasor diagram will vary depending on the characteristics of the actual potential transformer and the connected load.

Recall the Errors in Potential Transformers

There are several types of errors that can occur in potential transformers (PTs), including:

1. Ratio error: This occurs when the actual turns ratio of the PT is different from the nominal turns ratio. Ratio error can be caused by variations in the number of turns in the primary and secondary windings or by changes in the magnetic properties of the core.
2. Phase angle error: This occurs when the phase angle between the primary voltage input and the secondary voltage output is different from the ideal value of zero degrees. Phase angle error can be caused by the magnetising current and core losses.
3. Burden error: This occurs when the load impedance connected to the PT is different from the rated value. Burden error can cause a change in the secondary voltage output and affect the accuracy of the PT.
4. Saturation error: This occurs when the magnetic core of the PT becomes saturated, leading to a distortion of the primary voltage waveform and errors in the measurement of the primary voltage.
5. Frequency error: This occurs when the frequency of the primary voltage is different from the rated frequency of the PT. Frequency error can cause a change in the magnetic properties of the core and affect the accuracy of the PT.

Recall the Ratios of Instrument Transformers

Instrument transformers are used to step down or step up current and voltage levels for accurate measurement and protection purposes. The commonly used instrument transformers are current transformers (CTs) and voltage transformers (VTs) or potential transformers (PTs). Here are the ratios typically associated with these instrument transformers:

1. Current Transformer (CT) Ratios:

Primary Current (Ip) to Secondary Current (Is) Ratio: This ratio defines the relationship between the primary current flowing through the CT’s primary winding and the secondary current flowing through the CT’s secondary winding. CTs are designed to step down high primary currents to a lower secondary current suitable for measurement or protection devices. Typical CT ratios include 100:5, 200:5, 400:5, etc. For example, a 200:5 CT will step down a primary current of 200 Amperes to a secondary current of 5 Amperes.

1. Voltage Transformer (VT) or Potential Transformer (PT) Ratios:

Primary Voltage (Vp) to Secondary Voltage (Vs) Ratio: This ratio defines the relationship between the primary voltage applied to the VT’s primary winding and the secondary voltage measured across the VT’s secondary winding. VTs are designed to step down high primary voltages to a lower secondary voltage suitable for measurement or protection devices. Typical VT ratios include 11kV/110V, 33kV/110V, 66kV/110V, etc. For example, an 11kV/110V VT will step down a primary voltage of 11,000 Volts to a secondary voltage of 110 Volts.

It’s important to note that the ratios mentioned above are just examples, and the actual ratios used in practice depend on the specific application, system requirements, and standards followed in a particular region or industry. Instrument transformers are designed and selected based on the voltage or current levels to be measured, the accuracy requirements, and the burden or load connected to the secondary side.

Recall the Ratio Correction Factor of Instrument Transformers

The ratio correction factor (RCF) is a factor that is used to correct for the errors in the ratio of instrument transformers. The RCF is determined by comparing the actual turns ratio of the transformer with the nominal turns ratio.

For CTs, the RCF is calculated as follows:

RCF = (Nominal Turns Ratio) / (Actual Turns Ratio)

For PTs, the RCF is calculated as follows:

RCF = (Actual Turns Ratio) / (Nominal Turns Ratio)

The RCF is used to adjust the output of the transformer to the correct value and to ensure that the instrument or metre connected to the transformer provides accurate measurements.

Describe the Burden of an Instrument Transformer

The burden of an instrument transformer refers to the electrical load or impedance connected to the secondary winding of the transformer. It represents the total impedance that the secondary circuit of the instrument transformer presents to the measuring or protective devices connected to it.

The burden is typically specified in terms of volt-amperes (VA) or ohms (Ω) and is an important consideration in the design and selection of instrument transformers. It is crucial to ensure that the burden connected to the secondary winding does not exceed the rated burden of the instrument transformer, as it can affect the accuracy and performance of the transformer.

The burden of an instrument transformer includes both the actual load connected to the secondary winding, such as measuring instruments, relays, or meters, and the inherent impedance of the secondary winding itself. The burden impedance can be resistive, inductive, or capacitive, depending on the nature of the connected load.

The rated burden of an instrument transformer is specified by the manufacturer and represents the maximum load that the transformer can handle while maintaining its specified accuracy. It is important to choose an instrument transformer with a rated burden that is suitable for the connected load and to ensure that the burden impedance does not exceed the specified limits.

The burden of an instrument transformer can impact various parameters, including voltage or current transformation ratio, accuracy, phase angle error, and frequency response. Therefore, it is essential to consider the burden requirements and match them with the capabilities of the instrument transformer to ensure accurate and reliable measurement or protection in the system.

Recall the Wilson’s Compensation Method

Wilson’s compensation method is a technique used to compensate for the inductance and resistance of the test leads in AC bridge measurements. It is commonly employed in bridges such as the Maxwell-Wien bridge and the Hay bridge to eliminate the effect of the lead inductance, which can introduce errors in the measurement.

In Wilson’s compensation method, an additional set of compensating or auxiliary windings is added to the bridge circuit. These windings are connected in series with the test leads and are designed to produce an equal and opposite reactance to that of the test leads. The compensating windings are adjusted to balance out the inductance of the test leads, effectively canceling out their contribution to the overall impedance of the bridge.

By using Wilson’s compensation method, the inductance and resistance of the test leads can be effectively nullified, allowing for more accurate measurements of the unknown impedance or component being tested. This technique is particularly useful when working with high-precision

AC bridge measurements, where small errors due to lead inductance can significantly affect the measurement accuracy.

It is important to note that Wilson’s compensation method requires careful adjustment and calibration of the compensating windings to achieve accurate compensation. The technique relies on the assumption that the reactance of the test leads is predominantly inductive and can be effectively canceled out by the compensating windings. If the test leads have significant capacitive reactance, additional compensation techniques may be necessary.

Overall, Wilson’s compensation method provides a means to compensate for the inductance and resistance of test leads in AC bridge measurements, enhancing the accuracy and reliability of the measurement results.