Burden of current transformer. The secondary load of a current transformer is usually called the "burden" to distinguish it from the load of the circuit whose current is being measured. The burden, in a CT is the (largely resistive) impedance presented to its secondary winding.
Burden of CT can be specified as Volt-Ampere absorbed at certain Power Factor i.e the VA that can be consumed by the load. The burden can also be expressed as total Impedance in terms of ohms connected on secondary of CT i.e. pilot conductor and instrument burden (I2 x R=VA).
For example, assume that the current rating of the primary winding is 100A. The secondary winding has the standard rating of 5A. Then the ratio between the primary and the secondary currents is 100A-to-5A, or 20:1. In other words, the primary current is 20 times greater than the secondary current.
The turns ratio is defined as the ratio of turns of wire in the primary winding to the number of turns of wire in the secondary winding. Turns ratio can be expressed using Equation (13-1).
Potential Transformer ratio is the ratio of the primary rated voltage of the PT divided by the secondary rated voltage of the PT. A 480:120V rated PT will have a PT ratio of 4. When potential transformers are applied at a voltage different from its rated voltage, the secondary voltage needs to be calculated.
The ratio of the transformers primary and secondary windings with respect to each other produces either a step-up voltage transformer or a step-down voltage transformer with the ratio between the number of primary turns to the number of secondary turns being called the “turns ratio” or “transformer ratio”.
High voltage and current are not directly measured by measuring instruments, higher values are steped down with a known ratio of PT or CT and then multiply measured value with ratio of transformer to get accurate value of voltage or current.
To test a transformer with a digital multimeter (DMM), first turn off power to the circuit. Next, attach the leads of your DMM to the input lines. Use the DMM in AC mode to measure the transformer primary.
Volts per Turn.
Another way to consider transformer voltages is by volts/turn; if the 100 volts applied to the 1000 turn primary produces 100/1000 = 0.1 volts per turn, then each single turn on the 100 turn secondary winding will produce 0.1V so the total secondary voltage will be 100 × 0.1V = 10V.Voltage transformers (VT), also called potential transformers (PT), are a parallel connected type of instrument transformer. They are designed to present negligible load to the supply being measured and have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering.
Whereas PTs are used to measure voltage means potential difference & to get a potential difference you have to connect it with two different wire having different potential,so we have to connect it in parallel.
An ideal transformer is an imaginary transformer which has. - no copper losses (no winding resistance) - no iron loss in core. - no leakage flux. In other words, an ideal transformer gives output power exactly equal to the input power.
To prevent secondary circuits from reaching dangerous potential, the circuits shall be grounded. A voltage transformer, with the primary winding connected between two phases, shall have the secondary circuit, which has a voltage lagging the other terminal by 120 degrees, grounded. Windings not in use shall be grounded.
A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). It produces a current in its secondary which is proportional to the current in its primary. Current transformers, along with voltage or potential transformers, are instrument transformers.
There are two types of instrument transformer: a 'voltage transformer' (known as'potential transformer' in North America), and a 'current transformer'. They are used with high-voltage systems, where it is hazardous to connect instruments directly to such systems.
Potential transformer is a voltage step-down transformer which reduces the voltage of a high voltage circuit to a lower level for the purpose of measurement. These are connected across or parallel to the line which is to be monitored. In common, the potential transformers are abbreviated as PT.
A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. It can be designed to "step up" or "step down" voltages and works on the magnetic induction principle. When voltage is introduced to one coil, called the primary, it magnetizes the iron core.
Advantages of Instrument Transformers
The large voltage and current of AC Power system can be measured by using small rating measuring instrument i.e. 5 A, 110 – 120 V. By using the instrument transformers, measuring instruments can be standardized. Which results in reduction of cost of measuring instruments.The potential transformers are used in the protecting relaying scheme because the potential coils of the protective device are not directly connected to the system in case of the high voltage. Therefore, it is necessary to step down the voltage and also to insulate the protective equipment from the primary circuit.
Back to the Basics – Potential Transformer Testing (PT/VT)
- Perform insulation resistance tests winding to winding and each winding to ground. '
- Perform a polarity test on each transformer.
- Perform a turn's ratio test on all tap positions if possible.
Types of Transformer. The different types of transformer are Step up and Step down Transformer, Power Transformer, Distribution Transformer, Instrument transformer comprising current and Potential Transformer, Single phase and Three phase transformer, Auto transformer, etc.
Difference Between Current Transformer (CT) & Potential Transformer (PT) One of the major difference between them is that the current transformer converts the high value of current into low value whereas the potential or voltage transformer converts the high value of voltages into low voltage.
A single-phase transformer is a type of power transformer that utilizes single-phase alternating current, meaning the transformer relies on a voltage cycle that operates in a unified time phase. The ratio of primary (input) windings to secondary (output) windings determines the change in current.
