Voltage transformer
The PT is typically described by its voltage ratio from primary to secondary. A 600:120 PT will provide an output voltage of 120 volts when 600 volts are impressed across its primary winding. Standard secondary voltage ratings are 120 volts and 70 volts, compatible with standard measuring instruments
Burden and accuracy
Burden and accuracy are usually stated as a combined parameter due to being dependent on each other. Metering style PTs are designed with smaller cores and VA capacities than power transformers. This causes metering PTs to saturate at lower secondary voltage outputs saving sensitive connected metering devices from damaging large voltage spikes found in grid disturbances. A small PT (see nameplate in photo) with a rating of 0.3W, 0.6X would indicate with up to W load (12.5 watts[1] ) of secondary burden the secondary current will be within a 0.3 percent error parallelogram on an accuracy diagram incorporating both phase angle and ratio errors. The same technique applies for the X load (25 watts) rating except inside a 0.6% accuracy parallelogram
Markings
Transformer winding primary (usually high-voltage) connecting wires are of many types. They may be labeled as H1, H2 (sometimes H0 if it is internally designed to be grounded) and X1, X2 and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) (and third (Z1, Z2, Z3) may also be available on the same voltage transformer. The primary may be connected phase to ground or phase to phase. The secondary is usually grounded on one terminal to avoid capacitive induction from damaging low-voltage equipment and for human safety
Types of voltage transformers
There are three primary types of potential transformers (PT): electromagnetic, capacitor, and optical. The electromagnetic potential transformer is a wire-wound transformer. The capacitor voltage transformer (CVT) uses a capacitance potential divider and is used at higher voltages due to a lower cost than an electromagnetic PT. An optical voltage transformer exploits the Faraday effect, rotating polarized light, in optical materials
A capacitive voltage transformer (CVT), also known as capacitor-coupled voltage transformer (CCVT), is a transformer used in power systems to step down extra high voltage signals and provide a low voltage signal to the actual VT (voltage transformer) used for operating metering/protective relays.
In its most basic form, the device consists of three parts: two capacitors across which the transmission line signal is split, an inductive element to tune the device to the line frequency, and a voltage transformer to isolate and further step down the voltage for metering devices or protective relay.
The tuning of the divider to the line frequency makes the overall division ratio less sensitive to changes in the burden of the connected metering or protection devices.[4] The device has at least four terminals: a terminal for connection to the high voltage signal, a ground terminal, and two secondary terminals which connect to the instrumentation or protective relay.
Capacitor C1 is often constructed as a stack of smaller capacitors connected in series. This provides a large voltage drop across C1 and a relatively small voltage drop across C2. As the majority of the voltage drop is on C1, this reduces the required insulation level of the voltage transformer. This makes CVTs more economical than the wound voltage transformers under high voltage (over 100 kV), as the latter one requires more winding and materials.
The CVT is also useful in communication systems. CVTs in combination with wave traps are used for filtering high-frequency communication signals from power frequency.[5] This forms a carrier communication network throughout the transmission network, to communicate between substations.
What is Potential Transformer?
A potential transformer, also called a PT, is an instrument transformer used in power systems for voltage transformation. It converts higher voltage values to lower voltage values for measurement and protection purposes. By presenting a minimal load, it maintains an accurate voltage ratio and phase relationship, ensuring precise secondary connected metering.
Potential Transformer Definition
A transformer used to step down high voltage levels to lower, safer levels is a potential transformer. It is a type of instrument transformer. Most commercial voltmeters are suitable for measuring the output voltage from a potential transformer. They are designed to produce a negligible load from the measured supply. They have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering
Potential Transformer Working Principle
The Potential transformer working principle is based on Faraday's law of electromagnetic induction. Two primary windings and one secondary winding make up a potential transformer. The high voltage line that needs to be stepped down is connected in parallel to the primary windings. The secondary winding is linked to a measuring device, such as a voltmeter or wattmeter, to measure the voltage or power in the circuit.
A magnetic field is produced around the primary winding when a high AC voltage is applied to it. This magnetic field induces a voltage in the secondary winding. A large number of turns of a conducting wire are built into the secondary winding, which aids in stepping down the voltage to a lower level
The ratio of the turns in the primary winding to the turns in the secondary winding is known as the voltage ratio of a potential transformer. A potential transformer, for instance, with a voltage ratio of 1000:1, will step down a primary voltage of 1000 volts to a secondary voltage of 1 volt.
The accuracy of a potential transformer depends on several factors, such as the quality of the magnetic core, the number of turns in the windings, and the accuracy of the measuring instrument connected to the secondary winding. Power transmission and distribution networks frequently employ potential transformers to precisely and securely measure high voltage levels
Construction of Potential Transformers
A potential transformer (PT), also called a voltage transformer (VT), is an instrument transformer used to detect high voltages in electrical power circuits. The construction of a potential transformer includes the following components:
Core: The transformer's core is built of laminated magnetic material, such as iron or steel, to create a low reluctance path for the magnetic flux. Hysteresis and eddy current losses are minimized by the design of the core.
Primary Winding: The high voltage line connected in parallel with the transformer's primary winding requires periodic monitoring. The primary winding is typically built of a few rounds of thick copper wire or busbars to accommodate large currents
