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RVTs are used for residual earth fault protection and for discharging capacitor banks. The secondary residual voltage winding is connected in open delta. Under normal conditions of operation, there is no voltage output across the residual voltage winding. When there is an earth fault, a voltage is developed across the open delta winding which activates the relay. 3 phase RVTs typically have 5 limb constructions.
Tests
A number of routine and type tests have to be conducted on VT s before they can meet the standards specified above. The tests can be d. classified as:
Lighting Transformers are designed to supply power to lighting equipments in a commercial / industrial / domestic unit. Its primary supply connected to higher voltage 3Ph source and secondary is to be connected to load.
Lighting transformers serves as isolation between primary and secondary, it also restricts any high voltage spikes and EMF coming with the raw mains incoming power. Lighting Transformers are very useful when system neutral is not available or faulty i.e. where incoming supply is 3 phase 3 wire and lighting load is 220 Volt single phase. It also restricts short circuit current in the load and saves major accidents.
Power House lighting transformers are made with CRGO laminations and Electrolytic copper super enameled winding wires. They are available for wide ranges of inputs out are in the range and in capacity from 1K VA to 100 KVA.
Construction Lighting Transformers are two winding transformers with both windings made of copper. Electrolytic grade super enameled wires with best insulations are used. Core materials are CRGO Strip cores. Experienced hands with best machines for winding ensures best results with minimum size. Appropriate classes of insulation materials are used. Transformers are impregnated in insulating varnish for long lasting performance. Input and out are terminated on Glass Epoxy board with proper size of terminals. For Lighting Transformers greater than 30 KVA, usually a separate M.S Enclosure with temperature indicator is provided as shown in the drawing.
The purpose of a distribution transformer is to reduce the primary voltage of the electric distribution system to the utilization voltage serving the customer. A distribution transformer is a static device constructed with two or more windings used to transfer alternating current electric power by electromagnetic induction from one circuit to another at the same frequency but with different values of voltage and current. The standard Transformation Ratio of Distribution Transformers is from 11000/433 V or 22000/433 V or 33000 / 433 V. Sometimes an offload tap changer with tapings of +/- 2.5% or +/-5% is also provided to adjust to the local site conditions. Depending upon the requirement, an on load tap changer can also be provided.
Our advanced high voltage Distribution transformers incorporate state-of-the art engineering, optimally designed to give you high efficiency & reliability. Our transformers are manufactured in our comprehensive in house facilities, with full control on raw material, design & manufacturing process.
Our Range of Transformers
In cases where the neutral point of three phase system is not accessible like the system connected to the delta connected side of an electrical power transformer, an artificial neutral point may be created with help of a zigzag connected Earthing Transformer. This is a core type transformer with three limbs. Every phase winding in zigzag connection is divided into two equal halves. One half of which is wound on one limb and other half is wound on another limb of the core of transformer. End point of all three windings ultimately connected together and forms a common neutral point. Now if any fault occurs at any of the phases in delta connected system, the zero sequence fault current has close path of circulating through earth as shown in the figure. In normal condition of the system, the voltage across the winding of the Earthing Transformer is 1/V3 times of rated per phase voltage of the system.
But when single line to ground fault occurs on any phase of the system, as shown in the figure, zero sequence component of the earth fault current flows in the earth and returns to the electrical power system by way of earth star point of the Earthing Transformer. It gets divided equally in all the three phases. Hence, as shown in the figure, the currents in the two different halves of two windings in the same limb of the core flow in opposite directions. And therefore the magnetic (lox set up by these two currents will oppose and neutralize each other. As there is no increase in flux due to fault current, there is no extra dcp/dt means no extra voltage induced across the winding and no choking effect occurs to impede the tow of fault current. So it can be concluded like that, the zigzag type Earthing or grounding transformer maintains the rated supply voltage at normal current as well as when a solid single line to ground fault current flows through it.
The rated voltage of an Earthing or grounding transformer is the line to line voltage on which it is intended to be used. Current rating of this transformer is the maximum neutral current in Amperes that the transformer is designed to carry in fault condition for a specific time. Generally the time interval, for which transformer designed to carry the maximum fault current through it safely, is taken as 9 second.
A 3 Ph Star connected Auto transformer with a changeover switch forms an Auto Transformer starter. Initially the stator of the induction motor is supplied with reduced voltage through the Auto Transformer using a suitable tap. This limits the starting current to a safe value. Usually 3 tapings per phase (50%, 65%, 80% or 60%, 70%, 80%) or as per customers requirement are provided to give that % of line voltage across motor terminals. When the motor attains about70 to 80% of normal speed, then the motor is connected in delta connection which cuts off the Auto Transformer from the circuit and full voltage is applied across motor terminals. The loading the system and power loss involved are less in this type and it can be used in delta as well as star connected motors. The provisions of several tappings make the adjustments possible to suit local conditions.
Relation between starting torque and full load torque of an Auto Transformer Let Transformation ratio = N2/N1 = K Starting Current 1st (motor) = K Isc lsc= Starting current at normal voltage So current drawn from the mains = 1st (Mains) =K(Klsc) For motor, Tst/Tf= (Ist/lf) * (Ist/lf) * Sf = K*K*(lsc/lf)*(lsc/lf)*Sf. Sf=Slip at full load Thus with Auto Transformer starting for starting torque of K*K of that obtainable by direct switching Only K x K of short ckt current is taken from the supply. Also the torque obtainable can be varied be selecting suitable tap. The provision of taps helps in adjusting to local conditions. Auto-transformers for motor starting duty, both LT upto 1200 HP/ H.T upto 2500 HP, oil and air cooled constructions are manufactured. The typical duty cycle is 6 starts per hour and less than 20 seconds per start. Duty cycles can be offered to suit various motor characteristics.
There are following types and subtypes of Current Transformers.
1) L.T Tape wound/Cast Resin type CT.
i. Ring/window type and
ii. Wound primary type
2) L.T/ H.T Cast Resin CT.
i. Ring/window type and
ii. Wound primary upto 33 KV with Indoor/ Outdoor application.
3) H.T Indoor / Outdoor Oil Immersed type C.T/ P.T upto 66KV.
Function of Current Transformer
The main function of the Current Transformer is to reproduce the Primary current on its secondary side. The Current Transformer isolates the devices connected to their secondary circuit from the network system. Current Transformers are specially designed for measuring and protection applications.
Standard Terminology used for CT:
Rated primary current:
The Current to be measured and transformed to lower value
Rated secondary current:
The current on the secondary side of the CT Typical values of secondary current are 1A or 5A. For WTI Current Transformers 3 A secondary current is also specified. For special differential protection, secondary currents of 1/ Rt 3 A and 5/ Rt 3 A are also specified.
Accuracy class:
For metering CT s, accuracy class is typically 0.2, 0.5, 1 or 3. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The metering CT has to be accurate from 5% to 120% of the rated primary current at the specified power factor.
Class PS/ PXCT:
In balance systems of protection, CT s with a high degree of similarity in their characteristics is required. These requirements are met by Class PS /PX CT s. Their performance is defined in terms of a knee-point voltage (KPV), the magnetizing current (Imag) at the knee point voltage or 1/2 or 1/4 the knee-point voltage, and the resistance of the CT secondary winding corrected to 75C. Accuracy is defined in terms of the turns ratio.
Knee point voltage:
That point on the magnetizing curve where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current).
Exciting Current:
The current measured in mA@ Knee point Voltage
Secondary Resistance:
The secondary resistance of C.Tr generally measured @ 75 deg C.
A General table indicating the Application of Tests
A number of routine and type tests have to be conducted on CT s before they can meet the standards specified above. The tests can be classified as:
There are following types and Subtypes of V.Tr.
Function of Voltage Transformer
The main function of the Voltage Transformer is to reproduce the Primary Voltage on its secondary side. The Voltage Transformer isolates the devices connected to their secondary circuit from the network system. Voltage Transformers are specially designed for measuring and protection applications.
Standard Terminology used for Voltage, Transformers:
