Er. K. MOUNAGURUSAMY Design of a Potential transformer is similar to that of a power transformer, but the loading of the Potential transformer is only a few VA. Accuracy in design is more important than efficiency.
Due to short circuit fault, the transient D. C. component in the line voltage is less and that too occurs when the A. C. component is low, therefore a Potential transformer is not required to be oversized to handle the transient conditions.
Voltage drop in the secondary leads should be well taken into account. This can seriously falsify the accuracy of measurement. Take the case of V. T. with burden of 300 VA
in 57.7 V secondary and the lead resistance (for Exp. 100 m, double core 2.5 mm2 RL= 1.44 ohms)
Current in P. T. = 300/57.7 = 5.2 A •
• • Voltage drop = 5.2 x 1.44 = 7.5 V.
This accounts to 13%
Such drops will make the distance relays over reach by equal percent. A serious problem. Hence 6 Sq.mm cables are advised for P. T. circuits.
CAPACITANCE VOLTAGE TRANSFORMER
PERFORMANCE POINT OF VIEW:
The transient response of the CVT depends on the following : 1. Point on the primary voltage wave where the fault occurs.
2. The value of equivalent capacitance, which is dependent on three items namely Capacitor rating, Tap position, Turns ratio of the Intermediate voltage transformer.
3. Magnitude and power factor of the burden.
Composition and connection of the burden for the same burden and power factor the burden can be made up of parallel and series components.
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4.Type of ferro resonant damper circuit.
For well designed transformer the exciting current of intermediate transformer is purposely kept low of the order of 2 to 3 ma and does not adversely affect the response.
To see the effects of various important parameters, it is assumed that a single phase ground fault occur at the CVT location itself. Only resistance burden is considered. Two extreme conditions (a) fault at peak of voltage and (b) fault at zero point on the primary voltage wave.
i1 (O) = V1max/R4
Vc (O) 1 0
On analysis, the following conclusion are arrived.
For fault at the peak of voltage the transient component decays very fast.
For fault at zero point on the voltage wave transient will decay slowly.
The magnitude of the transient output voltage (for fault on zero point of voltage wave) will decrease with increase in equivalent capacitance value but duration will be increased due to corresponding decrease in value of equivalent conductance value. With the increase in burden (decrease in the magnitude of the resistine burden) CVT performance will further worsens for faults at the zero point of the wave.
The voltage transformers are normally connected phase to earth. In the event of disturbance in the network the voltage across the VT’s (CVT’s ) will be increased in the healthy phases.
IEC specifies the voltage factors:
19 for systems not having solidly earthed 15 for solidly earthed system.
The saturation is specified to be 30sec. for systems with tripping earth fault protection and 8 hours if no earth fault tripping protection is used.
The VT’s must not be saturated at the voltage factor.
For metering cores a high accuracy for voltages in range (80 - 120%) of nominal voltage is required.
For protection where disturbance information must be transferred to the secondary side, a lower accuracy is required but a high capability to transform voltages to allow the protection to operate and disconnect the fault. Class is 3p.
The 3p class will have measuring error of 3% and an angle of 120 min.
The voltage transformer winding can be given a continued class ie 0.5/3p which means that metering accuracy is fulfilled for 80-120% of nominal voltage but the requirement for 5%
of nominal voltage and the transient response requirement from protection cores is also fulfilled.
A good transient response is required to the protection transformers and this is a problem for CVT’s where the energy stored in the capacitive voltage divider and the interposing voltage transformer will result in a transient voltage oscillation on the secondary side. The transient oscillation consists of a low frequency components. (2-15Hz) and high frequency oscillation (900-4000Hz). The time constant for the high frequency part is short where as the low frequency part has long time constants. The amplitude is decided by the fault inception angle. Higher capacitances in the voltage divider gives lower amplitude of the low frequency oscillation. The secondary value, one cycle after the short circuit should be lower than 10%.
Ferro resonance can occur in circuits containing a capacitor and a reactor incorporating an iron core (a non-linear inductance). Both the CVT and a magnetic VT can be involved in Ferro resonance phenomenon.
Ferro resonance in a magnetic VT is an oscillation between the inductance of the VT and the capacitance of the network. Ferro resonance can only occur at ungrounded networks, but note the risk that some part becomes ungrounded under certain circumstances.
An oscillation is normally triggered by a sudden change in the network voltage.
Ferroresonance phenomenon can occur both with sub-harmonic frequencies or with harmonic frequencies. Generally it is difficult to state when a risk of ferro resonance occurs but as soon as a system with a voltage transformer is left ungrounded under some circumstances, preventive actions should be taken (also consider the risk of capacitive charged systems with a VT). The damping of Ferro resonance is normally done with a 27 – 60 ohms 200 W resistor connected across the open delta winding. The resistor value should give a current as high as possible but a current below the thermal rating of the voltage transformer.
The CVT with its capacitor and IVT is by itself a ferro-resonance circuit. The phenomenon is started by a sudden voltage change. A sub – harmonic oscillation can be started and must be damped to prevent damage to the transformer. The CVTs must be provided with ferro resonance damping devices, normally this consists of a saturating reactor and a resistor in each phase.
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COMMISSIONING INSTRUCTION FOR CVTs:
Test Equipments :- Meggar 0.5 or 1 KV
Battery box (with atleast three voltage levels).
Multimeter class 1.5 Phase sequence meter Test leads etc.,
SAFETY
PRECAUTIONS:-The transformer to be tested is to be checked for primary earthing before commencing the test. If it is not earthed, disconnected bus bars and lines can attain dangerous high voltage levels due to induction from energised parallel tying busbars or lines.
TEST
PROCEDURE:-1. Data and serial numbers, given in the rating plates of the transformer are to be entered in test record. Ensure that the serial number of each unit of the capacitor voltage divider is the same as that given on the main rating plates of the each respective transformer.
The accuracy classes given for the transformers apply only if the units, that are jointly trimmed at the workshop are mounted together at the site.
Check the oil level in the VT.
Check the connection to the voltage direction of any carrier frequency equipment. The coupling unit of the PLC equipment should be connected to the terminal HF which is the low voltage terminal of the capacitor stack. On delivery of the transformer this terminal normally earthed.
If PLC equipment is to be connected to the line, the direct earth connection is to removed and the voltage divider earthed via the connection unit.
When a transformer is energised, the earthing switch must be closed before any work is done in the marshalling kiosk. It is extremely dangerous to open the low voltage earth connection of an energised transformer. If no PLC equipment is connected, make a check to ensure that the low voltage terminal is earthed.
The secondary terminal box is to be visually checked for
1. That any spare winding that are not connected to the common marshalling kiosk, are open and earthed at one point.
2. That the correct terminals and connected in the core of multi terminal secondary windings.
POLARITY
CHECK:-The check is made by connecting a battery having predetermined polarity, for short between transformers primary terminal and earth. The polarity of the voltage, induced in the secondary, is checked with the aid of a directionally sensitive voltmeter. The test is similar to the polarity test of CTs. ie by checking the deflection of the pointer in the secondary connected voltmeter when the switch is operated in the primary side.
CHECKING INSULATION
RESISTANCE:-Checking the insulation resistance of the windings and of the voltage circuits between the transformers and the marshalling kiosk.
Check the phase to earth and the phase to phase insulation values when checking the phase to earth resistance, only the earthing terminal block, if the winding being tested os to be open. All phase terminal blocks of the voltage transformers are to be closed.
CHECKING PHASE
RELATIONSHIPS:-To be performed when the new voltage transformer and the voltage transformer of the reference group have separate infeed on the primary side if they are not connected together.
To be performed when the new voltage transformer and the voltage transformer of the reference group is connected together on the primary side in the sub station.