ADVANCED VOLTAGE AND CURRENT TRANSDUCERS BASIC INFORMATION

Advanced state-of-the-art loss measuring systems utilize a number of voltage and current sensors that have very low or zero phase angle error. Modern Voltage sensors utilize standard compressed gas capacitors connected with various active feedback circuits to minize the phase angle error of the voltage.

Although the compressed gas capacitors are known for stability and extremely low loss, the electronics associated with the divider must be designed to limit drift to acceptable levels in order to meet the accuracy requirements of the standards.

Also, sensing of the current for accurate scaling for transformer loss testing can be done by utilizing one of the following concepts:

a) Zero flux passive design current transformers

b) Two-stage current transformers

c) Amplifier-aided two stage current transformers.

These current transformers operate on the principle of reducing the flux in the active core of the CT to or near zero; thereby reducing the phase angle error associated with the flux into CT core. The use of high accuracy solid state transducers combined with digital readout can improve overall measurement accuracies due to the following factors:

a) Random error due to the limited resolution of analog instruments is virtually eliminated by the use of digital instruments.

b) Technology, such as solid state time division multiplier techniques for measurement of power, can improve accuracy over conventional electrodynamometer type wattmeters.

The accuracy is also improved because of reduced burden on the instrument transformers and reduction in internal phase shifts. Compensation for lead losses can be designed into these devices.

c) Judicious use of electronic circuits, aided by operational amplifiers, can ensure operation of transducers in their optimal operating ranges. This minimizes the error that is dependent upon the input magnitude as a percent of full scale.

d) Computing circuits for summing and averaging of three-phase measurements can be included in the system design to minimize calculation errors. Errors due to incorrect signs and errors due to selfheating are also minimized by these circuits.

PARTS OF VOLTAGE TRANSFORMERS (VT) USED IN GAS INSULATED SUBSTATION (GIS) BASICS AND TUTORIALS

PARTS OF VOLTAGE TRANSFORMERS (VT) USED IN GAS INSULATED SUBSTATION (GIS)
What Are The Parts Of Voltage Transformers (VT) Used In GIS?


VTs are inductive types with an iron core. The primary winding is supported on an insulating plastic film immersed in SF6.

The VT should have an electric field shield between the primary and secondary windings to prevent capacitive coupling of transient voltages.

The VT is usually a sealed unit with a gas barrier insulator. The VT is either easily removable so the GIS can be high voltage tested without damaging the VT, or the VT is provided with a disconnect switch or removable link

69 kV VOLTAGE TRANSFORMER SPECIFICATION EXAMPLE TUTORIALS

69 kV VOLTAGE TRANSFORMER SPECIFICATION EXAMPLE 
Specification Example of 69 kV Voltage Transformer


VOLTAGE TRANSFORMER

Brand:
Rated Voltage (kV): 72.5kV
Rated Frequency (Hz): 60
Type:         EMFC 84
Design:         Outdoor
Basic Insulation Level (kV) : 350kV
Rated primary voltage and ratio 69,000 Grd Y/40,250
Ratio: 350/600 & 350/600:1
Accuracy Class at Standard Burden: 0.3
Thermal Burden Rating: 200 VA @ 300 ambient

15 kV VOLTAGE TRANSFORMER SPECIFICATION SAMPLE TUTORIALS

15 kV VOLTAGE TRANSFORMER SPECIFICATION SAMPLE 
Specification Sample of 15 kV Voltage Transformer



VOLTAGE TRANSFORMER

Brand:
Rated Voltage (kV): 15kV 15kV
Rated Frequency (Hz): 60 60
Type: VEF 24-03 (Single Bushing) VRM-24
Design: Outdoor Outdoor
Basic Insulation Level (kV) : 125 kV 125 kV
Rated primary voltage and ratio 8,400 for 14,400Y (8,400:120) 8,400 for 14,400Y (8,400:120)
Ratio: 70:1 70:1
Accuracy Class at Standard Burden: 0.3 W, X 0.3 W, X
Thermal Burden Rating: 690 VA @ 300 ambient Temp. 690 VA @ 300 ambient Temp.

69 kV COMBINED VOLTAGE AND CURRENT TRANSFORMER SPECIFICATION EXAMPLE TUTORIALS

69 kV COMBINED VOLTAGE AND CURRENT TRANSFORMER SPECIFICATION EXAMPLE
Specification Sample Of Voltage-Current Transformer (69 kV)


Description:
Combined CT-VT consists of a current transformer portion located at the top of the unit and a voltage transformer portion in a tank at the base. External insulation consists of a one piece post type insulator.  All external parts are of corrosion – resistant material. The combined CT–VT shall be hermetically sealed.

For Oil-Filled Type:
The unit shall be hermetically sealed and of the minimum oil-filed type and compact design. All sealing shall be located below the oil level. The expansion room shall be of a gas cushion type filled with nitrogen. Oil level should be of the reflection type and without moving parts. Primary terminals shall be suitable for connection of copper or aluminum connectors. The external ferrous parts shall be of hair pin type insulation consisting of oil-impregnated paper and capacitor layers for voltage grading. It should be preferably provided with a capacitance voltage tap through out thru an insulated, factory grounded, bushing for checking the condition of its primary insulation. It should have a high seismic withstand capability of 0.5G. The unit must be able to be tilted to 60 deg. C.

For Gas Type:
The primary and secondary winding of the SF6 Gas Insulated shall be housed in a non-corrosive cylinder hermetically sealed type, preventing any contact of the SF6 with the atmosphere by means of separate flexible diaphragm.

The instrument transformer shall have the following built-in devices:
• A Pressure Relief Device that is set to trigger at the designed over-pressure value.
• An SF6 Gas Pressure Indicator with pressure graduations showing the Normal and High/Low danger levels of SF6 Gas Pressure.
• Pressure switches with two (2) sets of NO/NC contacts for  connection with the Substation Alarm/Protection Circuits.
   - One (1) sets of contacts calibrated to trigger at Low Pressure Alarm
     Level.
- One (1) sets of contacts calibrated to trigger at Low Pressure Danger
  Level.
Type Outdoor Type, Minimum Oil-filled or Gas Insulated
Cooling Oil-immersed/ Gas insulated, self-cooled
Model Combined VT/CT
Construction Single phase, inductive type, single bushing
Termination Line-to-ground
Accuracy Class 0.3 or better thru burden W up to Y for both cores for VT, Extended Accuracy Range
0.3 or better thru B-0.1 up to B-1 for  both cores for CT
Compliance to Standard ANSI/IEEE C57.13 or applicable IEC
Nominal System Voltage, KV 69
Maximum Continuous System Voltage, KV 72.5
BIL 350 KV, 60 Hz
Minimum Creepage Distance 1380 mm
Number of Core for VT Two (2)
Single Ratio (L-G) 40250V : 115V
PTR 350 : 1
Rated Secondary Voltage 115V
Number of Core for CT Two (2)
Rating Factor 1.5 at 30oC
Accuracy Range 1% up to 150 %
CTR (Double Ratio) (To be computed based on the actual load)
Rated Secondary Current 5 A
Rated withstand and test voltage, KV
• Low frequency withstand, KV RMS
• Impulse  lightning  withstand, KV crest

140

325
Short Time Rating Current (per IEC)
1. Thermal Ith, KA
2. Dynamic, Idyn, KA
22
55
Mounting Pedestal
Rated Frequency, Hz 60
Post Insulator Characteristics
1. Voltage Class, KV
2. Color
3. Creepage Length, mm
72.5
Chocolate brown (preferred)
1380

Accessories
• 1-set Primary Connectors, clamp type terminal of nickel-plated brass for horizontal connection of round aluminum or copper conductors.
• Secondary Connectors, 1 set- M10 split studs with 3.2 mm slot suitable for conductors of 8 mm2 across section with nuts for connecting cable lugs
• Grounding Connectors split studs, 1 set-Earthing clamp for a round conductors or line of 5-16 mm diameter.
• There should be provisions for the installation of security seals on the secondary terminal box. (e.g. seal holder, etc.)
REQUIREMENTS/CONDITIONS
• Factory test results and Type Approval Test Report of the combined CT-VT from an independent regulating body or international organizations.
• Field reference, catalogues, drawings, hardware and instruction user’s manual.
• Declaration and proof that the manufacturer should have been in the business of manufacturing the equipment of not less than ten (10) years.

CAPACITOR INRUSH/ OUTRUSH REACTORS BASIC AND TUTORIALS

CAPACITOR INRUSH/ OUTRUSH REACTORS BASIC INFORMATION

What Are Capacitor Inrush/ Outrush Reactors?

Capacitor switching can cause significant transients at both the switched capacitor and remote locations.

The most common transients are:

• Overvoltage on the switched capacitor during energization

• Voltage magnification at lower-voltage capacitors

• Transformer phase-to-phase overvoltages at line termination

• Inrush current from another capacitor during back-to-back switching

• Current outrush from a capacitor into a nearby fault

• Dynamic overvoltage when switching a capacitor and transformer simultaneously

Capacitor inrush/outrush reactors (Figure 2.9.15) are used to reduce the severity of some of the transients listed above in order to minimize dielectric stresses on breakers, capacitors, transformers, surge arresters, and associated station electrical equipment.

High-frequency-transient interference in nearby control and communication equipment is also reduced. Reactors are effective in reducing all transients associated with capacitor switching, since they limit the magnitude of the transient current (Equation 2.9.5), in kA, and significantly reduce the transient frequency (Equation 2.9.6), in Hz.

where

Ceq = equivalent capacitance of the circuit, F

Leq = equivalent inductance of the circuit, H

VLL = system line-to-line voltage, kV

Therefore, reflecting the information presented in the preceding discussion, IEEE Std. 1036-1992, Guide for Application of Shunt Power Capacitors, calls for the installation of reactors in series with each capacitor bank, especially when switching back-to-back capacitor banks.

Figure 2.9.16 shows a typical EHV shunt-capacitor installation utilizing reactors rated at 550 kV/1550 kV BIL, 600 A, and 3.0 mH

550-kV capacitor inrush/outrush reactors.

CONSTANT VOLTAGE TRANSFORMER BASICS AND TUTORIALS

CONSTANT VOLTAGE TRANSFORMER BASIC INFORMATION

What Is a Constant-Voltage Transformer?

A well-known solution for electrical “noise” in industrial plants has been the constant-voltage transformer, or CVT.

The typical components of a CVT are shown in Figure 2.8.2. The magnetic shunt on the central core has the following effects on the core’s reluctance. It reduces the reluctance of the core.

 This can be thought of as introducing more resistance in parallel to an existing resistance. The magnetic shunt in the CVT design allows the portion of the core below the magnetic shunt to become saturated while the upper portion of the core remains unsaturated.

This condition occurs because of the presence of the air-gap between the magnetic shunt and the core limbs. Air has a much higher reluctance than the iron core.

Therefore, most of the flux passes through the lower portion of the core, as shown by the thick lines in Figure 2.8.2.

In terms of an electrical analogy, this configuration can be thought of as two resistances of unequal values in parallel.

The smaller resistance carries the larger current, and the larger resistance carries the smaller current.

The CVT is designed such that:

• The lower portion of the central limb is saturated under normal operating conditions, and the secondary and the resonating windings operate in the nonlinear portion of the flux-current curve.

• Because of saturation in the central limb, the voltage in the secondary winding is not linearly related to the voltage in the primary winding.

There is consonance between the resonating winding on the saturated core and the capacitor. This arrangement acts as a tank circuit, drawing power from the primary. This results in sustained, regulated

oscillations at the secondary with the applied line frequency.