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.

TRANSFORMER INRUSH EFFECTS STUDIES DOWNLOAD LINK

DOWNLOAD LINK OF STUDIES ON TRANSFORMER INRUSH EFFECTS
Transformer Inrush Studies Link


Introduction
In a typical UK wind farm a series of radial 33kV collector circuits run from the main switchboard and link together individual wind turbine generator (WTG) transformers. At the design stage it is necessary to determine the maximum number of WTG transformers that can be energised simultaneously from the 33kV system.

One of the factors to be considered is the voltage dip experienced at the point of common coupling (PCC) or interface between the electrical system of the wind farm and the utility company. The UK standard applied is the Electricity Council’s Engineering Recommendation P28, which allows a 3% voltage dip. This article describes wind farm transformer inrush analysis studies the Glasgow based power systems consultants Mott MacDonald have undertaken using PSCAD to demonstrate compliance with P28.

Transformer inrush
When a transformer is energised, it may draw a high magnitude transient current from the supply causing a temporary voltage dip. This current, characterised as being almost entirely unidirectional, rises abruptly to its maximum value in the first half-cycle and then decays until the normal steady-state magnetizing conditions are reached. The magnitude and duration of the inrush current depends upon the following all of which can be represented using a PSCAD model:

the point on the voltage wave at the instant the transformer is energised (i.e. switching angle);
the impedance of the supply circuit;
the value and sign of the residual flux linkage in the core;
the non-linear magnetic saturation characteristic of the core.

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