Page 65 - impiantistica_3_2015

Table 2 - Technical features of the input step-down isolation power transformer

Design kV converter transformer
Type Hz oil immersed
Duty type % continuous S1
Windings material V copper
Cooling type kVA Onan
Primary winding rated voltage 33
Rated frequency % 50
Tap changer / type % ±4 × 2.5 / off-load
Secondary winding rated voltage dB(A) 4 × 1800
Rated power °C 10400
Connection group °C Dd0 / d0,5 / y11 / d11,5 (or equivalent)
Short circuit voltage mm 7
Efficiency at full load °C 98,5
Noise level 75
Temperature rises oil 45
Temperature rises windings 50
Overall dimensions: L x W x H 4800 × 4300 × 3800
Installation area / max design temperature Outdoor - safe / +55
Oil type Mineral

Fig. 5 - Variable frequency converter, type SVTN, mounted inside the Local Equipment On its front end section, the converter receives
Room (LER) the power supply from the medium voltage line by
the secondary winding of a step-down insulation
power transformer through a 24 pulse diode
rectifier. On its motor side, the IGBT based inverter
section uses a NPC voltage source configuration
controlled by PWM technology.
The converter primary coolant is deionized water,
flowing in closed loop by redundant pumps through
a water-to-water heat exchanger. Its secondary
coolant is cooled down to 25 °C by an external fully
redundant chiller.
The control unit includes all the electronic devices
and cards required for the digital control of the
system: power supplies, main CPU (Control Power
Unit) board, modulator and interface boards (IGBT
firing pulses generation, currents and voltages
transduction etc.) serial communication boards,
integrated PLC (Programmable Logic Controller)
etc. Control circuits are fully insulated from power
circuits: Hall effect transducers are used for current
and voltage feedback signals while firing signals are
sent to the controlled rectifiers by optical fibers.
The main electrical and mechanical features of the

Impiantistica Italiana - Maggio-Giugno 2015 63