Systems & Electronics

The protection relay ZFD8N belongs to SIGMA-N digital protection line and it performs one or more of the functions listed below:

All the functions related to the ANSI codes 67-50-51 , 67N-51N , 59N-59Vo and 79 are available and activable at the

same time to guarantee with a single protection relay all the functions required for:

• protection of ring mains
• protection of MV feeders

The directional earth fault function is used in electrical systems with:

• unearthed neutral
• neutral solidly earthed
• neutral earthed through a resistor
• via earthing transformer
• neutral earthed through Petersen coil

The under-impedence directional or non-directional function (ANSI 21) can be activated as an alternative to the previous functions.

All the functions of the relay are fully programmable by a front panel keyboard or through a RS485 serial interface using a personal computer with a set-up program; furthermore the relay can be programmed through a remote control and monitoring system.

Thresholds – the ZFD8N relay manages the following thresholds:

• 3 directional or non-directional overcurrent thresholds
• 3 directional or non directional earth fault thresholds (earth current and residual voltage)
• 1 residual overvoltage threshold

or

• 3 directional or non-directional under-impedence thresholds
• 3 directional or non directional earth fault thresholds (earth current and residual voltage)
• 1 residual overvoltage threshold

The first threshold related to 67-50-51 functions and to 67N-51N functions can be programmed as definite time or

dependent time in compliance with BS-142 and IEC 255-4 specifications (the time delay is a function of the measured

current and of the related threshold value). The remaining thresholds are definite time.

Each definite time delay can be combined with an additional timer controlled by the digital inputs (protection schemes for ring mains). The trip of the relay is shown and memorized by

LEDs and by a message on the display