Basler Electric FGD-2 Field Ground Detector

Introduction

The FGD-2 Field Ground Detector Module is a transducer that continuously

monitors the resistance of the field circuit of a synchronous machine with

respect to ground. It applies a fixed voltage with respect to ground which

causes a small leakage current to flow. The FGD-2 provides a low voltage

output signal proportional to the leakage current.

The voltage output is measured by a compatible product and used for

field ground protection. Two versions of the FGD-2 are available. A low-

voltage version, rated for 800 Vdc maximum generator field voltage, is

provided for brush or brushless exciter systems. A high-voltage version,

rated for 1.750 Vdc generator field voltage, is provided for static exciter

systems.

The metal enclosure of the FGD-2 offers electric shock protection from

incidental contact. A compatible footprint and identical functionality

make for simple replacement of a Field Ground Detector (FGD) with the

FGD-2.

Installation

The FGD-2 is typically preinstalled in an excitation control system.

If replacement is necessary, carefully label and document each of the

connections to the FGD-2 in order to properly reconnect the new module.

FGD-2 connectors and indicators are illustrated in Figure 1. Locator

letters in Figure 1 correspond to the lettered descriptions in Table 1.

When a replacement FGD-2 is installed in a DECS-2100 or ECS2100

system, the system calibration values may need to be updated.

Retrofitting an FGD

Although the FGD-2 is smaller in size than the FGD, it may still be mounted

in place of an FGD using only four of the six existing mounting holes.

Either the upper four or the lower four mounting holes may be used.

FGD-2 Connections

Connectors with screw-down compression terminals are used for all FGD-2

connections. The connectors, and the headers that they plug into, have a

dovetailed edge that ensures proper connector orientation. Similar-sized

connectors and headers are uniquely keyed to ensure that a connector

mates only with the correct header to prevent damage to the FGD-2.

However, care must still be taken to ensure that the proper connector is

inserted into the appropriate header. Connector screw terminals accept a

maximum wire size of 8 AWG. The maximum screw torque is 7.1 in-lb or

0.8 N•m. Figure 2 illustrates typical FGD-2 connections.

The FGD-2 was designed to be connected between F+ and ground, but

it is also compatible with applications where only F– is available.

Figure 3 illustrates this alternate connection option.

Operation

The FGD-2 continuously monitors the machine field circuit for ground faults.

Ground faults are detected through the measurement of leakage current to

ground which is determined by measuring the voltage drop across a shunt

resistor within the ground detector source. The Basler device connected to

the FGD-2 receives a voltage signal from the ground detection source that

is proportional to the level of field leakage current. The excitation

capabilities and analog input ranges of different Basler products will limit

the maximum compatible field voltage for accurate field ground resistance

measurements. Refer to Table 2 for more details.

AIOM-2

In a DECS-2100 system, the AIOM-2/AIOM digitizes the signal and transmits

it via fiber optic cables to the ECM-2. In an ECS2100 system, the digitized

signal is received by the ECM/SIM. The value of leakage current appears as

an input to the BESTCOMS™Pro FLDGND logic block. Within the FLDGND

logic block, a value of resistance to ground is calculated from the voltage

applied and the level of leakage current measured. Typically, this calculated

resistance value is transmitted to a display panel via the PMOUT logic block.

In the event of a low field ground resistance value, the FLDGND logic block

issues a Low Field Ground Resistance alarm. During detection of low field

ground resistance, an alarm issued by the DIGROUT logic block can be used

to energize a relay within the system’s Digital I/O Module (DIOM-2 or DIOM).

This annunciation may be used to provide a dry relay contact for a remote

alarm.

BE1-FLEX

The FGD-2 output can be connected to any analog input of the BE1-FLEX.

Field ground protection settings can be configured in BESTCOMSPlus.

Ground Detection Source

The ground detection source supplies approximately –60 Vdc to the positive

side of a machine field and measures the level of leakage current to ground.

The positive side of the field bus is held negative with respect to ground.