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.