The development and qualification of the insulation monitoring device (IMD) for subsea industrial application is presented in this paper. This device can be employed with the required high integrity electrical/electronic components and manufacturing processes demanded of the subsea equipment domain. Further, in order to meet the challenges of improved operational reliability, the requirement to detect and diagnose electrical faults within this environment is of utmost importance with the improved safety and operational availability.

The insulation monitoring device measures insulation levels between a non-grounded single or three phase 50 or 60 Hz supply network and its protective earth, regardless of the actual networks state of operation; is live or non-live (standby). Insulation measurement is based on the principle of injecting a direct current (DC) voltage through a shunt into the alternate current (AC) line through a high impedance serial choke that "insulates" the AC from the internal DC voltage. The 16 bit analogue to digital converter (ADC) reads the voltage balanced over the shunt resistor and gets a raw value that follows logarithmically the earth fault resistance from measurement line to protective earth (PE). The device continuously measures the changes in insulation level and any insulation breakdown is anticipated by monitoring the trend data. Thus, it is an important tool for preventive maintenance and asset management strategies.

The IMD device is capable of communicating with the topside systems through the Modbus communication protocol and provides output in terms of 4–20mA and 0–10V instrumentation signals. The IMD complies with the low voltage directive, subsea standard ISO13628-6 and subsea relevant electromagnetic compatibility standard. Moreover, this device fulfils the criteria of IPC-A-610E class 3.

A nonlinear mathematical transfer function has been developed through the offline estimation technique in SIMULINK/ Matlab® to represent the behaviour of IMD card in terms of insulation resistance and 4 – 20 mA output current.

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