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Modeling of VSC-based multi-terminal DC (MTDC) grids for modal analysis and stability studies is reported considering bipolar converters and DC cable network with metallic return path. The proposed model is flexible enough to accommodate different converter grounding options and unbalance on the DC side due to cable and/or converter outage. A simplified version of the benchmark test system for the envisioned offshore MTDC grid in the North Sea is studied. Modal participation factor analysis under nominal and converter outage conditions is used to ascertain the nature and root cause of the dynamic responses. Comparison of the time-domain simulation results using the proposed averaged model in Matlab/SIMULINK against a detailed switched model in EMTDC/PSCAD confirm the accuracy of the modeling approach.