Due to the complicated structure of conventional signed directed graph(SDG)model of complex thermal system

problems like combinatorial explosion in search paths and difficulty in distinguishing multiple fault sources occur easily during the diagnosis.Thus

the hierarchical probability signed directed graph(SDG)model was proposed

which uses the layered method to simplify the search paths and describes the causal relationship between failures by applying a combination method of qualitative and quantitative method.Moreover

a new fault diagnosis method based on the hierarchical probabilistic signed directed graph was put forward.This method takes advantage of the established SDG model to reduce the search space first

and then employs the compatibility and Bayesian inference to diagnose the faults

at last

it calculates the probability of failure and possible sequence of the source.Finally

this method was applied in failure diagnosis of high pressure heater system of a 300 MW unit.The results show that

this method not only improves the fault source resolution but also reduces the search space

which avoids the "combinatorial explosion".