Transportation electrification involves the wide utilization of power electronics based DC distribution networks and the integration of a large amount of power electronic loads. These power electronic loads, when tightly controlled, behave as constant power loads (CPLs) and may cause system instability when interacting with their source converters. In this paper, a composite nonlinear controller is proposed for stabilizing DC/DC boost converter feeding CPLs by integrating a nonlinear disturbance observer (NDO) based feedforward compensation with backstepping design algorithm. First, the model is transformed into the Brunovsky's canonical form using the exact feedback linearization technique, to handle the nonlinearity introduced by the CPL. Second, the NDO technique is adopted to estimate the load power variation within a fast dynamic response, serving as a feedforward compensation to increase the accuracy of output voltage regulation. Then a nonlinear controller is developed by following the step-by-step backstepping algorithm with strictly guaranteed large signal stability. The proposed controller not only ensures global stability under large variation of the CPL, but also features fast dynamic response with accurate tracking over wide operating range. Both simulations and experiments are conducted to verify the proposed strategy.

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Constant failure rate model used in the conventional reliability assessment of power systems cannot reflect the impacts of the various operating conditions such as transformer loading, ambient temperature, weather on component reliability. A hybrid conditions-dependent outage model (CDOM) of a transformer is proposed in this paper to include those impacts. The CDOM is the combination of three failure models: the aging failures due to the loss of mechanical strength of conductor insulation; the random failures considering weather conditions, and the outages caused by the direct trips of the overload protections. The component reliability using the proposed model has been tested under different operating conditions. The model is also applied in power system operational reliability assessment. The reliability indices using CDOM are compared with that using the condition-independent outage model. The reliability indices using the CDOMs of transformers can provide useful information for operators to understand possible system and component failure risk in real-time operation and to make important alleviation decisions.Index Terms-Condition-dependent outage model (CDOM), operational reliability, power system, reliability assessment, transformer.

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