A modular approach to handle asynchronous heterogeneous measurements in distribution system state estimation

State estimation in power systems is a fundamental task, and the state estimator has become an essential component in power system monitoring, even at the distribution level. Due to the limited number of measurement devices in the system, the available number of measurements is quite low. To address this issue and make real-time state estimation feasible and less complex at the distribution level, measurements from various types of sensors, such as micro-phasor measurement units ($\mu$PMUs), remote terminal units (RTUs), and smart meters (SMs), need to be utilized. However, the measurements received from these different types of devices are asynchronous, meaning they are not synchronized to a central clock. In this paper, a modular two-level state estimation algorithm is proposed, which processes hybrid measurements from $\mu$PMUs, RTUs, and SMs, taking into account the asynchronous nature of the measurements. Additionally, the developed algorithm considers the different refresh rates of these measurement devices when combining the measurements to evaluate the states of the system. The effectiveness of the algorithm is then verified using the IEEE 33-bus and 123-bus systems.