power at bus $i$. The variable $G_{ik}$ is the real part of the element in the bus admittance matrix corresponding to the $i$-th row and $k$-th column, and $B_{ik}$ is the imaginary part of the element. In Equations (\[networkflow1\]) and (\[networkflow2\]), $\phi_{ik}$ is the voltage angle between the $i$-th bus and the $k$-th bus, while $|v_i|$ and $|v_k|$ are the voltage magnitudes at bus $i$ and bus $k$, respectively. The stress on the voltage and frequency imposed by concentrated charging at EV charging station is expected to be significant. However, good models to calculate and penalize this stress are not yet considered.

In [@tong]-[@eric], the authors have proposed different frameworks to coordinate EV charging to ensure stable and economical operation of power grid. In this work, we will consider how to alleviate the “stress" added to the power grid by EV charging when determining the optimal charging station deployment.

Note that the voltage and system frequency are the two important factors of power quality in the power grid. The imbalance of the active power will lead to frequency drift, while the imbalance of the reactive power will cause the voltage variation [@powerstability]. Specifically, if the active power needed by the 