A voltage $V_1$ is measured $100$ times and its average and standard deviation are $100$ V and $1.5$ V respectively. A second voltage $V_2$, which is independent of $V_1$, is measured $200$ times and its average and standard deviation are $150$ V and $2$ V respectively. $V_3$ is computed as: $V_3$=$V_1$+$V_2$. Then the standard deviation of $V_3$ in volt is_____.