How to remove the equation number from both lines when equation is split in two lines?

Question

I am writing an optimization problem for a scientific paper using IEEE tran conference library. I am splitting long equations into 2 lines but then, latex is putting equation number on both lines. Below is the MWE related to my problem. Eq 1b and 1c is just one equation; similarly 1d and 1e is just one equation. I want to write them in two lines but want equation number only one the second line.

\documentclass[conference]{IEEEtran}

\usepackage{amsmath}
\usepackage{amssymb}
\begin{document}

\begin{subequations}
\begin{alignat}{2}
& (P1) \hspace{0.5em}\!\min_{\vartheta}  &\qquad& f(\vartheta)    \label{eq:OF}\\
&\text{subject to} &      & V_i^\varphi I_i^{*\varphi} =     (P_{g_i}^\varphi-P_{l_i}^\varphi)+j(Q_{g_i}^\varphi-Q_{l_i}^\varphi),\label{eq:PBG}\\
&                  &      & \forall \varphi \in \phi_{i}, i \in G\\
&                  &      & V_k^\varphi I_k^{*\varphi} = (-P_{l_k}^\varphi -jQ_{l_k}^\varphi)+jy_{c_k}^\varphi|V_k|^2,\label{eq:PBL}\\ 
&                  &      & \forall \varphi \in \phi_{k}, k \in N\backslash \{G\}\\
&                  &      & (\underline{V}_k)^2 \leq |V_k^\varphi|^2 \leq (\overline{V_k})^2, \forall \varphi \in \phi_{k}, k \in N,\label{eq:Vol}\\
&                  &      & \underline{P}_{g_i} \leq P_{g_i}^\varphi \leq \overline{P}_{g_i}, \forall \varphi \in \phi_{i}, i \in G,\label{eq:GenP}\\
&                  &      & \underline{Q}_{g_i} \leq Q_{g_i}^\varphi \leq \overline{Q}_{g_i}, \forall \varphi \in \phi_{i}, i \in G,\label{eq:GenQ}
\end{alignat}
\end{subequations}
\end{document}

Answer 1

Does this, with the aligned environment, solve your problem? I added some improvements to the overlines and underlines, and had to use a smaller space between the two column to fit line width:

\documentclass[conference]{IEEEtran}

\usepackage{mathtools}
\usepackage{amssymb}

\begin{document}

\begin{subequations}
\begin{alignat}{2}
& (P1) \;\min_{\vartheta} &\quad& f(\vartheta) \label{eq:OF}\\
&\label{eq:PBG}\mathrlap{\begin{aligned}[b]\text{subject to} \quad\, V_i^\varphi I_i^{*\varphi} & = (P_{g_i}^\varphi-P_{l_i}^\varphi)+j(Q_{g_i}^\varphi-Q_{l_i}^\varphi),\\
 &\phantom{{}=(} \forall \varphi \in \phi_{i}, i \in G
\end{aligned}}\\[0.5ex]
& & & \label{eq:PBL}\begin{aligned}[b] V_k^\varphi I_k^{*\varphi} & = (-P_{l_k}^\varphi -jQ_{l_k}^\varphi)+jy_{c_k}^\varphi|V_k|^2,\\
 & \phantom{{}=(}\forall \varphi \in \phi_{k}, k \in N\setminus \{G\}
\end{aligned}\\[0.5ex]
& & & \label{eq:Vol} (\underline{V\!}_{\mkern2mu k})^2 \leq |V_k^\varphi|^2 \leq (\overline{V}_{\mkern-5mu k})^2, \: \forall \varphi \in \phi_{k}, k \in N,\\[0.5ex]
& & & \underline{P\!}_{\mkern2mu g_i} \leq P_{g_i}^\varphi \leq \,\overline{\! P\mkern1mu}_{\!g_i}, \:\forall \varphi \in \phi_{i}, i \in G,\label{eq:GenP}\\[0.5ex]
& & & \underline{Q\mkern-2mu}_{\mkern1mu g_i} \leq Q_{g_i}^\varphi \leq\mkern2mu \overline{\mkern-2mu Q}_{g_i}, \:\forall \varphi \in \phi_{i}, i \in G,\label{eq:GenQ}
\end{alignat}
\end{subequations}

\end{document}