Improved Three-phase Power Flow Model for Ungrounded Distribution Network Considering Controllability of Line-to-line Voltage

Ungrounded distribution networks have two main features: 1) there is no path for zero-sequence current; 2) the endpoint element either has no neutral point or, even if with one, is without a neutral line. Because of the first feature, zero-sequence input current of an endpoint must be equal to 0, and because of the absence of neutral point or neutral line, the endpoint element is characterized by immeasurability of the phase-neutral point voltage or inadvisability of control on the phase-to-ground voltage. Therefore, the endpoint of power source in an ungrounded distribution network usually takes the line-to-line voltage as the objects of measurement and control rather than the phase-to-ground/phase-to-neutral point voltage of the endpoint in an actual ungrounded distribution network, which is called the line-to-line voltage controllability. By taking both features into consideration, this paper tries to improve the existing three-phase power flow model for ungrounded distribution network. First, an equivalent neutral point of the slack node is obtained and treated as the common reference point for phase voltages to serve as an improved model for the slack node. Then an improved model for active power-voltage node with the total active input power and line-to-line voltages as control objects is proposed based on the two watt-meter method. Finally, the improved three-phase power flow model with line-to-line voltage controllability and wye-connected/delta-connected load is proposed. The modified IEEE 13-bus distribution network is tested to verify the improved three-phase load model proposed. © 2017 Automation of Electric Power Systems Press.