Control of Smart Distribution Networks for Voltage Correction and Transmission Network Support

By Claude Ziad El-Bayeh and Khaled Alzaareer

Distribution networks (DNs), are generally operated using traditional techniques without any smart or monitoring devices. With the large - scale integration of distributed generation (DG) in the distribution power grids, many new opportunities have been evolved for voltage control. Moreover, due to the flexible operation of DG units, active distribution networks are requested to provide ancillary services by exporting reactive power to the transmission networks. In this regard, applications of smart grid technologies are urgently required for voltage control in DNs to achieve the best services presented by DG units while maintaining safe system operation.

The necessity to include new technologies for voltage control in DNs and transmission network supply can be summarized as the following reasons:

• To avoid the uncertainty in load demand and power produced by DG units.
• To easily capture any disturbance in the network.
• To successfully implement DG units in real-time voltage control without operational conflicts with traditional voltage control devices such as On-load Tap Changer and capacitors.
• To reach the target voltage level of DNs and meet the requested amount of reactive power transfer between transmission and distribution networks in an effective manner.
• To meet the requirements imposed on the connection between the two networks.

Many voltage control schemes have been proposed for voltage regulation in DNs. Optimization-based methods are one of the main control techniques, which are used to obtain the optimal operation of DNs such as, loss minimization and voltage correction. Some researchers have proposed optimization problems to adjust the outputs of DG units for voltage control in DNs in a 24-hour schedule. To take into consideration the uncertainty in load demands and power generated by renewable energy units, the concepts of state estimation of voltages and forecasting the weather and demand are included in the voltage control schemes.

Although the system, with the aid of estimator, attempts to meet the requested change in voltage control variables, load and generation uncertainty, and network disturbances still require performing additional actions to regulate the network voltages and reactive power exchange. The lack of monitoring tools reduces the ability of DNs and DG units in voltage correction while complying with security requirements. Therefore, introducing smart grid technologies in DNs and feeding the voltage control scheme with real-time measurements help network operators to avoid any violence in system constraints.

Regarding the transmission network support, there is still limited research on utilizing DG units for controlling the reactive power exchange between transmission and distribution networks. Some of the researches are focused on maximizing the reactive power output by DG units. However, this method solves the problem with no guarantee that the requirements at the connection point between the two networks will be met.

Transmission network support, by DNs, can be achieved either by maintaining the voltage at the boundary bus within a certain voltage margin or by controlling the reactive power exchange between the networks to maintain its value in a specified range. Some of the known networks of the system operators define some requirements at the connection point between transmission and distribution networks. For example, the reactive power transfer should be inside a range depending on the transfer power capability. Accordingly, DNs are required to regulate network voltages and meet the requested amount of reactive power transfer to the high voltage side while satisfying system constraints.

In this context, a new real-time short-term voltage control scheme can be proposed for DNs. This control scheme is a centralized scheme and can be implemented in a control center for a global decision. The controller performs control actions depending on the collected real-time measurements. The reactive power output by DG units along with other traditional voltage control devices can be used to reach the target state.

The controller can follow an economic or security purpose while satisfying the following constraints:

• Control variables constraints: Defined depending on the capabilities of DG units and voltage control devices.
• Distribution voltage constraints: Selected based on the fact that the distribution voltages must remain within a specified range.
• Transmission operation constraint: the external grid requests the DNs to keep the voltage at the boundary bus or the request reactive power exchange inside the acceptable limits.

The requirements for this type of real-time voltage control are:

• Voltage measurement units are required to be installed at critical load buses, the connection point with transmission network and the terminals of DG units.
• Power flow measurement unit is required to be installed at the connection point between the two networks.
• Monitoring units to count the taps of traditional voltage control devices.

The solution provides set-point values for DG units and traditional voltage control devices.

This letter shows that active DNs have the ability to support high voltage networks by improving the power factor at the boundary bus. The idea of this letter can be extended to study the interaction between various DNs and high voltage network during normal and abnormal conditions.