Integration of Solar Photovoltaic (PV) generation into an existing distribution system has many impacts on the system, with the power flow being one of the major issues. This impact is not generic for any network, but it may manifest itself either positively or negatively, depending on the grid configuration, interface control modes, operation mode, and load profile. Grid-connected PV systems have three control options of the local voltage controller of the interface DC-AC converter. These control modes are Power Factor control, voltage control, and Droop Voltage control. This paper aims at evaluating and comparing the impacts of those control modes on the grid power flow. A set of evaluation criteria and indices is defined and mathematically formulated. Based on the requirements of the used program (Power Factory Dig Silent V14.1.3), a computation plan (algorithm) has been proposed. The algorithm has been applied to a typical weak network and a wide range of simulations has been carried out. Simulation results have been thoroughly discussed and important findings have been concluded.
For nearly a century, most countries have relied on just one model of power distribution: sending electricity over huge transmission grids from big central generating plants to customers in their homes, offices and factories [
Due to the significant advantages of grid-connected PV systems, this paper focuses on this type of DG tech- nology. Grid-connected PV systems
・ will reduce the power bill as it is possible to sell surplus electricity produced to the local electricity supplier, comparatively easier to install as they do not require a battery system.
・ are comparatively easier to install as they do not require a battery system.
・ have the advantage of effective utilization of generated power because there are no storage losses involved.
・ are growing rapidly due to accelerated cost reductions and associated growth in production. As these tech- nologies mature they have the potential to provide an increasing share of electricity demand.
However, integration of Solar Photovoltaic (PV) generation into an existing distribution system has many im- pacts on the system, with the power flow being one of the major issues. This impact is not generic for any net- work, but it may manifest itself either positively or negatively, depending on the grid configuration, interface control modes, operation mode, and load profile [
The paper quantifies the influence of the three control options on the bus voltages and power losses of the host distribution network. For achieving this objective the power flow approach has been used, which is consi- dered as the optimal tool for analyzing the steady state operation of the network.
Modern distribution systems were designed to accept bulk power at the bulk supply transformers and to distri- bute it to customers. Thus the flow of both real power
The power flow approach in distribution networks hosting PV generators differs from the conventional one mainly in two issues; the selection process of the slack bus and the modelling of the generation buses. In a transmission network, a bus with a large generating unit is usually chosen as the slack bus. In a distribution network, the bus where the distribution system under investigation is connected to a higher voltage level is usually chosen as the slack bus. The complex voltage is specified at slack bus; the voltage angle is usually set to zero [
The PV generator bus is modelled through one of three models depending on the control mode of the converter. Three control modes are considered [
- Power Factor (PF) Control
The PF control of the converter is enabled, i.e. the converter operates at constant power factor. The used bus model is PQ type, i.e. the injected
where
This operation mode has two constraints: the rated MVA of the converter may not be violated, and the PF should be maintained constant.
- Voltage Control
The voltage control of the converter is enabled; that means, the converter operates at variable power factor operation. The used bus model is PV type, i.e. the injected
- Droop Voltage Control
For this control mode the voltage is controlled according to a specified droop (e.g. droop = 2%) and set vol- tage. The droop is defined as:
The generation bus model for this mode is DV type, which is new for power flow calculations (i.e. not in- cluded in the conventional power flow). The generator can be set to control the local voltage at its terminal to a specified set point. With droop control the set point is not reached in any case because the set point is moved (by DV droop) as more reactive power is needed to reach the original voltage set point of the PV generator. The ad- vantage of the droop control is that more than one machine at one bus could control the voltage as well as the participation of the single machine could be configured with the setting of the droop value. When set to voltage control, a droop value can be entered. The voltage at the local bus is then controlled according to the following equations [
where
For achieving a systematic methodology to quantify the required impact, a set of evaluation criteria and indices are defined and mathematically formulated.
The penetration level of distributed generation (DG) on the distribution network is measured against total load demand or the percentage of DG power referred to the rated power of the network [
It is defined as the ratio of the number of PV buses to the number of load buses:
This index points out to the geographic dispersion of PV generators and not its capacity.
Voltage rise index (VRI%) for a bus is the ratio of the difference between bus voltage with PV generation and bus voltage without PV generation, to the bus voltage without PV generation:
The German VDEW code allows VRI% = 2% as a maximum. This figure is suitable for strong grids. But for weak grids, this index is useful only as an indicator for the voltage support degree by PV. Therefore I suggest an alternative index VRI2%, which indicates the voltage rise relative to the nominal voltage:
It is defined as the ratio of the power loss (
This index shows the reduction or increase of total losses caused by PV inclusion relative to the original losses, but it gives no information about the loss rate relative to the input power (
Several simulations have been carried out using a typical distribution grid with the voltage level 20/0.4 kV. The test network is shown in
Based on the mathematical models and the requirements of the used program Dig Silent Power Factory (Dig Si- lent-PF) V14.1.3, which is a computer aided engineering tool for the analysis of electrical power systems, an algorithm has been developed. The algorithm is designed so that it can be applied to any active distribution grid with multiple voltage levels and different PV configurations. This algorithm will be adopted to investigate the test network. The developed approach is shown as a flow chart in
Initially a power flow was carried out for the distribution network without PVs (Base Case). The results show that the network has two problems: Significant total
To evaluate the impact of PV on power flow of base case, several scenarios was simulated. These scenarios include connecting PV units to the grid with different capacities (i.e. different PL%) and locations (i.e. different DI%).
The connection of a single PV generator at bus 8 with the three control modes has, in general, improved the network performance, but to an extent that is depending on the control mode. The major impacts are:
- The voltage profile at all buses has been improved and became acceptable (
- The power flows are reversed on several lines.
-
- The
A PV generator of a much more capacity (300 kVA) is connected to the same bus 8. The simulation results show the following impacts:
Ø The impact on bus voltages are shown in
Ø Concerning the
- For PF control,
- For both
These negative impacts on voltages and power losses occurred mainly because of the high penetration level (about 100%); so it is recommended to avoid that as it is explained in the next section.
With the interconnection location being fixed (i.e. DI% is fixed), the impacts of PV with PF control mode on power flow with different capacities are investigated.
loss in the host grid. From the trend lines plotted on the basis of simulation results for PF control mode, it is ob- vious that for low PV penetration levels
The simulation results show that the connection of a single PV generator at a bus with the three control modes has, in general, improved the network performance, but to an extent that is depending on the control mode and penetration level (PL%). The power factor (PF) control gives the highest voltage rise index (VRI) for buses which are adjacent to the PV connection bus. The Droop voltage control gives the next best average value of VRI, while the voltage control gives the lowest VRI. With a higher penetration level (about 60% - 70% PL), the PF control causes that the voltages of the PV connection bus and its adjacent buses violate the upper voltage limit.
For PF control mode with low PV penetration levels,
It is recommended to determine the maximum PV capacity allowed to be added to the desired bus for the concerned network, in order to avoid the negative impacts on the network performance.