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Impact of PV on Distribution System Voltage Control

This author no longer works for DNV GL.

The integration of PV on distribution systems can have significant impacts on system operations, particularly voltage control.  In general, the magnitude of these impacts heavily depend on the PV capacity deployed, the load level in the electrical neighborhood, the types of voltage control assets deployed, and the way they are operated.  Some effects of integrating PV cause technical issues requiring mitigation strategies, while others offer potential benefits, particularly if the utility has control over the PV site inverter.

Some of the most commonly discussed issues with PV penetration are the unpredictability of diurnal solar patterns due to the weather, and the variation between seasonal patterns, which are longer trends and relatively more predictable as far as their impact on system operations is concerned.  The introduction of distributed energy resources, such as PV, adds a level of complexity to systems operations in that the control set points must be determined based on a greater number of monitoring points (i.e., at the Point of Common Coupling and downstream from the PV site) to account for a new set of potential situations arising from the integration of the PV sites.  The impact of PV generation on the net demand perceived at a substation is illustrated in Figure 1.

Figure 1: PV Generation Impact on Demand

Figure 1: PV Generation Impact on Demand

In most cases, as a PV site generates power, the devices upstream from the point of interconnection experience a net reduction in loading.  This translates into a reduction in current magnitude, and therefore technical losses, and a levelling of the voltage profile.  If the voltage control assets are dynamically controlled, such as with an Integrated Volt/VAR Control (IVVC) scheme, this flattening may allow for voltage regulators to be set to a lower tap, which causes a marginal benefit in the form of further reducing the power demand for loads responsive to voltage levels (i.e., particularly resistive loads such as lighting and certain types of heating).  In systems where the voltage regulator settings are fixed (i.e., conventional system operations), these marginal benefits cannot be harnessed.

If the power generated is greater than the loading downstream from the PV site the power flow is reversed causing a rise in voltage from the substation to the PV site.  This type of situation can cause certain nodes to experience overvoltage, which can be detrimental to sensitive loads.  Reverse power flow can have serious safety implications, particularly if protection devices are not coordinated to handle such unusual circumstances.  This, however, is typically accounted for in impact studies during the interconnection process.

Another set of issues can arise from a PV site going offline suddenly during a period of high power generation relative to the load being served.  Such events can cause flickers or low voltages, both of which can be detrimental to sensitive loads.  Voltage control assets operate on a delay, which means low voltage levels may take a few seconds to a few minutes to be rectified, which is quite a long time in electrical terms.

The fact that the average customer does not feel the impact unpredictable and intermittent power sources as PV sites can cause is due to the diligence and thoroughness of system planners and operators who bring to bear the devices and systems technology innovators develop such as smart inverters and other controls with remote communications capabilities and systems such as ADMS and D-SCADA.

This blog post also received contributions from DNV GL’s Dennis Flinn, principal consultant.

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