While solar energy systems are at the heart of the renewable energy transition, another crucial issue arises in ensuring sustainable energy quality: reactive power management. Photovoltaic (PV) systems, by their nature, tend to generate reactive power, particularly capacitive power, when connected to the grid. This can create voltage imbalances, energy losses, and potential grid damage in systems operating with inductive loads.
Reactive Power Problem in Solar Power Plants
DC power from PV panels is converted to AC via inverters. During this conversion process, a capacitive reactive power component is generated due to the inverter’s structure. On the other hand, the vast majority of industrial loads (e.g., asynchronous motors) are inductive in nature and therefore draw reactive power from the grid. If these two opposing reactive characteristics are properly controlled, a balancing mechanism can be established. The capacitive reactive power generated by solar power systems can suppress the reactive power drawn by inductive loads.
Why is reactive power important in photovoltaic (PV) systems?
Unlike traditional motor-driven loads, photovoltaic systems do not directly generate inductive load. However, capacitive reactive power can occur due to the structure of the inverters and the loads they are connected to. Furthermore, in PV systems operating in parallel with the grid, a certain percentage of reactive power support has become mandatory to ensure grid stability (e.g., according to TEİAŞ and TEDAŞ regulations).
Reactive Power Problems in Solar Energy Systems:
Capacitive characteristics originating from the inverter
Voltage surges occurring over long cable distances
Extra load on the transformer due to reactive power imbalance
Reactive power penalty fees charged by distribution companies
Reactive Power Management in Solar Power Plant and Grid Interaction: Three Basic Scenarios
Scenario 1: The Facility’s Active Power Demand is Higher Than the Solar Power Generation.
When a business’s instantaneous active power requirement exceeds the solar power plant’s (SPP) production capacity, a portion of the load is met by the SPP while the remainder is supplied directly from the grid. In this scenario, since the system is still considered a grid-connected consumer, compliance with the current reactive power limit values continues.
However, as SPP production increases and the active power drawn from the grid decreases, the Qend/P ratio can rapidly rise, exceeding the 20% limit. This exposes the business to penalty tariffs due to inductive reactive energy limit violations. This situation becomes particularly critical during daylight hours when SPP production reaches its maximum. In the industry, this negative effect is often referred to as the “daylight effect” or, more strikingly, the “daylight nightmare.”
Scenario 2: The Situation Where Solar Power Generation Exceeds Plant Demand.
If the solar power plant’s (SPP) production capacity fully meets its operating load and the surplus is transferred to the grid, the plant acts as both a consumer and a producer. In this case, if the active power consumption is close to zero or negative, the method of measuring reactive power via the meter becomes more complex.
In this situation, reactive power compensation must be managed dynamically depending on which direction the meter is measuring (internal load or grid). Otherwise, a fixed compensation system setting can lead to exceeding limits and incurring penalties.
Scenario 3: Hours When the Solar Power Plant is Not Producing Energy (Night)
At night, when there is no solar radiation, solar power generation drops to zero, and the entire energy needs of the facility are met from the grid. However, as long as the inverters remain operational, the capacitive character of the system becomes dominant. In this case, if the compensation system is still operational and does not activate appropriate inductive loads, the system may exceed the capacitive reactive energy limit and be subject to penalties.
Therefore, considering the night operation scenario, dynamically configuring the compensation panel according to both day and night conditions is critical for system safety and eliminating the risk of penalties. Shunt reactors or inductive loads should be activated for reactive power balancing when necessary.
How is compensation performed in solar power plant (SPP) applications?
Compensation applications in PV systems differ from those in conventional industrial facilities. The primary goal here is to balance both capacitive and inductive loads. The following methods are used:
a) Automatic Compensation Panels
An automatic reactive power compensation panel installed at the PV connection point (switchyard or MV cell) activates capacitor or reactor stages according to the load condition. The reactive power relay operates to maintain the cosφ value at 0.99.
b) Harmonic Filter Compensation
The switching structure of PV inverters generates harmonics. To prevent these harmonics from resonating with capacitors, harmonic filter compensation should be applied. Typically, detuned reactors adjusted to the 189 Hz – 210 Hz range are used.
c) Shunt Reactors
If capacitive power is dominant in the system, cosφ > 1 may occur when the active load is low. In this case, capacitive reactive power balancing is performed using a shunt reactor. Especially at night, when there is no load in solar power plants, even though the inverter is not generating energy from DC, the capacitive character may persist on the AC side.
d) Thyristor Controlled Compensation
In PV-Diesel hybrid systems with sudden load changes or grid instabilities, fast switching thyristor compensation systems (TCR) are preferred.