Contemporary industrial facilities frequently satisfy their energy demands not solely from the utility grid but also through the integration of on-site solar power plants (SPP). While this hybrid renewable energy supply model presents substantial benefits concerning environmental sustainability and operational cost reduction, it simultaneously introduces novel technical demands regarding power quality management and power factor correction.
In scenarios where a facility both draws energy from the grid and injects active power back into it, the bidirectional power flows become inherently more intricate. Consequently, the deployment of four-quadrant power factor correction systems is crucial for effective system operation.
The following discussion will examine the topic in more detail from the perspective of power factor correction:
According to prevailing national regulations, the permissible limits are as follows:
- When reactive inductive energy / Active energy ratio is 20%
- Reactive capacitive energy / Active energy ratio is 15%
The formula demonstrates that active energy constitutes the denominator. Consequently, increased solar energy generation reduces the demand for grid-supplied energy, thereby diminishing the denominator. This reduction in the denominator leads to an elevated ratio, potentially resulting in reactive energy penalty charges.
To mitigate these adverse conditions, the implementation of four-quadrant power factor correction systems in industrial facilities is crucial for both effective reactive power management and enhanced energy efficiency.
The following section will provide a detailed analysis of each of the four operating quadrants within the context of a single-shift industrial facility.
Quadrant I Operation (P > 0, Q > 0):
In this prevalent scenario, the facility operates at full load, with inductive loads, such as motors and lighting systems, drawing both active and reactive power from the electrical grid.
The active power and reactive power are perpendicular vectors. The apparent power form an angle φ with active power and the Inductive reactive current lags the voltage by 90°.
In the absence of power factor correction system in industrial plants, the value of cosφ is typically approximately 0.8, which corresponds to a value of tanφ of around 0.75.
Industrial facilities typically exhibit inductive reactive characteristics. To counteract this, capacitive power factor correction systems, such as capacitor banks, are often integrated, with the required compensation determined through power triangle calculations.
The term ‘ Quadrant I Operation (P > 0, Q > 0)’ is used to describe the condition of an industrial facility when it is operating at its maximum capacity. In this state, the facility draws both active and inductive reactive energy from the electrical grid. In this operational mode, a variety of production line components, including motors, fans, pumps, and lighting systems, are in active use, thereby necessitating a supply of active power from the grid. Inductive loads, such as asynchronous motors and transformers, inherently consume reactive power for magnetization, thereby imposing an inductive reactive energy demand on the grid.
Consequently, the apparent power is conceptualised as the vector sum of these active and reactive components, typically illustrated by the power triangle. This condition is characterised by the current phasor lagging the voltage phasor, indicative of a lagging system characteristic and a positive phase angle. In such industrial contexts, the power factor is usually in the range of 0.75 to 0.85. However, the activation of a central power factor correction system enables a reactive power controller to systematically engage capacitor steps, thereby elevating the power factor to approximately 0.98–0.99.
For instance, on an overcast day when a rooftop solar power plant cannot fully satisfy the factory’s energy requirements and the grid remains the primary source of active power, the facility operates in the first quadrant with active compensation. In this particular scenario, both active and reactive power components are positive, and the power factor is displayed as a positive value on the reactive power controller.
Quadrant II Operation (P > 0, Q < 0):
In the context of a factory’s single daytime shift, the term ‘active power’ (P) denotes the amount of energy consumed by production lines, motors, lighting, and associated loads. This energy is drawn from the grid during periods of peak demand. Despite the presence of a substantial rooftop solar power plant that has been installed to address a significant proportion of the demand, the facility continues to draw active energy, thereby ensuring that the price remains above zero. However, when capacitor banks are operating at full capacity and thereby producing more capacitive energy than the facility demands, the excess reactive energy is fed back to the grid. This results in negative reactive power (Q < 0), causing the system to operate in the second quadrant.
This phenomenon is often attributed to a process of overcompensation. For instance, in the event of a sudden decrease in motor loads or production demand, the capacitor banks may remain active, thereby generating more capacitive energy than is required. The reactive energy that is produced in excess is then introduced into the grid in the form of capacitive reactive power.
Consequently, the load characteristic becomes capacitive: the current precedes the voltage ( leading ), and the phase angle becomes negative, indicating the current phasor is ahead of the voltage phasor. In this scenario, the active power remains positive, while the reactive power is negative. Consequently, the power factor appears negative on the reactive power controller’s display. It is evident that the billing process is frequently contingent on the parameter known as the “tan φ ” ratio. In instances where a sustained injection of capacitive reactive energy is introduced into the grid, particularly in conjunction with a minimal active energy draw, there exists a possibility of a substantial deterioration in the aforementioned ratio. This is attributed to the reduction in the denominator, consequently leading to an augmentation in the likelihood of incurring a reactive power penalty.
Quadrant III Operation (P < 0, Q < 0):
In a single-shift industrial facility operating exclusively during daylight hours, when the rooftop solar power plant’s output surpasses the factory’s instantaneous consumption, the energy flow reverses, and the facility exports active energy to the grid. Here, active power (P) is negative, signifying that energy is flowing towards the grid. To illustrate this phenomenon, consider a scenario where solar energy production peaks during midday hours. Under such conditions, with the factory load remaining relatively low, power factor correction remains active. Consequently, the reactive power (Q) is negative, and the factory injects capacitive reactive energy into the grid. Thus, the industrial facility operates within Quadrant III. The load becomes capacitive due to the combined effects of power factor compensation and the SPP system. Even when the factory load is moderate, the SPP system’s high production ensures the facility remains largely independent of grid energy; instead, excess energy is fed back into the grid, contributing to a net positive energy balance. In this scenario, the current precedes the voltage (leading), indicating a negative phase difference, where the current phase leads the voltage phase.
When an industrial facility sells active energy to the grid while simultaneously injecting capacitive reactive energy, the active energy consumption meter may register near zero or show no increase in consumption. Energy sales, however, are measured by a separate meter. Conversely, the capacitive reactive meter continues to show an upward trend. When active energy consumption is minimal, the denominator in the tan φ equation (tanφ = |Q| / |P|) becomes very small. This leads to a substantial increase in the ratio of capacitive energy supplied to the grid relative to active energy, causing tan φ to exceed prescribed limits, which may result in the imposition of a capacitive reactive power charge.
Quadrant IV Operation (P < 0, Q > 0):
In a single-shift industrial facility during daytime, when the rooftop solar power plant (SPP) generates more energy than the instantaneous factory load, the facility exports active power to the grid (P < 0). However, if the factory’s load remains predominantly inductive, the reactive power demand stays positive (Q > 0), meaning the facility continues to draw inductive reactive power from the grid. Inductive equipment—such as motors, pumps, and transformers—requires reactive power for magnetization, which persists even when active power is being exported.
Under these conditions, the facility operates in Quadrant IV: active energy is supplied to the grid, while inductive reactive energy is drawn from the grid. The current lags the voltage, resulting in a positive phase angle, characteristic of a lagging system. This scenario is common at midday, when solar production peaks but motor loads are still active.
From a billing perspective, although active energy is exported, the inductive reactive meter continues to record Q > 0. When active power is minimal or negative, the tanφ ratio (|Q| / |P|) can become very large, potentially exceeding permissible limits. In such cases, the facility may incur inductive reactive power penalties, despite exporting active energy.
Of course, in a single-shift industrial facility with a solar power plant (SPP), it is necessary to monitor the daily meter readings. In the electrical room, the area in which the facility is currently operating can be observed on the display of the microprocessor-controlled relay. The table below summarizes the four-zone operation we have described, making it easier to understand the concept by referencing the real-time relay screen.
Typically, a microprocessor-based reactive power control relay displays a negative value when reactive energy is supplied to the grid. This phenomenon is observed particularly frequently in Quadrant II and III. In Quadrant II, the factory draws active energy from the grid while supplying reactive energy to it. In this scenario, the load is capacitive, and the relay shows a negative value. In Quadrant III, both active and reactive energy are supplied to the grid. Again, the load is capacitive, and the relay displays a negative sign. In accordance with prevailing regulations, this can lead to a capacitive reactive charge payment situation. The reactive power control relay must be microprocessor-based, and its algorithm must support four-quadrant operation.
Didem Ergun Sezer
Electrical Engineer
Ergun Elektrik
