6 PCS Shutdown History Checks to Investigate When Power Output Is Low

When those responsible feel that a solar power system’s "generation is low," many first suspect weather, season, panel soiling, or shading. However, when output struggles to increase relative to irradiance conditions, or when only a certain block within the same plant shows reduced generation, it is important to check the PCS shutdown history. The PCS is the central device involved in plant operation, converting the DC power generated by the solar panels into AC power. Therefore, a history showing that the PCS temporarily stopped, had its output curtailed, or repeatedly operated protection functions can be a factor making generation appear low. By carefully reading the shutdown history, it becomes easier to distinguish whether the cause is weather-related, a device abnormality, or an influence from site conditions.

Table of Contents

• Before checking the PCS shutdown history, compile the overall picture of the decline in power generation

• Check 1: Cross-check the shutdown time with the period of reduced power output

• Check 2: Classify the reasons for stoppage and the error contents to grasp the direction of the cause

• Check 3: Verify the reboot history and the number of repeated shutdowns

• Check 4: Detect local anomalies from stop differences for each PCS

• Check 5: View solar irradiance, ambient temperature, grid status, and shutdown history overlaid

• Check 6: Confirm post-restoration output recovery and verify unrestored sections

• How to Leverage PCS Downtime History in Daily Management

• Summary: When power output is low, narrow down the cause by starting from the shutdown history

Before viewing the PCS shutdown history, assemble the overall picture of the decline in power generation

Before checking the PCS shutdown history when power generation is low, it is important to first assemble the overall picture of the generation decline. A low-generation condition can be viewed in several ways: being lower than expected on a daily basis, output not rising during certain times of day, output dropping only for specific PCS units, or the entire plant's output falling at once. If you look only at the shutdown history without distinguishing these, you may misjudge something that is actually due to weather as an equipment fault, or conversely fail to notice a PCS shutdown.

In practice, you first select the target day and check that day's power generation, generation curve, solar irradiance, and comparisons with past similar days. If, despite the day being near clear skies, there are changes such as a delayed rise in the generation curve, a sudden drop to near zero around midday, or output not recovering until the evening, cross-checking with the PCS shutdown history is effective. On the other hand, on days that are cloudy from the morning and have low solar irradiance, low power generation is not necessarily primarily caused by PCS shutdowns. Verifying the shutdown history is important, but as a rule you should not make a judgment based solely on the history without looking at the background behind the drop in generation.

PCS stoppages can include not only complete shutdowns but also temporary pauses, standby, protective actions, output curtailment, and apparent stoppages caused by communication loss. The way these are displayed on monitoring screens and in records varies by equipment, but it is easier for operational personnel to organize checks by focusing on "Was there any period with no power generation?", "Is the reason for the stoppage recorded?", and "Did it return to normal operation after the stoppage?" In particular, if the timestamps of the stoppage history and the generation data are misaligned, you need to align and check the history recording time, the monitoring device clock, and the data aggregation interval.

Also, when reviewing the PCS stoppage history, it is useful not to rely only on isolated entries but to include several days before and after. Even if power generation appears low on a single day, the PCS may actually have been stopped since the previous day, or it may have been repeatedly stopping for short periods over the past few days. Anomalies that are hard to detect from the daily report alone can reveal patterns when you view the stoppage history continuously. To narrow down the cause of low power generation quickly, it is essential to check the power generation figures, the generation curve, and the PCS stoppage history together.

Check 1: Correlate the stop time with the time period of reduced power generation

What you should check first in the PCS outage history is whether the time the outage occurred matches the period when power generation actually decreased. When looking at data for days with low generation, you may find that output drops only during part of the daytime. If the outage history contains records of outages for that same time period, it becomes easier to consider PCS outages as a possible cause of the generation decrease. Conversely, if the outage history shows outages only at night or in the early morning and there are no outages during daytime generation hours, the impact of those outages on daytime generation decline is likely limited.

When reconciling, it is important not to look only at the daily total generation. A low daily total alone does not tell you in which time period the losses occurred. The impact varies depending on whether the PCS stopped in the morning, during the midday high-output period, or during the low-generation evening hours. In particular, if it stopped during periods of strong sunlight, even a short interruption can have a large effect on the daily total.

If the output on the power generation curve suddenly falls to near zero and then recovers in a stepwise fashion, it may be related to PCS shutdowns or protection actions. On the other hand, output reductions caused by passing clouds often show similar changes across nearby PCS units or the entire plant, and may not leave shutdown logs. By overlaying the shutdown times with the generation curve, it becomes easier to distinguish whether the change is due to natural variability or an equipment-side shutdown.

Also, attention must be paid to the aggregation interval of monitoring data. If power generation data are aggregated in 30-minute or 1-hour intervals, stoppages of a few to a dozen minutes can be difficult to detect in daily reports. However, short interruptions may remain recorded in the PCS stop history. If short stoppages occur repeatedly, they can result in losses that are not negligible in the daily total. When investigating the causes of low generation, it is advisable to cross-check not only the daily report but also higher-resolution output data and stop histories as much as possible.

When checking the time, also confirm that the time settings of the monitoring system, the PCS unit, the data logger, and other devices are consistent. If the clocks are off by several minutes to several tens of minutes, you may misinterpret the relationship between reduced power generation and shutdown history. When acquiring data from multiple devices, verify which device’s time is being used as the reference and, if necessary, consider correcting the time during on-site inspections.

Check 2: Classify reasons for stopping and error details to identify the direction of the cause

The PCS shutdown history may record not only the time the shutdown occurred but also the reason for the shutdown and the error details. To narrow down causes of low power generation, it is important to categorize and read these shutdown reasons. For example, depending on the history contents, the areas to check change — input-side abnormalities, output-side abnormalities, grid-side abnormalities, temperature rise, insulation warnings, communication abnormalities, manual shutdowns, and so on. If you judge only that "the PCS stopped" without looking at the shutdown reason, the scope of on-site inspection can become too wide, and it may take longer to restore operation and identify the cause.

If there are many shutdowns or warnings related to the input side, it may be associated with the solar panels, junction boxes, strings, DC wiring, or similar components. If anomalies are concentrated in a specific input circuit, the workflow should include checking that circuit for open wiring, poor contacts, loose terminals, or effects from shading or soiling. However, avoid concluding the cause based only on the name in the history/log. You need to combine on-site safety checks, measurements, visual inspections, and comparisons with past records to make a determination.

When there are frequent stoppages related to the output side or the grid side, check the components that transmit power from the PCS to the AC side, the substation and transformer equipment, grid voltage, and the operation of protection devices. Voltage fluctuations in the surrounding area or grid-side conditions can cause the PCS to carry out protective operations. In such cases, it is not necessarily a fault of the PCS itself; conditions of the power plant’s overall equipment or the external environment may be involved. If multiple PCS units experience similar trips at the same time, you should consider common equipment or grid conditions rather than individual PCS problems.

If stoppages or output reductions related to temperature are recorded, check the ventilation around the PCS, the cooling fans, the intake and exhaust vents, and the installation environment. If power generation is low in summer or on hot days, the PCS may enter a protection mode due to temperature rise and reduce output despite strong solar radiation. The site environment should also be checked, such as whether weeds or materials around the PCS are obstructing airflow, whether dust has accumulated inside the enclosure, and whether ventilation paths are blocked.

When there is a history of communication faults, you need to distinguish whether the PCS actually stopped or whether the monitoring system failed to acquire data. Even if the monitoring screen shows generation as zero or missing, the PCS unit may have continued generating. Before concluding that generation is low, it is important to cross-check with the export meter and other measurements to confirm whether generation itself had stopped or whether there was only a problem with data acquisition.

Check 3: Verify the reboot history and the number of repeated stops

When checking the PCS shutdown history, confirm not only that a shutdown occurred but also whether it subsequently restarted and how many times it has cycled between shutdown and restart. On days with low power generation, the PCS may have either remained stopped after a single shutdown or repeatedly experienced short shutdowns and restarts. Both lead to reduced generation, but the way you interpret the causes differs. If it remained stopped and did not recover, it may be that it is still in a protective state, that manual recovery is required, or that the on-site conditions for recovery are not being met.

On the other hand, if the system is repeatedly stopping and restarting, the PCS may meet the startup conditions and begin operating once, but then enter a protection mode again under certain conditions. In such a state, the power generation curve may fluctuate unnaturally, or the system may stop when output exceeds a certain level. When investigating the cause of low power generation, it is important to check the number of stops, the intervals between stops, and the time until recovery, and to consider under what conditions the stoppages are occurring.

For example, if it stops repeatedly during morning startup, that provides an opportunity to check the startup conditions and the state of the input side. If stops are concentrated during midday high-output hours, temperature, grid voltage, load conditions, protection settings, and so on may be involved. If stops increase in the evening, check whether they are within the normal range as input fluctuations caused by a decline in solar irradiance or as behavior at the end of power generation. By looking at the time distribution in the history, it becomes easier to tell whether the stops are merely sporadic incidents or reproducible under specific conditions.

In the restart history, whether the recovery was manual or automatic is also a point to confirm. If it recovered automatically, the PCS protection function may have operated temporarily and the conditions may have returned afterward. If manual recovery was required, generation losses may have continued until on-site inspection or operations were performed. If you rely only on remote monitoring to check the recovery status, you may think recovery has occurred while some PCS units remain unrecovered. It is necessary to check the restart history together with the current operating status.

Even if the number of outages is small, long downtime can have a large impact on power generation. Conversely, even if downtime is short, if outages occur frequently during peak generation hours, losses will accumulate. Therefore, when generation is low, it is necessary to look not only at the number of outages but also at the total downtime and the time periods in which they occurred. In daily management, recording the number of outages, the duration of outages, and the response time until restoration helps analyze the causes of generation losses and make improvements.

Check 4: Detect localized anomalies from stop differences per PCS

In power plants equipped with multiple PCS, it is very important to compare the shutdown history of each PCS. When generation is low, whether the entire plant is declining in the same way or only some PCS are declining greatly changes the likely causes to suspect. If all PCS stop at the same time, it is necessary to check common grid-side factors, power receiving and transformation equipment, communication equipment, and external conditions. On the other hand, if only specific PCS are stopped, it is more likely that the cause lies with that PCS unit itself, the connected strings, or the surrounding environment.

In comparisons by PCS, we check not only the ranking of power generation but also, side by side, the presence or absence of downtime history, the number of shutdowns, the reasons for shutdowns, and the recovery status. For PCS units of the same capacity, generation on sunny days tends to be similar. Of course, differences in azimuth, tilt, shading, connected capacity, and number of panels will cause variation, but if a single PCS among those with the same conditions shows lower generation and also has many downtime records, it should be prioritized for detailed inspection.

When identifying localized anomalies, it is also important to link outage history with site layout. For example, if a particular PCS is located at the edge of the plant and installed in a place prone to surrounding vegetation, drainage issues, snow accumulation, fallen leaves, or wind exposure, environmental conditions different from other PCS units may underlie its outages. On-site conditions such as water pooling around the PCS cabinet, a tendency for higher temperatures, easy ingress by insects or small animals, and poor ventilation should also be checked.

Also, even if the same shutdown reason occurs across multiple PCS units, if the occurrence times are staggered, individual environmental factors may be involved. Conversely, if the same shutdown reason is recorded simultaneously across units, prioritize checking shared equipment or external conditions rather than individual PCS problems. In analyzing low power generation, distinguishing between individual and common abnormalities is a key point.

To understand differences between PCS units, it is advisable to have daily generation data available on a per-PCS basis. Relying solely on the plant-wide total can allow the shutdown of individual PCS units to be overlooked. This is especially the case for large plants, where the failure of a single PCS may be difficult to notice from aggregate values alone. Regularly checking per-PCS generation, outage history, and alarm history helps enable early detection of declines in generation.

Check 5: Overlay solar irradiance, ambient temperature, grid status, and shutdown history

To correctly interpret PCS stop history, it is important to cross-check it with external conditions such as solar irradiance, ambient temperature, and grid conditions. The cause of low power generation is not necessarily limited to PCS stoppages alone. Even when PCS stops are recorded as the result, their background may involve sudden changes in irradiance, rising temperatures, grid voltage fluctuations, or changes in the site environment. If you look only at the stop history and conclude equipment failure, you may not address the underlying root causes.

When comparing with solar irradiance, check whether the PCS has stopped during periods when irradiance is sufficient. It is natural for generation to be low during periods of low irradiance, but if output plunges during high-irradiance periods and a stop log is recorded at the same time, a loss caused by PCS stoppage is suspected. Also, on days when irradiance fluctuates rapidly, power output varies significantly. To avoid confusing these fluctuations with PCS stoppages, it is useful to view the irradiance data and the generation curve on the same time axis.

Do not overlook the relationship with ambient temperature. A PCS can be affected by both ambient and internal temperatures. In high-temperature environments, it may reduce output or shut down to protect itself. If days with low power generation are concentrated in midsummer or on particularly hot days, check the PCS installation location, ventilation, cooling conditions, and the history of internal cabinet temperatures. In particular, if there is a pattern where output drops from the early afternoon despite strong solar radiation and then recovers in the evening as temperatures fall, it is worth suspecting a relationship with temperature conditions.

When checking the grid status, confirm whether multiple PCS have stopped at the same time and whether any grid-side alarms or protection actions have been triggered. Because photovoltaic power plants operate connected to the grid, PCS may stop operating due to grid-side conditions. If the entire plant has stopped simultaneously, it is important to check AC-side equipment and grid conditions before deciding to replace individual PCS units. Also checking the voltages that can be verified on site, protection device histories, and the condition of the substation/transformer equipment will help narrow down the likely cause.

When overlaying weather and environmental conditions with shutdown history, compare not just single-day judgments but days with similar conditions. Check whether shutdowns occur only on clear, hot days, also on cloudy days, increase after rainy periods, or rise during times of heavy morning dew or high humidity. This makes it easier to identify which factors—temperature, humidity, insulation, ventilation, grid conditions, etc.—are likely involved. Interpreting low power generation by reading the combination of shutdown history and external conditions makes it easier to prioritize on-site inspections.

Check 6: Verify output recovery and unrecovered sections after restoration

After checking the PCS shutdown history, you need to confirm not only whether it has been restored but also whether the output has returned to normal after restoration. Even if the shutdown history indicates that it has been restored, actual power generation may not have sufficiently recovered. For example, there are cases where the PCS has returned to an operating state but input from some strings is low, output limits remain, or communications appear normal yet the generation curve is abnormal. Cross-check the restoration history with generation performance and verify the extent to which output has recovered compared with before the shutdown.

After recovery, comparing with other PCS during the same time period is useful. If a PCS that had been offline returns to about the same output as other PCS under the same conditions, it becomes easier to judge that operation has been restored for the time being. However, if output is still clearly low after recovery, the cause of the shutdown may not have been fully resolved. When low generation persists, don't be reassured by the recovery indication in the history alone; check the actual output values and the generation curve.

Checking for unrestored sections is also important. When the power plant's overall generation has somewhat recovered, it can be difficult to notice that some PCS units or some strings remain unrestored. Especially in facilities with multiple PCS units, looking only at the power plant's total output can make things appear close to normal. However, at the PCS level you may find that a single unit remains offline or that only part of the input is reduced. If such unrestored conditions are overlooked, generation losses will accumulate day by day.

When performing recovery work, it is advisable to record the time of the work, what was checked, the output after recovery, and whether the issue recurred. With only the stop history, it may be unclear who checked what and when, and which action restored the system. In practice, linking history data with work records speeds up response when the problem recurs. If the same reason for stoppage repeatedly occurs, past recovery methods serve as useful references, and if the recovery is only temporary, they provide material for deciding whether to pursue a root‑cause fix.

Do not limit post-recovery monitoring to the day of recovery. After a PCS has shut down once, it may operate normally for several hours and then stop again the next day. If the cause is environmental conditions such as temperature or humidity, the same symptoms can recur when those conditions are reproduced. After recovery, prioritizing monitoring of the same PCS’s shutdown history and power generation for at least several days will help you detect recurrence trends early. To prevent repeated periods of low power generation, it is important to regard recovery confirmation not as a single point in time but as continuous monitoring.

How to Utilize PCS Shutdown History in Daily Management

PCS downtime history should not be checked only when power generation is low. By checking it regularly as part of daily management, you can detect early signs of declining generation. In practice, it is easier to limit losses by identifying sooner increases in short-term stoppages, repeated identical warnings, or output decline trends in specific PCS units than by responding after an obvious outage occurs. Downtime history should be treated as an important record that indicates the health of the power plant.

In daily operations, it's effective to establish a workflow that consolidates checks of power generation, per-PCS output, stop history, and alarm history. Rather than searching for data only after receiving an inquiry that power generation is low, setting up daily or weekly checks for abnormal trends speeds up root-cause investigations. Especially when managing multiple plants, relying solely on manual monitoring of stop history becomes burdensome. It's important to prioritize anomalies and have a system that allows you to start by checking histories that lead to generation loss.

When reviewing stoppage history, it’s important to consider not only the number of entries but also their impact on power generation. Nighttime stoppage records or entries that are similar to normal end-of-generation behavior may not directly translate into daytime generation losses. Conversely, stoppages that occur during daytime periods of high solar irradiance can cause significant losses even if brief. Operations personnel need to evaluate not just the quantity of events but also the time of occurrence, duration, affected PCS, and the reason for the stoppage.

Also, to make use of PCS shutdown history in on-site inspections, it is important to standardize how records are interpreted among personnel. Even when viewing the same history, differing judgments by staff can delay responses or increase unnecessary inspections. If the classification of shutdown reasons, procedures for on-site verification, criteria for assessing urgency, and methods for recording in reports are organized in advance, the quality of operations will stabilize. In particular, the problem of low power generation involves multiple stakeholders—equipment management, maintenance personnel, power producers, and on-site workers—so it is important to have shared criteria for judgment.

In power plant management, it is necessary to manage the entire process from detection of anomalies through response, recovery, and prevention of recurrence. PCS shutdown histories serve both as the starting point of that process and as material for verification. By finding anomalies in the shutdown history, narrowing down causes through on-site checks, confirming the effect by post-recovery power generation, and tracking whether the same anomaly has recurred, the ability to respond to declines in power output is improved. It is important not just to store histories, but to use them in a way that leads to the next decision.

Summary: When power generation is low, narrow down the cause by starting with the shutdown history

When power generation is low, the PCS shutdown history is an important clue for narrowing down the cause. By matching shutdown times with the periods of reduced generation, you can confirm when the generation loss occurred. Classifying shutdown reasons and error details helps you identify which area to check—input side, output side, grid side, temperature, communications, etc. Looking at the restart history and the number of repeated shutdowns makes it easier to determine whether the stoppage was temporary or a recurring abnormality.

Also, by comparing the differences in shutdowns for each PCS, you can distinguish whether the issue is plant-wide or limited to a specific section. Overlaying solar irradiance, ambient temperature, grid conditions, and shutdown history makes it easier to identify the conditions behind PCS stoppages. Furthermore, by checking post-recovery output restoration and sections that remain unrecovered, you are less likely to miss cases where the history shows recovery but generation has not returned. By performing these checks in sequence, you can determine the cause of low generation based on data rather than intuition.

In solar power generation facilities, a drop in output may end after just one day, or it may continue unnoticed for an extended period. Routinely checking PCS stop histories and managing them in conjunction with generation curves and on-site conditions leads to early detection of losses and prevention of recurrence. Especially when managing multiple plants or a large number of PCS units, a system is needed to efficiently review stop histories and address the highest-priority abnormalities first.

When addressing the problem of low power output, it is important not to rely solely on solar irradiance or weather; instead, start from the PCS shutdown history and verify, one by one, the equipment condition, site environment, and recovery status. By combining shutdown history with daily generation data, you can detect the causes of generation decline early and more easily put in place a management system to prevent recurrence.