Validation of Standalone Photovoltaic Power System Designs

The design verification of the independent photovoltaic power generation system should be carried out after completing the design, equipment procurement, installation and commissioning. This is responsible for the quality of the photovoltaic system built and directly related to the interests of users.

Design verification should be carried out carefully in accordance with relevant standards. In October 2004, IEC published the IEC62124 independent photovoltaic system design verification standard. This standard establishes the procedures for testing independent photovoltaic system designs, as well as the technical requirements for system design verification, which can be used to evaluate the overall performance of the system. At present, the only standard in China is GB/T19064-2003 “Technical Conditions and Test Methods for Household Solar Photovoltaic Power System”, and the relevant national standards may be under formulation.

The technical performance test methods and procedures included in the IEC62124 standard are applicable to independent photovoltaic power generation systems. An independent photovoltaic system consists of multiple components. Even if the components meet the technical and safety standards, further verification is required to determine whether the technical indicators of the entire system meet the design requirements. This standard verifies the design and performance of the system and evaluates the system performance. Only the required system test instruments and equipment are verified here. and performance tests, etc.

  1. Instruments and equipment used in the system test

It is necessary to carry out systematic testing of the following instruments and equipment.
· DC voltage and DC current measuring instruments;
· DC ampere hour meter or some other means of monitoring;
· Time points table or some other means of monitoring;
·Select and calibrate the PV standard device according to the requirements of GB/T6495.2-1 to match the optical harmonics
Responsive array component tests;
·Temperature Sensor;
Equipment that checks whether the standard device and the array are in the same plane (within ±5°);
· Automatic data acquisition system for system monitoring;
·Environmental monitor, etc.

A technical description of the data acquisition system. The data logger should use at least 12-bit A/D conversion and the input range should be greater than the estimated maximum positive and negative voltages. Data acquisition systems must be reliable. If data is lost for more than 4h during the test, or important data is lost due to power failure, the test will restart.

The sample rate of the data logger is determined by the type of charge controller. For the switch controller, the sample rate of the data logger is at least twice as fast as the switch time of the charge controller. For example, if the voltage regulation circuit operates every 10s, then the sampling rate is 5s or faster.
Charge controllers use constant voltage or pulse width modulation circuits, and switching times may be milliseconds, not seconds. The sampling frequency of the data acquisition recorder is at least twice the switching frequency of the charge controller. If the sampling rate of the data logger is not fast enough, one approach is to add an integrator/filter circuit at the input of the data acquisition system to sample once per second. The time constant of the integrating/filtering circuit should be at least twice the sampling period, and the oscilloscope can determine the type of controller and switching frequency. every trial. Data should be stored once an average of 5 minutes.
The parameters in Table 1 will be tested and determined separately.

Table 1 - Test Parameters
Table 1 – Test Parameters

The technical specifications of the sensor are as follows:
The voltage sensor range should exceed the maximum expected voltage and have a resolution of at least 0.0lV or better. The range of the current sensor should exceed the maximum expected positive and negative current and have a resolution of at least 0.01A or better. Except that the accuracy is within ±1%FS, the measuring instruments of DC voltage and current should comply with GB/T6495.1-1996 “Measurement of Photovoltaic Current-Voltage Characteristics”. The temperature sensor range should exceed the expected maximum positive and negative system and ambient temperatures and have a resolution of at least 1K or better. The accuracy of the temperature sensor should be ±2K or better. The pyranometer should have a suitable range. The accuracy of the pyranometer is at least ±5% of the reading.

  1. System performance test

The performance test of the system shall be carried out according to the test procedure of the relevant standard. During the test, the tester shall strictly follow the manufacturer’s instructions for operation, installation and connection. The performance test can be carried out both outdoors and indoors. If the outdoor test conditions at the test site are similar to the simulated outdoor conditions in the standard, the outdoor test can be carried out. If the difference is big. Indoor tests are recommended. Test conditions can cover the major climatic regions in which the system is designed and used. The test needs to take two samples from the same type of system. If one system fails in any kind of test, the other system that meets the requirements of the standard will be re-accepted for a related test. If this system fails, then the design will be deemed to fail verification requirements.

2.1 Three stages of system performance test
The system performance test is divided into three stages: preconditioning, performance test, and suitability for load operation at maximum voltage.

(1) Preprocessing
The purpose of the preconditioning test is to determine the HVD (the voltage when the battery is fully charged and disconnected) and the LVD (the voltage when the battery is under-voltage disconnected) during normal operation of the system. The battery shall be preconditioned in accordance with the manufacturer’s instructions before the test (if it is stated in the system documentation that the battery does not require preconditioning, this work shall not be performed). If the photovoltaic module is amorphous silicon, the light-induced degradation test should be carried out.

(2) Performance test
The performance test has the following 5 steps.
①Initial capacity test (UBC0): After the system is installed according to the standard requirements, the battery is charged and discharged, and the capacity of the battery is measured to obtain the initial usable capacity of the battery (UBC0).
②Battery charge cycle test (BC): Recharge the battery.
③ System function test (FT); it mainly verifies whether the system and load are running normally.
④The second capacity test (UBC1); by charging and discharging the battery, measure the first available capacity of the battery (UBC1) and the independent operating days of the system,
⑤Recovery test (RT): To determine the recharge ability of the photovoltaic system to the battery that has been discharged:
⑥Final capacity test (UBC₂), by charging and discharging the battery to measure the second available capacity (UBC₂) of the battery. After the six steps of the performance test are completed, the system characteristic curve is prepared according to the test data to determine the system balance point .and derive the minimum average irradiance at the installation site for the system to function properly.

(3) Applicability of load operation at maximum voltage
The load operation test at maximum voltage verifies the suitability of the load when operating at the maximum voltage value at high irradiance and high state of charge. Under these conditions the load will operate without damage to the surface, and the system performance tests from functionality, stand-alone operation and battery The recovery ability after the over-discharge state has been comprehensively tested, so as to give a reasonable confirmation that the system will not fail prematurely. The qualified basis of the performance test is as follows.

①The load must keep running during the whole test, unless the charge controller is separated from the load in the over-discharge state of the battery (if LVD occurs, this data should be indicated).
②The decrease of battery capacity shall not exceed 10% during the whole test period.
④Recovery: The system voltage should show an upward trend in the recovery test. During the whole recovery test, the ampere hours (A·h) of the battery should be greater than or equal to 50% of UBC1.
Notes on system performance test For the verification of system design, the performance test can be divided into indoor and outdoor. Most manufacturers do not have the equipment conditions for indoor testing, and this part is usually entrusted to a qualified specialized testing and testing organization. The outdoor test part must be done. Here, we mainly introduce the things that must be kept in mind for outdoor testing.

(1) Standard exposure and system classification
Derive the annual average level of solar exposure and exposure range from weather stations close to the installation site.
The irradiance range (Hrange) is the difference between the monthly average daily irradiance in the highest irradiation month and the monthly average daily pot irradiance in the lowest irradiation month [unit length W·h/(m2·d)]. Each site corresponds to an exposure level (Table 2).

Table 2 - Radiation dose registration
Table 2 – Radiation dose registration

(2) Load specification
The manufacturer shall supply the system with the actual load supplied by the system in the design. In the case of multiple loads, the manufacturer should specify the switching sequence. In this case, there shall be markings on the switch panel or in a suitable location obvious to the end user to indicate the necessary switch sequence. In all tests, all loads were operated simultaneously. The manufacturer shall specify the number of hours per day that the system can supply power to the load under the relevant standard (daily operating hours) test conditions. This data should be derived according to exposure class II as defined in the table above.
For testing purposes. When the PV modules have been connected, the load should not be operated during the day or with irradiance greater than 50W/m2.

(3) System installation and preprocessing
Install the system according to the manufacturer’s installation instructions, and for outdoor testing, ensure that the solar array is not obstructed by any objects, such as buildings or plants, during the test. For indoor testing, a class C solar simulator or an electronic power supply to simulate components can be used. During system assembly, data acquisition equipment can be easily installed depending on the configuration of the system. The tester cannot modify or add to the system under test: only the original system sent is installed and tested according to the regulations in the document. If the cable is cut during installation, the tester shall use the full-length cable received with the system.

Note that the charge controller must be installed carefully and should be connected in strict order to avoid damage.
For the system to work, the electrolyte should be added and the battery preconditioned according to the manufacturer’s instructions. If it is stated in the system documentation that the battery does not require preconditioning, the system will accept:
·At least 5 cycles from HVD to LVD in outdoor test;
• At least 5 cycles at C10 in a laboratory test.

Some advanced charge controllers take days or several cycles to find the best settings that match the system design. The manufacturer should account for this, and performance testing will exclude previous cycles.
According to IEC61646. Photovoltaic modules (such as amorphous silicon) with light-induced degradation characteristics will receive initial light exposure. Mount the pyranometer (standard device) on the plane of the array. The pyranometer will be as close to the array as possible. Do not shadow the array. The pyranometer should be mounted on the same plane as the array, within ±5° of the inclination of the phalanx.

Data acquisition was programmed, the measured data was monitored, and stored on average every 5 min. Notes on installing the temperature sensor:
·The ambient temperature sensor must be installed in a vented or double shaded hood.
·The temperature sensor on the back of the module must be installed in the center of the battery in the middle of the module. Secure with thermal adhesive and cover with insulating material and foil. The battery temperature sensor must be installed as close as possible to the temperature compensation sensor. If the temperature compensation is in the charge controller, a temperature sensor should be added to the controller in addition to the battery temperature sensor. Install voltage sensors on the PV array and load. Install the battery voltage sensor on the battery terminals.

The load is part of the system. Its size is a very important design parameter. For testing purposes. All loads shall be installed and operated simultaneously. Verify that the load starts and runs normally.
When you have multiple loads in the system, observe if a single load can be up and running while all the other loads are running. In this test, it is sometimes necessary to run the load long enough to confirm that it is functioning properly. For example, it usually takes 15 minutes to light a low pressure sodium lamp until it is at its brightest.

Several keywords used in outdoor performance tests are as follows:
UBC0 (Initial Available Capacity of Battery): Initial Capacity Test – After the system is installed, the battery is charged and discharged to measure the battery capacity (UBC).
Vreg: The voltage level at which the battery is fully charged as determined by the controller.
BC (Battery Charge): Recharging of the battery prior to the functional test.
FT (Functional Test); run a functional test to verify that the system and load are operating normally.
UBC1 (one-time usable capacity of the battery): The second capacity test and days of independent operation – charge and discharge the battery. Measure the available capacity of the battery. Determines the number of days the system will operate independently.

RT (Recovery Test): Determines the ability of the photovoltaic system to recharge a battery that has been discharged.
UBC2 (secondary available capacity of battery); final capacity test – charge and discharge the battery to observe the available capacity of the battery. Various test sequences were used in the test to verify low discharge, battery recovery, functional operation and the ability to achieve HVD during normal operation under full sun conditions after full discharge.

Determination of outdoor test conditions: During the test, the temperature of the battery and the charge controller should be kept at 30℃±3℃; during the test, the temperature of the components should be monitored. On a day basis, hourly averages should be calculated and a graph of the average irradiance values ​​made over the same period. At the end of each day, these values ​​will be compared with the values ​​in Table 3. data if it falls between the values ​​listed in the table.
It can be calculated by linear interpolation.

Table 3 - Determining Acceptable Component Temperature Ranges Based on Irradiance
Table 3 – Determining Acceptable Component Temperature Ranges Based on Irradiance

Note: This procedure guarantees that the energy output of the module array does not exceed ±5% for both methods compared to the indoor measurement method (bulk silicon cell) under harsh conditions.
If the average hourly temperature of the module exceeds the following ranges, all tests shall be repeated. If it is necessary to simulate days of low solar radiation, such as in a functional test, the only option is to tilt the PV array to reduce the input energy to obtain simulated harsh climatic conditions. The PV modules are not allowed to be disconnected after reaching the required energy input under full power conditions.

The tests are carried out in the following order.
(1) Initial capacity test
Disconnect the load. Use the photovoltaic array to charge the battery. Once the system reaches the specified state, allow the system to maintain this state for 72h (cumulative), and it can be considered that the battery has been charged to the test target.

Disconnect the photovoltaic array, make the load work continuously, and allow the battery to discharge to the LVD state. When the LVD is reached, the battery can be considered to be discharged. Keep the battery in the 1CD state for at least 5 hours, and record the A·h of battery discharge and the temperature range of the battery. This is the initial battery available energy (UBC0).

(2) Battery charging cycle
Disconnect the load and use the photovoltaic array to charge again to reach (HVD), which is allowed to remain in this state for a maximum of 0.5h.

(3) System function test
This test verifies that the system can power the load as designed.
Connect the photovoltaic array and load according to the manufacturer’s requirements, and let the system work normally for 10d. The test cycle should include at least 2 consecutive low radiation doses and at least 3 significantly different daily doses. It is necessary to draw a system characteristic diagram with these three radiation quantities, and from this, the system equilibrium point can be deduced. Therefore, two irradiance doses are required corresponding to a higher irradiance dose than the equilibrium point of the system.
The average daily exposure for 10 days should be 4kW.h/(m·d)±0.3kW·h/(m·d).
If 2d of test l0d does not meet the requirements and the radiation dose of 4kw·h/(m·d) is not met, it needs to be extended by 20d. Until the previous 10d meets the requirements, if it still fails to meet the requirements, restart the test.

(4) The second capacity test
Disconnect the load after the functional test. Turn on the PV array, charge the battery again to reach HVD, and keep it for 0.5h at this point. Disconnect the PV array from the load and let the system discharge until LVD. Determine the discharge of the battery (A h) and the total discharge time, which is the usable capacity of the second battery (UBC1). Keep the system at the LVID point for at least 5h, but not more than 72h.

(5) Recovery test
Connect the PV array and disconnect the load. The load should be connected according to the manufacturer’s definition when the amount of irradiated radiation reaches 5kW·h/m2. Note 1: The system may still be under low voltage protection at this time. Note 2: The system does not have to receive 5kW·h/m2 of radiation in one day.
The combination of the charging phase with a total radiation dose of 5kW·h/m2 and the continuous duty phase according to the manufacturer’s defined load is called the recovery test cycle. These recovery test cycles were repeated until the total radiation from the system was 35kW·h/m2. If the system reaches HVD, record the number of recovery test cycles required for the battery to reach HVD. Record which recovery test cycle load is initiated.
Measure the (A h) of charging the battery and discharging the load in 7 recovery test cycles.

(6) Final capacity test
Disconnect the load after resuming the test cycle and wait until the system reaches the specified state of charge. Once the system reaches this state, it remains in this state for 72h, at which point the battery can be considered fully charged.
Disconnect the PV array from the load and allow the system to fully discharge. The battery is considered to be fully discharged when it reaches the LVD state, and it remains in this state for at least 5 hours. Record battery discharge (A.h) and battery temperature range. This is the final battery capacity (UBC2).

(7) Operate at maximum voltage
Verify the suitability of the load when operating at high irradiance and high state-of-charge at maximum voltage values. It will run for 1h under these conditions. The load should not be damaged. Any abnormal events should be recorded throughout the test, including unexpected short or open circuits, data acquisition system failures, etc.

Read more: What is the energy payback period and CO2 reduction potential of photovoltaic systems?