Measurement of power conversion efficiency of photovoltaic power generation system

Overview

In photovoltaic power generation systems, power conditioning systems (PCSs) and charge-discharge controllers are used to convert direct current output from solar panels into usable alternating current. The parameters that indicate the performance of a power generation system include actual amount of generated power and conversion efficiency.

The conversion efficiency of commercial solar panels ranges from 10% to 20%. Minimizing the loss when converting the solar panel output to AC leads to higher efficiency of the entire power generation system.

In recent years, PCSs for storage batteries and hybrid PCSs that can control storage batteries have also been introduced to the market for residential photovoltaic power generation systems, and a power generation system linked to storage batteries has become the mainstream type.

The conversion efficiency of PCS ranges from 95% to 98%. In order to improve the efficiency by 0.1%, evaluation using a precision power analyzer is required.

Configuration with a hybrid PCS that controls storage batteries

Challenges / Demands

Measurement methods for conversion efficiency of PCSs are specified in IEC 61683, EN 50530, JIS C 8961 and other standards. Not only the maximum efficiency but also Euro efficiency*1 and CEC efficiency*2, which are close to the efficiency in actual use conditions, need to be measured. Euro/CEC efficiency is determined from efficiencies at different load levels ranging from light to rated load that are weighted according to the load factor.

Measurement of PCS conversion efficiency

Measurement of MPPT control*3

Measurement of storage battery charge and discharge

Measurement of PCS output power quality

 

*1 Euro efficiency (Euro-eta, partial load efficiency): Efficiency calculated by weighting conversion efficiencies at different load factors (based on European climate patterns) and adding them together. Euro efficiency is said to a conversion efficiency close to that under actual operating conditions.

*2 California Energy Commission (CEC) efficiency: Energy efficiency regulations established by the California Energy Commission, USA. Products distributed in California must comply with these regulations.

*3 Maximum Power Point Tracking (MPPT) control: A function incorporated in PCSs and other devices to track and control the combination of voltage and current to maximize the PV power at different solar radiation levels. MPPT control is mainly used in large-scale systems.

Solution / Proposal

YOKOGAWA's WT5000 and WT1800E provide optimal measurement solutions for measuring power conversion efficiency of photovoltaic  power generation systems.

Since the WT5000 has a maximum of 7 inputs and the WT1800E has 6 inputs, a single WT5000 or a single WT1800E can measure input/output, voltage, current, power, frequency, and efficiency of a PCS and display the measurements on the screen. The WT5000 and WT1800E can measure not only the maximum conversion efficiency but also Euro efficiency and CEC efficiency.

Fluctuations in power or efficiency are displayed as numerical values or on a trend graph to allow users to understand the fluctuations intuitively.

Use of an optional external current sensor allows measurement of much higher currents up to 2000 A.

Harmonics generated by a PCS can be measured according to IEC standards.

The voltage peak value, current peak value, and maximum instantaneous power value (+ side, - side, respectively) as well as voltage, current, and power values are useful in evaluating MPPT control.

In the evaluation of battery charging/discharging characteristics, the integration function is used to calculate each power amount  from the instantaneous positive and negative power values.

 

WT5000 example of trend display of efficiency η1/η2/η3

 

WT5000 simultaneous measurement of seven channels

Waveforms of input signals can be displayed without using an oscilloscope, and advanced settings, such as filter setting, can be made while checking the actual input signal.

In addition, the WT5000 Data Streaming (/DS option) allows you to continuously stream waveform data synchronized with power parameter measurements to a PC at up to 2 MS/s without any gaps. Power parameter measurements and voltage/current waveform data of the same measurement period based on which the power calculations are made can be synchronized and streamed. You can check noise on waveforms or a change in control state and can perform a detailed analysis to see how the noise or the change affects the power value and parameters.

A current sensor has a through type structure, and the current flowing through the primary wiring is detected by the winding of the electromagnetic core of the current sensor. It is necessary to pay attention to the following points.

When an AC/DC current sensor or current clamp probe is used, its  amplitude error and the phase difference between the voltage  and current signals can be corrected to enable more accurate power measurement.

Null is a function that resets the offset value to zero while wired, including an external current sensor. Null can be individually set to ON, HOLD, or OFF for each input. It is recommended that you execute zero-level compensation (a function to compensate for the zero level in the internal circuit) before enabling the null function.

Simple wiring for high-current equipment measurement

The WT5000’s 760903 current sensor element is an input element dedicated for connection to an AC/DC current sensor or current clamp probe required for measurement of high-current equipment. With this current sensor element equipped with a DC power supply for sensors,  an external DC power supply and time-consuming wiring preparation are no longer required, and you just need a single WT5000 and sensor/probe for large current measurements.

Combined with the /PD2 option (current sensor power), a dedicated cable, shunt resistor box, and sensor, a WT1800E can measure high currents.

WTViewerE application software

 

Example of measurement screen

Integrated Software Platform IS8000

Synchronous display of DL950 waveform data and WT5000 power data

AC/DC Current Sensors

Choose a model based on the AC/DC current to be measured and the measurement bandwidth.

 

WT series Line-up