introduction
In recent years, the photovoltaic industry has developed rapidly and its role in power supply has become increasingly prominent. More and more distributed photovoltaic power generation systems are being connected to the distribution network, posing new challenges to the traditional distribution network and placing higher demands on photovoltaic power generation technology. Only by outputting high-quality current before grid connection can the adverse effects of photovoltaic power on the distribution network be minimized.
The main impacts of photovoltaic power on the power distribution network include: increasing the voltage at the connection point, causing voltage fluctuations, injecting current harmonics and DC components, DC side grounding faults, and abnormal islanding.
This article mainly explains the causes, hazards, and effective methods to suppress DC injection.
I. Causes of DC Component
The fundamental reason for the DC component is that the high-frequency SPWM wave output by the inverter contains a certain DC component. The reasons for the DC component in the inverter's output pulse width modulation wave can be attributed to the following points:
(1) Given a sinusoidal signal wave containing a DC component
This situation often occurs in analog-controlled inverters, where the sinusoidal command signal is generated by analog devices. Due to differences in the characteristics of the components used, the command signal itself contains a small DC component. Using closed-loop waveform feedback control, the output current waveform is basically the same as the command waveform, resulting in the output AC current also containing a certain degree of DC component.
(2) DC component caused by pulse distribution and dead zone formation circuit
The SPWM signal generated by the control system needs to be phased and dead-timed by the pulse distribution and dead-time forming circuit, and then isolated and amplified by the drive circuit to drive the switching transistors. The dispersion of component parameters will cause different dead times, that is, the loss in each transistor's conduction time is inconsistent, so the inverter output contains a DC component.
(3) Inconsistent characteristics of switching transistors
Even if the pulse width modulation wave generated by the control circuit is completely symmetrical, the differences in the characteristics of the power switching transistors in the main circuit, such as different saturation voltage drops when conducting and inconsistent storage times when turning off, will cause the output SPWM wave to be asymmetrical, resulting in the output current containing a DC component.
(4) DC component caused by zero-point drift in the control system feedback channel
1) Zero-point drift of sensing elements: Inverters inevitably employ various sensing elements when introducing output feedback control, the most common being voltage and current Hall sensors. These sensors generally exhibit zero-point and temperature drift, which causes the output current to contain a DC component.
2) Zero-point drift of the A/D converter: In a fully digitally controlled inverter, the output current detected by the sensing element needs to be converted from analog to digital by an A/D converter, and then processed by the processor according to a certain control law. Like the sensing element, the A/D converter also suffers from zero-point drift, which will cause the output AC current to contain a DC component.
II. The Dangers of DC Components
The DC component mainly has adverse effects on transformers, current-type residual current circuit breakers, current-type transformers, and metering instruments in the distribution network. Since these devices all contain magnetic cores, the presence of the DC component can easily cause magnetic circuit saturation, leading to malfunctions of current-type residual current circuit breakers, saturation and overheating of transformers or instrument transformers, generation of harmonics and noise, etc., resulting in operational failures or even paralysis of the entire system.
Suppressing DC injection is one of the key issues in photovoltaic grid connection. IEEE Std. 929-2000 stipulates that the DC component in the grid-connected current of a photovoltaic system must be less than 0.5% of the system's rated current. The US standard specifies that it should not exceed 0.5% of the rated current per phase. The Chinese national standard GB/Z19964-2005, "Technical Regulations for Photovoltaic Power Stations Connected to Power Systems," stipulates that the DC component should not exceed 1% of its AC rated value. Meanwhile, the "Technical Specification for 500kW Photovoltaic Power Generation Grid-Connected Inverters" requires that the DC current should not exceed 0.5% of the inverter's rated output current.
III. Suppression of DC Component
DC component suppression can be achieved through reasonable design parameter matching. Using an isolation transformer before grid connection is an effective method to suppress DC component.
However, in the last decade or so, due to technological advancements and cost pressures, the removal of the isolation transformer has led to higher efficiency and reduced production costs, resulting in the increasingly widespread application of photovoltaic power inverters without isolation transformers.
For systems without isolation transformers, photovoltaic power inverters that typically employ pulse-width modulation (PWM) technology can suppress the DC component output.
Selecting appropriate detection elements can also effectively reduce the DC component.
IV. Detection of DC Component
Closed-loop current sensors are generally used for DC component detection because they offer high accuracy, low linearity error, and minimal susceptibility to external interference. However, DC component detection differs from general current detection because the amplitude of the DC component superimposed on the AC signal is very small, placing higher demands on the sensor.
Current sensors typically exhibit zero-point error and temperature drift linearity error, both of which affect the DC component. Some of these errors can be mitigated through circuit design and software algorithms, while others are unavoidable. Selecting a suitable sensor and using it appropriately can effectively reduce the impact of the current sensor on the DC component.
Below, we will take the CASR 50-NP as an example to analyze the extent to which these errors affect the DC component, and how to eliminate and reduce their impact. The CASR series is a low-temperature drift, small-size sensor developed by LEM for photovoltaic applications, available in four specifications: 6A, 15A, 25A, and 50A.
Linearity error εl: ±0.1%, Zero-point offset Vout: 2.5±0.05, Zero-point temperature drift TCVout: ±0.7ppm/K, Gain temperature drift TCV: max±40ppm/K.
How much do these parameters affect the DC component when connecting distributed photovoltaic inverters to the grid? Let's analyze them one by one.
Linearity error: Since the measured effective current is an AC signal, the hysteresis curve in the magnetic circuit is a symmetrical curve. The linear errors generated in the positive and negative half-cycles are also symmetrical and can cancel each other out for the DC component. Therefore, the magnitude of the linear error will not affect the magnitude of the DC component injection. The hysteresis curve is shown in the figure below.
Zero-point drift value: Since the zero-point offset value of the sensor is a fixed value, in a photovoltaic power system, the zero-point offset value can be corrected or removed by software to eliminate its influence on the DC injection amount.
Temperature drift: This parameter changes with temperature and cannot be avoided. It directly affects the DC injection rate and must be considered in photovoltaic grid-connected systems.
Since the detected signal contains both AC and DC signals, the sensor's overall error only affects the AC signal. The DC signal is obtained through integration, and these errors cancel each other out during the positive and negative half-cycles, as shown in the diagram below. Based on the above error analysis, the main parameter affecting the DC injection amount in the CASR output signal is temperature drift error.
Using a grid-connected current of 30 Arms, and employing a CASR 50-NP sensor to detect the grid-connected current, the error affecting the DC component under 85°C conditions is:
Total error = TCV_OUT + TCG = 0.12% + 0.24% = 0.36%. This error is far better than the requirement of 0.5%, which can meet the inverter's requirements for DC measurement.
Conclusion: Based on the requirements and regulations for DC components in photovoltaic grid-connected power generation systems, in addition to employing pulse width modulation (PWM) technology, when selecting appropriate current sensors, besides considering parameters such as high measurement accuracy, small linearity error, and wide temperature range, temperature drift (zero-point temperature drift and gain temperature drift) is a particularly important parameter to consider. For sensors with the same nominal accuracy, the smaller the temperature drift, the less impact it has on DC component measurement. Therefore, selecting sensors with good temperature drift characteristics can effectively suppress DC injection and improve measurement performance.
Regarding LEM
LEM, a market leader in current sensors, provides customers with innovative technologies and high-quality current measurement solutions. Its core products are current and voltage sensors, widely used in driving and welding, renewable energy and power supplies, railway and rail transportation, high-precision applications, traditional automobiles, and new energy vehicles. LEM's strategy is to leverage the inherent strengths of its core business while developing new application areas and seeking growth opportunities in new markets. As a medium-sized global company, LEM has production centers in Beijing (China), Geneva (Switzerland), Machida (Japan), and Copenhagen (Denmark), with sales offices around the world providing comprehensive services to customers globally. The LEM Group was listed on the Swiss Stock Exchange in 1986 (stock code: LEHN). LEM Electronics (China) Co., Ltd. is a wholly-owned subsidiary of LEM Electronics in China, with sales offices in various regions to provide seamless global services to Chinese customers. For more information, please visit the LEM China official website www.lem.com.cn.
