Advanced Technology from Huawei to Ensure a Better Harmonic Suppression
The penetration of grid connected solar energy systems has been increasing in the Indian grids, from utility scale projects to solar-powered car ports for electric vehicles. Advanced smart energy systems are today the key for next generation energy distribution grids which will see a greater input of variable power from renewable energy.
With a rapid growth in variety, output and size of solar energy projects, generating power at regulated frequency and/or voltage for different load sizes, there is a growing emphasis on quality of power being generated. New demands for a) cost and size reduction, b) performance, revenue and quality improvement c) flexibility on power management and d) stricter regulations are bringing in rapid pace of advancements in grid-interface technologies such as solar inverters.
How Harmonics Impact Power Quality
Harmonics are integer multiples of the fundamental signal (voltage or current at 50 or 60 Hz) and have short and long term effects on grids and grid connected electronics and power electronics equipment such as malfunction, failure and losses. These reduce reliability, lifetime and efficiency of the electricity networks. Main drawbacks of power electronics systems are low (below 2 kHz) and/or high (above 2 kHz) frequency harmonics emissions.
As we know, in an electric power system, non-linear electric load will lead to harmonic voltage and currents. Harmonics in the power grid are a frequent cause of power quality problems. They increase the current and heating in the connected devices and conductors, and also produce harmonic voltage drops in the grid short-circuit resistance, so as to influence the voltage wave shape. The bigger the absolute value of harmonic current, the greater its influence to grid voltage distortion. Reduction of current and voltage harmonics is considered desirable.
According to IEC61727 standard, total harmonic current distortion should be less than 5% at rated inverter output. And in the solar industry most of the inverter vendors claim in their data sheet that the Total Harmonics is less than 3%。
Reducing Harmonics Risks Using Huawei String Inverter Technology
The issue of harmonic resonance is often overlooked while developing large and small scale solar power plants. Many times, these concerns are recognized only when construction of the projects are nearing completion. Due to this, the commercial operation if often delayed till harmonic problems are resolved placing the project developers under significant financial risk. Moreover, delayed recognition of the problems may lead to sub-optimal and high-cost solutions.
So, how can you suppress the harmonics to achieve a higher quality grids as per the standards? Huawei string inverters absorb and apply advance technologies to ensure a better performance in harmonic control. Here’s a look into how they achieve this.
(1) Intelligent Algorithms: From inverter algorithm, the output voltage is the sine wave. When there is any distortion to output PWM wave, inverter output harmonic wave and control will be influenced. Increase switching frequency and the number of output PWM level can reduce the distortion rate of PWM wave. The string inverter with high switching frequency and three-level topology will be better than the central inverter with low switching frequency and two-level topology.
(2) Higher Switching Frequency: The higher the switching frequency, the broader the control bandwidth and better the control on a wide range of current harmonic wave. To ensure the stability, the control bandwidth of the inverter is normally around 1/10 of its switching frequency. The switching frequency of string inverter (16 kHz) is much higher than that of central inverter (3 kHz for two-level topology inverter, 9 kHz for three-level topology inverter). The broader the control width, the better to control lower-order harmonic wave.
(3) Cutting Edge Filter Technology: The high frequency part in the current out of the bandwidth should be filtered by the inverter filter. Normally, the string inverter uses LCL filter featured with strong derating capability of high frequency harmonic wave and little influenced by on-grid resistance. Central inverters use LC filter instead to cut down the cost, which will influence the performance.
(4) Active Filter Mode: In one PV array, multiple string inverters are at different distances to the step-up transformer, so the line resistance will be different. The line resistance can equivalently change the LC inductance in LCL filter and the different filter parameter will change the phase of harmonic wave. When multiple inverters are parallel connected, the harmonic wave can be cancelled because of different phase so as to reduce the total harmonic wave. Except the passive harmonic wave cancellation, Huawei develops active filter mode which can actively reduce the harmonic wave for a smarter grid adaption.
Huawei: Meeting International Standards On Harmonics
So why do developers overlook the harmonic impacts of wind and solar plants? One of the primary reasons is that there exist multiple IEEE and IEC standards which stipulate strict limits on the amount of harmonic currents that can be produced by individual solar inverters. Globally, it is mandatory for inverter vendors to ensure that the harmonic injection at the device terminals is within prescribed limits. However, what is often overlooked is the fact that limiting the harmonic current injection at device terminals is not the same as limiting harmonics on the entire project output.
As mentioned in the IEC61727 standard, testing harmonics is very problematic, since voltage distortion may lead to enhanced current distortion. The harmonic current injection should be exclusive of any harmonic currents due to voltage distortion present in the utility grid without the PV system connected. Type tested inverters meeting the above requirements should be deemed to comply without further testing. To verify THD when SUN2000 inverter connected to grid, Huawei did both single THD test (as per IEC61727) and multi THD test on-grid system.
The Huawei Experience: Feeling the Difference on A 50MW Indian Project
On 23th August, an onsite test of harmonic waves was conducted on an Uttar Pradesh 50MW plant, India, witnessed by one of the biggest developers in south India and one of the major EPCs in New Delhi.
Below is the overview of the plant:
The test results are shown as the chart below:
The power load ratio during the test period are shown as below:
The field test curve shows that inverter THDi is ＜2%, much lower than IEC61727 standard.
So Why Do Huawei Inverters Stand Out?
Huawei communication network and energy equipment are used in more than 170 counties and regions, serving over one-third of the world’s population. Huawei continuously innovates and accumulates experience in system solutions, circuit topology, control algorithms, and structural design, and has realized the perfect integration of digital information technology and power electronic technology. As the representative of cross-domain integration, Huawei builds the smart PV solution that features “higher yields, smart O&M, safe & reliable” with the innovative ideology of simple, full digital, and global automatic operation.
Huawei’s PV plants have no redundant systems, like inverter rooms and DC solar junction boxes (SJBs), or vulnerable components, such as fuses and external fans, so that the delivery of simplified and standard solutions is achieved. All the components are able to adapt to various harsh environments, including high temperature, high humidity, dusty wind, salt mist and high altitude. Zero-touch maintenance for 25 years, guarantee the reliable operation, as well as simpler construction and O&M help PV plant owners and operators maximize their ROI.
Huawei string inverters adopt innovative algorithms and HiSilicon-developed chip technologies and can actively adapt to power grid changes by leveraging its advantages including high-speed processing capability, high sampling and control frequency, and control algorithms such as an advanced harmonic suppression algorithm, along with the with high control bandwidth, excellent LCL filter performance. It is worth mentioning that the SUN2000 can intelligently adapt to various power grids even in parallel mode, and can smooth the total harmonic along with the produced power that will be exported to the power grid. Compared with other central and string inverters that do not support the intelligent algorithm, Huawei string inverters better adapt to the various harsh working conditions of different power grids all over the world, help PV plants obtain better grid-connection performance, and better improve the grid stability and security.
The success of this test well proves the leadership of Huawei string inverter and smart PV solution. A PV plant with Huawei string inverters is grid friendly and a long-term stable performance.