Abstract: This paper introduces the harmonic sources and characteristics of port power distribution systems, analyzes harmonic data from actual measured harmonic sources, and proposes port harmonic mitigation measures.
Keywords: port, harmonic source, harmonic mitigation, active power, filter
Analysis of Harmonics in Harbourand Control Measures
Abstract：Thisthesisintroducetheharmonicsourcesandtheircharacteristicsofportdistributionsystem;Analysisofthedatathatactualmeasurementofharmonicsources;Portharmonicscontrolmeasures .
Keywords：HarbourHarmonicssourceHarmonicscontrolActivepowerfilter
1 Introduction
In recent years, power electronics technology, with its energy-saving, high-efficiency, and easy-to-control characteristics, has been widely applied in port power distribution systems. In particular, technologies such as rectification, frequency conversion, and energy feedback have been extensively used in mechanical equipment such as gantry cranes and container quay cranes. However, the use of these new technologies inevitably generates significant interference to port power distribution systems, especially harmonic interference, which has become an unavoidable problem. A company conducted power quality tests on its power distribution system. The tests revealed that most heavy equipment injects 5th and 7th harmonics into the system. High-order harmonics can cause various problems for the system, such as transformer overheating, increased noise, frequent capacitor bulging leading to low power factor, and severe cable overheating. This paper analyzes the characteristics of harmonic sources in ports based on actual test results and proposes corresponding mitigation measures.
2. Overview of Harmonics
The national standard GB/T14549-93, "Power Quality and Harmonics in Public Power Grids," defines harmonics (components) as: components whose frequencies are integer multiples of the fundamental frequency, obtained by Fourier series decomposition of a periodic AC component. Total Harmonic Distortion (THD), an important indicator of power quality, is defined as: the ratio (expressed as a percentage) of the root-mean-square (RMS) value of the harmonic content in a periodic AC quantity to the RMS value of its fundamental component. Voltage THD is denoted as THDu; current THD is denoted as THDi.
3. The impact of harmonics on port power distribution systems
(1) Equipment impact
Harmonics pollute power distribution systems, potentially affecting the normal operation of relay protection, computer systems, and precision machinery or instruments. This can reduce the lifespan of these devices and even cause relay protection malfunctions, leading to unnecessary accidents and varying degrees of impact and damage, particularly affecting induction-type energy meters. Related research indicates that induction-type energy meters exhibit a gradually increasing attenuation characteristic for harmonics of the second order and above, attenuating by more than 80% by the ninth order.
2) The impact of harmonic pollution on the power grid
Capacitors constitute the largest proportion of reactive power capacity in the power grid, with user capacitors accounting for approximately two-thirds of all capacitors. The design of these capacitors often only considers reactive power compensation, neglecting the actual power quality pollution at the installation point. Therefore, low power quality at the operating point frequently leads to accidents such as failure to engage compensation devices, reduced capacitor lifespan, blown capacitor fuses, and even series-parallel resonance, triggering harmonic overvoltages and overcurrents, potentially causing capacitor explosions. User capacitor management is still assessed based on the average power factor. However, due to harmonics, the power factor is also affected. Generally, the input power factor of equipment is:
As can be seen from this formula, when the current and voltage are distorted, the power factor will decrease.
4. Port survey data analysis
The following is an analysis of the test results of a company on the power distribution system in the port office area.
Figure 1 shows the current waveform at the measurement point and the current THDi value. Due to the influence of the load, the current waveform in this figure is severely distorted. This is caused by the superposition of various harmonics on the current under the sinusoidal waveform. The harmonic table shows the content of each harmonic in detail. Due to the large number of frequency converter loads in the port, the 5th, 7th and 11th harmonics in the power distribution system are too high.
Figure 1 shows the current waveform and the current THDi value.
Figure 2 shows the voltage waveform and its harmonic table. The voltage distortion is mainly caused by the distorted current generated across the line harmonic impedance. Because the current distortion is very severe, when the distorted current flows through the line impedance, it will produce a distorted voltage drop. According to Kirchhoff's voltage law, other equipment in this power distribution system must also be connected to the distorted voltage, thus being severely affected.
Figure 2 shows the voltage waveform and the current THDi value.
Figure 3 shows the reactive power demand trend of the port's gantry crane during operation and the power consumption of the office area. As can be seen from Figure 3, the reactive power requirements of these devices differ greatly and change rapidly under no-load and load conditions. The left figure shows that during gantry crane operation, there is a significant reactive power demand approximately every minute. Traditional reactive power compensation cabinets use contactors for switching and power factor controllers (RVCs) for control. The RVC has a set step switching time, typically between 10 and 40 seconds. For example, the default step switching time for ABB low-voltage RVCs is 40 seconds. Too long a switching time cannot keep up with the load's reactive power demand, while too short a switching time will accelerate the aging of components such as contactors and capacitors. Based on this characteristic, it is clear that traditional reactive power compensation cannot track rapidly changing load reactive power demand, resulting in a consistently low power factor. Since the power factor is affected by harmonics and rapidly changing loads, larger reactive power compensation systems or replacement with new reactive power compensation equipment are required, leading to increased costs.
As shown in Figure 4, the power factor is generally low due to the influence of harmonics, averaging only 0.84 .
Figure 3. Reactive power demand trend, power and electrical energy of port gantry crane during operation.
The above analysis shows that the power factor is affected by harmonics. According to the formula, computer simulation shows that when PF= 0.84 and THDu=5%, THDi is as high as 60%.
Figure 4 shows the curve of PF decreasing under the influence of THDi.
As shown in Figure 5, due to the presence of harmonics, the transformer impedance is...
When the system harmonics are large, the transformer impedance will increase while the capacitor impedance will decrease. A large amount of harmonics will flow into the reactive power distribution cabinet, and resonance may even occur at a certain frequency. These phenomena constantly threaten the stable operation of the port power distribution system.
Figure 5. Schematic diagram of system impedance in the presence of harmonics.
5. Harmonic mitigation solutions
5.1 Scheme Analysis
Based on the above analysis, the power quality of this power distribution system can be summarized. The port's machinery and equipment utilize numerous power electronic devices, generating a large number of harmonics. During operation, the reactive power demand of these machines varies greatly and changes rapidly, making it impossible for traditional reactive power distribution cabinets to compensate in a timely manner. When harmonics are present in the system, overcompensation by the reactive power distribution cabinets can amplify these harmonics, potentially leading to resonance and directly overloading the transformer and tripping the switchgear.
According to the relevant provisions of the national standard GB/T 14549-1993 "Power Quality and Harmonics in Public Power Grids" and Article 28 of the "Jiangsu Province Power Protection Regulations" which was officially promulgated and implemented on May 1, 2008, the power supply company may interrupt the power supply for users who seriously affect power safety under any of the following circumstances: "(1) When the harmonic current injected into the power grid by the user's nonlinear impedance characteristic electrical equipment connected to the power grid or when the sinusoidal distortion rate of the voltage at the point of common coupling exceeds the national standard, and the user fails to correct it after being notified by the power supply company; (2) When the user's impact load, fluctuating load, or asymmetrical load affects the power supply quality or interferes with or hinders safe operation, and the user fails to correct it after being notified by the power supply company." Harmonics have a great impact on both the power system and the user's electrical equipment. According to relevant regulations, it is recommended to carry out harmonic control for the power distribution system.
5.2 Main methods of harmonic control
Currently, methods for suppressing harmonic interference are mainly divided into passive and active methods. Their advantages and disadvantages are analyzed below.
5.2.1 Passive Harmonic Filtering Device
The main structure of passive filtering involves connecting a reactor and a capacitor in series to form an LC series circuit, which is then connected in parallel to the system. The resonant frequency of the LC circuit is set at the harmonic frequencies that need to be filtered out, such as the 5th, 7th, and 11th harmonics, to achieve the purpose of filtering out these harmonics. It is low-cost, but the filtering effect is not very good because of its inherent frequency limitations. If the resonant frequency is not set properly, it can easily resonate with the system, causing a surge in harmonic current and damaging precision instruments and equipment with high power quality requirements. Currently, this is the most common filtering method on the market, mainly because of its low cost and ease of acceptance by users, but the filtering effect is very poor. Since most small and medium-sized enterprises in my country are privately owned, owners often lack awareness of the dangers of harmonics and are generally unwilling to increase funding for harmonic mitigation. However, some enterprises have excessively high harmonic content, making conventional reactive power compensation ineffective, while power supply departments have very strict requirements for reactive power, resulting in fines for non-compliance. This leads many companies to focus only on reactive power compensation, spending a lot of money on faults where reactive power compensation cannot be used due to harmonics. Although the initial investment cost is low, the replacement and maintenance costs will increase exponentially over the years, which is a temporary solution that does not address the root cause.
① It can only suppress a fixed number of harmonics, and under certain conditions, a certain harmonic will resonate and amplify, causing other accidents;
② It can only compensate for fixed reactive power and cannot accurately compensate for changing reactive loads;
③ Its filtering characteristics depend on the power supply impedance, are greatly affected by system parameters, and its filtering characteristics are sometimes difficult to coordinate with voltage regulation requirements;
④ Due to the high requirements for the component parameters and reliability, and the fact that they cannot change with time and external environment, the manufacturing process of passive filters also has high requirements.
⑤ This method is not suitable for situations where the system load varies significantly;
⑥ It is relatively heavy and bulky.
5.2.2 Active Harmonic Filtering Device
Active harmonic filtering devices are developed from passive filtering. They offer superior filtering performance, achieving 100% filtering efficiency within their rated reactive power range. They primarily consist of circuits composed of power electronic components that generate a harmonic current with the same frequency and amplitude as the system's harmonics, but with an opposite phase, to cancel out the existing harmonic currents. Their manufacturing is significantly more complex than passive filtering devices, resulting in a slightly higher initial investment cost. Their main application is in computer-controlled power supply systems, particularly in office buildings and factories. Currently, due to the significant harm harm harms cause to power systems, more and more companies are focusing on harmonic mitigation, leading to a high level of acceptance of active harmonic filtering devices. Figure 6 shows the structure of a parallel active filter.
Figure 6 Parallel active filter structure
5.3 Proper Installation of Active Filters
In summary, active power filters are ideal supporting equipment for harmonic control. When installing active power filters, both the physical and logical locations need to be considered. The physical location requires taking into account the available space at the installation point. For new projects, it is generally recommended to install the active power filter at the bottom of the reactive power distribution cabinet, maintaining consistency with the cabinet type and color of other electrical cabinets. For later renovation projects, where no space is available, it can be placed as close as possible to the installation point, while also considering the overall aesthetics of the distribution room layout. In this project, the active power filter is installed at the very end of the distribution cabinet, with cabinet dimensions consistent with other electrical cabinets.
Logical location refers to the installation position relationship between the active power filter and other electrical cabinets in the power supply system, as shown in Figure 7. Based on the characteristics of capacitors, high-frequency current flows more easily into them. When the active power filter is logically located before the capacitor bank (closer to the transformer end), more high-frequency current flows into the capacitor bank, and the active power filter can only filter out a small portion. Furthermore, if the reactive power cabinet impedance is lower, the reverse harmonics emitted by the active power filter will also flow into the reactive power cabinet. Therefore, the logical location of the active power filter should be at the lower end of the capacitor bank. This not only filters out harmonics, but also allows the active power filter to quickly track and compensate for rapidly changing reactive power demands, achieving harmonic mitigation while protecting the reactive power cabinet.
Figure 7 Logic location of active power filter
Based on the above analysis, the active power filter for the port is installed at the end of the electrical cabinet and connected in parallel to the system via a cable. Figure 8 shows the installation location of the active power filter.
Figure 8. Installation location of active power filter
5.4 Equipment Selection
Based on the actual measurement results, refer to the following formulas (1), (2), and (3):
Where: is the total harmonic current; is the effective value of the fundamental current; is the effective value of the full-wave current; is the total harmonic current distortion rate. Since the full-wave current is relatively easy to measure and observe in actual measurement, and the difference between calculating the harmonic current using the full-wave current and the fundamental current is not significant (see equation (3)), the full-wave current can be used for estimation.
The harmonic currents required for compensation in each power distribution system can be calculated using the above formulas. Based on the harmonic currents, the required capacity of the active power filter can be selected. Table 1 shows the harmonic current content.
Table 1 Harmonic Current Content
power distribution system | |||
Phase A | Phase B | C phase | |
THDi(%) | 21:30 | 25.00 | 28.00 |
Full-wave current | 242 | 250 | 227 |
Harmonic current | 51.5 | 62.5 | 63.6 |
Based on the calculated data, an active power filter of appropriate capacity can be selected. When performing power management, active power filters can simultaneously manage harmonics, compensate for reactive power, and address three-phase unbalanced current. As the above analysis shows, the port's load demand for reactive power changes rapidly, resulting in a low power factor. Active power filters can track and compensate for this rapidly changing reactive power. When used in conjunction with existing reactive power control cabinets, it can significantly reduce retrofit costs and achieve excellent reactive power compensation. Since active power filters simultaneously generate reactive and harmonic currents, when selecting an active power filter, to ensure a significant filtering effect and additional reactive power compensation, a filter with a capacity slightly larger than the measured value should be chosen. Here, we recommend selecting the ANAPF75-380/BGC active power filter, which can compensate for 75A harmonics, for this application. Figure 9 shows a schematic diagram of the active power filter installation.
Figure 9. Schematic diagram of active power filter installation
5.5 Governance Effectiveness
After conducting actual tests on the power distribution system, an active power filter ANAPF75-380/BGC was installed. Measurements and comparisons after operation revealed a significant improvement in performance. The current waveform changed from a large spike to a smooth sine wave, and the neutral (N) line current decreased from 43A to 10A. THDi decreased from 21.3%, 25.0%, and 28.0% to 2.6%, 2.6%, and 2.6%. The power factor also increased from 0.85 to 1.00, perfectly resolving harmonics, imbalances, and low power factor issues in the power supply system. A comparison of the improvement effects is shown in Figure 10.
Figure 10 Comparison of treatment effects
6. Conclusion
Although the harmful effects of harmonics manifest in various ways, the root cause of these hazards is harmonic current—the harmonic current emitted into the power grid by nonlinear equipment during operation. Therefore, whatever the ultimate goal of harmonic mitigation, its essence is to reduce the harmonic current injected into the power grid by a load (possibly a group of loads), that is, to minimize the distortion of the current waveform. This is because harmonic current is the source of harmonic problems. While in some cases the goal of harmonic mitigation is to ensure that the voltage distortion rate of the power grid meets national standards, it ultimately comes down to controlling harmonic current.
The optimal location for harmonic mitigation is at the power inlet of the nonlinear load. This effectively transforms the nonlinear load into a linear load, resolving all harmonic-related problems. Because the harmonic source is eliminated, the original power distribution system operates as if under traditional linear load conditions, without any hidden dangers. For designers, due to harmonic mitigation, both power distribution and manufacturing system designs can adhere to traditional standards without considering the risks posed by harmonics. Most developed countries implement harmonic mitigation using this strategy. Management measures to achieve this include requiring equipment procurement to meet the GB17625 standard.
While mitigating harmonics at the power input of nonlinear loads is the optimal solution, this approach can be costly. Depending on the specific system requirements, a more flexible approach can be adopted. Typically, local harmonic mitigation combined with partial harmonic mitigation creates a cost-effective solution. For high-power harmonic source loads (such as frequency converters), active power filters can be used for local harmonic mitigation to reduce the harmonic current injected into the grid. For lower-power, more dispersed nonlinear loads, unified mitigation can be implemented on the busbar. The design scheme should be tailored to the specific conditions of the power distribution system to achieve a perfect harmonic mitigation effect.
Source: *Electrical Drive Automation*, Issue 6, 2014
References
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About the author:
Zhao Hongjun, male, Bachelor's degree, Acrel Electric Co., Ltd., major research interests: active power filters and harmonic mitigation.
Email: [email protected] ,Tel: 15000539641 ,QQ: 2880157856
