A Brief Discussion on Power Grid Harmonics

1 Introduction

With the rapid development of power electronics technology, the types of power loads are increasing day by day. In particular, motor systems driven by frequency converters are increasingly being used because of their advantages such as convenient process control, significant energy saving, simple maintenance, and networking. Regardless of the type of frequency converter, high-order harmonics will be generated at its input and output ends during the frequency conversion process. The harmonics at the input end will affect the public power grid through the input power line, and the harmonics at the output end will have a great negative impact on loads such as motors. As a result, the power grid generates more and more harmonics under steady-state operation. Where do harmonics come from, what are their effects, how to detect them, and how to suppress them have become issues of concern to us today.

2 Harmonic sources

Harmonics are generated from various harmonic sources. The so-called "harmonic sources" usually refer to various specific electrical equipment, that is, nonlinear electrical equipment or nonlinear power loads. Among them, power electronic equipment in the power grid accounts for the main component. A large number of transformer groups, iron core reactors, arc furnaces, variable frequency drives or thyristor rectifier DC drive equipment, computers, uninterruptible power supplies (UPS) used for important loads, energy-saving fluorescent lamp systems, household appliances and electronic equipment are all harmonic sources. These nonlinear loads will cause grid pollution, power quality degradation, cause power supply equipment failure, and even cause fire accidents in serious cases.

3 Harmonic hazards

当电力电子装备容量达到所接电网短路容量的0.3至0.5倍以上时，或电网参数引起较低谐波次数的谐波谐振时，可能在某一范围内易引起“谐波不稳定”。5、7次谐波含量占总谐波量比重较大，其中5次谐波含量占谐波总含量的45％，有甚高达80％。谐波危害主要表现在以下几方面：

(1) When the skin effect of high-order harmonic current on the motor transformer is serious, it is easy to cause local high temperature and insulation damage. When the harmonic content is low, it increases the copper loss ratio of the transformer.

(2) The interaction between harmonic current and fundamental magnetic field generates adverse mechanical vibration, which accelerates shaft and bearing fatigue and shortens their life, and also increases transformer noise.

(3) For motors that are subjected to large harmonics, the harmonic shaft voltage can easily break through the bearing lubricating oil film, causing discharge and damaging the bearing.

(4) A large amount of third harmonics in the power grid causes the power capacitors to "bulge" and "explode" quickly and stop operating. When the third harmonics flow through the neutral line, they can cause the line to overheat and even cause a fire.

(5) Under certain conditions, the instantaneous spike voltage generated by the operation of power electronic equipment is an important reason affecting the partial discharge and life of power cables, causing cable insulation failure and breakdown and damage.

(6) Harmonics may also cause malfunction of protection and automatic devices.

(7) Harmonic signals have the characteristics of high frequency, high energy, and the possibility of capacitive coupling, electromagnetic induction, and electrical conduction, which inevitably interfere with communications.

(8) Excessive harmonics can easily damage electronic components or shorten their lifespan.

(9) Harmonics can cause local parallel resonance and series resonance in the public power grid, thereby amplifying the harmonics. This greatly increases the above-mentioned hazards and may even cause other serious accidents.

4 National Standards

GB/T14549-93 "Power Quality - Public Grid Harmonics" is shown in Table 1.

Table 2 shows the permissible values ​​of harmonic currents injected into the common connection point.

Table 1 Power quality - public power grid harmonic standards

Table 2 Allowable values ​​of harmonic current injected into the common connection point

5 Harmonic Measurement

The harm of the above harmonics makes it an important indicator of the quality of low-cost power. It should be detected and monitored in the daily operation of the power grid when necessary. Due to the complexity of the power grid harmonic problem, it is difficult to accurately reflect the actual situation of the power grid by using certain theoretical calculations. Usually, the interference of the actual measured power grid harmonics is used to ensure the safe operation of the power grid and high-quality power supply.

At present, according to the performance and test purpose of the instrument, it can be divided into: harmonic detector alarm, harmonic spectrum analyzer, and power quality comprehensive analyzer. Developed countries are more advanced in the development and use of harmonic analyzers. The instruments are fully functional, with a wide measurement range, convenient and reliable to use, and relatively high prices. However, the performance of domestic instruments is still relatively poor, and the data collection is not ideal, but the price is relatively low. For example: domestic dxj series harmonic detectors, American fluke power quality analyzers, Japanese digital power analyzers, domestic gxf series harmonic spectrum analyzers, etc.

6 Harmonic Control

The main way to suppress harmonic damage is to take measures on the harmonic source itself and on the propagation path. The basic ideas are: first, install harmonic compensation devices to compensate for harmonics; second, modify the power electronic device itself to generate less harmonics and control the power factor to 1; third, take appropriate measures to suppress harmonics in the mains network. The specific methods are as follows:

6.1 Measures for harmonic suppression

(1) When selecting power electronic equipment, try to choose a converter transformer with a larger pulsation number or a certain phase shift angle.

The thdv is about 10%-15%, and the thdv of 18-phase pulse rectification is about 3%-8%. The disadvantage is that a special transformer is required, which is not conducive to equipment modification and has a high price.

(2) When the equipment itself cannot be improved, the method of installing an active power filter near the harmonic source to absorb the harmonic current is more advanced, but the compensation capacity is small and the cost is high;

(3) Devices with harmonic complementarity should be concentrated, otherwise they should be dispersed or staggered, and working modes with large harmonic amounts should be appropriately limited;

(4) Add a series reactor at the user's incoming line to increase the electrical distance from the electrical system and reduce the mutual influence of harmonics;

(5) Find out the cause of imbalance from aspects such as power supply voltage, line impedance, load characteristics, etc., and improve the imbalance of the three-phase power supply;

(6) Install reactive power compensation devices to suppress voltage fluctuations, flicker, and three-phase imbalance;

(7) Increase the power supply capacity and reduce the harmonic content;

(8) Improve the performance of equipment or devices and enhance the anti-interference ability of electrical equipment;

(9) Separate signal lines from power lines and use twisted pair cables as much as possible to reduce common mode interference;

(10) When using communication electronic control systems, appropriately add software filtering to the detection signal and output control part when compiling the software to enhance the system's own anti-interference ability.

The passive power filter is simple to install, small in size, and has good reactive power compensation effect, which is suitable for steady-state harmonic places. The active filter installed in parallel automatically detects the harmonic current generated by the nonlinear load and the voltage distortion at the connection point between the filter and the system, and outputs a harmonic current that is equal to the harmonic current generated by the load and opposite in phase, which plays a role in compensating harmonics. It has good dynamic performance, short response time (15μs), and low power loss of three-phase compensation harmonic current (less than 3% of the rated power of the equipment), which is suitable for transient harmonics or places with complex harmonic components, fast changes, and strong randomness.

6.2 Application Examples

At present, factories widely use variable frequency speed regulation, most of which are 6-pulse triggered inverters, with a high 5th harmonic content. Our factory has a 980kw/690v three-phase asynchronous motor driven by a Siemens 6-pulse inverter. A Nokia passive power filter is installed in parallel at the input end of the inverter, which is designed to mainly absorb the 5th harmonic generated by the inverter. The 5th harmonic distortion rate was 28.8% before commissioning, and it became 1.7% after commissioning, and the compensation rate exceeded 90%. The 5th harmonic interference of the system is basically eliminated, and the service life of components is extended. The capacitor in the passive filter structure improves the power factor at the end of the line to 100%, and the statistical results of energy saving show that more than 8,000 degrees of electricity are saved after commissioning compared with the month before commissioning, winning good economic benefits.

7 Conclusion

Harmonics have caused a certain degree of damage to the public power grid and have become one of the sources of power grid pollution. The various methods currently used have certain disadvantages, so analyzing and studying how to suppress harmonics will become a very important topic.