Nowadays, with the rapid development of science and technology and the continuous improvement of medical level, it is also accompanied by the introduction of various large-scale advanced medical equipment, which produces a large number of harmonics in these medical facilities, which brings serious harm to the electrical safety and normal work of medical equipment. Active filter device has become the key device to solve this problem.
1.1 Medical Equipment
There are a large number of power electronic components in medical equipment, and these devices will produce a large number of harmonics during work, causing pollution. The more common equipment are MRI (nuclear magnetic resonance instrument), CT machine, X-ray machine, DSA(cardiovascular contrast machine) and so on. Among them, RF pulse and alternating magnetic field are generated during MRI operation to generate nuclear magnetic resonance, and both RF pulse and alternating magnetic field will bring harmonic pollution. The rectifier bridge of the high-voltage rectifier in the X-ray machine will produce large harmonics when it is working, and the X-ray machine is a transient load, the voltage can reach tens of thousands of volts, and the original side of the transformer will increase the instantaneous load of 60 to 70kw, which will also increase the harmonic wave of the grid.
1.2 Electrical Equipment
Ventilation equipment in hospitals such as air conditioners, fans, etc., and lighting equipment such as fluorescent lamps will produce a large number of harmonics. In order to save energy, most hospitals use frequency conversion fans and air conditioners. Frequency converter is a very important harmonic source, its total harmonic current distortion rate THD-i reaches more than 33%, will produce a large number of 5, 7 harmonic current pollution power grid. In the lighting equipment inside the hospital, there are a large number of fluorescent lamps, which will also produce a large number of harmonic currents. When multiple fluorescent lamps are connected to a three-phase four-wire load, the middle line will flow a large third harmonic current.
1.3 Communication Equipment
At present, hospitals are computer network management, which means that the number of computers, video surveillance and audio equipment is a lot, and these are typical harmonic sources. In addition, the server that stores data in the computer network management system must be equipped with backup power such as UPS. The UPS first rectifies the mains power into direct current, part of which is stored in the battery, and the other part is converted into regulated AC power through the inverter to supply power to the load. When the mains terminal is supplied, the battery supplies power to the inverter to continue working and ensure the normal operation of the load. And we know that the rectifier and inverter will use IGBT and PWM technology, so the UPS will produce a lot of 3, 5, 7 harmonic current at work.
2. Harm of harmonics to medical equipment
From the above description, we can find that there are many harmonic sources in the distribution system of the hospital, which will produce a large number of harmonics (with 3, 5, 7 harmonics as the most) and seriously pollute the power grid, causing power quality problems such as harmonic excess and neutral harmonic overload. These problems can affect the use of medical equipment.
2.1 Harm of harmonics to image acquisition equipment
Due to the impact of harmonics, medical staff often experience equipment failures. These faults can cause data errors, blurred images, information loss and other problems, or damage circuit board components, resulting in medical equipment can not continue to work normally. In particular, when some imaging equipment is affected by harmonics, the internal electronic components may record fluctuations and change the output, which will lead to overlapping deformation or ambiguity of the waveform image, which is easy to cause misdiagnosis.
2.2 Harm of harmonics to treatment and nursing instruments
There are many electronic instruments used in treatment, and the surgical instrument is the most damaged by harmonics. Surgical treatment refers to the treatment of laser, high-frequency electromagnetic wave, radiation, microwave, ultrasound, etc. alone or in conjunction with traditional surgery. Related equipment is subject to harmonic interference, the output signal will contain clutter or directly amplify the harmonic signal, causing strong electrical stimulation to patients, and there are major safety risks when treating some important parts. Nursing instruments such as ventilators, pacemakers, ECG monitors, etc., are closely related to the lives of the guardians, and the signal of some instruments is very weak, which may lead to incorrect information collection or even failure to work when subjected to harmonic interference, causing heavy losses to patients and hospitals.
3. Harmonic control measures
According to the causes of harmonics, the treatment measures can be roughly divided into the following three kinds: reducing the system impedance, limiting the harmonic source, and installing the filter device.
3.1 Reduce the system impedance
To achieve the purpose of reducing the impedance of the system, it is necessary to reduce the electrical distance between the nonlinear electrical equipment and the power supply, in other words, to improve the supply voltage level. For example, the main equipment of a steel mill is electric arc furnace, which originally used 35KV power supply, and was respectively set up a 35KV special line power supply by two 110KV substations, and the harmonic component was higher on the 35KV bus bar. After the use of a distance of only 4 kilometers 220KV substation set up 5 35KV special line power supply, the harmonics on the bus significantly improved, in addition to the plant also used a larger capacity synchronous generator, so that the electrical distance of these nonlinear loads greatly reduced, so that the plant generated harmonic reduction. This method has the largest investment, needs to be coordinated with the power grid development planning, and is suitable for large-scale industrial projects, and hospitals require uninterrupted continuous power supply, generally powered by two or more substations, so this method is not a priority.
3.2 Limiting harmonic sources
This method needs to change the configuration of harmonic sources, limit the working mode of generating harmonics in large quantities, and concentrate on using devices with harmonic complementarity to cancel each other. The frequency of characteristic harmonics is increased by increasing the phase number of converter, and the effective value of harmonic current is greatly reduced. This method needs to rearrange the equipment circuit and coordinate the use of instruments, which has high limitations. The hospital can adjust slightly according to its own situation, which can reduce the amount of harmonics to a certain extent.
3.3 Installing the Filter Device
At present, there are two commonly used AC filter devices: passive filter device and active filter device (APF). The passive filter device, also known as the LC filter device, uses the principle of LC resonance to artificially create a series resonance branch to provide a very low impedance channel for the specific number of harmonics to be filtered out, so that it is not injected into the power grid. The passive filter device has a simple structure and obvious harmonic absorption effect, but it is limited to the harmonics of the natural frequency, and the compensation characteristics have a great influence on the grid impedance (at a specific frequency, the grid impedance and the LC filter device may have a parallel resonance or series resonance). Active filter device (APF) is a new type of power electronic device, which is used to dynamically suppress harmonics and compensate reactive power. It can collect and analyze the current signal of the load in real time, separate each harmonic and reactive power, and control the converter output with harmonic and reactive current equal amplitude and reverse compensation current through the controller to offset the harmonic current in the load, so as to achieve the purpose of harmonic control. Active filter device has the advantages of real-time tracking, rapid response, comprehensive compensation (reactive power and 2~31 harmonics can be compensated at the same time).
4 Specific application of APF active filter device in medical institutions
With the continuous improvement of people’s living standards and the acceleration of population aging, the demand for medical services is steadily increasing, and the medical service industry is about to enter a period of rapid growth, and the most significant and important representative of the medical industry is the hospital. Because of the special social value and importance of hospital, the solution of its power quality problem is urgent.
4.1 APF selection
The benefits of harmonic control, first of all, is to ensure the personal safety of patients and medical personnel, that is, to reduce or eliminate the adverse impact of harmonic control on the distribution system, to ensure the normal operation of transformers and medical instruments; Secondly, it directly reflects the economic benefits, that is, to ensure the normal operation of the low-voltage capacitance compensation system, play its due role, reduce the harmonic content in the power grid, and improve the power factor, reduce reactive power loss, and extend the service life of the equipment.
The harm of harmonics to the medical industry is very great, a large number of harmonics will affect the performance and use of precision instruments, and may endanger personal safety in serious cases; It will also increase the power loss of the line and the heat of the conductor, reduce the efficiency and life of the equipment, so the importance of harmonic control is self-evident. Through the installation of active filter device, the purpose of harmonic control can be well achieved, so as to ensure the safety of people and equipment. In the short term, harmonics control needs a certain amount of capital investment in the early stage; However, from the long-term development point of view, the APF active filter device is convenient to maintain in the later period, and can be used in real time, and the economic benefits brought by it to control harmonics and the social benefits of purifying the power grid are also obvious.
Post time: Jun-30-2023