Modern medical institutions to improve the level of medical services, the introduction of new and complex various advanced medical equipment such as nuclear magnetic resonance, CT machine, X-ray machine, hemodialysis machine, while a variety of energy-efficient lighting equipment, inverter air conditioner, elevator equipment Put into use. These devices are non-linear devices, in the course of the operation will produce a large number of high harmonics, the distribution system and medical equipment caused some interference at the same time these advanced medical equipment has a high-end computer parts and a large number of highly sensitive micro Electronic devices, the power supply power quality requirements are high, the voltage fluctuations and power harmonics are very sensitive to serious power harmonic problems can lead to damage to medical equipment and even lead to the occurrence of medical malpractice..
Harmonic Sources and Harmonic Harm in Power Supply System.
◇ harmonic source
|
Main classification |
Facility name |
|
Construction facilities |
Elevator equipment |
|
Central air conditioning system |
|
|
Fire pump |
|
|
lighting device |
|
|
Medical Facilities |
Operating room, laminar flow chamber system |
|
ICU, delivery room, neonatal care room, hemodialysis center |
|
|
Preparation, sample, blood preservation and refrigeration equipment |
|
|
Hospital office automation facilities |
|
|
Medical diagnostic equipment |
The main diagnostic equipment, such as spiral CT machine, color Doppler, nuclear magnetic resonance, laparoscopy, EEG, chromatograph, a variety of functional testing equipment |
|
The main treatment equipment, such as ventilator, hyperbaric chamber, all kinds of monitors, laser treatment and so on |
.
◇ harmonic characteristics.
|
Harmonic source load type |
Harmonic current times |
THDi |
|
Lighting, computer and so on |
3、5、7、9,（2n+1）次 |
（7-40）% |
|
Electronic testing equipment, operating room, gamma knife and so on |
3、5、7、9,（2n+1）次 |
（10-15）% |
|
CT, magnetic resonance, DSA and so on |
3、5、7、9,（2n+1）次 |
（30-40）% |
|
Accelerator, X-ray machine, gastrointestinal machine |
3、5、7、9,（2n+1）次 |
（50-60）% |
|
UPS, variable frequency ventilation equipment, elevators and so on |
5、7、9、11,（6n+1）次 |
（25-35）% |
From the above table analysis shows that the hospital's low-voltage power distribution system has a large number of harmonic source load, will produce a large number of 3,5,7 times and other harmonics, serious pollution of the grid.
◇ Harmonic harm
① Harmonic damage to medical devices leads to synchronization disorders and data errors in devices that rely on microprocessor technology.
② harmonic current also flows through the grid, the harmonic current flow leads to harmonic voltage, direct damage to the internal electronic circuit equipment safety, resulting in breakdown or discharge.
③ harmonic current mainly through the capacitor.
.
Reactive power compensation scheme selection
.
The current low-voltage distribution system, the common reactive power compensation there are three:
Pure capacitance compensation;
Non - tuning compensation of 7% reactance coefficient in series;
Non - Tuning Compensation of 14% Coefficient in Series.
The application of the three compensation schemes
① pure capacitance program: the system harmonic voltage total distortion rate THDU <5%, the harmonic current within the provisions of the national standard, does not affect the normal operation of equipment. Such as residential quarters;
② 7% reactance rate compensation program: low voltage system total harmonic voltage distortion rate THDU> 5%, high harmonics mainly 5 times, 7 times, 9 times the main, the compensation circuit to be series reactor, the reactance rate of 7% (tuning Frequency 189Hz). Such as petrochemical, steel, nonferrous metals, automobile manufacturing and other industrial projects;
③ 14% reactance rate compensation program: low voltage system total harmonic voltage distortion rate THDU> 5%, high harmonics mainly to 3 times, 5 times, 7 times, etc., where the system third harmonic current of not less than 62A, to ensure that no The safety, reliability, and reactivity of the compensation are 14% (tuning frequency 134Hz). Such as hospitals, theaters, shopping malls, data centers and other public utilities;
For the specific circumstances of the hospital, the choice of 14% reactance rate of non-tuned compensation program reasons:
⒈ technical point of view - third harmonic frequency of 150Hz, 14% of the reactance rate of the capacitor / reactance tuning frequency of 134Hz (<150Hz), the impedance characteristics of the interference to prevent 3 times and above harmonic current intrusion protection capacitor long-term stable operation.
⒉ reliability - 14% of the non-harmonic compensation coefficient of the non-harmonic compensation unit by the 525V capacitor and 14% reactance rate of the reactor. Visible capacitor voltage level is much higher than the system 400V voltage, redundant, can easily cope with harmonics caused by over-voltage, and grid voltage fluctuations and flicker.
⒊ economy - 14% of the non-tuning compensation coefficient of the unit, to increase the service life of the capacitor, improve the power factor, reduce reactive power loss, an important means of saving electricity costs. ⒋ reduce maintenance and replacement costs, as well as power outage time.
Do not use pure electric compensation program, 7% reactance rate compensation program reasons:
⒈ technical point of view
① pure capacitance solution without harmonic suppression. By the formula
Ic = U / Xc = 2πfCU,
It can be seen that the capacitance of the harmonic is a low impedance channel, so a large number of harmonic current will flow into the capacitor, resulting in component overload, severe heat, serious and even capacitor explosion.
At the same time the capacitor voltage level (nominal voltage of 450V), can not cope with system voltage fluctuations and flicker and harmonic overvoltage.
② 7% reactance rate compensation program, through the previous analysis we can see: can suppress 5 times and above harmonics, but the third harmonic powerless.
Because at 150Hz, the impedance characteristics of the capacitive, will amplify the third harmonic current. With prolonged operation, the capacitance value is rapidly attenuated, and even the risk of resonance.
⒉ reliability
① pure capacitance compensation, the capacitor will be put into the moment of inrush current, remove the over-voltage shock, overload heat, the rapid decline in capacitance value.
② 7% reactance rate compensation program, due to the third harmonic caused by the circuit overload heating problem persists, reactor vibration, serious noise, component insulation strength.
⒊ economy
(Pure capacitance scheme <7% reactance rate> 7% reactance rate), but the reliability, stability, energy saving and operating cost (pure capacitance scheme> 7% reactance rate) Compensation scheme> 14% reactance rate compensation scheme).
⒋ maintenance
Maintenance of the consumption of manpower, material resources, and the resulting power distribution failure, but also to the normal production and life challenges.
.
Harmonic governance
.
Non-tuned compensation scheme, but to protect the capacitor from harmonic intrusion, to ensure reliable operation of the reactive power compensation system, but the low-voltage network harmonics still exist. The harm to other medical equipment still exists. In the harmonic pollution of the hospital low-voltage distribution system, can be used active filter professional governance.
The purpose of the control of harmonic pollution is to ensure the safety of patients and health care workers, that is, through targeted harmonic pollution control, reduce or even eliminate its adverse effects on the distribution system to ensure that the transformer, cable, medical equipment, normal operation ; Followed by direct economic benefits, that is, to ensure the normal operation of low-voltage capacitor compensation system, to play its due role.
