Harmonic hazards and solutions

Harmonic Harm

Due to the use of a large number of non-linear load devices (inverter air conditioners, elevators, X-ray machines, CT machines, nuclear magnetic resonance machines, energy-saving lamps, and EPS rectification devices) in the hospital, the harmonic pollution of hospital power supply systems is generally more serious. Harmonic pollution can cause serious hazards and hidden dangers to electrical equipment and power supply systems. The major ones are:

Interfere with the normal operation of electronic equipment;

Influencing the performance of equipment such as nuclear magnetic resonance, causing noise interference and image disturbances;

Caused malfunction of electrical automatic devices, resulting in blackouts;

Make the precision test equipment vibration and noise increase, shorten the service life;

As a result, the power loss of the power supply system increases, causing grid resonance, accelerating the insulation aging, lowering the power factor of the system, and increasing system risks.

solution

There are many harmonic sources in the hospital, and the distribution is relatively dispersed. Harmonic distortions produced by different devices are different, and the number of harmonics is also different. Harmonic currents and harmonic voltage components are also relatively large. Therefore, it is necessary to effectively implement harmonics in hospitals. Governance requires rational design of primary and secondary power supply systems.

1 A system design

First of all, the system must be designed rationally. For example, a K-factor transformer with D and yn11 connections, a harmonic source device with dedicated line power supply, and three-phase load balancing are used.

2 Harmonics Measurements and Harmonic Control

Real-time and effective monitoring of system harmonics, harmonics measurement and analysis are prerequisites for secondary treatment of harmonics.

Transformer low-voltage side switches are equipped with harmonic monitoring and analysis instruments, real-time monitoring of the power system current and voltage harmonic distortion data, and can set the harmonic distortion limit alarm, can be configured APF active filter device for centralized compensation;

The harmonic distortion rate of power supply loops with more harmonic sources such as medical technology buildings is monitored and APF active power filter devices are configured to perform partial or on-site compensation;

Harmonics monitoring is performed on electrical circuits in Class 2 locations such as operating rooms and ICU wards. If the harmonics exceed the standard, a small-capacity APF active power filter device is used for local control to ensure power safety.

Compensation method

In the hospital power supply system, APF active power filter device can use three kinds of filter compensation methods.

1 Concentrated compensation on the outgoing side of the transformer

When the number of non-linear loads is large, the distribution is relatively dispersed, and the harmonic distortion ratio is not too high, an APF active power filter device can be centrally installed on the transformer low-voltage power distribution bus for centralized compensation, such as more energy-saving lamps. Lighting load transformer. This compensation method can effectively improve the operating performance of the transformer and improve the overall power quality of the system.

2 Partial branch partial compensation

When non-linear equipment is concentrated in certain branches, an APF active filter device can be installed on the branch to perform local compensation.

3 On-site compensation for specific equipment

Single-unit capacity non-linear medical equipment with stable operation, obvious frequency spectrum characteristics, low natural power factor, and harmonics generated by harmonic sources are relatively concentrated in three consecutive or less (such as three times, 5th and 7th harmonics treatment can compensate harmonic sources in situ, such as MRI, X-ray machine, ECT, central air conditioning, etc. This method can achieve very good compensation and energy-saving effect. It will affect the branch and the entire system.

4 On-site compensation

When designing a harmonic control solution, the above three harmonic control methods can be flexibly adopted according to the harmonic monitoring data of the distribution network, and the harmonic pollution of the hospital can be better managed, and the hidden troubles of the power supply system can be eliminated and the harmonics can be eliminated. Wave pollution minimizes the impact on the hospital's entire system and equipment, avoids affecting the normal operation of important medical equipment, and ensures the safety of power supply in the 2 types of places.

Harmonic control equipment selection calculation

Active Filter Equipment Selection Calculation

When the active filter is selected, according to the position of the installation and the object to be compensated, the estimation and calculation of the compensation current may be performed. The calculation is mainly based on the type of customer load, which is calculated from the typical data of this type of load and the user load current calculation.

Centralized compensation capacity calculation

The harmonic current that needs to be compensated for by the centralized compensation method can be estimated by the transformer capacity, and the load factor of the transformer needs to be considered. The specific formula can be estimated according to the following formula:

K1 is the load ratio: that is, the ratio of the load to the rated capacity of the transformer, which is usually taken as 0.6 to 0.8, where 0.6 is taken;

K2 is the harmonic coefficient, and the centralized compensation field is suitable for occasions where the overall harmonic pollution is not high but scattered, taking 12%;

ST is the rated capacity of the transformer in kVA;

US is the low voltage system voltage, which generally takes 0.4.

According to the above formula, a transformer with a capacity of 2000 kVA has an output side harmonic current of ITHD=206 A. An active power filter cabinet with a harmonic compensation capacity of 200 A can be selected for compensation.

Branch compensation capacity calculation

For local compensation of specific branches with relatively concentrated harmonic source loads, the calculation formula can be calculated based on the calculation formula of centralized compensation. For example, the branch of medical technology building with a capacity of 250 kVA, most of the equipment is medical equipment, and current harmonic distortion The rate is selected according to 20%, the load ratio is 0.8, and the ITTD is 56A. A 75A active power filter cabinet can be selected for local compensation.

Specific equipment on-site compensation capacity calculation

Single large-capacity harmonic source equipment can achieve the best compensation effect for on-site compensation. The harmonic content of various typical harmonic source equipment is shown in the following table:

APF active power filter compensation effect

Before using the APF active power filter device, the harmonic pollution of the circuit was serious, the waveform was seriously distorted, and the three-phase current was also seriously unbalanced. After the APF device is used to compensate the harmonic source device in situ, the measured waveform data is better compensated.