The healthcare environment comprises of various
combination of sensitive electronic loads and other
commercial loads. Hence maintaining quality power is
essential and critical.As the
medical devices are sensitive, poor quality power affects
their performance. Electrical disturbances will lead to lock
up of the devices. Power quality problems may arise due
to non-linear loads, injection of harmonics, and interaction
between medical equipment. Common sources of power
quality problems found in hospitals . Since power quality problems are
cumulative; power quality events (detectable in an
audit) can lead to loss-of-life and eventually premature
equipment failure. An attempt was made to
identify the power quality levels to assess the quality of
service received from the utility and locates the areas
within the hospital where problems arising from instability
exists or may develop in the near future.
The hospital under study has the following types of power
supply.
Normal Supply (NS) is a direct utility supply
(HESCOM) for non-essential areas.
Essential Supply (ES) is used for areas of medical
importance that are not critical to patients in the case of supply interruption.
The essential supply is backed up
by an emergency generator.
Uninterruptible Power Supply (UPS)/stabilizers is
used for operating theaters, patient monitoring and other
equipment that is important to the well being and safety
of patients.
The hospital complex under study has incoming supply of
11kV from the utility. Transformer with Δ-Υ connection
and solidly grounded is used to buck the voltage to 440
volts. The critical medical loads are: X-ray, CT, MRI,
ICUs, operation theaters which contains surgical suits etc.
Case study
The methodology employed was taking the measurements
of voltage, current, % voltage THD, % current THD at few
medical load points and other miscellaneous loads using an
Energy audit kit (Fluke) at Reputed College of
Medical Sciences & Hospital, Chennai, The data obtained were compared against the IEEE 519
standards for analysis.
X-ray
Here X-ray equipment is fed from essential three phase
supply. But no stabilizer is employed. X-ray circuit consists
of primary and secondary sections. Primary will get normal
three phase supply and the secondary will step up the
voltage in terms of kV which will inturn builds up the
capacitor to charge when the circuit is closed (when
exposure button is pushed) . When pre-selected charge is
reached, the capacitor completes the circuit & sends the
charge to the x-ray tube. The voltage THD was found to be
5.2%, which is more than IEEE 519 standard.
Computed tomography (CT)
CT scan uses powerful x-ray technology that circles the
body as the patient, lying on a moving, flat table, passes
through a very shallow tunnel, commonly referred to as the
donut. The fast moving parts of the x-ray source and
detectors are within the donut and out of sight. As the Xray assembly rotates around the patient's body, the scanner
produces images in thin slices which a computer
reconstructs into sharp, three-dimensional (3D) images of
the scanned body part. CT is fed from UPS in this hospital.
The THD for current under operating condition was 6.01%,
voltage THD was found to be within limits.
MRI (Magnetic Resonance Imaging)
An MRI (or magnetic resonance imaging) scan is a
radiology technique that uses magnetism, radio waves, and
a computer to produce images of body structures. The
MRI scanner is a tube surrounded by a giant circular
magnet. The patient is placed on a movable bed that is
inserted into the magnet. The magnet creates a strong
magnetic field that aligns the protons of hydrogen atoms,
which are then exposed to a beam of radio waves. This
spins the various protons of the body, and they produce a
faint signal that is detected by the receiver portion of the
MRI scanner. The receiver information is processed by a
computer, and an image is produced.
MRI is fed from UPS in this hospital. A phantom
bottle assy and a head coil was used as a simulator instead
of patient. The voltage THD was found to be closer to the
limits, but the current THD was 7.9%, and was more than
current distortion limit for the system’s ISC/IL ratio. The
ratio is 20 for this system.
Cathlab
Cardiac Catheterization is a complex medical procedure
that is used to diagnose and treat a wide variety of heart
conditions. During cardiac catheterization, a narrow tube
called a catheter is inserted into the femoral artery near the
groin with a plastic introducer sheath. The catheter is
guided through the blood vessel to the coronary arteries
with the aid of an x-ray. The procedure is performed by a
cardiologist in a specially equipped room know as a
Catheterization laboratory (cath lab). The most used
cathlab equipment consists of an x-ray generator, contrast,
and catheters The CATHLAB was fed from UPS. The voltage THD
was found to be 4%, which is exceeding the standards
prescribed for hospital premises.
Surgical suits
The supply to the surgical suits was highly distorted. Both
current and voltage distortion were beyond limits. Another
observation was found to be of unbalance in three phases.
This results in neutral current to flow back to the
transformer.
Other loads
These loads consisted of pharmacy, administration blocks,
store, lift panel, supply to surgery OPD. This implies that
load is of mixed type. Lot of current harmonic distortion
was observed along with unbalanced load current. The
neutral current was high. At the main panel also, current
distortion was found to be 7%.
Observations
The analysis of the system highlighted some aspects. They
were high THD a unbalanced load currents. These both
contribute to high neutral current which will flow back to
transformer neutral resulting in excess heating of the
neutral cable. As the THD increases, the neutral current
also increases
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