Common Power System Architectures for Health Care Facilities


in the previous Topic; Electrical Distribution systems for nursing homes and residential custodial care facilities we talk about the electrical design methods for such buildings.

You can review the following previous topics for more information and good following:

Today, i will explain the Common Power System Architectures for Health Care Facilities as follows. 


Common Power System Architectures for Health Care Facilities:


the health care facilities have (4) common Power System Architectures as follows:
  1. Radial – Generator.
  2. Dual Source and Radial Secondary – Generators.
  3. Secondary Selective – Generators.
  4. Dual Source and Secondary Selective – Generators.


1- Radial – Generator


This is the simplest system to operate and has the lowest first cost. It is also very easy for maintenance people to understand. System can have outages for faults and maintenance. If utility source A is lost, the entire system is lost for 10 seconds until generators can be brought on-line. This system is normally used on smaller healthcare facilities.

Radial – Generator Power System Architecture
For smaller healthcare facilities, small nursing homes and residential custodial care facilities, a single service-entrance switchboard, and a small generator can be the major components of the electrical system. These facilities usually have several normal power panelboards fed from the normal power system. If motor loads are to be energized by the generator, their restoration under emergency conditions can be delayed by using a time-delay relay in the automatic transfer switch or by adding a time-delay relay in the motor starter, which is energized by auxiliary contacts within the transfer switch.

Where there will be significant emergency power requirements, several transfer switches may be used to increase reliability. The transfer switches used for the equipment system can be adjusted to transfer sequentially, thus minimizing generator inrush requirements.


Advantages:

  • Low first cost. 
  • Simple operation and understanding by personnel. 
  • Easily expanded. 
  • Radial ground fault. 

Disadvantages:
  • Low Reliability. 
  • Maintenance causes outages. 



2- Dual Source and Radial Secondary – Generators


A double-ended substation should be considered where transformation will exceed 750 kVA.

System has the ability to switch between two utility sources via auto throw-over scheme. Electrically operated medium voltage breakers are required for this transfer system. If both utility sources are lost, generators back up essential electrical loads via transfer switches. System design is reliable and used for medium to large size hospitals.

Dual Source and Radial Secondary – Generators  Power System Architecture

As health care facilities become larger, additional feeders for normal power loads and essential electrical power systems will be required. The essential electrical system will thus require three or more transfer switches, some of which could be non-automatic. Using several smaller transfer switches, in lieu of a large switch, will contribute to system stability and reliability.


Further, reliability can be obtained by placing the transfer switches as close to the ultimate load as possible.

For example, fig (1) shows two schemes for distributing power through vertical risers.

Fig (1)
When an outage occurs in the normal power supply, the transfer switches will direct an alternate power supply to the essential electrical system panelboards. However, if a specific normal feeder outage occurs, only Scheme A can sense the specific outage and restore power to the affected panel via the alternate power supply. Scheme A is more reliable than Scheme B, but is a more costly arrangement. Loss of normal power is sensed at the transfer device to initiate start-up of the emergency generator via auxiliary contacts. When the voltage and frequency of the emergency power is in proper limits, and stable, then relays initiate switching to the alternate power source. Finally, restoration of normal power causes the automatic transfer switches to return to the normal power source while initiating a shut-down of the emergency generator.

Advantages:
  • Protection against loss of primary source. 
  • Possible paralleling of sources. 
  • Choice between Sources A or B. 
  • Reliable. 

Disadvantages:
  • Additional cost of equipment. 


3- Secondary Selective – Generators 


The secondary selective system allows the transfer of load from one transformer to the other with the use of electrically operated main and tie-breakers via auto throw-over scheme. This is important if one transformer fails or needs maintenance. If both utility sources are lost, generators backup essential electrical loads via transfer switches. Transformer sizing is critical if all secondary loads are to be serviced from one transformer. This is usually accomplished by either loading the transformers to 50%, or by using the transformer forced air (fan) rating with temperature controllers.

Secondary Selective – Generators  Power System Architecture
This system may require complex ground fault solution if both 480 Y/277 V transformers have separate grounds or if 3 pole transfer switches are used for 4 wire loads. This causes circulating current on the neutral busses.

This system is common in large cities where the utility companies use transformers, located in vaults, to service hospitals.

Advantages:
  • Normal operation as radial system with stand-by generators. 
  • Isolation of cable or transformer for faults or normal maintenance. 
  • Feed other side with use of transfer scheme and electrically operated breakers. 
  • Main and tie breakers can be interchanged for maintenance to keep outages to a minimum. 
Disadvantages:
  • Additional cost. 
  • Transformer load monitoring. 
  • May require complex ground fault system if neutrals are tied together and sources have multiple grounding points. 


4- Dual Source and Secondary Selective – Generators


This system combines the advantages of both primary sources and secondary selective systems used with backup generators. It not only provides the more reliable system, but also one of the most costly system.

Dual Source and Secondary Selective – Generators  Power System Architecture
 
Evaluation of the probability of total downtime costs will be necessary to justify the additional first cost. This system may require complex ground fault solution if both 480 Y/277 V transformers have separate ground points or if

3 pole transfer switches are used for 4 wire loads. This causes circulating current on the neutral busses. This power system is popular for large hospital complexes.

Advantages:
  • High Reliability. 
  • Combined advantages of both sources and generators. 
Disadvantages:
  • Higher initial cost. 
  • May require complex ground fault system if neutrals are tied together and sources have multiple grounding points. 


in the next Topic, I will explain the Major Types of Electrical Loads. so, please keep following.




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