Types of Power Distribution System in a Data Center
Power to the data center is usually supplied by the utility to the electrical yard at a predetermined and fixed voltage. The distribution of the voltage within the data center can be easily done by a simple radial system. It is a system where the power is received at the substation at the utility voltage and it is stepped down to a voltage as desired by the consumer.
There are cases when the utility service voltage is higher than the utilization voltage. In such cases, serious damage can be done to the equipments depending on the difference in the magnitude of the two voltages. The more the difference, greater is the damage. The engineer may use a number of power distribution systems as explained extensively by Eaton in one of its resources. Some of these systems are briefly described below.
1. Simple Radial System
As stated before, in this system, the utility voltage is supplied to a single substation where it is stepped down and the power is supplied throughout the building by using a main voltage bus. In the event of loss of power source, fault in transformer or any other device, the circuit breaker will isolate the entire building until the problem is rectified. This system is simple, and cost effective. It is easy to coordinate with and there are no idle parts in the system. The source voltage, transformer and the primary breaker are managed by the utility thus relieving the data center of power maintenance concerns. But in case of an outage, the data center has to wait for the fault to be rectified by the utility company. Also, this system raises issues with voltage regulation and efficiency.
2. Loop-primary System-radial Secondary System
This system is effective when the data center is being supplied with power from two different service lines from the utility. It consists of one or more primary loops with more than two transformers connected in the loop. Care is taken to ensure that both the service lines are not operational at the same time by using switches. By operating the appropriate switches, it is possible to isolate or reconnect any one section of the loop from the system. A key interlocking mechanism is recommended to prevent closing all sectionalising devices in the system at the same time. Although this system is slightly more expensive, it is more reliable and quicker restoration of power is possible in the event of utility outage, line fault or overload occurs. In this basic system, the loop may be normally operated with one of the loop sectionalizing switches open as described above or with all loop sectionalizing switches closed. If a fault occurs in the basic primary loop system, the single loop feeder breaker trips, and secondary loads are lost until the faulted conductor is found and eliminated from the loop by opening the appropriate loop sectionalizing switches and then reclosing the breaker.
3. Primary selective system- secondary radial system
This system operates with two primary feeder circuits in each load area so that if one primary feed fails, the other can easily take over. The secondary feeder should be capable of supporting the full load of the primary feeder. Half of the transformer is connected to each feed such that if the primary fails, only half the data center will be affected until the secondary takes over. Because of the additional power and protective equipments, the cost of the system is obviously more than the radial system. It depends on the load that the secondary feeder is expected to take. But power can be quickly restored in case of an outage in this system. There is reduction in the amount of load lost when the primary feeder outs.
4. Two-source primary- secondary Selective System
This system is based on the principle of duplicate sources from the power supply point using two primary main breaker and primary tie breaker. Each transformer secondary is follows a double substation unit arrangement. This system grants quick restoration of power to all loads but requires extensive use of thick cables to withstand the load in case one system fails. Each of the subsystem will be capable of supporting the load in case one of them fails. this system provides improved voltage regulation as compared to the other systems.
5. Sparing Transformer System
This system came into use to reduce the cost of the two-source primary system. It replaces two ended transformer with a single transformer, sparing one transformer and all its respective connections. Hence the name. Not more than five substations are on a sparing loop. It makes use of an automatic throwover system to restore utility power to the data center. In this, each transformer carries its own load and also takes up less space. It adopts a close transition scheme which makes power transfer without interruption. But this makes use of an external secondary tie system limiting the location of the substation.
6. Simple Spot Network
This system has been made use of to deliver power to high density areas of cities. A slight modification will make it applicable to data center power supply. One major advantage of this system is the continuous supply of power irrespective of any fault on the primary system. This system can also maintain flexibility to meet load change at minimum cost and minimum interruption in service to other loads on the network. It also provides a consistently efficient voltage regulation and is cheaper. In this, a network provider is connected to the secondary leads of each transformer in place of secondary main breaker. All secondary of the transformers are connected to each other by switchgears. Spot networks are economical to the buildings that have a heavy concentration of loads covering small areas.
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