Friday, May 4, 2018

Critical Facilities & Data Center Design Consideration: Generator Systems Design (1)

Let's recap the basic concept of mission-critical facilities:-

(1) Uptime Four-Tier Levels
(2) Critical Supply Diagrams and Configurations

The terms “N, N+1 and 2N”, typically refer to the number of power supply and cooling components that comprise the entire data center infrastructure systems. An “N” system is not redundant at all. N+1 and 2N, represent increasing levels of component redundancies and power paths, roughly mapping to the Tiers 2-4.

A paralleled generator system uses two or multiple generators to form a large-capacity generator system. Paralleling multiple generators is achieved by synchronizing the output of the generators and connecting them to a paralleling switchgear (PSG) common bus. Synchronizing the output of the generators requires all of the paralleled generators to have the same voltage, frequency, and phase rotation.

With closed transition back to the utility, PSG will parallel the generators and synchronize the generator output with the utility source for a short duration before transitioning back to utility power. When connecting the generators in parallel or synchronizing with the utility, the following criteria must be met:

  • Matched / proper frequency
  • Matched / correct phase rotation
  • Phase voltages in phase and within specified voltage range

Typical parameters that determine synchronization include a voltage difference of less than 5%, a frequency difference of less than 0.2 Hz, and a maximum phase angle of 5 electrical degrees between the sources.

Closed transition is used when it is desirable to transfer loads with zero interruption of power when conditions permit. It is used when the generator system transfers back to the utility and when load testing the generators with building loads. Closed transition can be either a soft load transfer or a make-before-break transfer. The PSG soft-load transfer synchronizes and operates the generators in parallel with the utility and transfers loads in increments between the two sources, thereby minimizing voltage or frequency transients on the generator plant and utility distribution system.

The typical soft-load-transfer overlap time is around 2 seconds. The make-before-break transfer will parallel the generators and perform a transfer of load from the generator to the utility. This can be the transfer of one large block load or the transfer of multiple block loads having time delays between the block loads. Time-delay transfer can either be programmed through the PSG or the downstream automatic transfer switches (ATS). Typical ATS make-before-break transition overlap time is usually less than 100 milliseconds.

To simplify the design of a paralleled generator system, identical generators should be used with the same manufacturer, ratings, type, output rating and alternator pitch.

If paralleling of dissimilar generators is required because of existing onsite conditions, the design of a paralleled generator system becomes much more complex. Each generator configuration must be evaluated and dissimilar components, such as speed control, voltage regulation, and alternator, must be retrofitted to match.

"Pitch" is the term used to define the mechanical design characteristics of the alternator. Paralleling a generator of 2/3-pitch alternator design with a generator of 5/6-pitch alternator design will result in circulating neutral currents. The circulating current will be disruptive to protective device operation and may damage alternators.


The electrical loads must be classified into emergency loads, required standby loads, and/or optional standby loads that classified loads are separated, and the generator sets are sized so one generator can serve the emergency and required loads purpose.(see the following Figures)

One generator supplies emergency power to emergency loads, required standby loads and optional standby loads.

Multiple generators in parallel supply power to emergency loads,  required standby loads and optional standby loads.

Kindly note paralleled generator systems that rely on a single master control for signals to start and close to a paralleled bus actually replace one failure point with two, as the master control and the communication link between the master control and the generator systems each represent Single Points of Failure. A well-engineered paralleling system will have dual hot-backup control systems, redundant communication pathways, redundant best battery select dc power supplies, multiple breakers, multiple power pathways, and a well-documented procedure for system recovery whenever a component fails.

Benefits of Parallel Generator Systems

Paralleling multiple sources provides increased reliability, flexibility in load management, and maintenance capabilities with little to no disruption. Multiple generators paralleled to a common bus can better serve emergency and business critical loads, particularly for system response time and dynamic load response once in operation. However, more complex, parallel generator standby systems have significant advantages with respect to reliability and redundancy. These advantages include redundancy, efficiency and ease of maintenance and serviceability.


The redundancy inherent in the parallel operation of multiple generators provides greater reliability than a single generator unit for critical loads. If an N+1 configuration is adopted, one generator can be offline for maintenance while serving the required loads. Furthermore, providing a running spare, an N+1 generator configuration will increase the reliability of the generator system from 98% to 99.96% reliability.


Variations in power demand can cause a single larger generator to run at loads of less than 30% of capacity. The optimum operational point is between 75% and 80% of its rated value. The paralleling control system can be equipped with a generator load control that can add and remove generators in response to the actual load/demand of the system. If the load changes and demand reach 90% of running capacity, for example, an additional generator can be started, synchronized, and paralleled to the bus with no time delay.

Maintenance and Serviceability:

Maintenance can be performed without interrupting the availability of the generator system because one generator can be removed from the system to undergo scheduled or unplanned maintenance while the other generators are available to supply the loads.

Go to the next articles:
Generator Systems Design (2) - Generator Ratings
Generator Systems Design (3) - Generator Sizing

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(13) Generator Systems, etc.

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