EXCITATION SYSTEM VOLTAGE AND CURRENT SPECIFICATION TUTORIALS

Rated Current

The continuous current rating should be specified to be equal to or larger than the maximum required by the synchronous machine field under any allowed continuous operating conditions. Note that some machines have a continuous overload rating.

In addition, ANSI C50.12-1982 [2], ANSI C50.13-1989 [3], and ANSI C50.14-1977 [4] allow all machines to operate at rated MVA and within ± 5% of rated terminal voltage. Some machines may require an even wider operating range.

The need for off-frequency operation must also be considered in establishing the rating. Some machines, such as combustion-turbine-drive units, have a variable rating depending on ambient air temperature.

The excitation system for these machines may require a variable rating based on ambient air temperature. In the past, some excitation systems have had a small continuous negative current rating. This was utilized when the machine was operated as a synchronous condenser and the negative field current allowed a slightly greater transmission line charging capability.

The negative current was relatively easily supplied from commutator-type exciters, with little additional complexity. Modern exciters employing solid-state rectifiers do not normally have inherent capability for negative currents.

It could be obtained with great complexity, which cannot justify the associated minimal performance improvement (except for synchronous condensers). Some modern exciters will generate a transient negative voltage to force the decay of field current toward zero.

This should not be confused with the concept of a continuous negative current rating, which is mentioned above. For exciters that are specified with redundant current paths or cooling elements, the continuous rating should apply with the redundant parts out of service. The exciter efficiency and losses should be measured at the rated current and voltage point with all redundant parts in service, since this is the normal operating mode.

Rated Voltage

The continuous voltage rating of a system should be such that the voltage is sufficient to supply the necessary continuous current to the synchronous machine field, with the field at its maximum temperature under rated load conditions.

In addition, the continuous voltage capability should allow operation of the synchronous machine at rated MVA and within ±5% of rated terminal voltage unless otherwise specified.

In determining the required voltage for the continuous as well as the transient ratings mentioned later, all voltage drops, including interconnecting bus or cable run voltage drop, up to the field winding terminals should be considered. Any brush drop voltage should be considered part of the synchronous machine field circuit.

ELECTRICAL SPECIFICATIONS BASIC INFORMATION AND TUTORIALS

What Are Electrical Specifications?


Electrical specifications for buildings or projects are written legal descriptions of the work to be performed by the electrical contractor, subcontractors, and electric power utilities and the responsibilities and duties of the architect/engineer, general contractor, and owner. Electrical specifications and electrical drawings are integral parts of the contract requirements for the performance of electrical work.

Because specifications are a significant part of a legally binding contract, typically involving expenditures of thousands or even millions of dollars, it is important that they be mutually compatible with the drawings and as free as possible of errors or discrepancies.

It has long been known that even minor errors in wording or intent or the presentation of incorrect data or measurements can result in expensive repairs or replacements of hardware, lost time in the completion of the schedule, and serious project cost overruns due to delays and the need for additional labor and supervision.

In most engineering and architectural firms, regardless of size, specifications writers are skilled persons with technical backgrounds who report to a responsible project supervisor. The preparation of an error-free specification is a time-consuming task calling for the writer’s patience and the ability to deal effectively with complex technical details.

The process might call for many drafts and revisions following the review, comments, and corrections made by persons within the architect/engineering organization with specialized know ledge and experience in each of the trades involved in the project. As with drawings, all responsible reviewers are expected to sign the final version that is released for bid.

Nevertheless, this does not relieve specifications writers of their responsibilities, because they are expected to have sufficient knowledge of the project to make them capable of finding and resolving any discrepancies between the specifications and the drawings. Discrepancies are most likely to occur when

A generic master or prototype specification is used without making all of the modifications necessary to reflect what is actually shown on the working drawings. Revisions that should have been made in a previously prepared drawings are indicated only by a note in the revision block, leaving the drawing unchanged.


Revisions in items that are listed both in schedules on the drawings and in the written specifications are made on only one of these documents. For example, there is a discrepancy if the specification calls for one load center but the drawing has been revised to show two load centers and this change is not reflected back to the specifications.

Such a discrepancy could result in unnecessary costs, unless caught in time. For this reason, it is not good professional practice to duplicate the same information on both specifications and drawings. It is preferable that the required information be placed on the document on which it is most logically found to assure compliance, with perhaps a reference to its location on the other document.

If for some reason duplication of information occurs in both drawings and specifications, and it is not practical to delete it from one of the documents, the project supervisor should add a note to the contract before it is put out for bid stating whether the specifications or drawings take precedence.