A protective system should be designed to recognize certain system abnormalities which, if undetected, could lead to damage of equipment or extended loss of service. The design and specification of the system components is an important part of the protective strategy, and power systems are designed to withstand the usual operating contingencies that accompany load changes and line switching operation. The coordination of insulation is an important design consideration, and protective devices are commonly installed to protect expensive apparatus from damaging over voltages.
These problems, although a part of the overall system protection, are outside the scope of protective relaying field. Our concern here is the detection, clearing, and restoration of circuits from damaging abnormal conditions, which we usually callfaults. This requires a knowledge of the types of faults that are likely to be experienced and the kinds of protective devices that can be used to recognize faults and initiate action to clear the fault from the system. The protective system designer must develop a strategy to accomplish this within the framework of available protective equipment while optimizing the restoration of the system to the normal state.
There are several design considerations that must be weighed against cost in devising a protection strategy. We have mentioned an optimum restoration, but this does not necessarily imply the fastest possible restoration. For example, if the protective system intelligence determines that a fault is permanent, there is no point in repeating the reclosing of the circuit before a repair crew has located and repaired the difficulty. Only then can the circuit be restored to its normal state. Thus every occurrence has a unique optimum pattern for returning to normal following a disturbance and this may involve human intervention, such as a physical repair.
Normally, there is no human intervention in the protective system action, however, as this would cause the abnormal condition to persist for an extended time. In cases where this is feasible, the protective system issues an alarm, following which the operator can analyze the situation and manually take any action that is required.
The protective system should also be designedfor minimum loss-of-load, There is no need, usually, to de-energize the entire system because of an isolated fault. The system should have selectivity to isolate the fault such that the minimum interruption occurs. Often this requires automatic reclosing following a circuit opening, since experience has shown that the large majority of faults are temporary and that reclosures are very often successful. Minimum loss-of-load may also require that alternate circuits be available to serve important loads. For example, bulk transmission systems usually have the capability of serving all load with one or more major circuit components out of service.
The protective system should also be designed with due regard for its own unreliability. This means that backup protective systems should be installed to operate in case of primary protective equipment failure so that system damage can be minimized and restoration of normal service can be achieved quickly. It is also important that the protective system be designed such that the system can perform under normal operating conditions. The protective equipment senses system voltages and currents and from these measurements computes a relaying quantity which is compared to a threshold or trip value. This threshold must not be set too low or the protected circuit may be interrupted unnecessarily. Furthermore, threshold values must be periodically reviewed to make certain that these settings are satisfactory for current system loading conditions. This is an operating as well as a design problem.
The operation of protective equipment must be accurate and fast. Bulk power system reliability standards require that systems survive severe fault conditions without causing a system collapse. This in tum requires fast, reliable protective system operation. Thus, there is a direct dependency upon the protective system to achieve a given level of system reliability.
This adds to the challenge of designing an effective protective system and a reliable power delivery system.
Source: Power System Protection - P. Andreson
No comments:
Post a Comment