1113System Protection

To protect distribution system equipment from damaging overloads, that equipment contains protective devices. This protection can be manifest in a

* A common way to denote three-phase service is to list the line-to-neutral followed by the line-to-line voltage. Three-phase circuits are defined by the line-to-line voltage, with its line-to-neutral voltage multiplied by J3 .

variety of forms, but usually through fuses or circuit breakers. The devices sometimes employed to provide the logic to decide when to trip a circuit breaker are referred to as protective relays, or sometimes simply relays. Relays can be programmed for a variety of functions, including (but certainly not limited to) overcurrent, over- and under-voltage, over- and underfrequency, differential current, and reverse current.

The primary objective of protective devices is to de-energize equipment when conditions warrant. To protect equipment or maintain safety, these devices typically respond to faults (e.g., a short circuit) by isolating appropriate equipment. The goal of good protective coordination is to limit the isolation to as small a portion of the system as possible. This contains the disruption and prevents adjacent portions of the system from being affected.

Also, protection schemes need to account for the failure in isolating devices (e.g., fuses, circuit breakers, etc.) by providing redundant protection capability. This is usually accomplished through protective zones whereby protective relays operate quickly for faults within their primary zones and more slowly for faults that are farther away in secondary or tertiary zones. Therefore, if a relay (or its associated circuit breaker) fails, another relay and its circuit breaker will operate as a backup, but, because this secondary system is farther away, it will disrupt a larger portion of the system. A simple example illustrates this principle. A short in a residential circuit will trip the circuit breaker associated with that circuit. Should that breaker fail to clear the fault in time, the main circuit breaker will trip and interrupt power to the entire panel. Similarly, there are other overlapping zones of protection upstream in the distribution system.

Because many faults in overhead transmission and distribution lines are transient in nature (e.g., lightning and small animal contact), the fault persists as long as the circuit is energized and the arc is sustained. When the fault is cleared (circuit is de-energized), the arc is extinguished. In those cases, it is safe to reclose (energize the circuit again). Therefore, most overhead circuits have reclosers that automatically re-energize the line after clearing a fault. If the line continues to trip, that means that a persistent fault exists, and the line goes into lockout (remains de-energized until a line crew inspects the line and manually resets the circuit breaker).

11.2 Operational Concerns

The overriding consideration regarding the interconnection of distributed resources to an electric utility system is ensuring the safety of the maintenance crews that must work on the distribution system. There must be absolute assurance that these systems cannot pose a risk to the safety of the utility linemen at any time. The most robust situation is where the generation device is physically prevented from backflow because it is connected through a transfer switch. A transfer switch connects the load to a normal source of power or an emergency source of power, but because of the construction of the switch itself, these two sources of power can never be connected to each other, even inadvertently. Use of a transfer switch also provides assurance that two sources that have not been synchronized cannot be inadvertently connected. With such a transfer switch configuration, there is no possibility that the generation can backfeed into the power system, and thus there are no adverse impacts to the power system.

However, operation of the on-site generation in parallel with the utility source is often desired. This can be a vitally important means of ensuring that adequate resources are available to meet peak load requirements in excess of generating capacity, or fulfilling a desire to maximize generation by generating either base-load (constant) or peaking power requirements. This could be driven by economies associated with the specific application or by resource availability, as is the case with certain renewable resources. In these cases, the generator may provide only a portion of the total on-site load, in excess of the on-site load, or combinations of the two at various times. The consequence of inadvertently connecting two sources out of sync is immediate and severe. Therefore, any time that it is possible to operate in parallel with the grid, appropriate synchronizing and sync check relays are absolutely necessary.

It is also imperative that the generator does not inadvertently feed the utility during a utility outage. This can be as simple as a directional relay that prevents any backflow. However, in many instances the design may call for the generator to feed back into the utility under normal conditions. This issue can be complicated if there are multiple generators or alternate sources of power generation (e.g., wind or solar generation with battery backup). In those cases, other types of protection such as under-frequency or undervolt-age will be necessary to determine when to isolate from the utility source. The specific design requirements can vary greatly because they depend on many factors.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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