Failures can occur in the electrical disconnect switches, fuses or circuit breakers that control, protect and isolate electrical equipment in electrical power systems, also known as switchgear. When a failure happens it can endanger the operation of an entire electrical power system, the equipment using that power and nearby personnel as well. This is especially true of short circuits.
Of the two types of short circuits typically encountered--the metal short circuit and the arc short circuit--the latter poses the greatest danger. If not detected and responded to in time, an arc flash can cause fires, damage equipment and even cause physical harm. Typical damage includes overheating copper wiring and melting aluminum rods, as well as toxic gasification. Overheating also causes dangerous increases in pressure that can cause switchgear to explode.
Switchgear uses several protection methods including fast bus protection, backup over current protection and arc flash protection. Fast bus protection is available only for high-voltage switchgear, and the backup over current protection method requires significant time to switch the circuit breaker. Therefore, low- and medium-voltage switchgear needs a faster detection system.
Response Time Is Key
An arc develops and becomes destructive within milliseconds. Failure to open a circuit breaker in time can result in enormous losses (Fig. 1). The damage resulting from an arcing accident relates directly to the amount of current flowing through the short and the time duration. However, of the two parameters, time duration is the more critical. Thus, to maximize protection, both the arc flash detector and the entire switchgear system must have a quick response time.
Figure 1: The graph shows the possible damage that can occur with longer arcing times.
Arc Flash Detection Methods
An arcing fault instantaneously releases large amounts of radiant energy, including both light and thermal energy. The light intensity resulting from an arc can be thousands of times higher than normal ambient light. An arc flash detection relay takes advantage of this phenomenon to achieve significantly faster response times—thereby affording significantly greater protection from damage—than the conventional relay. Thus, arc flash detection has become a critical requirement for all switchgear installations. However, light is only one of many indications that an arc flash has occurred.
Figure 2: This system diagram of a generic arc guard includes both optical and current detectors.
An arc flash dramatically increases light, pressure, heat, current and even sound, all of which a switchgear system can be designed to monitor and detect. Although detecting light is perhaps the easiest and fastest detection method, many systems detect two and sometimes a combination of three or more parameters (i.e., light, current and sound). Though the latter is more thorough, the additional cost and complexity of this method can be prohibitive. The most efficient, cost effective, and therefore the most commonly used method combines the detection of light and current. This method has quickly become the solution of choice for the realization of a reliable arc flash detection system.
Arc Flash Detection System
The primary components of an arc flash detection system (light and current detector) are the arc monitor unit, control unit, optical detector, current detector and current transformer. The control unit receives signals from both a high-sensitivity light detector and the upstream current transformer, enabling it to determine whether to trigger the circuit breaker. Clearly, this signaling process must be both fast and reliable to minimize danger and damage. Fiber optics, with its inherent speed and EMI immunity, make it a perfect medium for an arc flash detection system.
The optical detector unit includes an optical emitter and receiver, an optical sensor in the form of a bare fiber loop, and fiber optic cable. The optical sensor collects the flash light and transfers it via fiber optic cable to the fiber optic receiver, which converts the optical signal to an electrical signal that informs the control system when an arc flash is occurring. There are two types of optical sensors commonly used in such systems: the point sensor and the loop sensor. The point sensor approach uses a light sensor and an optical receiver to detect light in a given area, while the loop sensor ( Fig. 3) uses a loop of bare fiber positioned strategically throughout the equipment.
Loop sensor technology offers several advantages when compared to the point sensor. First, it dramatically reduces the cost of installation. A single optical fiber sensor can be as long as 200 feet, typically covering the same protection zone associated with conventional bus-differential protection but at a much lower cost than point sensors. Second, the loop approach eliminates any concerns about shadows from internal structures that might block the direct exposure to an arc flash. Third, if the fiber sensor is configured in a loop, it can provide regular self-checking of the sensor’s integrity and continuity, producing an alarm if a problem is detected.
Wavelength And Illumination
Generally, the wavelength range of an arc flash is 300-1500 nm compound light. Therefore, a 650-nm or 820-nm fiber optic receiver can be used to detect the arc flash light. Two types of fiber optic cables can be used within this wavelength range—plastic optical fiber (POF) or multimode glass fiber cable.
POF cable is best suited for 650 nm since it has the lowest attenuation at this wavelength. POF is also lower cost and easier to install than other types of fiber optic cables. Multimode glass fiber cable has lower attenuation at 820 nm than the POF cable has at 650 nm.
|Sun (summer, in room)||100-1000|
|Operating room light||1m||1500-3000|
Table 1 shows the typical illumination of various light sources. Arc flash illumination is usually from 5K to 50K lux, so it is easy to differentiate the arc flash light from ambient light.
Short circuits are one of the most common faults that occur in switchgear. If the switchgear is not equipped with detection and protection capability, the cost may extend beyond monetary losses to personal injuries. Therefore, it is imperative to ensure each system can detect a fault as soon as it happens to prevent a disaster. The arc flash detection system presented here is one of the most effective detection methods, both in terms of cost and performance.
Alek Indra ([email protected]) is Asia Pacific Marketing Manager in Avago Technologies. His experience and expertise has been in fiber optic products for various applications, like smart grid, drive, transportation and industrial automation. Over the last few years, he has dedicated his work in the areas of renewable energy (e.g Solar and Wind Energy) and train that use fiber optic solution to ensure reliable and safe system.