FIGURE 3.22: Typical Radio Transmitter Unit to Accommodate Up to 12 FCIs. Courtesy of
flashing LED is possible with a fiber optic cable and requires only a ¼-inch hole. The display mounts directly through the hole; there is no Plexiglas cover.
Remote displays allow restoration crews to trace fault indicators faster. This reduces outage time and improves system reliability. However, a determined vandal could break through the Plexiglas and gain entry into pad-mounted equipment. The flashing light indicator presents less risk of forced equipment entry. However, the cooperative engineer should investigate the durability of this device to be sure that it is very difficult to damage or remove. A ¼-inch hole is large enough to probe an object into the pad- mounted enclosure. In areas subject to vandal- ism, a display mounted on the sensor or a remote flashing light display should be consid- ered. In other areas, remote displays of either type are beneficial.
Acoustic annunciation is another specialized type of FCI output. This type of FCI has a bat- tery-powered speaker that emits a distinctive tone after the passage of a fault. Application of acoustic FCIs is generally limited to locations where the equipment could be obstructed by snow or vegetation, thus limiting the effective- ness of visual indicators. Acoustic indicators are usually time-reset with provisions for manual reset during circuit restoration.
Another type of FCI output is a contact suit- able for input to a distribution SCADA system. This approach might be useful in congested areas, such as shopping centers, where there are many fault indicators and an opportunity for communication circuits to connect several FCIs to a common SCADA remote terminal unit.
A final concern is that the display maintains its state during normal handling in the field. IEEE Standard 495 requires an impact resistance test. This test requires the display to maintain its
indication state when the transformer lid is slammed open or shut. This is particularly im- portant for indicators with mechanical flags.
OTHER CONSIDERATIONS Fault Current Withstand
FCIs are exposed to high fault currents. To be reliable, an FCI must continue to operate prop- erly after being exposed to these high current levels. The cooperative engineer should specify that all FCIs meet the Short-Time Current Test of IEEE Standard 495.
Maximum Continuous Current
An FCI must be capable of operation when ex- posed to the maximum continuous load current. Indicators with fixed pickup settings will give false indications if the load current exceeds their rating. Adaptive FCIs have the ability to accom- modate increasing load currents, but, in some cases, these changes in trip characteristics may impair coordination with system overcurrent protection.
Environmental Requirements
An FCI must operate in harsh environments including direct sunlight, earth burial, and intermittent or continuous water submersion. An FCI must also operate under a varying range of temperatures. IEEE Standard 495 requires that FCIs operate properly in an ambient tempera- ture range of -40 to 85°C. In addition, this standard requires the following design tests to ensure that FCIs will function in their harsh environments:
• Temperature cycling test,
• Water submersion test,
• Outdoor weathering of plastics test,
• Salt spray test, and
1. Fault current values should be available from system fault current study.
2. Sometimes the maximum interrupting rating of a protective device is rated in asymmetri- cal amperes but only a symmetrical fault current rating is available. UseEquations 3.1 and 3.2andTable 3.1to convert from symmetrical to asymmetrical.
3. When minimum fault is calculated, a fault re- sistance of zero to 10 ohms for underground cable and 30 to 40 ohms for overhead line is recommended. Zero ohms for underground and 30 ohms for overhead are less conser- vative and should be used only within the restrictions noted in theMinimum Available Faultsubsection and subject to good engineering judgment and knowledge of the system.
4. All load-carrying components should be rated to withstand maximum through-fault currents on the system. If this is not possi- ble, current-limiting fuses or circuit recon- figuration should be used to limit the fault. 5. Proper location of protective devices will
limit fault damage and the number of con- sumers affected by the fault and also help locate the fault. Recommended locations are the following:
(a) In substations,
(b) At the beginning of underground cable, (c) At transitions from underground to
overhead,
(d) On taps off main feeders and sub-feeders,
(e) On transformers, and (f) Within long feeders.
6. Use the cable damage curves inAppendix F to determine if a protective device protects a cable against through-fault damage. The short-circuit curves are normally used; how- ever, the emergency overload curves can be used for a more conservative approach or where the cable is normally operated near its continuous ampacity limit.
7. Where the neutral/shield is reduced in size or is jacketed, the temperature increase in the shield during faults may be more critical than the temperature increase in the phase
conductor.Equation 3.3andTables 3.2 through 3.6can be used to evaluate the temperature increase in the concentric neutral or shield during faults.
8. Table 3.8shows fault levels that may lead to destructive transformer failure for internal faults. If actual withstand levels of I2t values are known for a particular transformer, Equation 3.4should be used to calculate a corresponding maximum symmetrical fault level. Current-limiting fuses should be used to protect against destructive transformer failure in high-fault areas.
9. The magnetizing inrush current point for a transformer is estimated as follows:
Summary and
Recommendations
Transformer Size Magnetizing
Three-Phase Single-Phase Inrush Current
>3 MVA >1 MVA 12 × base-rated
full-load current for 0.1 seconds
≤ 3 MVA ≤ 1 MVA 8 × base-rated
full-load current for 0.1 seconds
Protective device curves should fall to the right of and above this point to prevent unnecessary tripping.
10. A good rule of thumb for cold-load pickup current is the following:
(a) Six times full-load current for one second, (b) Three times full-load current for up to 10
seconds, and
(c) Two times full-load current for 100 seconds up to 15 minutes.
Frequently, these points may be modified on the basis of the type of load and local climate. Protective device curves should fall to the right of and above these points to prevent unnecessary tripping. This coordination may not always be possible. 11. Several types of protective devices are
available for use on an underground system. Most of these are available in a pad-mounted type enclosure. Several of these devices can be operated remotely.