Electric Distribution – System Hardening and Modernization

Hardening and modernizing the electrical distribution system for any utility involve a number of related, yet separate actions. These actions are typically tied to:

• Effecting Repairs and Replacement,
• Improving Reliability,
• Meeting Load Projections and
• Modernizing the Grid

Most of these actions are already incorporated in existing energy delivery operating plans, but typically in an incremental and tactical fashion consistent with the need to drive towards system reliability performance targets, meet system loading requirements, and/or address asset health and condition concerns. However, with consideration to grid modernization, it is generally accepted that these actions need to be addressed in a more strategic and accelerated fashion.

Repair and Replacement of Critical Assets

Typically, repair and replacement actions originate from an overall maintenance and inspection program and they are prioritized and budgeted based on near-term operational considerations. Recognizing that any Grid Modernization strategy must acknowledge and account for these tactical investments, a successful strategy will provide the design criteria for these repairs and replacements in a manner that supports the overall scheme for grid modernization, Smart Grid and/or AMI.

The following are six distinct repair and replacement actions that serve to illustrate this point:

• Replace Gang Operated Air Break Switches (GOABs) at the Circuit Tie-Ins: The first device encountered in a substation is typically a disconnect switch, whose purpose it is to disconnect the substation from the incoming line (not to disconnect the transformer from the load). The GOAB is “gang operated” in that it has one switch for each of the three phases of a circuit, yet is operated as a group from a single control and “air break” because the switch operates in air rather than in another medium, such as oil. Current electric distribution networks have various GOAB switches that are unreliable and do not incorporate the technologies necessary to realize the full benefits of grid automation. Electric utilities should consider opportunistically installing new remote operated GOABs to address the maintenance concerns posed by the currently installed switches, potentially reduce the duration of outages by allowing for timelier back feeding, and position the electric utility for full-scale automation.
• Replace Load Tap Changers (LTCs): Load Tap Changers are mechanical switching devices that statistically cause more failures and outages than any other component of a power transformer. Their primary function is to regulate voltage (change turns ratio without interrupting the load current) preventing any power quality issues and precluding the need to drop load. Many of the currently installed LTCs require significant maintenance (commonly due to oil leaks), a situation that can be addressed by replacing them with a better product (.
• Replace U-Type Bushings: The U-Type Bushings act as an interface (insulator and conductor) to a device such as a transformer, circuit breaker, arrestor, etc. Many of the electric utilities are experiencing a high failure rate of these bushings, a situation that leaves the device unprotected and vulnerable to damage. Depending on the specific device and its role in the overall electric distribution system, these types of failures can have significant impact of overall system performance.
• Replace Spacer Cable:Spacer cable in non-shielded, non-tensioned, insulated conductors and they are supported in a close triangular configuration by insulating spacers from a high strength messenger/neutral. Though more expensive in terms of construction than open, bare-wire construction, this type of cable, used typically as an express feeder, is well suited for areas with tight clearances. The issue across the industry concerns the overall condition of pre 1984 spacer cable (some of which has seriously degraded). When replacing this cable, there are some areas where the more traditional standard cross-arm, open conductor construction will be more than adequate.
• Upgrade Circuit Breakers:Circuit Breakers protect substation equipment from destructive short-circuit power surges. Since substations are oftentimes supplied from a variety of transmission voltages and those voltages are transformed to various distribution voltages, there is a large variety of circuit breakers (models and types). Typically, the electric utilities are looking to replace the oil and air magnetic breakers and reclosers with vacuum breakers, coordinated with the installation of smart relays.
• Upgrade Relays: Relays are small fast acting automatic switches designed to protect an electrical system form faults and overloads. When a relay senses a problem it quickly sends a signal to one or many circuit breakers to open, or “trip”, thus protecting equipment as well as preventing personal injury. In order to attain the full value of network automation, electric utilities need to review their full complement of relays to ensure they can obtain the full information gathering capabilities of the newer technology relays.

Electrical Distribution Reliability Improvement

Most electric utilities acknowledge that additional sectionalizing (e.g. installation of fuses,  reclosers, normally open ties to nearby circuits) is for a key part of further hardening the system and mitigating the impacts of customer interruptions. Continued attention to pole replacement and closing out system inspection deficiencies will assist in eliminating outages. From both a scoping and impact perspective, the mitigating actions (installing fuses and reclosers) are definitely part of a more holistic and proactive asset management strategy. The following actions typify these types of actions:

• Install Reclosers:An electric recloser senses and interrupts fault currents and automatically restores service after a momentary outage. In the context of grid automation, any recloser installations should meet current planning guidelines regarding number of customers per sectionalized zone with a remote radio capability along with the necessary intelligence to sense over-currents to time and interrupt fault currents, and to re-energize the line by reclosing automatically; and, if a fault is permanent, lock open after a preset number of operations, isolating the faulted section from the main part of the system.
• Install Fuses: Fuses are the most rudimentary form of protection. During normal flow of electricity they allow the current to flow unobstructed but during an unsafe overload, they interrupt the flow of electricity. Placed at taps, fuses help isolate faults and thereby limiting the impact or scope of service interruptions. In terms of their role with respect to grid modernization, the proper fusing of taps allow for maximum value in terms of providing back feeds in the event of an outage, and certainly facilitates the locating of any faults.
• Reliability and Asset Health and Condition Programs: The actions related to these programs typically include various “Fix-It-Now” type items, Priority Pole Repair and Enhanced Tree Trimming, each categorized as basic “house keeping” types of  activities are necessary to maintain the health and condition of the electric distribution system assets.

Load Projections-Short and Long-Term Benefits

There are two areas of focus with respect to capacity and load projections and their impact on both system reliability and gird automation: substation transformer capacity and the flexibility to automatically back feed circuits.

• Substation Transformer Capacity: Most electric utilities meet their non-coincident summer peak levels in terms of transformer capacity; limiting concerns to specific substations in identified growth areas. In order to accommodate any projected shortfall in capacity, may have adopted Mod Subs as the solution, which serve to alleviate any pending capacity challenges and act as a system sectionalizer.
• Increase Circuit Capacity: When evaluating the ability to “back feed” a specific circuit temporarily (a viable approach to service restoration), the actual capacity of the back fed circuit to receive added load is critical. Already a critical element of any reliability performance-based plan, the impact of this capability in an increasingly automation-oriented environment is even more pronounced.

Additional Actions to Maximize the Benefits of Distribution Automation

The following actions are tied exclusively to grid modernization as without an overall drive to achieve full-scale distribution automation; they would not be justified on a cost-benefit basis. However, in the context of grid automation, they are of paramount importance in any electric utility realizing its full benefits.

• Install Gang Operated Air Break Switches (GOABs) at the Circuit Mid-Points as well as at additional Circuit Tie-Ins: This action supports grid automation, recognizing that additional GOABs installed mid-circuit will provide increased fault isolation capabilities; and similarly in select locations where additional circuit tie-ins are required.
• Upgrade Recloser Panels: In addition to installing reclosers complete with remote radio capability as part of any reliability improvement initiative, there are usually installed reclosers with control panels that need to be upgraded to the current Form 6 specification (a pre-positioning action for distribution automation).
• Add Additional Modular Subs: Assuming a plan to implement full-scale system automation and the inherent need for reserve capacity implied in such a plan, electric utilities should consider adjusting its loading limit to a pre-specified percent of substation capacity (sufficient to support any need to back feed the related circuits). The use of Mod Subs may prove to be a cost-effective alternative to constructing additional substations.
• Install Remote Terminal Units (RTUs): As electric utilities modernize and ultimately automate their grid, they invariably must confront the challenge of obsolete RTUs. These units collect data (alarms, events, measurements, etc.), and act as an interface between the sensing devices and a SCADA master controls. Most utilities are installing master controllers that will ultimately coordinate communication between all sensing and controlling devices in the substation and communicate with a central Energy Management System (EMS-linkage to Transmission from an overall coordination perspective). These master controllers will possess the necessary logic to ensure the relays are coordinated, and control certain automatic functions. Should the distribution automation architecture be decentralized, these master controllers will be able to control switching and feeder reconfiguration in response to faults, while the smart relays themselves will control the first response to faults.
• Install Capacitor Controllers: Capacitors play an important role in offsetting the impact of stress on the electric distribution system (particularly during the peak periods of usage) by preventing fluctuations in voltage levels. Having too may capacitors, however, can also be detrimental to system operation. And, though time clocks have been effectively used to turn capacitors on and off based on anticipated peak periods of usage, a more effective method is to install automated controls to call upon capacitors only when needed to manage these stresses.