Every day, utilities face challenges that affect the safe and reliable operation of the distribution system. High-density loads create the need to provide multiple distribution feeders in a confined area. In contrast, rural areas tend to have very low-density loads distributed over many miles. These varying load and distance characteristics create special challenges for utility protection schemes and the ability to maintain adequate voltage supply to customers. The challenges become more complex when small generators want to interconnect to rural distribution systems.

System Status and Approach

Alabama Power Co. (APC; Birmingham, Alabama, U.S.) is an investor-owned subsidiary of the Southern Company, providing electric service to the southern two-thirds of the state of Alabama. APC serves approximately 72,100 miles (116,033 km) of distribution lines, provides service to more than 1.37 million customers in the 44,500 sq miles (115,255 sq km) of service territory, and had total territorial sales of more than US$3 billion in 2003. APC has about 75 MW of generation from small generators connected to the distribution system. Of that 75 MW, approximately 10 MW is from small generators that are interconnected and operate in parallel with the distribution system.

If a customer requests interconnection of a small generator (no more than 20 MW) to the distribution system of APC, engineering staff must determine if the distribution system can continue to safely and reliably operate with the addition of the new resource. Initially, a determination must be made as to the applicability of the Small Generator Interconnection Rule (FERC Order No. 2006). If this rule applies, the proper steps must be taken to comply with the procedures of the order. To determine the impact of the proposed small generator, an interconnection system impact study is performed. The system impact study consists of steady-state voltage, thermal and short-circuit analyses. The customer provides detailed information concerning the machine characteristics, protection information and technical parameters of any transformation installed by the customer, as well as the proposed location of interconnection to the APC system. APC models its system based on the characteristics of the feeders (line construction, conductor types, conductor sizes, voltage-regulating devices, capacitors and protective devices) affected by the proposed location of the small generator. APC's distribution planning staff uses CymDist Distribution System Analysis Software.

Customer Request and Interconnect

APC was approached with a proposal to connect two 1000-kW, three-phase, synchronous generators with an output voltage of 460 V to a common customer bus. These generators met APC's minimum requirements for reactive power support. In addition, the facility was equipped with a protection system that contained relays with the capability to meet APC's requirements for voltage, frequency and current protection. APC was requested to provide transformation to connect to its 12,470-V, 3-phase, 4-wire distribution system. The customer had availability of a methane gas fuel source at a specific location in a rural section of APC's service territory and did not have the flexibility to relocate the generator to an area with heavier load.

The customer's requested point of common coupling (PCC) was located 49,943 ft (approximately 9.5 miles) from the distribution substation. The substation transformer that provides the source for the area is a 25-MVA, 3-phase, 115/12.47-kV transformer. The substation contains two separate feeders on separate breakers from a straight bus configuration on the low-voltage (distribution) side of the transformer with voltage regulation provided by a ±10% 3-phase load tap changer attached to the transformer. The majority of the feeder that the customer requested to connect is a distribution line that was converted from a 44-kV transmission circuit. The radial circuit consists of three #1/0 ACSR primary conductors and one 3/8-inch steel-shield wire acting as neutral with wooden cross-arm construction. Due to the extensive length and exposure of this circuit, an in-line electronically controlled 3-phase oil circuit recloser (OCR) was installed to protect and isolate the line. In addition, voltage regulation (three 150-A, 10% 7.6-kV regulators) was installed approximately 18,000 ft (5486 m) from the substation in order to maintain proper voltage at the end of the feeder. A capacitor bank (600 kVAR) was also located on one of the taps for voltage support.

APC determined that a 2500-kVA, 3-phase, 12,470/480-V pad-mounted transformer was required to serve this customer. The customer will be required to install conductors to the low-side terminals of the pad-mounted transformer, which will be the PCC. Figure 1 shows the substation feeder single-line drawing along with the proposed location of the generator.

Analyzing the Performance

The varying load characteristic of the feeder significantly impacts the performance analysis. Figure 2 graphs the daily minimum and maximum loading on this feeder based on historical data from the previous year. The graph denotes a significant daily and seasonal variation of load on this circuit. The maximum loads on this feeder coincide with APC system peaks, and the minimum loads are coincident with the APC system valleys. In addition, the majority of the customers on this circuit have single-phase loads and the individual phase loadings vary as well.

Previous voltage analysis performed by system planning engineers determined that adequate steady-state voltage supply (±6% of nominal voltage) was available for all of the phases on this circuit under these measured minimum and maximum load conditions. Figure 3 graphically depicts the voltage bandwidth as measured at the substation bus with historical data from the previous year.

The voltage and thermal analyses for this specific request were modeled with system parameters as previously described, and studies were performed under both peak and valley conditions. The generator was modeled in these studies at full output. Each phase of the circuit was reviewed, graphed and analyzed in order to determine if any thermal overloads occur or if voltage variations can occur in excess of acceptable limits. It should also be noted from Fig. 2 that under certain minimum loading conditions, the 2-MVA output of the generating facility is in excess of the load on the feeder. Thermal studies of equipment and conductors showed no problems with the proposed interconnection of the generators on this circuit. However, under valley conditions, the studies indicated that the circuit voltage near the PCC exceeded the maximum allowable voltage limits even when the down-line regulators reduced supply voltage to their maximum-programmed capability. Removal of these regulators mitigated the problem while the generator was operating but created a low-voltage dilemma when the generator was not operating. The only feasible solution on this circuit was to remove the down-line regulators and restring the entire circuit with larger primary conductors. The cost to perform this work was estimated to be more than $1 million and would take almost one year to complete.

Studying Alternatives

Additional studies to determine the impact of possible backfeed into the transmission system during minimum loading conditions were not performed due to the extensive upgrades required to mitigate the voltage variations. Short-circuit analysis identified only minor problems associated with maintaining safe operation of equipment.

Since the results of the system-impact study identified such high cost and extended time to modify the specific feeder in order to safely and reliably connect the generator at this location, other avenues were explored in order to find a more economical accommodation for interconnection. Engineers determined that a circuit from another substation could be extended several miles to the proposed generator's PCC without significant costs. The proposed alternative circuit was more heavily loaded and contained larger primary conductors. The system-impact study was re-evaluated using the alternative feeder and substation. Estimated cost for completion of the new line and minor modifications to the alternate feeder was less than $200,000. In addition, the new line would create a tie that would allow some load to be shifted in the distribution system during abnormal conditions.

In conclusion, the introduction of a generation source on lightly loaded distribution circuits will often create voltage conditions that are costly to mitigate. Many rural circuits serve highly dispersed small loads, and feeder circuits are not typically designed to accommodate a reversal of current or reactive flows. In this case, a more economical option existed; however, most rural circuits will not have alternative proposals to provide better solutions. The utility engineer's ability to study and recognize these conditions is vital to provide reliable service to all customers.

John W. Bowen is a senior engineer at Alabama Power Co. (APC) and has worked for more than 31 years as a distribution engineer. He presently works in the methods and systems department with primary responsibility of distribution system planning and system coordination technical and software support at APC. He has a BSEE degree from Auburn University, an MSEE degree from the University of Alabama in Birmingham, an MBA degree from Samford University, and is a registered professional engineer in the state of Alabama. jwbowen@southernco.com

Carl T. Wall is a senior engineer at Alabama Power Co. with more than 32 years of distribution experience. His primary responsibilities are for underground standards and distributed generation attachments to the distribution system. He has a BSEE degree from the University of Alabama in Birmingham and is a registered professional engineer in the state of Alabama.