POWER DISTRIBUTION--Smart Grids

Over the last several years, the term ''smart grid'' has taken the electric power industry by storm, with its use being further cemented in the power industry lexicon with the launch of the IEEE Transactions on Smart Grid Journal in 2010. While a new word, the smart grid actually represents an evolutionary advancement on the technological innovation that has been present in the power industry since its inception in the 1880s. Such pervasive innovation over more than a century resulted in electrification being named the top engineering technology of the 20th century by the U.S. National Academy of Engineering in 2000. The smart grid represents a continuation of this application of advanced technology into the 21st century to take advantage of near ubiquitous computing and communication.

As defined in, "A smart grid uses digital technology to improve reliability, security and efficiency (both economic and energy) of the electric system from large generation, through the delivery system to electricity consumers and a growing number of distributed-generation and storage re sources." Probably the best definition of the attributes of the smart grid is also given in with its listing of six key characteristics:

1. Enables informed participation by customers;

2. Accommodates all generation and storage options;

3. Enables new products, services, and markets;

4. Provides the power quality for the range of needs;

5. Optimizes asset utilization and operating efficiency;

6. Operates resiliently to disturbances, attacks and natural disasters.

While the smart grid covers large generation and high-voltage transmission, it's most germane to the distribution system and ultimately the end-use customer. The distribution system that, quoting from, ''has traditionally been characterized as the most unglamorous component'' of the power grid is suddenly front and center. Rather than just being a passive, radial conduit for power to flow from the networked transmission system, it will be the means for supporting a bi-directional flow of information and energy to customers who are no longer content to just receive a monthly electric bill. The large, new load potential of electric vehicles requires that the home electric meter and the distribution system become smarter, since the grid can not reasonably accommodate charging a large number of car batteries as people return to their garages in the early evening when the remainder of the electric load is at peak demand. In many locations distributed energy re sources, both fossil fuel-based and renewable, means new power flow patterns and continuing challenges for protection engineers.

Underlying the smart grid is the need for a trustworthy cyber infra structure. As more smart grid technologies are deployed, the result will be a power grid increasingly dependent on communication and computing. Disruptions in this cyber infrastructure, either due to accidents, bugs, or deliberate attacks could result in wide scale blackouts.

PROBLEMS

SECTION 2

1 Are laterals on primary radial systems typically protected from short circuits? If so, how (by fuses, circuit breakers, or reclosers)?

2 What is the most common type of grounding on primary distribution systems?

3 What is the most common primary distribution voltage class in the United States?

4 Are reclosers used on: (a) overhead primary radial systems; (b) underground primary radial systems; (c) overhead primary loop systems; (d) underground primary loop systems? Why? SECTION 3

5 What are the typical secondary distribution voltages in the United States?

6 What are the advantages of secondary networks? Name one disadvantage.

7 Using the internet, name three cities in the United States that have secondary network systems.

SECTION 4

8 A three-phase 138 kV_/13.8 kV Y distribution substation transformer rated 40 MVA OA/50 MVA FA/65MVA FOA has an 8% impedance. (a) Determine the rated cur rent on the primary distribution side of the transformer at its OA, FA, and FOA ratings. (b) Determine the per unit transformer impedance using a system base of 100 MVA and 13.8 kV on the primary distribution side of the transformer. (c) Calculate the short-circuit current on the primary distribution side of the transformer for a three-phase bolted fault on the primary distribution side. Assume that the prefault voltage is 13.8 kV.

9 As shown in Ill. 24, an urban distribution substation has one 30-MVA (FOA) and three 33.3 MVA (FOA), 138 kV_/12.5 kV Y transformers denoted TR1-TR4, which feed through circuit breakers to a ring bus. The transformers are older transformers designed for 55_ C temperature rise. The ring bus contains eight bus-tie circuit breakers, two of which are normally open (NO), so as to separate the ring bus into two sections. TR1 and TR2 feed one section, and TR3 and TR4 feed the other section. Also, four capacitor banks, three banks rated at 6 Mvar and one at 9 Mvar, are connected to the ring bus. Twenty-four 12.5-kV underground primary feeders are served from the substation, 12 from each section. The utility that owns this substation has the following transformer summer loading criteria based on a percentage of nameplate rating:

1. 120% for normal summer loading.

2. 150% during a two-hour emergency.

3. 130% 30-day emergency loading.

Determine the following summer ratings of this substation: (a) the normal summer rating with all four transformers in service; (b) the allowable substation rating assuming the single-contingency loss of one transformer; and (c) the 30-day emergency rating under the single-contingency loss of one transformer. Assume that during a two-hour emergency, switching can be performed to reduce the total substation load by 10% and to approximately balance the loadings of the three transformers remaining in service. Assume a 5% reduction for unequal transformer loadings.

10 For the distribution substation given in Problem 9, assume that each of the four circuit breakers on the 12.5-kV side of the distribution substation transformers has a maximum continuous current of 2,000 A/phase during both normal and emergency conditions. Determine the summer allowable substation rating under the single-contingency loss of one transformer, based on not exceeding the maximum continuous current of these circuit breakers at 12.5-kV operating voltage. Assume a 5% reduction for unequal transformer loadings. Comparing the results of this problem with Problem 9, what limits the substation allowable rating, the circuit breakers or the transformers? SECTION 5 11 (a) How many Mvar of shunt capacitors are required to increase the power factor on a 10 MVA load from 0.85 to 0.9 lagging? (b) How many Mvar of shunt capacitors are required to increase the power factor on a 10 MVA load from 0.90- to 0.95 lagging? (c) Which requires more reactive power, improving a low power-factor load or a high power-factor load? 12 Re-work Ex. 3 with RLoad = 40 _/phase, XLoad = 60 _/phase, and XC = 60 _/phase.

SECTION 7

13 Tbl. 10 gives 2010 annual outage data (sustained interruptions) from a utility's CIS database for feeder 8050. This feeder serves 4500 customers with a total load of 9 MW. Tbl. 10 includes a major event that began on 11/04/2010 with 4000 customers out of service for approximately six day (10,053 minutes) due to an ice storm.

Momentary interruption events (less than 5 minutes duration) are excluded from Tbl. 10. Calculate the SAIFI, SAIDI, CAIDI, and ASAI for this feeder: (a) including the major event; and (b) excluding the major event.

14 Assume that a utility's system consists of two feeders: feeder 7075 serving 2000 customers and feeder 8050 serving 4500 customers. Annual outage data during 2010 is given in Tbl. 6 and 10 for these feeders. Calculate the SAIFI, SAIDI, CAIDI, and ASAI for the system, excluding the major event.

SECTION 8

PW 15 Open PowerWorld Simulator case Problem 14_15 which represents a lower load scenario for the Ill. 22 case. Determine the optimal status of the six switched shunts to minimize the system losses.

PW 16 Open PowerWorld Simulator case Problem 14_16 and note the case losses. Then close the bus tie breaker between buses 2 and 3. How do the losses change? How can the case be modified to reduce the system losses? PW 17 Usually in power flow studies the load is treated as being independent of the bus volt age. That is, a constant power model is used. However, in reality the load usually has some voltage dependence, so if needed decreasing the feeder voltage magnitudes has a result of reducing the total system demand, at least temporarily. Open PowerWorld Simulator case Problem 14_17, which contains the Ill. 22 system except the load model is set so 50% of the load is modeled as constant power, and 50% of the load is modeled as constant impedance (i.e., the load varies with the square of the bus voltage magnitude). By adjusting the tap positions for the two substation transformers and the capacitor banks, determine the operating configuration that minimizes the total load plus losses (shown on the display), with the constraint that all bus voltage must be at least 0.97 per unit.

SECTION 9/18

Select one of the smart grid characteristics from the list given in this section. Write a one page (or other instructor selected length) summary and analysis paper on a cur rent news story that relates to this characteristic.

CASE STUDY QUESTIONS

A. What is a smart grid?

B. What provides the foundation for a smart grid?