*7.1. Case Study A*

The existing 22 kV Vagarai feeder consists of one standalone 1.5 MW wind turbine ( *W*1), one 1.5 MW wind turbine ( *W*2-*PV*) combined in a hybrid system with a 0.300 MW solar PV system and one standalone 2.8 MW wind turbine ( *W*3). In this case, study A, the cost analysis for the interruption period without using energy storage has been analyzed. Here, there is no generation during load curtailment and interruption periods of the system due to non-availability of battery storage. The results of case study A mainly show the impacts of higher load curtailment i.e., fault trip, main supply failure, shutdowns, etc. Table 1 lists the amount of energy really sold to the TANTRANSCO grid in KWHr and the actual cost of the electricity (\$/month).



The quantum of the energy really purchased from the TANTRANSCO grid in KWHr and the actual cost paid for the electricity (\$/month) is derived in Table 2. The MATLAB/Simulation proves the generation cost is optimum after consideringthe sale of excess stored power. The total price (\$/month) of electricity sold to grid monthly is denoted as *IImport*−*month*.

The export of energy from the wind turbine ( *W*1) is denoted as *EW*<sup>1</sup>*,* the export of energy from wind turbine ( *W*2) plus solar Photo Voltaic (*PV*) is denoted as (*EW*2-*PV*) and the export of energy from the wind turbine ( *W*3) is denoted as *EW3* (per unit of power purchased from TANTRANSCO in INR is 3.30 [\$ 0.48] for the *W*1 and *W*2 plus PV solar hybrid system and INR 3.70 for *W*3 [1 INR = \$0.015]. *C W*1 is the cost of the sale of power to grid due to the standalone 1.5 MW wind turbine. *C W*2 is the cost of the sale of power to the grid due to the hybrid 1.5 MW wind turbine combined with the 0.300 MW solar PV systems and *C W*3 is the cost of the sale of power to the grid due to the standalone 2.8 MW wind turbine. The import of energy from the wind turbine ( *W*1) is denoted as *IW*1, The export of energy from the wind turbine (*W*2) plus solar PV systemis denoted as *IW2 PV* and the export of energy from the wind turbine (*W*3) is denoted as *IW*3 and per unit of power purchased from TANTRANSCO in INR is 3.30 [\$0.48] for *W*1 and *W*2 plus solar PV hybrid system; and INR 3.70 is for *W*3 [1 INR = \$0.015]. Similarly, the import of energy for the wind turbine (*W*1) is denote as *IW*1, the import of energy for the wind turbine (*W*2) plus solar PV systemis denoted as *IW2 PV* and the import of energy for the wind turbine (*W*3) is denoted as *IW*3. Here, the export of energy from wind turbine (*W*1) is denoted as *EW*1 in column (2); export of energy from wind turbine (*W*2) plus solar PV is denoted as *EW*2 *PV* in column (3); and export of energy from wind turbine (*W*3) is denoted as EW3.The amountof energy really purchased by the grid through 22 kV feeders in the 110/22 kV Vagarai SS is tabulated in Table 2.


**Table 2.** Quantum of energy really purchased by the TANTRANSCO grid in KWHr and actual cost paid towards the electricity (\$/month).

The actual current flown at 01.00 hon 19 July 2018 in all segments like 22 kV side, LV 1, LV2, Bus 1 and Bus 2; 110 kV side, GC and 110 kV Udumalpet/Renganathapuram in the switch yard is shown in Figure 7.

**Figure 7.** Actual flowchart of both the 110 kV and 22 kV sides @ 01.00 hour on 19 July 2018 in the 110 kV SS.

The hourly readings of the five 22 kV feeders in the 110/22 kV Vagarai substation and the L.V 1 and L.V 2 of the power transformers1 and 2 on 19 July 2018 are tabulated in Tables 3 and 4.

The 22 kV Bus 2 is connected with three 22 kV feeders, namely (1) Hybrid Generation Regen feeder, (2) Agen Vijay feeder and (3) Appanuthu feeder; whereas the 22 kV Bus 1 is connected with two 22 kV feeders, namely (4) VepanValasu feeder and (5) Mill feeder in the 110/22 kV Vagarai substation.


**Table 3.** Readings of all the 22 kV feeders including the hybrid generation regen feeder in the 110/22 kV Vagarai SS on 19 July 2018.

**Table 4.** Readings of the 22 kV regen feeder, LV1 and LV2 of power transformers 1 and 2, respectively in the 110/22 kV Vagarai SS on 19 July 2018.


From Figure 7 and Table 3, it is seen that the required load was only 60 Amps (30 Amps each for the 22 kV Appanuthu and Agen Vijay feeders) on 22 kV Bus 2 and 75 Amps (10 Amps for 22 kV Mill feeder and 65 Amps for the 22 kV VepanValasu feeder).As the Bus coupler switch No. 12 was kept in an open condition, at 01.00 hon 19 July 2018, the power for an amount of 2.57 MW (75 Amps) out of 5.5 MW (160 Amps) from the hybrid feeder was stepped up to 110 kV by the power transformer (Pr. Tr)-2. And stepped down to 22 kV voltage level by power transformer (Pr. Tr)-1, (or, say from LV-2-Pr.Tr.2–Pr.Tr.1-LV-1) from the balance power of the 22 kV regen feeder, which is kept at 22 kV Bus-2.

Anamount of current of 100 Amps at the 22 kV side go<sup>t</sup> transformed through the power transformer, as 20 Amps at the 110 kV side; and only an amount of 15 Amps at 110 kV equivalent to 5 Amps for the 22 kV side required for Bus 1. The balance amountof 5 Amps would flow to the 110 kV GC and finally be exported through 110 kV Udumalpet/Ranganathapuram feeders. Or, one can say that the required load demand for the 22 kV Veppen Valasu and 22 kV turbinefeeders was only 75 Amps @ 01.00 h, and the balance 25 Amps was fed into the power transformer 1 and stepped up to110 kV.

The actual current flown on all the 22 kV feeders in the 22 kV side in both the 22 kV Bus 1 and Bus 2 are given in Table 4 and Figure 7. At 01.00 h, the demand of the 22 kV Agen Vijay and Appanuthu feeders was 30 Amps each, whereas the hybrid generation of the regen feeder was 160 Amps.

Similarly, duringeach hour of 19 July 2018, the actual incoming generation from the hybrid regen feeder and the load demand of the remainingfour22 kV feeders and power flow to/from the 110 kV GC are recorded and charted in Table 3.

The time vs. current (in Amps) graph is given in Figure 8. From the graph, it is understood that the wind generation alone on the 22 kV hybrid feeder balanced the entire load of four 22 kV feeders. The unutilized amount of energy had been exported through the 110 kV Udumalpet/Ranganathapuram feeders from 01.00 a.m. to 06.00 a.m. on 19 July 2018.

**Figure 8.** Time vs. current graph showing the actual current flowed with respect to hours of the day on each of the 22 kV feeders and LV1, LV2 and GC of the 110 Vagarai substation on 19 July 2018.

As the load increased during the day hours and as the hybrid generation could not cater to the load, 110 kV GC imported power through the 110 kV Udumalpet/Ranganathapuram feeders from 7.00 a.m. onwards. If the 22 kV regen feeder is kept connected only with the solar generation, then there would not be any green energy from the wind and it would not fulfil the requirement at least up to some early hours of the day. In addition, if an energy storage system were implemented in the HRES, it would be more economical and beneficial and facilitate the generation of green energy even during predictable interruptions and curtailment periods also using the interconnectivity of the same grid.

The following Table 5 is a summary of various interruptions occurred in the 110 kV Vagarai SS during the year of 2018.


**Table 5.** Summary of all interruptions like fault trip, 110 kV main supply failure, load shedding, line clear, hand trip, break down and shut down in 110 kV Vagarai SS during the year of 2018.

From the table, it is understood that seven various types of generation interruption have occurredfor a total of 212 hours and 27 min during the year 2018. The different types of interruptions are described as fault trip, (b) load shedding, (c) hand trip, (d) main 110 kV supply failure, (e) total shut down (f) line clear and (g) breakdown of other feeders and equipment. Asummary of all month wiseinterruptions during the year of 2018 in the 110/22 kV Vagarai SS is presented in Table 6. All the interruptions can be grouped into two segments, some interruptions were predictable, and others were not predictable.


**Table 6.** Summary showing all interruptions month wise during the year of 2018.

7.1.1. Interruptions That Would Not Be Predictable and Can Occur Suddenly

Interruptions like (a) fault trip, (b) load shedding (c) hand trip and (d) main 110 kV supply failure are categorized in this group. These interruptions could not be predicted. However, if a proper energy storage system is available; the energy can be stored by diverting the output through the breaker to the energy storage system. The number of occurrences of said unpredictable interruptions was 93 in the year 2018 but the total duration of the interruptions is only 27 h and 57 min, as given in Table 7.


**Table 7.** Summary of unpredictable interruptions during the year 2018 in the 110 kV Vagarai SS.

7.1.2. Interruptions That Are Predictable and Carried Out with Prior Intimation

Interruptions like (e) total shut down, (f) line clear and (g) proposed break down (BD) of other feeders and equipment are categorized in this group. The occurrence of the predictable interruptions during the year 2018, in the 110/22 kV Vagarai SS was 24 in number, but the duration was 184 h 30 min, as tabulated in Table 8.

**Table 8.** Summary of predictable interruptions during the year 2018 in the 110 kV Vagarai SS.


These interruptions are normally taken intentionally for maintenance of equipment in the yard of the substation, it can be predictable, and in fact, the details of the interruptions would be conveyed over phone before being carried out. The duration of the interruptions isvery largeand the green power from the renewable energy sources can be stored; if a proper energy system is incorporated. As there is an alternative supply available for running the wind turbines, it can be possible to harness the green energy and store it in the energy storage system to be sold to the grid in the future.
