0 MW 1 MW DG 2 MW DG 3 MW DG 4 MW DG
5 MW DG 6 MW DG 7 MW DG 8 MW DG
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Figure 4.8: Cable loading during off peak loading condition [5]
Figure 4.9 shows the case of a network contingency analysis where the Electricity Department DSS 5374 bus section was opened. In this case with 8MVA EG injection, the cable between Electricity Department DSS 5374 and 1/5 Intersite Avenue DSS 16541 loading increases to 127% of its thermal rating. This is also due to insufficient off peak loading at the injection substation to utilize the generated electricity. The electricity has to be transmitted back to 1/5 Intersite Avenue DSS 16541 and ultimately back to the bus bars at the Connaught Major Substation where it is then distributed to the other feeders. [5]
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Figure 4.9: Off peak loading with bus section open during contingency [5]
Figure 4.10 shows the cable loading during peak loading condition under normal operating condition indicates that none of the cables exceed their thermal ratings. However, there was a reduction in loading on certain feeders since the load is now supplied from the EG plant as opposed to the grid. [5]
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Figure 4.10: Cable loading during peak loading [5]
Three Phase Fault Level
Figure 4.11 shows the three phase fault level on the bus bars. The network three phase fault level rose as the number of EG units increase from 1 to 8 units (power output increased from 1MW to 8 MW). This shows the ability of the SG to contribute to a fault on the network. The fault level increase occurred on all feeder bus bars on the network connected to the transformer at the Connaught Major Substation with which the SG plant operates in parallel.
Network records indicate that the increased fault levels remained within safe breaking capacity of the circuit breakers on the network which varied from 250MVA for the older circuit breakers to 350MVA for the newer circuit breakers. The new increased three phase fault level of 193MVA occurred at the Connaught Major Substation front bus bar which the 8MW EG operates in parallel with. The highest increase in fault level was 51.623MVA and this occurred at the injection point. This is due to the fact that SGs have the ability to provide fault current up to 8 times its rated MVA capacity. [5]
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Figure 4.11: Three phase fault level with increased EG penetration [5]
Single Phase Fault Current
The earth fault was simulated at the injection point on the grid. Figure 4.12 shows the worst case earth fault current contribution as 3.095 kA by the generation plant 11kV bus bar, however a decision was taken to limit the earth fault current to 25A per 1MW SG. This was achieved by installing a 254-ohm neutral earthing resistor (NER) on the star point of each generator transformers which then reduced the earth fault to 203 A. [5]
Table 4.3 shows the reduction in Earth Fault current with the use of NER’s.
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Figure 4.12: Single phase fault current with 8 MW EG at the injection bus bar with/without a NER [5]
Table 4.4 shows the EG earth fault contribution for different EG ratings.
Table 4.4: Earth Fault Current limiting using an NER [5]
EG rating (MW) Fault Contribution from the EG (A)
1 25
2 50
3 75
4 100
5 125
6 150
7 175
8 200
Synchronous Generator Power Factor Varied
An assessment of the impacts of operating 4, 6 and 8 MW EG units at varying power factors from 0.9 lagging to 0.9 leading was carried out. The Bisasar Road Landfill gas to electricity project first stage was proposed to have an installed capacity of 4 MW with the plan to buy an additional two 1 MW engines whilst the site had a total generation potential and was designed for 8 MW. Hence I selected 4, 6 and 8 MW to represent the various potential stages of the project for this study. The impacts of the different power factors on the network voltage during peak and off peak periods were studied. [5]
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Figure 4.13 shows the off peak loading and Figure 4.14 shows the peak loading with 4 MW EG. No network violations are experienced.
Figure 4.13: Off peak loading with 4 MW EG [56]
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Figure 4.14: Peak loading with 4 MW EG [56]
With the installation of a 6 MW EG to the network, no voltage violations were experienced during the network peak period as shown in Figure 4.15 However voltage violations were experienced during the off peak period as shown in Figure 4.16 when the generator is operated at leading power factor below 0.98. This is due to two factors namely, lower network load demand during the off peak period resulting in a lower voltage drop on the network bus bars and the injection of reactive power from the synchronous generator when operated in leading Power Factor mode causing a voltage rise. It is therefore not advisable to operate the EG in leading power factor mode on the Springfield distribution network. [56]
133
Figure 4.15: Peak loading with 6 MW EG [56]
134
Figure 4.16: Off peak loading with 6 MW EG [56]
With the injection of 8 MW on to the Springfield distribution network, there were no voltage violations experienced during the peak period as shown in Figure 4.17. However, there were voltage violations experienced when the EG was operated at leading power factor during the off peak period as shown in Figure 4.18. This is due to two factors namely, lower network load demand during the off peak period resulting in a lower voltage drop on the network bus bars and the injection of reactive power from the synchronous generator when operated in leading Power Factor mode causing a voltage rise. It is therefore not advisable to operate the EG at leading power factor in the Springfield distribution network. [56]
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Figure 4.17: Peak loading with 8 MW EG [56]
Figure 4.18: Off peak loading with 8 MW EG [56]
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Landfill Gas to Electricity Generation Process
The construction of the Bisasar Road landfill gas to electricity project, the largest in Africa at the time then went ahead after receving all environmental clearances and the requred funding for the project. The process of extracting and utilizing the landfill gas to generate electricity is then explained below utilising the Bisasar Road landfill site construction to explain the process. The process required to extract the landfill gas from the site in order to be used as a fuel for the landfill gas-to-electricity generation units is shown in Figure 4.19 and entails the following process;
Figure 4.19: Schematic layout for landfill gas-to-electricity [5]
1. Vertical and horizontal extraction gas wells need to be constructed on the landfill site as shown in Figure 4.20 and Figure 4.21. Bisasar Road landfill site has a number of vertical gas wells (VGW) and horizontal gas wells (HGW) constructed.
Gas Collector
Wells in Landfill Site
Landfill Gas Pump &
Flare Station
Landfill Gas Engine and Electricity Generator
Step-up Transformer
Switchgear &
Control Room Generation of Landfill
Gas (CH4, CO2)
Destruction of
Methane (CH4) Generation of Electricity
Supply to Municipal Electricity
Grid
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Figure 4.20: Installation of horizontal gas well [41]
Figure 4.21: Installation of vertical gas wells [41]
2. “Gas collection pipe works are then installed from the extraction wells to the extraction plant from where it is distributed to the electricity generators and the excess to the flare.”
[41] Figure 4.22 shows the gas collection pipework.
138
Figure 4.22: Installation of the gas collection pipework [41]
The Bisasar Road landfill gas management system was designed with a 5000 m3/hr total flow consisting of two 450mm outside diameter (OD) high density polyethylene (HDPE) (nominal pressure up to 12MPa) which allows conveyance of extracted LFG from the landfill to the generation compound. The plant is equipped with two 2500 Nm3/hr variable speed drives operated centrifugal blowers that induce a negative pressure (-10 mbars) on the gas field and this is stabilised by a 2000 m3/hr flare and fed to General Electric Jenbacher (GEJ) spark ignition engine. The pipe work network (better referred to as “fuel carrier”) has had careful consideration given to condensate management with knock out pots and attention to pipe grades i.e. in direction of LFG flow allows for minimum 3% whilst in opposite LFG flow direction is typically minimum 5%. “Bisasar Road has to date some 70 vertical gas wells, 100 horizontal gas wells and 70 gas risers which have managed to deliver approximately 4200Nm3/hr of LFG.” [41]
3. “Centrifugal blowers installed for the sole purpose of extracting the gases from the extraction wells and supplying it to the generators and the excess to the flare” shown in Figure 4.23. [41]
139
Figure 4.23: Centrifugal gas blowers installed [41]
The landfill gas engine is coupled to a SG that uses the extracted gas from the landfill site as a fuel to generate electricity. They generator output from these generatoration modules are commonly available in 1 MW modules (e.g. Jenbacher type 320 engines) or 0.5 MW modules (e.g. Jenbacher type GS – LL 312 engines). Currently the Bisasar Road landfill site has 6 × Jenbacher type 320 engines and 1× Jenbacher type GS – LL 312 engines. The site has a total installed generation capacity of 6.5 MW which was built up in modules over time. Stage one of the Bisasar Road landfill gas to electricity project started with the arrival of the first four gas to electricity engine/generator units shown in Figure 4.24. These engines /generator are in containerised modules rated at 1 MW. The modules contained the engine and generator already assembled except the radiators and cooling equipment which arrived separately and had to be assembled on site. [41]
140
Figure 4.24: Off-loading of the generator modules [41]
4. A flare unit then needed to be installed at the site to flare of the excess extracted gases and to balance the extraction and generation system.
The flare unit was installed at the Bisasar Road landfill site to balance the extraction and generation system by burning up the excess gas which was not utilized by the engines. This also ensuring that the Methane was destroyed even when some of the engines were out of service. A flare unit of rated capacity of 2000 m3/hr was installed at the Bisasar Road landfill site as shown in Figure 4.25. [41]
141
Figure 4.25: Installation of the gas extraction system and the flare [41]
5. “Appropriate size cable and switchgear are installed on site to allow interconnection onto the local distribution network. The generated electricity is injected into the closest 11 kV DSS within the eThekwini Electricity Central distribution network.” [41] Step up generator transformers were installed on site to step up the voltage from the generators.
The output voltages from the generators are 400 V which is then stepped up to 11 kV via the generator transformers which was then fed into an onsite substation. Figure 4.26 shows the installation of the step up generator transformers whilst Figure 4.27 shows the 11 kV switchboard installed at the Bisasar Road generation plant. [41]
142
Figure 4.26: Step up generator transformers installed [41]
Each engine came containerised containing a 20 cylinder gas to electricity engine coupled to 4 pole lap wound synchronous AC generator. Each generator has its own step 0.4/11 kV step- up transformer which then injected the electricity on to a common bus bar on the onsite substation. Then a 300 mm2 paper insulated lead covered copper cable was then used to inject the power into the local Springfield distribution network. [41]
143
Figure 4.27: Bisasar Road 11 kV switchboard [41]
Commissioning of the Bisasar Road Landfill Gas to Electricity Project
Since the generator units arrived completely assembled, it took around three months for all the commissioning to be completed and the generators were finally switched on in mid- March 2008. Figures 4.28 and 4.29 shows the generation profiles from the Bisasar Road landfill site during commissioning in March and during operation in June 2008. [41]
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Figure 4.28: Generation profile for March 2008 (Bisasar Road LFS generators switched on during final commissioning tests) [41]
Figure 4.29: Generation profile for June 2008 at Bisasar Road Landfill Site [41]
The generation plant was subsequently increased to 6.5 MVA from 4 MW in August 2009.
This was achieved by the purchasing two additional 1 MW gas to electricity generators and transferring a 0.5 MW engine purchased for another landfill site where the gas quality and
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