Chapter 5 studies the controllability and operational flexibility of a proposed 10 MW PV farm on a closed portion of an active landfill site. An RMS model was created of the PV farm and the farm was tested for grid code Category B compliance according to the SAREGC. The purpose of the test was to understand the operational and control functionality of the PV farm as required by the SAREGC.
Solar PV in South Africa
Based on the number of bids for solar farms in the REIPPPP in South Africa, it can be seen in Table 4.12 that solar farms are set to play a substantial role going forward in the energy sector. In round one of the REIPPPP, 28 successful bidders were selected making up a total of 1416 MW of capacity. In round two, 19 projects were selected making up a capacity of 1045 MW. Whilst in round 3, 17 projects were selected making up a capacity of 1686 MW and in round 4, 19 projects of 2206 MW capacity was selected. Solar PV (2327 MW) makes up the second largest portion of the 6330 MW projects selected under round 1 to round 4 of the DOE REIPPPP. This indicates that solar farms make up 36.76% of the capacity from the total projects selected in the REIPPPP round 1 to round 4 as shown in Table 5.1. [41]
Table 5.1: Selected bidders in REIPPPP: Round 1 to 4 [41]
Technology MW Awarded
Round 1
MW Awarded
Round 2
MW Awarded Round 3-3.5
Total MW’s Awarded
Round 4
Total MW’s Awarded Round
1 - 4
Solar PV 632 417 465 813 2327
Wind 634 563 787 1363 3347
Solar CSP 150 50 400 0 600
Landfill Gas 0 0 18 0 18
Biomass 0 0 16 25 41
Small hydro 0 15 0 0 15
Total 1416 1045 1686 2206 6330
The average bid per kWh of energy from solar PV has reduced from R2.76/kWh in round one, R1.65/kWh (60% of round one prices) in round two to R0.88/kWh (32% of round one prices) in round three. The average round three price of R0.88/kWh is highly competitive to
159
the price per kWh that eThekwini Electricity purchases electricity from Eskom on the 275 kW Time of Use (TOU) Megaflex tariff structure.
Potential PV Farms Development in Durban: Installation of Solar PV on old closed Landfill sites
For the development of medium to large scale PV farms there need to be large plots of land available that these farms can be built on. These sites need to be also located close to the grid in order to keep the grid connection costs to a minimum. We investigate potential plots of land that can be utilized for the installation of PV farms. Large plots of land close to the grid connection is expensive and not readily available in Durban. However there are currently in excess of 20 old landfill sites within eThekwini Municipality that are owned by the municipality. These are large sites which has limited land use options due to the future settlement of the land. This land hence has very low property value and could be leased relatively cheap from the municipality. This provides excellent potential for medium to large scale PV installations. The nine most suitable sites were selected for further investigation of suitability of the 20 potential old landfill sites. The selection criteria was based on the locality of the site to the grid, sites with the largest area for PV installation, date of closure of the site, settlement of the site to date and safety and security of the equipment on the closed site.
Closed landfill sites offer opportunities for medium/large scale PV installations (>1MWp) because their geotechnical instability prohibits other developments, and the land has therefore no commercial value. In Europe and the US PV installations on closed landfill sites are becoming increasingly common. [38]
Grid Code Requirements from a 10 MW PV Plant
In accordance with the Electricity Regulation Act (Act 4 of 2006) in South Africa, the grid code requirements in the SAREGC shall apply to all RPPs seeking connection to the transmission and distribution system of the respective Network Service Providers (NSPs).
The 10 MW solar PV farm falls in Category B of the SAREGC and hence needs to comply with all the requirements for Category B of the SAREGC. Firstly lets identify the differences in the requirements of Category B compared to Category C which is shown in Table 5.2. The comparison will be between the Category B 10 MW PV farm vs the 25 MW wind farm (Category C) which is discussed on the wind farm Case Study in Chapter 6. [16]
Table 5.2: Difference in requirements between Category B and Category C RPPs [16]
Grid Code Requirements Category B Category C
160
Maximum Size MV connected up
to 20 MW
MV & HV Connected > 20 MW
Reactive Power requirements -0.28≤Q/P≤0.28 -0.33≤Q/P≤0.33
Power Factor Requirements -0.975≤Q/P≤0.975 -0.95≤Q/P≤0.95
PDelta Requirements
(Delta Production Constraint)
No Yes – Minimum of 3% of PAvailable except from PV
Voltage Control Same requirement
Absolute Production
Constraint
Same requirement
Power Gradient Constraint Same requirement
Frequency Response Only include over
frequency response
Includes both under and over frequency response
High Voltage Ride Through Capability
Not required Category C RPP plants are required to withstand voltage peaks up to 120%
measured at the POC for a minimum period of 2 seconds
Low Voltage Ride Through Capability
Same requirement
The difference in requirements between solar farms and wind farms technology according to the SAREGC is that solar farms do not need to provide PDelta requirements as opposed to wind farms. There is also not High Voltage Ride Trough Capability requirement from Category B RPPs.
For this case study, the RMS model of this 10 MW PV farm to be installed at the closed half of the Bisasar Road Landfill site is modelled and studied for Grid Code compliance. Firstly we need to check for a connection point for the PV farm on the eThekwini Electricity distribution network. The current 6.5 MW Bisasar Road Landfill site gas to electricity project feeds the generated electricity into the Connaught Major Substation. For this project, the generated electricity will be fed into the newly built Randles Major Substation. Load readings taken at the Randles Major Substation indicates that with reconfiguration, it is possible to absorb the generated electricity from the 10 MW PV farm as per Figure 5.1. Since this will be PV generation, generation only occurs during sunlight hours hence this will fit perfectly into the substation profile however should the technology been landfill gas to electricity generation then we would have had problems absorbing the generates electricity at certain times of the day.
161
Figure 5.1: Major Substation transformer load readings for one week
Grid Code Compliance Study for the 10 MW Solar PV Farm at Bisasar Road For the purpose of this case study, a 10 MW PV farm RMS model will be used to check grid code compliance. The purpose of this study is to better understand the SAREGC requirements from a 10 MW PV farm and to understand the control and operational functionality of the PV farm that can be utilised to operate the farm during normal and contingency condition in the eThekwini Electricity distribution network.
Compliance to the SAREGC is mandatory and provides minimum guidelines for RPPs to connect onto the transmission or distribution networks in South Africa. In the case of the proposed 10 MW PV farm to be installed at the closed part of the Bisasar Road Landfill, the grid connection will occur at 11 kV and hence based on the plant size and connection voltage level the plant will fall in the grid code category B as per Table 5.3. The plant will then need to comply with all the requirements of the SAREGC Category B.
Table 5.3: SA Renewable Energy Grid Code Categories [16]
Category Minimum Size (kVA) Maximum Size (kVA) Connections Voltage
B 0 20000 MV connected
The SAREGC provides minimum technical requirements that all RPPs needs to comply with prior to the Network Service Provider (NSP) allowing commercial operation on their grid. In
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0
03/13/2015 00:00:00 03/13/2015 05:00:00 03/13/2015 10:00:00 03/13/2015 15:00:00 03/13/2015 20:00:00 03/14/2015 01:00:00 03/14/2015 06:00:00 03/14/2015 11:00:00 03/14/2015 16:00:00 03/14/2015 21:00:00 03/15/2015 02:00:00 03/15/2015 07:00:00 03/15/2015 12:00:00 03/15/2015 17:00:00 03/15/2015 22:00:00 03/16/2015 03:00:00 03/16/2015 08:00:00 03/16/2015 13:00:00 03/16/2015 18:00:00 03/16/2015 23:00:00 03/17/2015 04:00:00 03/17/2015 09:00:00 03/17/2015 14:00:00 03/17/2015 19:00:00 03/18/2015 00:00:00 03/18/2015 05:00:00 03/18/2015 10:00:00 03/18/2015 15:00:00 03/18/2015 20:00:00
MVA
MAJOR SUBSTATION LOAD READINGS FOR A WEEK
162
this case the NSP is the eThekwini Municipality. For the purpose of this case study, the focus will be on the requirements for the connection of a Category B 10 MW PV farm on the eThekwini Municipality distribution network. To check grid code compliance, a 10 MW PV farm was built utilising a type tested RMS model of the plant to check how the plant reacts under different network conditions and set points for grid code compliance in the Digsilent Powerfactory power systems simulation package shown in Figure 5.2. The Digsilent RMS model comprise of 7 step up (1360 kVA, 11/0.4 kV) transformers shown as PTR1 to PTR7 on the model. Each transformer is then supplied by two 680 kVA central inverter from the solar farm. Each inverter is then supplied by multiple strings from the solar farm. The farm 11 kV bus bar connects via two cables onto a 11 kV gridbox which has 2 down stream circuits, circuit 1 which feeds 4 transformers and circuit 2 which feeds 3 transformers.
163
Figure 5.2: Type tested RMS model of PV farm utilised to study Grid Code Compliance SAREGC RPP Design Requirements
The SAREGC has many design and operation requirements from Category B RPPs which will be discussed in brief detail in this case study with some simplified testing methods which was developed and then utilised to test the proposed 10 MW solar PV farm to check Grid Code compliance.
Tolerance to Voltage Deviations
The SAREGC requires Category B RPPs to be designed in order to operate continuously within the POC voltage range specified by Umin and Umax in Table 5.4.
164
Table 5.4: RPP continuous operating voltage limits [20]
VNorminal (Un) [kV] UMin (pu) UMin (kV) UMax (pu) UMax (kV)
11 0.90 9.9 1.08 11.88
Voltage Ride Through Capability
The capability of the RPP to be able to ride through voltage disturbances often caused by faults on the network is very important on the local network to ensure that stability of the grid is maintained at all times. Voltage-Ride-Through-Capability (VRTC) assists with preventing loss of generation on the network when a voltage disturbance is experienced on the network. Hence the code requires the RPP to be designed to withstand voltage drops to zero measured at the POC for a minimum period of 0.15 seconds. This ensures that should there be a fault or disturbance on the network, the network protection has adequate time to operate and isolate the problem circuit without the plant shutting down. The required voltage operating capability of the RPP is shown in Figure 5.3 whilst Figure 4.38 shows the reactive power requirements from the RPP based on a function of the voltage. [18]
Figure 5.3: VRTC for Category B RPP [18]
165
The SAREGC requires the RPP to either supply or absorb reactive current based on the function of the POC voltage (LVRT or HVRT) level following a network incident. It looks at two cases, a case of over voltage and a case of under voltage at the POC. The 10 MW PV farm only needs to comply to the LVRT requirements as per the SAREGC. Figure 5.4 shows the Area A which is normal operating area (0.9 ≤V≤ 1.1), Area B (0.2 ≤ V< 0.2), and Area E (V<0.2), where reactive current support is required to help in stabilizing the voltage. [18]
Figure 5.4: Reactive power requirements during voltage drops or peaks from Category C RPP [18]
Low Voltage Ride Through (LVRT) has to be tested via a power systems simulation package such as Digsilent Powerfactory to simulate the appropriate low voltages for the specific
166
duration as per the SAREGC. It is not possible to test this during functionality during site testing. The purpose is to ensure that the plant remains connected to the grid in the event of a disturbance on the network. HVRT is not a requirement from Category B RPPs and will not be considered in this case study. The purpose of the VRTC study is to check if the 10 MW PV farm remains connected to the grid for Area A and Area B in Figure 5.4 whilst it can disconnect in Area C. The applicability, purpose, test procedure and acceptable pass criteria is described in Table 5.5. The tests carried out to prove grid code compliance is shown in Table 5.6. [18]
Table 5.5: Test Criteria for Voltage Ride Through [16]
Parameter Description
Simulations of fault ride though voltage droops and peaks.
APPLICABILITY
All new RPPs coming on line and after major modifications or refurbishment of related plant components or functionality.
Routine test/reviews: None.
PURPOSE
To confirm that the limits for all power quality parameters specified is met.
PROCEDURE
By applying the electrical simulation model for the entire RPP it shall be demonstrated that the RPP performs to the specifications.
1. Area A - the RPP shall stay connected to the network and uphold normal production.
2. Area B - the RPP shall stay connected to the network. The RPP shall provide maximum voltage support by supplying a controlled amount of reactive power within the design framework offered by the technology, see Figure 5.
3. Area C - the RPP is allowed to disconnect.
4. Area D - the RPP shall stay connected. The RPP shall provide maximum voltage support by absorbing a controlled amount of reactive power within the design framework offered by the technology as required by the SAREGC.
ACCEPTANCE CRITERIA
1. The dynamic simulations shall demonstrate that the RPP fulfils the requirements specified.
Submit a report to the SO, NSP or another network operator three month after the commission.
167
Table 5.6: VRTC tests carried out on the 10 MW PV Farm [18]
Test Fault Type Voltage Dip (pu) Duration (Seconds)
Test 1 Single Phase 0.00 0.15
Test 2 Two Phase 0.00 0.15
Test 3 Three Phase 0.00 0.15
Test 4 Three Phase 0.20 0.59
Test 5 Three Phase 0.50 1.24
Test 6 Three Phase 0.70 1.67
Test 7 Three Phase 0.85 20.0
For Test 1, the case of a single phase fault is simulated for a duration of 150 miliseconds where the voltage drops to zero pu. Figure 5.5 shows the results of a voltage ride through test done on the RPP. This indicates the plant passing the VRTC test as the plant remains connected to the network during the simulations. The tests also reveal that the plant supplies reactive power as required during a low voltage on the network to try and assist in stabilising the network voltage. In Figure 5.5 to Figure 5.11, the Y axis represent the voltage in pu whilst the X axis represent time in seconds.
Figure 5.5: Test 1 - Single phase fault with 0 pu LVRT for 0.15 seconds
168
For Test 2, the case of a two phase fault is simulated for a duration of 150 milliseconds where the voltage drops to zero pu. Figure 4.40 shows the results of a voltage ride through test done on the RPP. This indicates the plant passing the VRTC test as the plant remains connected to the network during the simulations. The tests also reveal that the plant remains connected and supplies reactive power to try and assist in stabilising the network voltage.
Figure 5.6: Test 2 - Two phase fault with 0 pu LVRT for 0.15 seconds
For Test 3, the case of a three phase fault is simulated for a duration of 150 milliseconds where the voltage drops to zero pu. In this test, the PV farm remains connected to the grid with the POC voltage at 0 pu for 0.15 seconds active power reducing close to zero as shown in Figure 5.7. The plant then supplies maximum reactive power to assist with stabilizing the POC voltage as required as required. The plant hence pass the 0 pu LVRT test.
169
Figure 5.7: Test 3 – Three phase fault with 0 pu LVRT for 0.15 seconds
For Test 4, the case of a three phase fault is simulated for a duration of 0.59 seconds where the voltage drops to 0.2 pu. In this test, the PV farm remains connected to the grid with the POC voltage at 0.2 pu for 0.59 seconds whilst supplying about 20% active power as shown in Figure 5.8. The plant also supplies close to maximum reactive power to assist with stabilizing the POC voltage as required. The plant hence pass the 0.2 pu LVRT test.