SA grid code compliance for medium-high voltage renewable power plants

Transcription

1 SA grid code compiance for medium-high votage renewabe power pants by Sanjeeth Sewchurran, Jay Kaichuran, and Sandie Maphumuo, ethekwini Eectricity Renewabe energy with its short ead times has become an attractive option for the South African energy sector. The Department of Energy's Independent Power Producer Procurement Programme has resuted in great interest shown by Independent Power Producers (IPPs) to buid renewabe power pants in South Africa. This has resuted in a arge number of renewabe energy projects that have come up over the recent few years. The South African Renewabe Energy Grid Code (SAREGC) was then created as a mandatory requirement that a renewabe power pants need to compy with prior to commercia operation on the grid. This paper discusses the requirements of the SAREGC and testing methods for renewabe power pants to ensure compiance to the SAREGC. Energy security, economic and environmenta benefits have produced an increased interest in the widespread usage of Distributed Generation (DG) wordwide [1]. In South Africa there have been an increasing number of requests to connect DG onto Eskom and oca municipa distribution networks. This is driven in part by the prevaiing energy shortages, oad shedding, rising eectricity prices, ower DG technoogy costs, proposed carbon taes, reduction in greenhouse gas emissions (GHG) and carbon footprint, the Department of Energy (DOE) Renewabe Energy Independent Power Producer Procurement Programme (REIPPPP) and DG abiity to aeviate network congestion and to improve overa eectricity security in South Africa [2]. Hitherto, South Africa s renewabe energy (RE) poicy (2003) has been argey driven by the Integrated Resource Pan (IRP) 2010, approved and pubished in May 2011 by the DOE, outines the proposed energy mi for South Africa. The IRP 2010 seeks to increase the overa contribution of new RE generation to MW by This is 42% of a new-buid generation. Based on the approved IRP 2010, on 2 Juy 2011, the Minister of Energy issued a determination for the IPP procurement programme in accordance with Section 34 (1) of the Eectricity Reguation Act, The energy reguator concurred with the ministeria determination on 7 Juy On 19 December 2012, the Minister of Energy made a new determination for the procurement of an additiona 3200 MW capacity to the previous determination of 3750 MW. The tota capacity to be procured is currenty 6925 MW [3]. In accordance with the Eectricity Reguation Act (Act 4, 2006), it is mandatory for a renewabe power pants (RPP) connecting on the Eskom/municipaity transmission or distribution grid to compy with the requirements of the SAREGC. The SAREGC was first pubished in 2010 and has evoved into the current version 2.8 of the code. In the SAREGC, the minimum technica requirements for an RPP is specified and determined based on the size of the pant and the connection votage eve. The grid code is appicabe to a RE technoogies, namey: photovotaic pants, concentrated soar power pants, sma hydro power pants, andfi gas power pants, biomass power pants, biogas power pants and wind power pants. To date, the SAREGC has been used to certify compiance of a medium and arge scae REIPPPP projects connecting onto the oca South African grid. Fig. 1 shows the aocation of REIPPPP RPP ocations in each province from round 1 to round 4 with a cumuative power output of 6330 MW. Size and connection votage eve of an RPP is then assigned to a category ranging from Category A to Category C as defined in Tabe 1. This paper focuses on the requirements for the connection of medium and arge scae (Category B and C) medium votage/ high votage (MV/HV) connected RPPs. The SAREGC requires a testing of RPP compiance to be done at the Point of Connection (POC) and not at the generator terminas as required by some internationa grid codes. SAREGC RPP pant design requirements The SAREGC has many design and operation requirements from Category B and C RPPs. Toerance to votage deviations The SAREGC requires Category B and C RPPs to be designed in order to operate continuousy within the POC votage range specified by U min and U ma in Tabe 2. Votage ride through capabiity Fig. 1: REIPPPP projects in South Africa [4]. The capabiity of an RPP to be abe to ride through votage disturbances often caused by fauts on the network is very important on the oca network to ensure that stabiity of the grid is maintained at a times. Votage Ride Through Capabiity (VRTC) assists with preventing oss of generation on the network 44 65th AMEU Convention 2016

2 Category Minimum size (kva) Maimum size (kva) Type A1 0 13,8 LV connected A2 13,8 100 LV connected A LV connected B MV connected C > MV/HV connected Tabe 1: SA Renewabe Energy Grid Code Categories [5]. Fig. 2: VRTC for Category B and C RPP [5]. Nomina (Un) [kv] U min (PU) U ma (PU) 132 0,90 1, ,90 1, ,90 1, ,90 1, ,90 1, ,90 1, ,90 1,08 Tabe 2: RPP continuous operating votage imits [5]. Study case Number of affected phases Faut duration Retained votage U rt [PU] 1 1 0,15 0, ,15 0, ,15 0, ,59 0, ,24 0, ,67 0, ,85 Tabe 3: Case studies to be carried out for LVRT studies. Fig. 3: Reactive power requirements during votage drops or peaks [5]. when a votage disturbance is eperienced. Hence, the code requires the RPP to be designed to withstand votage drops to zero measured at the POC for a minimum period of 0,15 seconds. Category C RPP pants are further required to withstand votage peaks up to 120% measured at the POC for a minimum period of 2 seconds. The required votage operating capabiity of the RPP is shown in Fig. 2 whie Fig. 3 shows the reactive power requirements from the RPP based on a function of the votage. Both Low Votage Ride Through (LVRT) and High Votage Ride Through (HVRT) RPP capabiity are tested using a power systems simuation package (e.g. DigSient Powerfactory) to simuate the appropriate ow and high votage durations and scenarios to ensure that the pant remains connected to the grid in the event of a disturbance on the network. To check compiance, the IPP is required to provide the Network Service Provider (NSP) with a type tested, manufacturer specific RMS mode of their pant which can then be used to check how the pant behaves for different under and over votage conditions on the network. Checks need to be done to ensure that no disconnection of the pant occurs as ong as the POC votage remains within the curve in Area A, B and D in Fig. 2. The SAREGC requires the RPP to either suppy or absorb reactive current based on the function of the POC votage (LVRT or HVRT) eve foowing a network incident. Two cases are considered: first, a case of over votage; second, the case of under votage at the POC. Fig. 3 shows the Area A which is norma operating area (0,9 V 1,1), Area B (0,9 <V 0,2), and Area E (V<0,2), where reactive current support is required to hep in stabiising the votage whist Area D (V>1,1) requires reactive current absorption to assist in reducing the votage [5]. LVRT studies need to be carried out for the seven case studies defined in Tabe 3 and three scenarios for each case study defined in Tabe 4. Fig. 4 shows an eampe of a LVRT study carried out for a three phase faut with a zero retained votage for 150 ms. Fig. 4 shows the RPP reduces its active power and increases its reactive power during the faut as the votage drops to zero and remain connected for the 150 ms duration as required by the SAREGC. Toerance to frequency deviations The RPP is required to be designed to operate continuousy from Hz and the pant must be abe to withstand phase jumps of up to 20. However if the frequency is higher than 52 Hz for more than 4 seconds or ess than 47 Hz for greater than 200 ms, the pant is aowed to disconnect from the network as shown in Fig. 5. This simuates an over frequency and under frequency event on the grid. Frequency response requirements Fig. 6 shows the requirements for Category 65th AMEU Convention

3 Parameter Magnitude (Hz) f min 47 f 1 f 2 f 3 As agreed with the SO As agreed with the SO As agreed with the SO f 4 50,5 f 5 51,5 f 6 50,2 f ma 52 Tabe 5: Required frequency defaut settings [5]. Fig. 4: Eampe of a LVRT test for a three phase faut with zero retained votage for 150 miiseconds. Set vaue of P Deta % of P Avaiabe Set vaue of Droop 1 4% Start frequency 50,0 Hz 1st test Set point 1 49,85 Hz 2nd test Set point 2 49,5 Hz 3rd test Set point 3 49,0 Hz 4th test Set point 4 48,0 Hz 5th test Set point 5 50,0 Hz Tabe 6: Test for under frequency response [7]. Set vaue of Droop 2 Start frequency 8% 50 Hz 1st test Set point 1 50,50 to 50,55 Hz 2nd test Set point 2 51,00 to 51,05 Hz 3rd test Set point 3 51,10 to 51,20 Hz 4th test Set point 4 51,35 to 51,45 Hz 5th test Set point 5 50 Hz Tabe 7: Test for over frequency response [7]. Fig. 5: Minimum RPP pant frequency operating range [5]. Scenario Active power P Reactive power Q Votage at POC a P = P n Q = Q Ma U = U n b P = P n Q = 0 U = U Ma c P = P n Q = Q Min U = U Ma Tabe 4: Scenarios for initia network parameters for LVRT. B RPPs. The RPP is required to curtai its active power output once the network frequency eceeds 50,5 Hz. Shoud the network eceed 52 Hz for more than 4 seconds, then the RPP may disconnect from the network. In Fig. 7, frequency f 2 to f 3 forms a dead band where no action is required from the pant whist f 1 and f 4 form a contro band. Once the frequency eceeds f 2, indicating an under frequency event (the oad eceeds the network generation) and the pant is required to inject P Deta into the network to assist in stabiising the frequency. The pant is required to foow the Droop 1 setting on the network. Droop is defined as a percentage of the requency change required for an RPP to move from nooad to rated power or from rated power to nooad. A RPPs are required to be equipped with frequency controed droop settings which sha be adjustabe between 0 and 10%. During an over frequency event, the network frequency wi eceed f 3 (there is more generation than oad on the network), the pant is required to foow the Droop 2 setting. This dictates the reduction in power required from the RPP for a change (increase) in frequency. Fig. 7 and Tabe 5 show the required defaut pant frequency settings. Frequencies f 1, f 2 and f 3 shown in Fig. 7 and Tabe 5 wi be set and agreed upon by the IPP and the system operator (SO). To prove grid code compiance on site to the frequency response curve in Fig. 7, a frequency generator is required to inject the frequencies shown in Tabe 5. This is carried out by simuating an under frequency event on the grid to check if the RPP responds in accordance to the requirements of Fig. 7. P Deta is not required from PV RPPs. To begin, seect a vaue for P Deta (P Deta sha be minimum 3% of P Avaiabe ) which is a percentage of P Avaiabe and a suitabe Droop 1 and Droop 2 (vaue range from 0 to 10% athough Droop 1 is usuay seected at 4% and Droop 2 at 8% for testing purposes). Cacuations of the droop settings are shown in Fig. 8. Five tests are carried out as shown in Tabe 6 and the resuts are recorded. Compiance of the tests is determined if the recorded resuts after 10 s is within ±2% of the set point vaue or ±5% of the rated power, depending on which yieds the highest toerance. The net step is to simuate an over frequency event on the grid to check if the RPP behaves according to the requirements of Fig. 7. Seect a suitabe Droop 2 and use a frequency generator to simuate the frequencies in Tabe 7 during the onsite compiance testing. Five tests shoud be carried out as shown in 46 65th AMEU Convention 2016

4 Fig. 6: Power curtaiment during over-frequency for Category B RPPs. Fig. 8: Cacuation of the droop [6]. Fig. 7: Frequency response requirement for Category C pant. Fig. 9: Reactive power requirements for Category B [5]. Tabe 7 and resuts recorded to ensure that the RPP response within the required time and accuracy to check compiance. Contro functions required for the RPP The RPP is required to have the foowing contro functions as shown in Tabe 8 per the respective category. Reactive power capabiity The grid code specifies the reactive power requirements from: Category B pant [ 0,228 (Q/P Ma ) 0,228] and Category C pant [ 0,33 (Q/P Ma ) 0,33] This is measured at the POC, and shown in Fig. 9 and Fig. 10. To check onsite grid code compiance of RPPs with regards to reactive power requirements, tests and measurements need to be carried out in accordance with Tabe 9 which is for the case of U = 1 pu. If U is not equa to 1 pu then the pant sha operate in accordance to Fig. 9 or Fig. 10 (depending on RPP category). The measured vaues are recorded after 30 s upon receipt of the set point to a measured accuracy to the higher vaue of either ±2% of the set-point vaue or ±5% of maimum reactive power. As per Fig. 9 and 10, if the votage at the POC is U Min then the pant ony needs to suppy reactive power whist if the POC votage is at U Ma then the pant is ony required to absorb reactive power. Fig.11 shows the simuations resuts of the reactive power contro test shown in Tabe 9. The simuations were carried out on a Category B RPP at U = 1 pu. The resuts show that the RPP responds to the set points issued and compies by suppying and absorbing the correct amount of reactive power within the correct accuracy and timeine required. Power factor contro function Category B: Sha be designed to operate from 0,975 agging to 0,975 eading, measured at the POC from 20% and above of the rated power. Category C: Sha be designed to operate form 0,95 agging to 0,95 eading, measured at the POC from 20% and above of the rated power. The RPP is required to respond within 30 seconds of receipt of the set point to a measured accuracy of ±0,02 in order to pass the test. The test that needs to be carried out is shown in Tabe 10 which tests the pant's Contro function Category B Category C Frequency contro - Absoute production constraint Deta production constraint Power gradient constraint Reactive power (Q) contro Power factor contro abiity to meet the required maimum power factor vaues in Fig. 12 or 13. The pant must be abe to provide the required power factor from P 20%P Ma. Fig. 14 depicts the simuation of the test set points shown in Tabe 10 for a Category B RPP. Votage contro functions - Votage contro Tabe 8: Contro functions required for RPPs [5]. The votage contro function (VCF) for RPPs is shown in Fig. 15. If the RPP votage set point is to be changed, a set point is issued and the 65th AMEU Convention

5 6 5thA M E U C on v e n t i o n Fig. 10: Reactive power requirements for Category C [5]. Fig. 11: Q contro test set points for testing a Category B RPP at U=1 pu. Test 1 P = Avaiabe Reactive power contro fied Cos (ϕ) Set point 1 Q = 0 Mvar PF set point 1 Set point 2 Qma Set point 3 Q = 0 Mvar Set point 4 Qma Set point 5 Q = 0 Mvar Test 2 Carry out Test 1 at P = 20% Pma 0,228 Pma (overecited) Category B 0,33 Pma (overecited) Category C 0,228 Pma (underecited) Category B change needs to be impemented within 30 s with the accuracy of ±0,5% of VNormina whist the accuracy of ±2% of the required injection or absorption of reactive power according to the droop characteristic defined. The tests to be carried out are shown in Tabes 11 and 12 using 4% and 8% droop. Fig. 16 depicts a simuation of the test set points that the RPP needs to achieve with a votage droop of 4%. Fig. 17 depicts a simuation of the test set points that the RPP needs to achieve with a votage droop of 8%. Power quaity Power quaity (PQ) is required to be monitored at the POC and the foowing parameters sha be monitored: PF set point 4 Rapid votage change compiance against vaues given to the IPP by the NSP. The PQ imits given by the NSP to the IPP are apportioned vaues which take the PQ imits given in NRS 048 and apportioned to incude the upstream contribution together with current and future customer s contribution imits. If the pant vioates the PQ imits, then the IPP wi need to design fiters to be instaed to ensure compiance. Active power constraint function For reasons of system security, the RPP may be requested to curtai active power output when requested by the SO. Hence the RPP sha have the foowing Active Power Constraint functions shown in Fig ,95 overecited Category C ony 1 0,975 under ecited Category B ony 0,95 under ecited Category C ony 1 Tabe 10: Power factor contro function test [7]. Reactive power contro Q(U) characteristic Test 1: Set the droop to 4%: (Qma)/4% Un Reactive power testing at different vaues of active power Set point 1 Nomina votage Set point 2 1,02 of Un Set point 3 0,98 of Un Set point 4 Nomina votage Tabe 11: Votage contro function test with 4% droop [7]. Absoute production constraint Deta production constraint Set point 1 Power gradient constraint Set point 2 1,04 of Un Set point 3 0,96 of Un Set point 4 Nomina votage Harmonics An absoute production constraint (APC) is used to constrain the output active power from the RPP to a predefined power MW imit at the POC. This is typicay used to protect the network against overoading. In order to check compiance of the RPP to the APC function, the pant sha be tested as per Tabe 13. The measured vaues sha be recorded after 30 s after receipt of the set point to a measured accuracy to the higher These power quaity parameters can be checked utiising the type tested, manufacturer specific mode in a power systems simuation package prior to the construction of the RPP. After construction of the RPP, on site PQ meters can be instaed to gather the data which can then be utiised to check Category B ony Absoute production constraint function Unbaance votage and current PF set point 5 0,975 overecited Reactive power contro Q(U) characteristic Test 2: Set the droop to 8%: (Qma)/8% Un Reactive power testing at different vaues of active power Ficker PF set point 3 0,33 Pma (underecited) Category C Tabe 9: Reactive power Q contro test [7]. PF set point Nomina votage Tabe 12: Votage contro function test with 8% droop [7]. vaue of either ±2% of the set-point vaue or ±5% of the rated power for each set-point. If the pant meets the required set-point within the time period and accuracy imit, then the pant passes [5]. 65th AMEU Convention 2016

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7 Fig. 12: Power factor (PF) requirements from RPP Category B [5]. Fig. 14: Set points to power factor contro on a Category B RPP. Fig. 13: Power factor (PF) requirements from RPP Category C [5]. Fig. 15: Votage contro for RPPs [5]. Fig. 19 shows a simuation of the resuts from the APC function test depicted in Tabe 13. Deta production constraint function A deta production constraint (DPC) function is used to constrain the active power from the RPP to a required constant vaue in proportion to the possibe active power. It is typicay used to estabish a contro reserve for contro purposes in connection with under frequency. To check compiance of the RPP to the DPC function, the pant sha be tested as per Tabe 14. The measured vaues sha be recorded after 30 seconds after receipt of the set point to a measured accuracy to the higher vaue of either ±2% of the set-point vaue or ±5% of the rated power for each set-point. If the pant meets the required set-point within the time period and accuracy imit, then the pant passes [5]. Power gradient constraint function A power gradient constraint (PGC) function is used to imit the RPP maimum ramp rates by which the active power can be changed in the event of changes in primary renewabe energy suppy or the set-points for the RPP. A PGC is typicay used for reasons of system operation to prevent changes in active power from impacting the stabiity of the network. The test to check compiance is shown in Tabe 15. The measured vaues sha be recorded after 30 seconds after receipt of the set point to a measured accuracy to the higher vaue of either ±2% of the set-point vaue or ±5% of the rated power for each set-point. If the pant meets the required setpoint within the time period and accuracy imit, then the pant passes [5]. Fig. 20 shows the simuation of the PGC test at a 2 MW/min ramp rate. Signa, communication and contro requirements Tabe 17 shows the signas that are required from the RPP pant. This wi then assist the network controer to manage the RPP connection together with the network more effectivey. Each signa ist made up of a number of signas. Fig. 21 shows a typica screen of signas brought back to the network contro room via SCADA from an RPP. Testing of SCADA compiance The foowing tests sha be performed from the Network Service Providers Contro Room to the RPP Power Park Controer on the day of the grid code compiance tests [5]: Check capabiity to remotey open the breaker at the POC from the respective NSP SCADA. Capabiity to change the mode of operation at the RPP. Check capabiity to change the set-point in any mode of operation such that the RPP adjusts accordingy. The faciity sha be subjected to a sef-isanding condition to determine the response of the anti-isanding protection function. Foowing this test, the RPP s automated response to the synchronisation function to the network at the POC sha be evauated th AMEU Convention 2016

8 Fig. 16: Simuation of votage set points for 4% votage droop. Fig. 17: Simuation of votage set points for 8% votage droop. Seect a P reference vaue in MW P set point 1 P reference to 80% P reference P set point 2 80% P reference to 40% P reference P set point 3 40% P reference to 20% P reference P set point 4 20% P reference to 10% P reference P set point 5 Increasing imit to 30% P reference P set point 6 30% P reference to 50% P reference P set point 7 50% P reference to 80% P reference P set point 8 P reference to 0% P set point 9 RPP sha go back to norma operation Tabe 13: Tests to check operation of constraint function [7]. Deta contro (P Avaiabe >20% of P Ma ) Time for Test: 10 Minutes Test Step 1 Check P Deta contro enabed Step 2 Send e.g. 10% of P Avaiabe (>1 MW) Check if power reduces to set point Step 3 vaue on Park controer, SCADA or better measurement system. Step 4 Step 5 Step 6 Hod for at east 10 min Further tests as optiona. For e.g. onger period if the primary energy does not change during the 10 min test period or other setting ike P Deta of 3% woud be tested Disabe P Deta contro Tabe 14: Tests of deta production constraint function [7]. which do not meet the harmonic emission imits and some which coud not meet the reactive power capabiity imits. Concusion and recommendations Hitherto, there is on-going work being done on the medium and arge scae (MV and HV) connected RPP, which has been captured in the SAREGC Version 2.8. This code is currenty undergoing further changes to incude other interna standards used to cover detaied protection requirements and power quaity requirements. The SAREGC has been Fig. 18: Required RPP active power contro functions [5]. used to certify grid code compiance of a REIPPPP projects to date. However certain pants such as concentrated soar power have encountered probems meeting grid code requirements due to the sow response of the pant. This is going to be addressed in the net version of the SAREGC. Most technoogies easiy compy to the SAREGC, however there has been requests for eemptions from pants This required power quaity meters to be instaed on site to gather data to correcty design harmonic fiters and the instaation of suitabiity sized capacitor banks or SVCs to provide the required reactive power. If a connected RPP to the grid are Grid Code compaint, it wi make operating and managing the network easier for the SO at Eskom Nationa Contro or at the NSP Contro Room. With a pants compying with the SAREGC, the SO wi have both contro and visibiity of these RPP pants making it possibe to despatch and contro. To date there is ecess of 2000 MW of RPPs connected onto the nationa grid in South Africa with a number of RPPs sti in design, impementation or commissioning stages. This paper has provided fundamenta information to stakehoders and utiities regarding the SAREGC requirements and testing methods, which can be utiised to certify compiance to the grid code. 65th AMEU Convention

9 Fig. 19: Absoute production constraint function test. Fig. 20: PGC function 2MW/min ramp rate test. Signas ist List 1 List 2 List 3 List 4 List 5 Description Genera pant data and set points RPP avaiabe estimate RPP MW curtaiment data Frequency response system settings RPP meteoroogica data Tabe 17: Signa required from the RPP pant [5] Fig. 21: Eampes of signas brought back via SCADA [5]. Test 1: Ramp rate = (0,4 P reference The active power has to set to P reference before the start of 1st test. 1st Test: Down ramp rate Ramp rate: (0,4 P reference P reference to 20% P reference 2nd Test: Up ramp rate Ramp rate: (0,4 P reference 20% P reference to P reference The RPP is aowed to go back to norma operation Tabe 15: First test to check PGC function [7]. Test 2: Ramp rate = (0,2 P reference 1st Test: Down ramp rate Ramp rate: (0,2 P reference P reference to 20% P reference 2nd Test: Up ramp rate Ramp rate: (0,2 P reference 20% P reference to P reference The RPP is aowed to go back to norma operation Tabe 16: Second test to check PGC function References [1] DQ Hung, N Mithuananthan and RC Bansa: An Optima Investment Panning Framework for Mutipe Distributed Generation Units in Industria Distribution Systems, Appied Energy, Vo. 124, pp , [2] MM Beo, R Smit, C Carter-Brown and I E Davidson: "Power Panning in a Smart Grid Environment A Case Study of South Africa". In Proceedings of the IEEE Power Engineering Society (PES) 2013 Meeting, Vancouver, BC, Canada, Juy IEEE Epore Digita Object Identifier: /2013. [3] Nationa Energy Reguator of South Africa, Sma-Scae Embedded Generation: Reguatory Rues Consutation Paper, 25 February [4] [5] Grid Connection Code for Renewabe Power Pants (RPPs) connected to the Eectricity Transmission System (TS) or Distribution System (DS) in South Africa, Version 2.8, Nationa Energy Reguator of South Africa, Juy [6] J Möer: Presentation to Renewabe Energy Technica Evauation Committee on Grid Integration of Renewabe Energies, [7] Renewabe Energy Technica Evauation Committee: Renewabe Power Pant Grid Code Compiance Standard Test Procedure Rev Contact Sanjueeth Sewchurran, ethekwini Eectricity, Te , sanjeeth.sewchurran@gmai.com 52 65th AMEU Convention 2016