Overcoming Barriers to Power Purchasing from Very Small Renewable Energy Producers

Transcription

1 from Very Small Renewable Energy Producers W. *, W. Limpananwadi**, B. Kamonsin** and P. Sripattananon** * Department of Electrical Engineering, Chiang Mai University, Chiang Mai THAILAND worawit@doe1.eng.cmu.ac.th ** Provincial Electricity Authority, 200 Ngam Wong Wan Road, Bangkok 10900, Thailand. Abstract This paper presents the regulatory and technical issues concerning the purchasing of electric power from very small power producers (VSPP. It emphasizes on the use of renewable energy which is abundant in Thailand. The paper also includes the description of VSPP s characteristics, the qualified generating processes and conditions. In addition, other details such as the current status of the VSPP plants connected to distribution system of the Provincial Electricity Authority (PEA) and power purchasing price are also briefly presented in this paper. Finally, the paper presents a case study to identify and overcome technical and regulatory barriers encountered in implementing the VSPP.. 1. INTRODUCTION Since 1992 the Royal Thai Government (RTG) has been promoting private sector investment in the electric power generation business in order to increase the efficient operation, to promote the use of indigenous by-product energy sources and renewable energy for electricity generation and to alleviate the government s investment burden of generation and distribution system. Power purchases from small power producers (SPP) with selling capacity not exceeding 90 MW is one of the important measures employed to achieve the target. As of 2003, there were 48 SPPs interconnected to PEA s distribution network with total generation of 2,168 MW selling power to grid at, and 115 kv (PEA, 2003), (Limpananwadi and, 2003). However, SPPs renewable energy were employed far less than their potential. On May 8, 2002 the government approved the regulation of power purchasing from VSPP using renewable energy with selling capacity not exceeding 1 MW ( 5 MW limit is being reviewed). And the guideline for VSPP interconnection to distribution network (EPPO, 2002). The net metering and time of use (TOU) principles are used in this case. More recently in August 2003, the Ministry of Energy had launched a Strategic Plan to Promote New and Renewable Energy Technology Development. An increase of renewable energy share in the country fuel mix from 0.5 % in 2002 to 8% by 2011 is set as a target (Lertsuridej, 2004) PEA is one of the two state enterprises distribution utility that will be directly influenced by this policy. Like distribution utility all over the world, PEA will soon be facing interconnection of DGs with its network. Hence, it deemed timely to investigate impacts of DGs on PEA distribution network. This case study aimed to investigate impacts of multiple DGs on a distribution network. In the past, most papers studied the impacts of a single DG connected to a radial system. In addition, DGs are always connected to distribution networks, which typically have voltage levels from 230/400 V to 145 kv (Jenkins et al, 2001). Widespread use of DG has both advantages and disadvantages. The positives of DG connection on distribution system are voltage level improvement, power loss decrease and system efficiency increase (Ijumba et al, 1999). The impacts of a single DG on voltage profile, power losses, power quality, islanding condition and network reliability were studied by Baker and Del Mello (2000). The positive impact of DG is dependent on the location and power output and reliability of DG. Similarly, the effects of DG penetration on the voltage profile and power losses are subject to location and size of DG (Salman, 1996). The impact of wind turbine connecting on the voltage stability

2 was investigated by Jenkins and Strbac (1997), which was determined by the ratio of DG size and system fault level. Increasing fault level and islanding condition were some of DG s negatives (Woodworth, 1996), including the impact of DG location and size on the voltage, power flow and power losses. In this paper, methodologies employed in this study and descriptions of distribution network changed over the study timeframe are presented. Then, the impacts of multiple DGs are discussed. Finally, the discussions are presented in details. 2. VERY SMALL POWER PRODUCER (VSPP) 2.1 VSPP Electric Generating Process Characteristics In this regulation, the distribution utilities will purchase electricity from VSPP with qualifying electricity generation processes as follows. (1) Electric generation using renewable energy such as solar, wind, mini and micro-hydro, biogas, etc. (2) Electric generation using the following fuels: Waste or residue from agricultural activities or waste from production process of industrial or agricultural products. Products derived from waste or residue from agricultural activities or from production process of industrial or agricultural products. Garbage. Dendrothermal fuels (e.g. tree plantations). (3) Steam residue from production process of industrial or agricultural products in 1 or 2. Any SPP using the above fuels may use commercial fuels such as oil, natural gas and coal as supplementary fuels. However, thermal energy produced from such supplementary fuels for each year does not exceed 25% of the total thermal energy used in electricity generation in that particular year. 2.2 Conditions for Purchasing Electricity from VSPP (1) The distribution utilities will purchase electricity from VSPP with electric generating processes as specified in heading 2.1 (2) above. (2) The maximum power selling to grid of VSPP does not exceed 1 MW with voltage level less than 33 kv at the connection point. However, the power generation of VSPP will also be considered carefully to maintain the system security and to correspond with regulations for synchronization of generators to the utility distribution system. (3) To maintain the system security, the distribution utilities has the right to inspect and/or request the VSPP to check, rectify and improve its power distributing equipment whenever deemed necessary if it may affect the system. 2.3 Power purchasing price from VSPP The net metering and time of use (TOU) principles are used to define the power purchasing price from VSPP. The criteria for determining monthly electricity tariff are as follows; If electric energy used is more than that generated : Net Energy Consumption = Retail rate + F t Retail If electric energy used is less than that generated : Solar 2004: Life, the Universe and Renewables 2 of 11

3 Net Energy Production = Wholesale rate + F t Wholesale Where F t is an adjustment for uncontrollable factors such as fuel cost, currency exchange rate etc. 2.4 Current status of VSPP The current status of VSPP is shown in Table VSPPs have submitted the proposal to PEA. There are 21 VSPPs being reviewed by PEA. Two VSPPs had been connected to PEA grid. Currently, the total generation selling to the grid is 403.1kW. These VSPP are classified by fuel types into 4 categories, namely; solar, biogas, paddy husk and wood chips. The most common fuel type is solar, which has 19 VSPPs. The average and maximum capacity of solar VSPP is 2.7 kw and 42 kw respectively (PEA, 2003). Table 1: Current Status of VSPP (As of Sep 2003) Fuel Type No. of VSPP Proposed Power Sale to Grid (kw) Status 0.4 kv : 22, Non-connected : Connected Solar Biogas Paddy husk 1-1, Wood chips Total , A CASE STUDY This section presents a case study to identify and overcome the impacts of multiple distributed generations on a distribution network. A case of the Fang radial distribution network, a small system located in Thailand s Chiang Mai province is investigated. 3.1 General Descriptions The Fang radial distribution system is located in the northern area of Chiang Mai province, Thailand. The system is supplied by three grid sources with considerable feeder length and multiple distributed generations using hydro, geothermal and heavy oil, which are installed close to the area load centre. There are mismatches of local power generations and demand during peak load and light load, as each hydro power plant is not centrally dispatched. Moreover, two hydro power plants are the run of river type with power generation is highly seasonal. For example, in winter, the hydro power plants cannot generate at full power output during November to February. Prior to 1997, there were four hydro power plants and a geothermal power plant connected to the system. After that, because of an increase in demand, a 9 MW small power producer (SPP) was connected to the system. Its size was apparently much bigger than previous power plants. It was expected that connecting SPP to the system would help the power balance and improve the system reliability. But as a matter of fact, the Provincial Electricity Authority (PEA), which is responsible for distributing power to general customers, had received many customer complaints about frequent supply interruptions. The system had been modified continuously to improve the system reliability. To date, the system reliability is much improved but it still doesn t meet PEA reliability standard. 3.2 Methodology The followings are methodologies to identify impacts of multiple DGs on a distribution network; Solar 2004: Life, the Universe and Renewables 3 of 11

4 (1) Data collection. Interview of power plant technical personnel and review of PEA documents to study: System configuration modifications. System performance e.g. reliability data System operations: annual energy losses, daily load curve, generation profile and SPP operation etc. (2) Study period consideration. Each study period will be identified based on major and significant system configuration modifications. (3) Performance analysis. Reliability, power losses and voltage profile of the system are analyzed during each period. (4) Discussion and conclusions 3.3 System Configuration From data obtained from PEA, the distribution network studied is configured as shown in Figure.1. Prior to 1997, the system was supplied by two grid sources: Chiang Rai (CRA) and Chiang Mai 2 (CMB) substation with considerable feeder length and five principal power plants using hydro and geothermal, G1-G5 as shown in Table 2. Since Aug 1997, a 9 MW SPP has been connected to the system as G6 in Table 2. In Feb 1998, Mae Chan substation (MCA) replaced CRA as a main grid source because of 115 kv transmission system construction in the CRA feeder right of way. The system load at present is ranging from appropriately 3 MW to 17 MW. During high water season, the hydro power plants produce total power of approximately 17 MW which reduces to 10.7 MW during low water season. Each generator feeds the power through its own step-up transformer to 22 kv distribution network. Each line capacity is dependent on line length and plug-in transformer rating (T1 and T2). The maximum power delivered from MCA and CRA are 6 MW and 3.5 MW respectively. The 2 MW power is fed from CMB to a small area of system. This part has not been modified. G2 Fang System, G3 G4 6 km 22 km CRA 1 96 km T2 S1 120 km CMB 2 G km G1 S2 S5 S3 9 MW G6 S4 33 km T1 54 km 3 MCA Figure 1. System configuration and modification Solar 2004: Life, the Universe and Renewables 4 of 11

5 Table 2 System installed generation capacity No Name of Power Plant Installed Capacity (kva) High Capacity (kw) Low Capacity (kw) G1 Mae Mao Hydro PP 2x2,750 4,100 1,000 G2 Mae Kum Luang Hydro PP 2x2,200 2, G3 Mae Jai Micro-Hydro PP 1x G4 Geothermal PP 1x G5 Ban Yang Micro- 2x70,1x Hydro PP G6 S P P, Heavy oil 2x6,000 9,000 9,000 Total (G1-G6) 23,160 17,170 10,700 S1 CRA 4,000 3,500 3,500 S2 CMB 2,000 2,000 2,000 S3 MCA 8,000 6,000 6,000 Total 37,160 28,670 22, Load and Generation Profiles An example of the system daily load curve (load with system power losses) during 1-7 Nov 2000 is shown in Figure 2. It indicates peak loads in evenings ( hrs), at dawns ( hrs) and in mornings (10.00 hrs). Daily Load Curve (Nov 2000) 14,000 12,000 Load (kw) 10,000 8,000 6,000 4,000 1/11/2000 2/11/2000 3/11/2000 4/11/2000 5/11/2000 6/11/2000 7/11/2000 2,000 Figure 2. Daily load Curve Time (hrs) There are two hydro power plants in the system which are run of river: Mae Jai and Mae Kum Luang. Mae Mao hydro power plant has a storage dam. The annual energy production of Mae Jai hydro power plant during is given in Figure 3. It can be assumed that energy generation of each hydro power plant is seasonal and dependent on water level similar to G3. Annual Energy Production of Mae-Jai Hydro Power Plant 700,000 Energy (kwh) 600, , , , , , Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Month Figure 3. Annual energy production of Mae jai Power Plant (G3) Solar 2004: Life, the Universe and Renewables 5 of 11

6 The power generation profile of power plants is illustrated as an example in Figure 4. It is shown that SPP generates constant power at 4.5 MW according to a firm contract. G2 and G3 always operate all day since they are run of river. G1, being a storage dam type, produces the power during hrs. 5,000 Power Generation Profiles on 6/11/2000 4,000 Power (kw) 3,000 2,000 1,000 SPP G1 G2 G ,000 Time (hrs) Figure 4. Generation Profile of main power plants as of 6 Nov Record of System Modification In 1997, before SPP connection, the system was supplied with four hydro power plants and a small geothermal power plant. Because of load growth, SPP was connected to the system in Aug After SPP connection, the system had more frequent outages. Therefore, the system configuration had been modified over the study period from 1997 to 2000 to improve system reliability as shown in Table 2. The first three system configuration modifications are demonstrated in Figure 1. And Figure 5 illustrates the present system. The highlight area of Figure 1 and Figure 5 is supplied with Chiang Mai2 substation, which is unchanged. Table 2 System configuration modifications No Period System modification Switch status 1 Prior to Aug 1997 No SPP S3,S4 opened 2 Aug 1997-Feb 1998 Interconnecte d SPP S4 opened 3 Feb Feb 2000 SPP & MCA S1 opened 4 Feb 2000-present Two parts G6 connected to MCA Remark: all switches in Figure.1 are closed if not otherwise specified. The followings are descriptions of the system configuration modifications which can be divided into five periods; 1. Period 1: The system was supplied by CRA and G1-G5. 2. Period 2: The system was supplied by CRA and G1-G6. 3. Period 3: The system was supplied by MCA and G1-G6 because PEA had constructed the 115 kv system overlapping CRA feeder. Thus, the system was not supplied from CRA during this period. 4. Period 4: The system was divided into two parts according to two grid sources. The first part is supplied by CRA and G1-G5. The second part is supplied by MCA. The 9 MW SPP had been connected to both grid sources but at different time in order to improve system reliability. 5. Finally, the 9 MW SPP was connected to MCA with reduced 4.5 MW power generation. Solar 2004: Life, the Universe and Renewables 6 of 11

7 G2 Fang System, G3 G4 6 km 22 km CRA 1 96 km T2 120 km CMB 2 G km G1 G6 4.5 MW 33 km T1 Part 1 Part km MCA Figure 5. Present system configuration 4. TECHNICAL BARRIER IDENTIFICATION 4.1 Network Reliability The system average interruption frequency index (SAIFI) and system average interruption duration index (SAIDI), and the reports of system configuration modifications obtained from PEA are employed to identify the network reliability impacts. The SAIFI and SAIDI as compared to PEA reliability standard are illustrated in Figure 6 and Figure 7 respectively. It indicates that there were substantial outages, above PEA standards, interruptions per customer in 2000 and the outages had decreased drastically to and in 2001 and 2002 respectively, after the system configuration was modified. The maximum outage duration was 5, minutes per customer in 1998 and was reduced to 1, minutes in The outage causes can be analyzed according to system configuration modifications as follows; In 1997: There were 49.6 interruptions and duration was 3, minutes caused by line faults occurred mostly in conserved forest area. In 1998: After SPP interconnection, the outage and duration increased to interruptions and 5, minutes per customer because of flash over of insulators reported by PEA. This year had highest duration because all power plants were connected to one part system, and considerable time was required to clear the faults and restore the system. In 1999: The outage and duration decreased to interruptions and 3, minutes after the insulators replacement carried out by PEA. In 2000: The outage and duration increased drastically to interruptions and 5, minutes as the system was modified by changing SPP connections to MCA grid sources. In 2001 and 2002: The system configuration was modified into two parts at the end of 2000, the outage and duration had decreased to interruptions and 2, minutes and interruptions and 1, minutes respectively. Before the system was modified into two parts at the end of 2000, one part system was interconnected with all power plants. As any fault occurred, outage was system wide and took more time to restore the system. Therefore, in 2001 the durations of outage was shorter than in 1999 though outage occurred more often. Solar 2004: Life, the Universe and Renewables 7 of 11

8 FANG SAIFI int/customer.year FANG SAIFI PEA SAIFI Std Figure 6. System Average Interruption Frequency Index (SAIFI) 6,000 5, FANG SAIDI minutes/customer.year 4,000 3,000 2, FANG SAIDI PEA SAIDI Std. 1, Figure 7. System average Interruption Duration Index (SAIDI) 4.2 Power Losses The system annual energy losses were calculated by PEA from the net energy of selling to customers and purchasing from all power plants. The system annual energy losses compared with other PEA s northern regional areas is shown in Figure 8. It is shown that the energy losses of the case study system are significant and higher than the other area every years. In the energy losses were 7.59% and 6.77% respectively. The highest energy losses were % in 2000 because the system configuration was modified into two parts with 9 MW SPP, which power generation was much higher than demand and power flowed to grid source. In 2001 the system energy losses was reduced to 7.16% when the power output of SPP was reduced from 9 MW to 4.5 MW. Energy Losses Energy losses (%) Fang PEA's Northern Regional Years Figure 8. Annual energy losses (%) comparison of Fang and PEA s Northern Regional area during Solar 2004: Life, the Universe and Renewables 8 of 11

9 After the system configuration was modified into two parts, Figure10 illustrated the power flow from CRA and MCA. The power generation profile and daily load curve on 6 November 2000 of the system are given in Figure 4 and Figure 9 respectively. The result of power flow into and out of the system shows that there is 1,000 kw power import from CRA grid sources during hrs and hrs and power export to CRA during hrs and hrs. The maximum power exported is about 1,600 kw. For MCA line, the power is always imported from MCA source because in this part the SPP generation of 4.5 MW is less than the demand in this area. These will increase losses due to long feeder. Thus, if the system power generation can follow the demand, the power losses will be reduced. Daily Load Curve on 6/11/ ,000 12,000 10,000 Power (kw) 8,000 6,000 4,000 2,000 Load+losses - -2, Time (hrs) Figure 9. Daily load curve Power flow in and out of the ystem on 6/11/2000 4,000 3,000 2,000 Power (kw) 1, CRA MCA -1,000-2,000 Time (hrs) Figure 10. Power flow in and out of the system 4.3 Voltage Profile The qualitative system voltage profile obtained from PEA documents was studied according to system configuration modifications. During period 1, total power generation was less than the demand, the system was supplied from CRA grid source which was quite far away. Therefore the system had undervoltage problem. After connecting SPP, during period 2-3, the bus voltage level had improved. As the system configuration was divided into two parts during period 4-5, there were some areas closed to SPP and the automatic tap changer transformer (ATCT) had experienced overvoltages. This was due to more power generation of SPP and inappropriate ATCT setting which was set for importing power from grid source with long line. When SPP power generation was decreased as given in Table 4, bus voltage can be kept within the limit (1.05 pu). For example, during hrs, the SPP power generation was reduced from 4.5 MW to 3.7 MW to maintain a voltage of 1.05 pu. However, because of ATCT, the voltage on the low side is higher than the specified limit during light load. On the contrary, during peak load if the power plant does not respond to demand increase, the voltage level will drop below the nominal voltage. Solar 2004: Life, the Universe and Renewables 9 of 11

10 Table 3 Abnormal power generation records of Military power plant Time SPP (MW) Voltage level at SPP bus (pu) > > DISCUSSIONS 5.1 Reliability The system configuration modifications during period 1 to period 4 did not improve the system reliability. However, after the system had been modified to be two parts in period 5, the system reliability has been dramatically improved. Therefore, if the system was isolated from the grid sources, the impact of line faults could have been totally eliminated. 5.2 Power Losses The power losses of the system during period 1 to 4, when the system was a one part, was less than period 5 because after the system was divided into two part, the second part of system is supplied with 9 MW SPP, which was much higher than the demand. Therefore, the power losses increase since the power has flowed out to MCA grid source. After decreasing SPP power generation from 9 MW to 4.5 MW, the power losses decreased dramatically as given in Figure.10. Thus, the power losses of the system will be decreased if the power generation and demand is balance and the system uses the power generated from all power plants located in the area to meet the demand. 5.3 Voltage Profile Prior to SPP connection, the system experienced undervoltage problem because the power generation is less than the demand. After SPP was connected to the system, the voltage level is improved. Therefore, connecting SPP to the system may improve the voltage level. On the other hand, SPP connection may result in system overvoltage. However, this is dependent on the system configuration and operating conditions. 6. CONCLUSIONS This paper has investigated impacts of multiple distributed generations in a radial distribution network. The reliability, power losses and voltage profile are identified as possible barriers to implementation of VSPP. It was found that if the system has power balance between power generation and local demand, the power losses and voltage profile impacts will be optimised. In addition, if the system is not dependent on the grid sources e.g. a micro-grid system, the reliability will be greatly improved. However, these possible technical barriers can be overcome with investment cost. Whether the utility and/or the power producers are willing to share this cost, is an important issue to be resolved. Further study is also required to look into an impact on distribution system protection which is one of major issues as far as the utility is concerned. 7. ACKNOWLEDGEMENTS This work was supported by the Joint Graduate School of Energy and Environment (JGSEE), King Mongkut's University of Technology Thonburi. In addition, the technical data obtained from the PEA Northern Area Office, Chiang Mai Province and Military power plant (Defense Energy, Ministry of Defense) and the Department of Energy Development and Promotion (DEDP, Ministry of Energy) are appreciatively acknowledged. Solar 2004: Life, the Universe and Renewables 10 of 11

11 8. REFERENCES Baker, P.P. and De Mello, R.W. (2000), Determining the impact of distributed generation on power systems: Part I. Radial distribution networks, IEEE Power Engineering Society Summer Meeting, 2000, Volume:3, pp Energy Policy and Planning Office, Power, (2002), VSPP-Regulation of Power Purchasing from Very Small Power Producers Using Renewable Energy, May Ijumba, N.M., Jimoh, A.A. and Nkabinde, M. (1999), Influence of distributed generation on distribution network performance, IEEE Africaon,1999, Volume:2, pp Jenkins, N., Allan, R., Crossly, P., Kirschen, D. and Strbac, G (2000), Embedded Generation IEE Power and Energy Series 31, The Institution of Electrical Engineer, London, United Kingdom Jenkins, N. and Strbac, G. (1997), Impact of embedded generation on distribution network voltage stability, IEE Colloquium on Voltage Collapse, pp. 9/1-9/4. Lertsuridej, P, Policy on New and Renewable Energy Technology Promotion in Thailand, Internatinal Conference PVSEV-14, Bangkok, THAILAND 2004, pp. PL-3 Limpananwadi, W. and, W, (2003), Impacts of small power producers on distribution network: Thailand case study, Power-gen Asia Salman, S.K. (1996), The impact of embedded generation on voltage regulation and losses of distribution networks, IEE Colloquium on the Impact of Embedded Generation on Distribution Networks., pp. 2/1-2/5. Small Power Producer Analysis Section, Research Division, Provincial Electricity Authority, (2003), Report: Current Status of SPP and VSPP connected to PEA s distribution network, September Woodworth, M. (1996), A co-generator's (CHP) viewpoint (embedded generation), IEE Colloquium on the Impact of Embedded Generation on Distribution Networks, pp. 8/1-8/7. Solar 2004: Life, the Universe and Renewables 11 of 11