CHAPTER 6 TRANSMISSION SYSTEM EXPANSION PLANNING AND APPLICATION OF FACTS CONTROLLERS

Transcription

1 108 CHAPTER 6 TRANSMISSION SYSTEM EXPANSION PLANNING AND APPLICATION OF FACTS CONTROLLERS 6.1 INTRODUCTION Planning and development of power system infrastructure to cater to the needs of the people is of great importance at any point of time. However, this task is getting increasingly difficult as the recent days are witnessing an increase in the gap between demand and supply in most of the states of India and due to the meteoric load growth. Developing the transmission corridor in time by the State Transmission Utility (STU) or CTU or Public Private Partnership (PPP) to evacuate the power from the projects and segregation of corridors viz. State sector, central sector, private sector with a provision to interconnect each is an absolute necessity to alleviate congestion and balance the reactive power and ensure availability of power to all the end-users. The goal of relieving the stress on the national grid due to the transfer of surplus power across the states would also be achieved by appropriate planning. In order to derive the benefit of FACTS controllers listed in the Appendix 5, the planners have to suggest the strategic location of FACTS controllers while evolving system expansion planning. To achieve this goal, it is essential to study the various aspects of Transmission System Expansion

2 109 Planning (TEP) in India and accordingly propose certain suggestions to incorporate FACTS controllers in the system expansion planning. 6.2 PROBLEM FORMULATION Several methods have been proposed by earlier researchers for TEP from different viewpoints such as minimizing investment, maximizing security and improving the optimal utilization of transmission assets. In the present research, the various aspects of TEP in India have been reviewed. Suggestions to incorporate FACTS controllers in strategic locations in the grid system during System Expansion Planning have been made based on the issues faced by the utilities in managing the grid. 6.3 FUNDAMENTAL CARE APPROACH FOR TEP Preparation of development plans for any transmission system requires attention on the following basic issues. Infrastructure Planning Horizon The plan period for basic infrastructure for all the sectors is 5 years in both the State and Central Governments of India. The Planning Commission of India and State Planning Commissions including the concerned Ministries review the progress of implementation of the developed plan every year. To this reference, the Power System infrastructure development is to be planned for a period of 5 years for the specific plan period with short term milestones to be achieved every year.

3 110 Flexible Approach The development of any system for a plan period is subject to changes during implementation due to the sudden changes in the internal and external environment of the system like green Energy development and regulatory and reform requirements. Therefore, any system development should be flexible to accommodate the possible changes that may occur during the course of the period of plan. Technology Advancement Approach The benefit of technology advancement such as considering higher capacity infrastructure in place of existing infrastructure and incorporation of FACTS controllers to optimally utilize the infrastructure before planning for a new one have to be explored by referring to international associations. Similarly, inviting global bids for the new infrastructure development and technology collaboration leads to a new way of doing the regular planning such as a new standard in capacity like 800 MW instead of regular 210 or 500 MW Generator. Thus, harnessing technology benefits is vital for the success of planning. Meeting Technical Standards In India, CEA provides guidelines for the development of transmission infrastructure with reference to the technical requirements. Similarly, Utilities have to maintain the system as per the IEGC (2010). Also, power system equipment manufacturers maintain certain design criteria specific to equipments such as MVA capacity, Amps capacity, MVAR capacity and Voltage levels. All such applicable standards compatible to the existing networks have to be chosen before planning the infrastructure.

4 111 State, Regional and Central Level Coordination Requirements The power generating stations are established nearer to the resources. However, power evacuation system and transmission system are developed based on the proportional share of the power to the constituents of the particular region and needs of the other regions. This necessitates proper coordination between states within and outside the regions and Central Utilities and authorities. The State and Central Utilities are committed to provide additional transmission infrastructure facility to facilitate power trading. 6.4 COMPONENTS OF TEP as given below: The formulation of TEP involves various interrelated components Power Evacuation System Planning Network Expansion Planning Reactive Power Management Congestion Alleviation Management Uncertainty Management 6.5 POWER EVACUATION SYSTEM PLANNING Renewable Energy (RE) Projects and its Critical Issues In recent years, Government of India is giving special focus to promote renewable energy projects. RE projects are normally small capacities in nature and distributed. This attribute of RE projects facilitate the evacuation of power from these projects at medium voltage level network, connected to the nearby 110 kv Substations. These RE projects supplying

5 112 power to meet the local demand, reduces the system losses due to the transfer of power from distant generating stations. However, during non-peak demand periods, the EHV transmission infrastructures are under loaded due to the dispersal of RE at the local load centres at the low and medium voltage level leading to the problem of high voltage in the transmission grid. Indian State Utilities are facing the problem of high voltage especially during non-peak hours where the large wind power is injected into the grid. However, the renewable rich Tamil Nadu has an installed capacity of about 7200 MW which is more than 40 % of total Wind installed capacity of India, is experiencing low voltage problem due to large integration of wind power. This is mainly due to the high reactive power requirement of induction generators based WEGs, overloading of lines and transformers at downstream of the grid and the significant reactive power requirement of the system at EHT level. As the wind power is infirm in nature, managing Grid with wind power variability is the major concern for STUs. The major tie lines transferring power from wind concentrated region to distant load centers at EHT level are experiencing high variations in power flow and changes in the direction of power flow based on the conventional power mix with wind energy and demand at that time. For large size group of Wind Projects and Solar Parks, the power evacuation system encounters the problem of under utilization during off- season resulting in over voltage issues. Apart from the above, the gestation period for establishing WEGs is shorter than for the establishment of transmission infrastructure for power

6 113 evacuation. Wind farm promoters are establishing their WEGs in a specified time frame but the utility faces many difficulties in developing transmission system within the specified time frame due to various constraints. This results in non-availability of grid for free wind power. To remove this problem to the extent possible, the promoters are requesting temporary connectivity of their WEGs in the nearby grid till the time of establishment of specified connectivity. This leads to congestion and resulting backing down of wind power Application of FACTS controllers to address RE Issues As per the Electricity Act of 2003, Section 10(1), the WEGs are establishing substation at 110 kv and 230 kv for connecting their cluster of WEGs, there by connecting their 110 kv or 230 kv substations to the grid system. To mitigate the low voltage problem that is encountered, adequate shunt compensation is to be proposed in these substations by the WEG promoters to enable them to meet their reactive power requirements. Hence, the utilities should enforce adequate VAR compensation to be provided by the promoters at the evacuation substations. The problem of high Voltage faced by the utilities during injection of wind power into the grid is to be mitigated by proposing the placement of suitable Switchable/Controlled Reactor at higher voltages above 230 kv. This can be installed at wind farm pooling substations. Thus, the pooling station to be established for the evacuation of RE power has to take care of high voltage issues by the placement of Thyristor controlled Reactors (TCRs). In order to mitigate the high or low voltage problems during wind power injection into the grid and to provide instant VAR support during disturbance, the STATCOMs are largely used in wind concentrated regions.

7 114 Further, STATCOMs shall also be provided based on the VAR requirements of future wind capacity addition. For maintaining the grid stability and to address the above said voltage issues, the dynamic reactive compensation through SVC at strategic locations have to be identified by conducting power flow studies of different scenarios reflecting real time generation mix with different demand conditions. The Strategic locations for the placement of FACTS controllers include major intra-state tie lines and inter-state lines to manage the grid. Thus, as and when integration of wind power into the grid through CTU lines is proposed to export surplus power during high wind seasons, the planners have to take care of the dynamic reactive compensation requirement through SVCs. For large size group of Wind Projects and Solar Parks the power evacuation system has to be designed in such a way that the utilization of the Green Corridor during the off-season can be taken care by properly connecting the nearby lines in order to avoid the condition of minimum load in the corridor during the off-season resulting in issues of over voltage. CEA is sanctioning the schemes proposed by the STU to evacuate wind power by establishing a separate corridor with the 400kV substations at selected locations. Here, the power from clusters of Wind farm Substations are pooled and connected to the proposed 765 kv substations to be executed by CTU which forms a part of national grid. This corridor will facilitate the power export during high wind season. However, utilization of this corridor and overvoltage condition during the off-season has to be taken care of by exploring the possibility of power transfer through this corridor to the nearby load centre and installing line reactors to mitigate over voltages.

8 Conventional Generation Projects and its Issues State Sector Projects The transmission system for the evacuation of power from the new generation projects of the State GENCOs as well as replacement of old obsolete stations shall be planned and developed in accordance to the commissioning schedule of the generation project. Similarly, transmission system for the evacuation of power from the Generation Expansion Projects of the existing plants have to be designed by optimally utilizing the existing transmission system and right-of-way by converting Single Circuit to Double Circuit or Multiple Circuit or Ultra High Voltage (UHV) system. Central Sector Projects The Central Sector Projects are located at different States and the power from these projects is shared among the constituents and the power transfer from one state to the other is done through the method of displacement. This minimizes the power transfer through ISTS lines. A special attention should be given to ensure the availability of ISTS lines for the transfer of power under different practical situations such as additional allocation from unallocated power in the Central Pool, sudden reduction of drawal from the allocated share due to renewable energy injection and sudden changes in the allocation under emergencies and also due to the changes in network conditions. Competitive Bidding based Projects The purchase of power in India is largely done through competitive bidding which is also called the Case 1 and Case 2 bidding. Case 1 is an open bid where the developer has to decide for fuel and location and compete against any developers in general. In Case 2 bids, the developer is expected to

9 116 bid on the basis of specific fuel and specific location where the specifics are provided by the Central/State government which is calling for the bids. Currently, the major power purchases are contracted through competitive bidding by the DISCOMs. The development of transmission system for these Case 1 and Case 2 generation projects are not routine, as the purchasing DISCOMs and quantum of purchase are not known during the development of generation projects. This needs special attention while developing the transmission system. This has to be seen differently by analyzing the demand-supply gap of the State where the project is proposed and the possible other purchasers, by analyzing the demand-supply gap of the other States. Accordingly, the CTUs and STUs have to develop a transmission system to meet the proposed generation addition for the plan period. Power Pool to Evacuate Power from Large Capacity Plants Unlike traditional Utilities, Merchant Power plants (MPPs) compete for customers and absorb the full market risk. There is no guarantee that they will have a minimum off-take of their output. MPPs operating competitively assure efficient production and delivery of power. They must respond to the market needs. Hence, while developing the evacuation system for MPPs, large demand-supply gap has to be factored into the transmission planning for deciding the strengthening of the regional grid to export to other regions apart from possible consumption through the State grid where the generator is located. Generally, different MPPs are connected to 765/400kV pooling station which in turn is connected to the regional grid to fully evacuate the power without constraints. Interconnection of Projects The possible interconnection of projects shall be studied for availing start-up power and for improving the reliability of power evacuation

10 117 during outage of any of the evacuation feeders. This will also enhance the utilization of evacuation feeders and reduce the necessity of more bay provision at substations and avoids laying of new lines for every expansion project/new adjacent project. Issues of Transmission Projects Delays Though planning of transmission schemes are done well in advance, there might be unexpected delays during the execution of the schemes due to various unforeseen reasons and the consequent postponement of the commissioning of the projects. Further, the gap between the building infrastructure by the CTU and STU leads to power delivery constraints between the boundaries of States. The delay in building adequate transmission system forces the system operator to operate the system in critical conditions such as low voltage and overloading which leads to load shedding Application of FACTS controllers to tackle Delays in Transmission Projects The planners have to identify the possible ways of applying FACTS controllers in Transmission Expansion Planning to overcome the problem of delays. The major outcome of the delay is overloading of the transmission lines and under voltages in the system. This can be improved to a certain extent using FACTS controllers appropriately at the optimal location, thereby overloading can be minimized and under voltage can be improved with limited restriction and control measures on load to be supplied.

11 118 The re-locatable FACTS controllers could be utilized at optimal locations based on the studies during delay in commissioning of infrastructure. The modular type and container mounted FACTS controllers are available in the market which are compact in nature and easily relocatable. The studies shall be conducted by considering the delay in the commissioning of one of the power evacuation lines as a critical line and contingency analysis shall be carried out for N-2 condition to create the worst possible scenario and to find out the severity of overloading of the system. Based on this, the application of FACTS controllers for the reduction of SOL and voltage control under contingencies can also be applied. The TCSC, TCPAR and SVC can be utilized for this application. 6.6 NETWORK EXPANSION PLANNING The network expansion planning has to take care of the following multiple objectives to manage the grid efficiently, Manage maintenance outage planning. Avoid total blackout by developing islanding scheme. Achieve special objectives like loss reduction. Manage peak and off peak loads with energy storage options. Meet variation in seasonal demand and supply. Facilitate purchase of power customers. and accomodation of OA In the network expansion planning, it is possible to identify the weak points where the system requires strengthening and development and the area where the utilization has to improve to the optimum before thinking

12 119 of the new infrastructure. This can be achieved by conducting multiple scenario load flow analysis with different objectives as mentioned above. The transmission planning shall be developed giving due consideration to enhancing the transmission capacity margin and meeting certain regulatory requirements such as loss reduction or improvement of the quality of supply and the schemes for operational flexibility through system reconfiguration. The requirements of the above such objective are to be factored into the network expansion planning using FACTS controllers such as TCSC, SVC and STATCOM so as to utilize the existing system more optimally by improving loadability and also improving the system security. 6.7 CONGESTION ALLEVIATION MANAGEMENT Though planning of the network is based on a careful study of the forecasted demand and anticipated capacity addition, congestion may arise during evacuation of power due to unforeseen outages. Hence the carefully planned network should not collapse but deliver the power through alternative arrangement which is to be part of the proactive planning schedule. The inter-area congestion has to be identified by checking the power flow of intra-state tie lines from one major load centre to the other major load centre. Thus, though there is no congestion in the power evacuation lines of the generating stations, congestion may arise in the interarea lines of the next stage based on the load in that area. Further, the removal of congestion in one area may or may not reduce the congestion level in the other area. This condition shall be

13 120 considered while preparing the plan for the strengthening of the system to avoid the unnecessary investments to create a well balanced network. On identifying the congested lines through multiple scenario power flow studies, the power flow can be re-routed through underutilized lines by the placement of TCSCs so as to alleviate or minimize the possible overloads in the system. 6.8 REACTIVE POWER MANAGEMENT Analysis on VAR Billing to Place FACTS controllers Injecting or drawing of reactive power is done preferably on the distribution load point itself, in addition to meeting the reactive power requirement of network at the appropriate point. This will minimize the need for the reactive power exchange between the ISTS and STU boundaries. The voltage at the particular bus in the boundary points is taken as the reference for determining the VAR drawal charges to the recipient. This discourages the recipient to draw the reactive power from the ISTS end. Similarly, VAR injection into the ISTS boundary points is supported with reference to the voltage at the particular bus in the boundary points. STU is billed at the rate of 10 paise per kvarh for VAR drawal when voltage at the metering end is below 97%. In the same way, STU is earning revenue for drawal of VAR when voltage is above 103%. VAR injection when voltage is above 103% and VAR injection when voltage is below 97% is considered equally for billing/payment. Generally, STU has to minimize the reactive power drawal at the boundary point when the voltage is below 95% at the boundary point, and is

14 121 to avoid injecting VAR when the voltage is above 105%. By analyzing this billing, one can fairly arrive at the additional reactive power support needed by the existing system and plan the system improvement accordingly. Thus, STATCOM or SVC can be proposed to absorb or deliver VAR instantly needed by the existing system RPC and RLDC Reports for the Placement of FACTS controllers In India, the Regional Power Committee (RPC) annually conducts the shunt compensation study to assess the shunt compensation requirement for the next year for each constituents in the region based on the current year data during regional peak load condition occurring during the month of March obtained from the Regional Load Dispatch Centre (RLDC) and the Regional Constituents. The data such as list of capacitors in service, likely load growth, new elements scheduled for commissioning during next year are supplied by the constituents of RPC. A weekly report available in the website of RLDC provides information on the performance of the regional grid for the previous 12 weeks. In that report, voltage profile of important substations and details of substations normally having low /high voltages are also available. From this report, the specific area of reactive power support devices such as SVC, STATCOM broadly to be located can be arrived at Analysis on Reactive Power through ICTs to place FACTS controllers Reactive power through Inter-Connecting Transformers (ICTs) shall be minimal. Normally, it should not exceed 10% of the rating of the

15 122 ICT. The analysis of the reactive power flows through the ICTs present in the system indicates preferable locations for placement of FACTS controllers. 6.9 UNCERTAINTY MANAGEMENT Listing Probable Uncertainties All the possible uncertainties arising from the future generation including distributed generation, demand growth, market players strategies, changes in policies in power such as Solar Policy 2012 by different Indian States, proposed infusion of Foreign Direct Investment(FDI), increasing environmental concern, inter-regional transmission constraints, long transmission lead time and judicial directions on public interest are to be analyzed in-depth in the particular State Factoring the Impact of Uncertainties in Transmission Planning The impact of these uncertainties have to be factored into the transmission planning by giving various alternate options with FACTS controllers by giving the budget provisions to tackle the uncertainties. Thus, the techno-economic transmission expansion plans with options for FACTS controllers are to be developed and the best option shall be chosen to meet the current and future environment CONCLUSION The multi-faceted TEP in India concerning various aspects have been reviewed. Based on the issues faced by the utilities in optimal power flow management and reactive power management, certain suggestions have been proposed to incorporate FACTS controllers in strategic locations in the grid system during system expansion planning.