KILOVAR BRIEFS Date FROM THE McGRAW-EDISON POWER SYSTEMS CAPACITOR PLANT June 1989 GREENWOOD, SOUTH CAROLINA issue 15 Capacitor Technology Advancement Enhance D e s i g n o f L a r g e Capacitor Banks Introduction A few of the difficulties facing utilities today include: All time high peak loads, little new generation coming on-line or being planned, and difficulty in obtaining new rights of way. To address these problems, many utilities are turning towards the installation of shunt capacitor banks, SVC's, and HVDC lines. These trends and the increasingly common practice of replacing old capacitor banks with fewer, consolidated installations, has resulted in an increase in the size of many new substation banks. Yesterday's large banks, c o m m o n l y 20 or 30 M V A R are dwarfed by new 100 or 200 MVAR installations. Also, with the increasing number of static var compensators (SVC's) being installed, these large banks, once relegated to transmission level voltages only, are now appearing in the form of large (200+ MVA R ) banks at voltages as low as 35 kV. Past capacitor technology was limited by two very important concerns; the tank rupture and energy handling characteristics of the capacitor. Although still valid concerns, modern power capacitors exhibit characteristics notably superior to that of older designs. These enhanced characteristics require the re-evaluation of many existing application rules. Figure 1 compares the tank rupture curves for 200 KVAR paper-film, conventional all-film, and high energy all-film capacitor designs. Figure1 ComparativeTank Rupture Curves for 200 k v a rCapacitors Paper-Film Technology: Although most manufacturers ceased the manufacture of paper-film capacitors in the late 1970's, many rules of thumb are still in use today which were based upon the limitations of paper-film capacitor technology. , Post Office Box 1224. Greenwood. SC 29648 The tank rupture curve of a paper-film capacitor exhibits two deficiencies in comparison to all-f ilm capacitor designs: - The tank rupture TCC is much further to the left, making coordination margins small. The tank rupture TCCs were probability curves. This was a result of the uncertainty involved in the high resistance characteristics of the failure location. This resistance results in high arc voltages being present which accelerates the development of gas producing arcs within the capacitor. The latter point explicitly reveals the main drawback in protecting paper-film capacitors; Any overcurrent protection which coordinated with the paper-film's tank rupture TCC still left a probability of case rupture. This uncertainty led to many of the design rules still in place today. Improvements With All F i l m C apacitors All-Film Technoloqy: The development of the all-film capacitor allowed s o m e manufacturers to achieve capacitor designs which adhered to a definite, not a probability, curve. This was essentially due to the low resistance characteristic of the failure location. However, the tank rupture characteristics of all kvar size and voltage rating capacitor units were not the same, requiring a family of curves. H i g h E n e r g y All-Film Technology: The advent of the high energy all-film capacitor has provided for previously unavailable tank rupture performance. The mechanically crimped extended foil internal construction a l l o w s the capacitors to follow one tank rupture curve encompassing all capacitor unit ratings, with up to 10 KA fault current capability. Table I summarizes the salient characteristics of these three designs. Table I Comparison of Tank Rupture Curve Figure 2 Parallel Energy Discharge into Failed Capacitor The amount of parallel energy discharge that a failed capacitor can handle is a critical limiting factor in the design of large capacitor banks. The amount of parallel connected Kvars in each series group of a substation bank is therefore limited by the energy handling capabilities of the failed capacitor. For ease of use, Table 2 converts this energy limitation into an equivalent number of parallel kvar. Table 2 Energy Handling Limitations of Various Capacitors Capacitor Technology Maximum Parallel Energy Equivalent Parallel Connected Kvar Per Series Group 3100 Kvar Paper film 10 kJ Conventional All Film 15 kJ Mechanically Crimped Extended Foil (EX-7) 4650 Kvar 30 kj 9300 Kvar Electrical Parameters Capacitor Technology Max* # of Curves Fault Curve Current Characteristics Needed Probability 1 to 4 4,000A All Film Definite* 4 5,500A High Energy Mechanically Crimped Extended Foil (Type EX-7 ) Definite 1 10,000A Paper Film *By some manufacturers * * For a 200 kvar, 7200 V unit at 1.2 cycles Parallel Stored Energy In substation applications, multiple capacitor units are connected in parallel to form each series group in a bank. When a capacitor unit fails, it fails to a short circuit. Therefore, before the fuse operates when a failure occurs, the energy stored in the parallel connected good capacitors will discharge through the failed unit. This situation is depicted in Figure 2. The ability of the mechanically crimped extended foil capacitors to be applied safely with up to 9,300 kvar of parallel connected capacitors per series g m u p is a significant breakthrough in relation to past practice. One rule frequently cited by utility personnel is that contained within the NEMA Standard CP-1 issued in 1976. This standard limits parallel connected kvar to 3,100. When use is made of the 9300 parallel kvar l i m i t available today, significant economic benefits may result. These benefits are best illustrated through use of an example. S a m p le Bank Design Using High Energy All-Film Capacitors For our sample installation, a 43,200 kvar, 69 kV L-L, grounded wye bank was chosen. Utilizing the high energy all-film capacitor design l i m i t of 9,300 k v a r , the following design is achieved. = 43,200 / 3 = 14,400 Kvar One phase parer Number of series groups = 2 (using 19.92 kV capacitor units) Power per Series Group = 14,400 / 2 = 7,200 Kvar = 400 kvar Power per Capacitor Number of Parallel Units = 7,200 / 400 = 18 Total number of Capacitors per bank = 3 x 2 x 18 = 108 (1) (2) (3) (4) (5) Figure 3 illustrates the normal mechanical configuration of this bank. Note that this bank fits entirely into one, three-phase assembly. Power per Split wye = 43,200 / 2 = 21,600 kvar (7) = 21,600 / 3 = 7,200 kvar (8) One phase power Number of series groups = 2 (using 19.92 kV capacitor units) (9) Power per Series Group = 7,200 / 2 = 3,600 MVAr (10) = 400 kvar (11) Power per Capacitor Number of Parallel Units = 3,600 / 400 = 9 (12) Total number of Capacitors per bank = 2 x 3 x 2,x 9 = 108 Figure 4 illustrates the normal mechanical configuration of this bank. Note that this bank consists of two structures, each similar in configuration to that required by the single wye bank. One Three Phase Stack Figure 3 Layout of 43.2 Mvar Bank Using High Energy All-Film Capacitors The Current-Limitinq F u s e Alternate Redesigning the bank to utilize conventional all-film capacitors presents a problem. From equation (3), it is apparent that the 7,200 kvar in parallel per series group is in excess of conventional capacitor's capability. The most direct solution is to replace the expulsion fuses with current-limiting fuses. This allows the bank configuration to remain the same. However, current-limiting fuses have two inherent drawbacks: - They are significantly more expensive. Typical costs for original or replacement current-limiting fuses are roughly 5 times the cost of the initial expulsion fuse assembly. Current-limiting fuses exhibit much higher power losses. The 2OT expulsion fuse link typically used in this application contributes a negligible amount of watts per kvar of losses. The 25 a m p current-limiting fuse contributes losses roughly equivalent to those generated by the capacitors themselves. This effect almost doubles the power losses of the installation. The initial expense of the current-limiting fuses adds approximately 10% to the price of the bank. The cost o f the additional losses due to the fuses must also be evaluated. In this case, they would add an additional 5% to 7% to the cost. The Split W y e Alternate To avoid the added expense and high losses of current limiting fuses, one common approach in conventional all film applications is to arrange the bank into a split wye, rather than a single wye, configuration. The following calculations would be used to arrive at this configuration . One o f Two Three Phase Stacks Figure 4 Layout of 43.2 M V A r Bank Using Conventional All-Film Capacitors In a Split Wye Configuration Depending on bank layout, the total substation area requirement increases by up to 15%. The cost of the additional substation land area required must be factored into the final price. Also, the extra structure and insulators required for this configuration would add approximately 10% to the price of the single wye configuration. Alternatives Using Additional Series Groups As the 4,650 kvar limitation is based on a per series group basis, another solution to the problem would be to leave the bank as a single wye, but increase the number of series groups per phase. The new design would be arrived at as follows: One phase power = 43,200 3 = 14,400 kvar (14) Max Power per = 4,650 kvar (15) Series Group Number of series groups = 14,400 + 4,650 = 3.1 (16) As three series groups would result in 4,800 kvar in parallel, the next larger number of series groups must be used. Number of Power per Power per Number of Series groups = Series Group = Capacitor = Parallel Units= 4 (using 9.96 kV units) 14,400 + 4 = 3,600 Kvar (17) 400 kvar (18) 3,600 400 = 9 (19) Total number of Capacitors per bank = 3 x 4 x 9 = 108 As each block, or frame sub-assembly, of a bank can electrically contain up to two series groups of capacitors, two blocks must be used per phase. Installing all six blocks in one bank assembly would result in a bank which would stand 35 feet high. This height is undesirable due to maintenance and mechanical stability concerns. Common practice is to limit capacitor banks to three or four blocks per structure. For banks using two blocks per phase, the most typical configuration is shown in Figure 5. Note that the bank now consists of three structures, one per phase. Table 3 Summaryof Alternative Bank Designs Required for Conventional All-Film Capacitors Conventional All-Film Type EX-7 Bank Single Wye Single Wye Double Wye Single Wye Charac- 2 Series 2 Series 2 Series 4 Series Groups teristics Groups Groups Groups Unit Kvar 400 400 400 400 Unit Voltage 19,920 19,920 19,920 9,960 Parallel Kvar 7,200 7,200 3,600 3,600 Total # of Units 108 108 108 108 Number of Structure 1 1 2 3 Current Limiting Expulsion Expulsion F u s eType Expulsion Figure 5 Layout of 43.2 Mvar Bank Using Conventional All-Film Capacitors In a 4 Series Group Configuration The added structural material required to assemble this bank will again add approximately 10% to the price of the bank. Once more, an enlarged substation area will be required for its installation. Table 3 summarizes the salient characteristics of the three bank design. Addition Required Substat. Area (1) 0% 0% 0-15% 15-75% Addition cost of Losses Due to C/L Fuses 0% 5-7% 0% 0% %Increase Initial Price of 0% 5-10% 10% 10-15% Bank The recent development of advanced dielectric systems and internal construction methodologies utilized in high energy all-film capacitor designs, such as McGraw-Edison's Type EX-7, has allowed for many improvements in the application of power capacitors. These include: 1. The replacement of probability tank rupture curves with definite TCCs. 2. The use of one tank rupture curve to represent all unit kvar sizes and voltage ratings, eliminating the need for a family of curves. . 3. Enhanced safety characteristics due to a significant increase in the coordination margins between the tank rupture c u r v e and fuse. 4. Ability to connect up to 9300 kvar in parallel per series g r o u p while being able to retain the use of inexpensive expulsion fuses. 5. A savings of 5 to 10% in the initial cost of substation banks utilizing designs which take advantage of the 9300 parallel kvar capability.