1 00:00:05,700 --> 00:00:07,470 Welcome back. 2 00:00:07,470 --> 00:00:14,600 Now we come to an important power electronic device that makes the modern PV system widely usable. 3 00:00:14,639 --> 00:00:18,520 I am talking about the solar inverter. 4 00:00:18,520 --> 00:00:25,520 To know what an inverter is, we must first understand its need in the PV system. 5 00:00:26,210 --> 00:00:31,750 All the discussions we have had so far on the PV electrical output in terms of current, 6 00:00:31,750 --> 00:00:36,579 voltage and power have all been DC in nature. 7 00:00:36,579 --> 00:00:38,980 What do I mean by DC? 8 00:00:38,980 --> 00:00:46,900 DC, or direct current is the unidirectional flow of electric charge. 9 00:00:47,100 --> 00:00:54,100 DC is produced by power sources like batteries, solar cells, dynamo's, etc. 10 00:00:55,220 --> 00:01:01,899 The term DC is broad enough to be used for any electric signal that is unidirectional. 11 00:01:01,899 --> 00:01:08,899 Thus, it is common to talk of quantities like DC voltage, power and current. 12 00:01:11,259 --> 00:01:14,999 The symbol for DC power is this. 13 00:01:14,999 --> 00:01:23,419 AC, or alternating current on the other hand, is the flow of electric charge such that it 14 00:01:23,420 --> 00:01:27,079 constantly reverses direction. 15 00:01:27,079 --> 00:01:32,840 The usual form of an AC power is a sine wave. 16 00:01:32,840 --> 00:01:36,679 The symbol for AC signal is this. 17 00:01:36,679 --> 00:01:43,679 As a thought, DC signals can be looked upon as a signal with zero frequency. 18 00:01:45,679 --> 00:01:49,090 So why do we need AC power? 19 00:01:49,090 --> 00:01:56,090 After a brief 'war of the currents' in the 19th century, AC power was chosen as the standard 20 00:01:56,259 --> 00:02:01,259 for central power generation, transmission and distribution. 21 00:02:01,259 --> 00:02:07,689 Till this day, almost all national electric grids are based on AC. 22 00:02:07,689 --> 00:02:14,689 Almost all households have what are known as the "AC mains sockets". 23 00:02:15,319 --> 00:02:23,500 Consequently, most household electric appliances expect to be fed AC power, even though sometimes 24 00:02:23,500 --> 00:02:28,209 the internal circuitry of the appliances might use DC. 25 00:02:28,209 --> 00:02:35,209 Nevertheless, the fact remains the solar power produced needs to be converted to an AC form, 26 00:02:35,519 --> 00:02:42,519 so that the solar power is more usable in the electricity framework we have today. 27 00:02:42,909 --> 00:02:49,909 How do we make this conversion from DC solar power to the easily usable AC power? 28 00:02:50,400 --> 00:02:57,060 We need a device which can simply convert a DC electric signal to an AC one. 29 00:02:57,060 --> 00:03:03,560 The device which can make this possible is an inverter. 30 00:03:03,560 --> 00:03:10,500 Given the time constraints, we will not go into the internal circuitry of the inverter. 31 00:03:10,500 --> 00:03:15,560 But we will talk about the features of the solar inverter and its application in the 32 00:03:15,560 --> 00:03:18,870 modern PV system. 33 00:03:18,870 --> 00:03:27,600 Now inverters could be classified based on their size, mode of operation, or implementation topology. 34 00:03:27,700 --> 00:03:34,750 Here I will talk about the classification based on the mode of operation; under this 35 00:03:34,799 --> 00:03:41,700 the inverters can be classified into 3 broad categories: stand-alone inverters, 36 00:03:41,700 --> 00:03:46,540 grid-connected inverters and bimodal inverters. 37 00:03:46,540 --> 00:03:52,689 Here, a typical off-grid or stand-alone PV system is shown. 38 00:03:52,689 --> 00:03:56,620 Consequently, the inverter is a stand-alone inverter. 39 00:03:56,620 --> 00:04:05,100 In this case, the PV system is stand-alone, the load can only depend on the PV system for power. 40 00:04:05,200 --> 00:04:11,819 So the inverter that supplies AC power to the load has to appear as a voltage source 41 00:04:11,819 --> 00:04:22,000 with a stable voltage and frequency, supplying power at 230 V_AC or 110 V_AC or as is the voltage 42 00:04:22,000 --> 00:04:24,920 standard at the location. 43 00:04:24,920 --> 00:04:29,780 Here, a typical grid-connected PV system is shown. 44 00:04:29,780 --> 00:04:34,220 Consequently, the inverter is a grid-connected or grid-tied inverter. 45 00:04:34,220 --> 00:04:38,410 In this case, the PV system is grid-connected; 46 00:04:38,410 --> 00:04:45,250 the load can depend on the PV system as well as the grid for power. 47 00:04:45,250 --> 00:04:50,780 There is an exchange of power between the grid, the inverter and the load. 48 00:04:50,780 --> 00:04:55,820 The inverter latches onto the grid frequency and voltage. 49 00:04:55,820 --> 00:05:01,880 The inverter's main task is to only pump power into the grid. 50 00:05:01,880 --> 00:05:08,550 So the inverter that supplies AC power to the grid as a current source, while the role 51 00:05:08,550 --> 00:05:14,470 of the constant voltage source in the system is fulfilled by the grid. 52 00:05:14,470 --> 00:05:17,340 Here we see a hybrid system. 53 00:05:17,340 --> 00:05:20,760 These configurations are much rarer. 54 00:05:20,760 --> 00:05:26,560 The PV system is capable of functioning either as a stand-alone system with battery storage 55 00:05:26,560 --> 00:05:30,810 or as a grid-connected mode. 56 00:05:30,810 --> 00:05:36,990 The inverter plays a crucial role, as it has the necessary hardware implementation to work 57 00:05:36,990 --> 00:05:41,930 in either mode, based on the operating conditions. 58 00:05:41,930 --> 00:05:47,940 In the stand-alone mode, the grid is disconnected, and the system might cater to some special 59 00:05:47,940 --> 00:05:54,370 backup loads with the inverter behaving as an AC voltage source. 60 00:05:54,370 --> 00:06:00,440 In the grid-connected mode, the inverter behaves as an AC current source. 61 00:06:00,440 --> 00:06:06,370 These bimodal inverters are usually more expensive, and are used less often. 62 00:06:06,370 --> 00:06:10,600 Now we already know the basic application of an inverter in the PV system: 63 00:06:10,600 --> 00:06:14,380 power conversion from DC to AC. 64 00:06:14,380 --> 00:06:18,880 Can the inverter perform any additional function? 65 00:06:18,880 --> 00:06:24,670 The answer is yes, and thanks to the advancements in power electronics, it is common to have 66 00:06:24,670 --> 00:06:32,400 inverters that implement an MPPT mechanism before inverting the voltage, thus ensuring 67 00:06:32,400 --> 00:06:38,640 that the PV modules or arrays are operating at their MPP. 68 00:06:38,640 --> 00:06:46,000 Apart from the modes of operation, inverters are also classified on the basis of the implementation topology. 69 00:06:46,000 --> 00:06:53,040 There can be 4 different categories under this classification. 70 00:06:53,660 --> 00:07:00,660 Central inverters, which are usually around several kW to 100 MW range. 71 00:07:01,220 --> 00:07:08,220 Module inverters or micro inverters, typically rated around 50 to 500 W. 72 00:07:09,780 --> 00:07:14,940 String inverters, typically rated around 500 W to a few kW. 73 00:07:14,940 --> 00:07:18,400 A string is nothing but a number of PV modules connected in series. 74 00:07:18,400 --> 00:07:28,400 And finally, multi-string inverters, typically rated around 1 kW to 10 kW range. 75 00:07:28,400 --> 00:07:30,250 Let's start with the central inverter. 76 00:07:30,250 --> 00:07:34,740 This is the most traditional inverter topology in use. 77 00:07:34,740 --> 00:07:41,740 As seen in the figure this is a simple implementation with one central inverter catering to all 78 00:07:41,740 --> 00:07:45,430 the PV modules in a PV system. 79 00:07:45,430 --> 00:07:51,180 While this inverter topology increases the ease of system design and implementation, 80 00:07:51,180 --> 00:07:55,400 it suffers from several drawbacks. 81 00:07:55,400 --> 00:08:00,870 In large systems, large amounts of DC power will be transferred over long cables to reach 82 00:08:00,870 --> 00:08:03,280 the central inverter. 83 00:08:03,280 --> 00:08:12,200 This increases DC wiring costs, and also decreases safety, as DC fault currents are difficult to interrupt. 84 00:08:12,420 --> 00:08:21,400 An MPPT implementation inside the central inverter will only cater to the entire system as a whole. 85 00:08:21,710 --> 00:08:28,500 If the various modules, strings are mismatched, let's say due to partial shading, the overall 86 00:08:28,500 --> 00:08:31,740 system output is drastically reduced. 87 00:08:31,740 --> 00:08:36,560 Also, the system is usually designed for a fixed power. 88 00:08:36,560 --> 00:08:43,000 There is little scope for extendibility of the system if more strings and modules need to be added. 89 00:08:43,960 --> 00:08:49,670 Next, let's look at the micro inverters or module level inverters. 90 00:08:49,670 --> 00:08:56,440 As the name suggests, each module has a dedicated inverter with an MPP tracker. 91 00:08:56,440 --> 00:09:03,420 Therefore the topology is more resilient to partial shading effects as compared to the 92 00:09:03,420 --> 00:09:06,120 central inverter topology. 93 00:09:06,120 --> 00:09:13,120 Clearly, the micro inverters provide the highest system flexibility, since extending the size 94 00:09:13,480 --> 00:09:18,000 of a system under this topology is far simpler. 95 00:09:18,000 --> 00:09:22,290 Furthermore, the DC wiring costs are greatly reduced. 96 00:09:22,290 --> 00:09:31,500 However, the investment and maintenance costs tend to increase, especially if the cost per Wp are compared. 97 00:09:32,010 --> 00:09:37,170 Then we have the string inverter concept, which seeks to strike a balance between the 98 00:09:37,170 --> 00:09:41,840 module level inverter and the central inverter topologies. 99 00:09:41,840 --> 00:09:47,880 The string inverter topology is more resilient to mismatch than the central inverter, because 100 00:09:47,880 --> 00:09:56,800 each string is independently operated at its MPP, thus guaranteeing a higher energy yield. 101 00:09:57,300 --> 00:10:00,810 String inverters are smaller than central inverters. 102 00:10:00,810 --> 00:10:06,090 However, the implementation is more complex than the module inverter. 103 00:10:06,090 --> 00:10:13,050 Also, the partial shading will have a greater influence over the string inverter topology 104 00:10:13,050 --> 00:10:16,390 than over the micro inverter topology. 105 00:10:16,390 --> 00:10:20,390 Finally, the multi-string inverters. 106 00:10:20,390 --> 00:10:26,510 This concept seeks to combine the higher energy yield of a string inverter with the lower 107 00:10:26,510 --> 00:10:28,920 costs of the central inverter. 108 00:10:28,920 --> 00:10:35,860 Each of the strings is pre-power-processed using low power DC-DC converters. 109 00:10:35,860 --> 00:10:42,860 Each string has its own MPP tracker implemented alongside the DC-DC converter. 110 00:10:44,460 --> 00:10:51,300 All the converters are connected via a DC bus to the inverter, and ultimately to the grid. 111 00:10:51,560 --> 00:10:58,560 Within a certain power range, only a new string with a dedicated DC-DC converter has to be 112 00:10:58,560 --> 00:11:02,370 included to expand the system size. 113 00:11:02,370 --> 00:11:06,770 We now have an overview about the solar inverters and their topologies. 114 00:11:06,770 --> 00:11:13,220 Of course, the choice of your topology for implementation would depend entirely on the 115 00:11:13,220 --> 00:11:17,240 system needs, size, and your system's budget. 116 00:11:17,240 --> 00:11:23,910 But in general, while choosing an inverter for your PV system, what are the requirements 117 00:11:23,910 --> 00:11:28,220 from a good solar inverter? 118 00:11:28,220 --> 00:11:34,240 There are several characteristics expected from a good solar inverter. 119 00:11:34,240 --> 00:11:41,600 As every power processing step expends power itself, the solar inverters are expected to 120 00:11:41,600 --> 00:11:45,470 be as efficient as possible. 121 00:11:45,470 --> 00:11:52,470 This is because we wish to deliver maximum PV generated power to the load or the grid. 122 00:11:53,730 --> 00:12:00,730 Typical efficiencies are in the range of more than 95% at rated conditions. 123 00:12:00,850 --> 00:12:07,850 Depending on the topology, it is expected that the inverters have in-built MPP trackers. 124 00:12:09,660 --> 00:12:15,770 Grid-tied inverters are expected to have active islanding detection capability. 125 00:12:15,770 --> 00:12:21,210 Islanding refers to the situation in which the inverters in a grid-tied setup continue 126 00:12:21,210 --> 00:12:28,210 to power the system even though the power from the grid operator has been restricted. 127 00:12:28,570 --> 00:12:34,510 Due to safety issues, islanding needs to be prevented. 128 00:12:34,510 --> 00:12:42,200 Therefore, inverters are expected to detect and respond by immediately stopping from introducing 129 00:12:42,290 --> 00:12:44,970 power into the grid. 130 00:12:44,970 --> 00:12:48,930 This is also referred to as anti-islanding. 131 00:12:49,660 --> 00:12:57,200 Since in a lot of situations, the solar inverters are exposed to ambient conditions, these must 132 00:12:57,200 --> 00:13:02,860 comply with the temperature and humidity conditions of the location. 133 00:13:03,160 --> 00:13:10,000 Since grid-tied inverters pump power into the grid, they are expected to maintain very 134 00:13:10,000 --> 00:13:15,779 high quality, so as to not corrupt the power flow in the grid. 135 00:13:15,779 --> 00:13:22,779 Thus inverters are expected to have very low harmonic content on the line currents. 136 00:13:25,380 --> 00:13:31,080 It is a work in progress to increase the lifespan of the inverter, the crucial power electronic 137 00:13:31,080 --> 00:13:33,779 device in the modern PV system. 138 00:13:33,779 --> 00:13:42,700 A good inverter will probably reach, under favorable conditions, around 10-12 years of lifetime. 139 00:13:43,080 --> 00:13:48,740 This is the bottleneck in the modern PV system's lifetime, especially considering the fact 140 00:13:48,750 --> 00:13:54,100 that PV modules can last over 25 years. 141 00:13:54,100 --> 00:13:59,149 We have discussed in this block the need for a solar inverter and also the different types 142 00:13:59,149 --> 00:14:02,690 and topologies of inverter implementation. 143 00:14:02,690 --> 00:14:09,690 However, there is one more crucial aspect that I have not talked about so far. 144 00:14:09,820 --> 00:14:13,279 That is the inverter sizing. 145 00:14:13,279 --> 00:14:19,250 This would largely depend on the system and the load requirements, and the size of the rest 146 00:14:19,250 --> 00:14:20,620 of the system. 147 00:14:20,620 --> 00:14:29,600 I will discuss the inverter sizing when we know more about the different types of PV systems next week. 148 00:14:29,640 --> 00:14:32,160 So, see you in the next block! 149 00:14:32,160 --> 00:14:34,160