10th October 2000 MICROGRID 1....... INTRODUCTION 2....... DESCRIPTION 3....... HISTORY 4....... SERIES LOOPS and TWO WIRE NETS 5....... TECHNOLOGY 6....... SHORTCOMINGS 7....... MARKET 8....... SCALE 9....... INTEGRATION 10...... LIST of INSTALLATIONS 11...... ELECTRONIC ENGINEERING 12...... COMMERCIALLLY AVAILABLE COMPONENTS 13 ......FAULT LOCATION and MAINTENANCE ################################################################# ####### 1/ INTRODUCTION Microgrid(TM) is a system for power transmission defined by its interfaces - electronics in both the generation and load points. All equipment plugged into a microgrid must comply with a set of performance definitions. Its key features are that it is bi-directional, sub lethal, self protective, easily repaired, easily altered, and fails gracefully. Microgrid has been successful in remote area power systems where several customers share one or more renewable generators. It has also been used as a "kit" electricity distribution system for temporary or portable systems in camps, festivals, and communities where the cost of 240v reticulation is not justified by the electricity demand. More recently it is seen as a way of providing mains backup to primarily Photovoltaic power systems as the cost of panels reduces but the cost of storage and backup generators does not. 2/ DESCRIPTION What Microgrid looks like on the ground is a system of 100v DC cables, typically shallow buried or conduited on fences or buildings. The layout or topology of the network is random and alterable. There are frequent junction boxes, but there is a notable lack of any protective devices. There may be any number of load points or generation points. At each load or generation point there is an electronics box which performs all the functions of protection, isolation, indication and microgrid system compliance. These boxes can be designed for sending power into the microgrid, getting it out of the grid, or both. The load points consist of an outlet of 12 or 24 volts DC which is not just voltage controlled, but has integrated battery charge control, and smooth current limiting. While designed to work with batteries connected at load points, the system will work without, and will tolerate very bad batteries. 3/ HISTORY There is nothing new about 100 volt DC distribution systems. Back two centuries Edison used battery centred DC systems, and Thury used distribution based on series DC using motor-generators as the converters. Before 1980 people in Nimbin used series loops to charge communities, simply connecting up several 12 volt houses in series and supplying a constant current feed. It was after several bad experiences with these systems that Microgrid was designed by Kali McLaughlin and the first system impelmented with help from Chris Dunne and Peter Helliger, and some dole money! There was a technological breakthrough when Power Mosfets became cheaply available, and there was great excitement when the first ones arrived from USA. Previously the transistors (and transformers) had been extracted from dead Television sets. By 1990 Microgrid was an established product range of Rainbow Power Company and many systems had been installed. As a preemptive move Kali published the key Microgrid ideas in "Soft Technology" magazine (now "Renew") and later in an ANZSES paper at the UNSW conference. This proved fortuitous as Butler Solar was granted a patent which covered Microgrid. After some blustering an accommodation was made. 4/ SERIES LOOPS and TWO WIRE NETS The series loop around a neighbourhood is alluringly simple, but practice proved it to be very difficult. These systems do not require any electronics, and share out power equally between clients. Both these advantages ended up being the opposite! The issue which finally plagued series loops was faultfinding. Any open circuit anywhere will stop the whole system, and reading the voltages to find the fault is difficult as there is no reference! Even the visibly equal power distribution was not good, as in low occupancy times neighbours could not use the uneeded power of their neighbours, and worse still, earthing problems caused battery discharge in some circumstances. Worse still the geographically simple loop was a wonderful lightning ariel, presenting a large area for inductive pickup as well as long wires for resistive/capacitive pickup with no way of shunting common mode voltages. While a Microgrid can be taken down by a short circuit between its two wires, in practice this is far more rare than open circuits, and the diagnosis can be made by the very intuitive process of disconnecting feeders until the system comes back up again. 5/ TECHNOLOGY The principle of Microgrid is that rather than constant voltage (the conventional grid), or constant current (Thury system), or constant impedance (coaxial cable networks), Microgrid has an intentionally "soft" voltage combined with current limiting. The detail is that no load is allowed to pull the line below 70 volts, and its consumption should fall with voltage. In symmetry, no generator is allowed to push the line past 130 volts, and its current should fall with voltage. Loads thus act like resistances on a platform and generators must be able to run open circuit. Because of the breakthrough in power electronics, the idea of using electronic interfaces is no longer a cost liability, and in fact expensive protection devices can be avoided. While the interface electronics in Microgrid is very similar to computer power supplies, the philosophy of design is very different. At a premium is efficiency and reliability. Some downconverters (microgrid to 12v boxes) have been running continuously for 15 years thanks to care with the failure mechanisms of converters- lightning, heat, component ratings, insects, dirt, condensation, protection against wrong connections. The efficiency also has exceeded 90% which while not spectacular is good enough and limited by practical necessities like isolation, filtering, wide operating windows, and small scale. 6/ SHORTCOMINGS Historically Edisons 100v DC power systems gave way to the ac systems for two compelling reasons: AC power can be easily fed through transformers to step the voltage up for transmission long distances, and AC power is far easier to interrupt with switches. The ability of ac power to drive motors without brushes was also a great advantage. Changes in technology have rendered all these advantages merely historical. Microgrid still has the disadvantage of transmission loss through being only at 100v. The matter of ac or dc is almost irrelevant to the loss. 120v is four times as bad as 240v in terms of transmission loss. That said, there are compensating effects that level the playing field. * surge and peak loads, the main cause of voltage instability in AC systems can be eliminated in DC systems by the addition of relatively small batteries. * Much of the cost of transmission lines is not the metal, but the insulation and protection systems! With 100 volt lines the lowered standards of protection easily cover the cost of increased amounts of metal conductor. * transmission loss has two different bad effects - energy wastage and voltage droop. There is still the energy wastage in a DC system, but the switchmode electronics can easily compensate for the voltage droop. The energy wastage can be reduced as well if batteries adequate to perform load levelling are installed. * Microgrids can be fed from multiple points with random timing and output, needing no synchronising or overall control. This means that average voltage drops can be reduced and topological techniques such as ring mains are feasible to utilise statistical effects in both the load and generation distribution. Another disadvantage of microgrid current is that being DC it forms very persistent arcs in line faults. This can cause heat and fire. In underground systems this is less of a worry, but where lines are in plastic conduit it is important that there are no high resistance joints. In high power systems it is attractive to use bare metal conductors inside channels and ducts to avoid this problem. A further problem of DC systems is that if wet the conductors will disappear through electrolysis. This might be expected to cause the arcing failures just mentioned, but the metal usually disappears into a pile of wet oxide which does not burn! Failures through electrolysis usually cause open circuits and cessation of current in that feeder is swift and harmless as the remaining metal fuses in the attacked area. * Conventional power distribution systems use transformers and sparkover arresters. This combination is quite good for limiting the effects of lightning. DC systems give problems with arresting an arc formed by lightning as there are no current zero points at which the arc can extinguish. Furthermore, the first components the lightning pulse hits are not the reactive coils of a transformer but the vulnerable semiconductors of a switcmode converter in a DC system. There are techniques to deal with the lightning problem, but it must be taken seriously. * Microgrid converters can be very noisy in the AM radio band. Recent design has concentrated on reducing this noise to background levels, but because of the inherent power switching harmonics to several megahertz and the inherently excellent performance of distribution cables as ariels at these frequencies, it is a big ask to get the noise levels below that of weak radio stations. That said, conventional power systems are not innocent of noise either. Large amounts of 50 cycle magnetic fields are radiated by transmission lines, and wideband noise up to the VHF band is created by arcing over insulators. Further, the earthing provisions necessary in AC distribution systems send earth currents through systems such as telephones. 7/ MARKET There are two different sorts of market for Microgrid. One is the original concept where a group of people clustered around a renewable source geographically fixed can share its output. Typical of this market are people sharing the output of a microhydro scheme, or a single wind machine enjoying the economies of scale. Another market niche is that of extension of mains power to consumers who for various reasons do not want a standard connection. A list of these reasons follows: * only a small amount of energy is required and 240 lines are not warranted. For example a data acquisition or repeater site. * There are terrain difficulties with 240 volt lines such as trees, rocky unstable slopes, environments which cant be disturbed by machinery, easement legalities, or lack of vehicular access. * The distribution system is temporary. A reticulation network from a very limited 240 volt outlet can be quickly set up with no complex current management over a random topology of feeders as typically occurs at shows, camps, field emergencies etc. The problem of finding tripped breakers, and the cause, can end up defeating the function of large AC networks. * The intrinsically lower safety hazards of the lower voltage and fault current make Microgrid installations preferable in areas of chaos. 8/ SCALE Microgrids so far installed have scale less than a kilometre across, and with power levels of only a couple of kW average. Transmission losses and costs get prohibitive above this. It is counterintuitive that microgrids can actually stretch further at 100 volts than 240v minigrids which theoretically have 1/5th the loss. Reasons for this are: a/ current in a microgrid is more averaged over the day b/ there is unity power factor c/ there are no surge loads to cope with d/ generation and load centres can be more overlapping e/ conductors can be quickly beefed up where the current is found to be heavy. f/ Cheaper insulation systems mean more money is available for conductor section g/ Heavy conductors suffer no skin or stray losses at DC h/ The topology is more free, allowing ring mains, no multiple wiring, no centralised protection systems. i/ generators can have their energy fed to load centres at 415v 3 phase and then rectified and controlled for the microgrid interface. j/ transmission losses occur at the "rough" end of the power system, before energy storage and inversion. Voltage maintenance is not a critical matter as it is in a 240 v minigrid. Higher losses are often justifiable in a microgrid where wind, hydro are the driving force - after all the fuel is free! Capital efficiency is more important than conversion efficiency. There have been feasibility studies done into far higher current microgrids than the usual ones with maximum currents in the order of 15 amps in the 100 volt lines. It appears economic to use aluminium busbars located in underground service conduits to transmit as much as 5 kW continuous. This would be enough power for a frugal village of 100 dwellings 9/ INTEGRATION The overwhelming strength of Microgrid is its ability to integrate a complex array of generators and loads that both are variable with the weather, and cover a geographic area which makes the typical battery centred, centralised remote area power system difficult. The attached picture shows wind, solar and microhydro power fed directly into a network rather than into a battery. This in distinction to the usual use of a micro hydro system which has to be set at higher power than the peak load, with power dumping to control the voltage. Power dumping and low priority loads can be also attached to a microgrid of course, and can use output from any of the multiple generation sources. Load management can be done by conventional methods like daylight switches (eg streetlight), or demand switches such as pumps, but many loads such as grey water aeration, hot water heating, ice making and so on can be passively controlled by local microgrid voltage. This requires a linear, rather than "on-off" switch, but the system stability that results is very satisfactory. As energy storage is the biggest problem in all remote power systems, it makes sense to relieve batteries of all but "UPS" type functions, and run all processes with end product storage as second priority. The local house batteries and downconverters can be viewed as glorified UPS or "EXIT light" systems! A feature supported by Microgrid is the integration of public and private generation. It is common for high capital and decentralisable generation like PV to be privately owned, while site specific and cheaper generation sources such as wind and Hydro to be community public works projects. Rather than dumping or switching out PV, surplus power can be sent back to the microgrid by consumers, as their downconverters can be made bi-directional. This avoids the need for a solar regulator and goes part of the way to reducing costs. There are many plans for more intelligent and optimised system management with SCADA comms overlayer, possibly simply modulated onto the cables - there are no transformers in the way after all! Currently this has no implementation, as the passive self levelling microgrid behaviour is reasonably close to optimal, but the idea of running phone or internet on the microgrid lines is appealing. 10/ LIST of INSTALLATIONS Malapiki, Tuntable Falls...once Hydro, now mains powered Heaven, Tuntable Falls....series loop, mains powered Trizardia, Tuntable falls...mains powered Wattle Ck, Tuntable Falls...mains plus solar Echo, Tuntable Falls.....mains plus solar Davies, Tuntable Falls...microhydro plus solar Moondani, Nimbin.....mains Harbour Pt, Eyre Peninsula...wind plus solar Adjinbilly, Killarney......Microhydro Kittani Sanctury.........mains plus solar Pretty Gully, Mt Warning.....microhydro Nymbinge, Nimbin........series loop, mains 11/ ELECTRONIC ENGINEERING The building block of microgrid is the switchmode converter, which performs functions of voltage change, voltage regulation and isolation. The principle of the device is to chop up the DC into AC and then pass it through a transformer, and then rectify it back to DC. This sounds like a highly involved and inefficient process, but converters using this concept can be made as efficient, and far smaller, than an equivalent 50 cycle AC transformer. The reason for this is that rather than the 50 cycle used by the mains to suit large (1 MW) motors and generators and transformers, a more convenient frequency of 20,000 cycles can be chosen which has many advantages in small sized equipment. Rather than the iron core used in 50 Hz transformers, ferrite can be used at 20KHz. Ferrite magnetic cores support lower magnetic fields than do iron ones, but the losses involved are very much less. To minimise other losses in the converter topology chosen for the downconverter, a single sided forward series arrangement is used. This maximises utilization of the switching transistors and also allows for only one diode pass in the rectification stage. The transformer utilization is not optimal, but there are no great prizes for miniaturisation in microgrid gear. The important design features are that there is only one high voltage switching block, and current passes through it more than 50% of the time, and in a roughly rectangular pulse to minimise FET losses. Several converter design problems are avoided by this design, such as symmetry, startup, smooth current limiting, operation into a short circuit, and voltage shifting of control circuits. The fundamental control, output voltage, is achieved through an optocoupler, thus allowing excellent separation of microgrid and consumer. Shottky rectifiers are used in the output of the 12 volt downconverters as these have an insertion loss of only about 0.4 volts, and do not exhibit "doorslam" spikes on turnoff. Being majority carrier devices there is no time delay as carriers are swept across the p/n junction, and so no ringing of inductive components. This makes the converters produce far less EMI. 12/ COMMERCIALLY AVAILABLE COMPONENTS At the current time (2000) Rainbow Power Company and Flowtrack both manufacture Microgrid components. Many types of generators can be bought to supply 100 volts, including the Flowtrack wind turbine and the RPC microhydro. Solar panels of course are remarkably easy to interface...six panels in series with a diode to the microgrid! The Downconverters are the biggest investment, being in essence a controlled battery charger, and priced accordingly. The RPC model has an LCD display which indicates total power consumed from the microgrid as a basis for billing. The Downconverter can perform some battery charge control functions such as float voltage, manual boost, protection, isolation, indication, cable junction etc. The Flowtrack version has very similar functional specifications but is different cosmetically and is more "mechanical." Less common parts of a microgrid such as a controlled rectifier from 240 volts, or deep well pump controller, or high efficiency backup engine set can be ordered specially. REFERENCES: K.F. McLaughlin Community Power Grids ANZSES proceedings, 1994 conference, SYDNEY .