Challenges to the Power quality of modern electricity grids

It's been a long time since my last post due to a busy semester in  university and my recently started internship but I am looking forward to write more posts in upcoming months since I have some free time due to summer vacation of university. Sharing the knowledge I got from my studies always help me appreciate what I learnt and I think it's my duty to share them as much as possible because in university I realised how important to share knowledge. In almost every unit we used world wide web to learn more often we use textbook. So I think by doing somebody might find it useful.
Today I am going to present you few points about power system quality. This is not a detailed post but rather an entry point to upcoming posts under this topic. I will talk about each and every topic in this post in upcoming posts.

Modern power transmission and distribution is done by using alternative current (AC).  There are several advantages using AC transmission over DC transmission like ability step down or step up voltage as required, Ability to use high voltages in long distance transmission and almost all generators (Synchronous generators) are AC machine is another reason that AC transmission is used. But in recent years HVDC (High voltage DC) technology proved it’s capability for long distance transmission and to connect grids that use different frequency levels are quite useful. We might see HVDC more in future but at the moment all our grids are used to transmit AC.  In an ideal case these currents (including voltages) would be purely sinusoidal waves which have a frequency of 50 or 60 Hz depending on the country.  But this is impossible to achieve as we all aware.  Since the advancements in electronic devices and large demand for power by numerous types of customers on daily basis the utility companies face numerous challenges to keep their power close to the ideal case mentioned above. There are several problems associated with quality of power.

•         Voltage dips
•          Voltage swells and spikes
•          Over  voltages
•          Harmonics
•          Variations in frequency
•          Voltage fluctuations
•          Voltage unbalance
•          Supply interruptions
•      Transient behaviour 

Voltage dips

Voltage dips are decrease in magnitude of the supply voltage for short time period. Voltage dips can happen due to various reasons such as,

•          Tripping of sensitive protection equipment.
•          Resetting of large computer systems.
•          Inductive loading
•          At the starting of a large inductive motor.

When activating large industrial loads such as large motors, they draw large currents. These are called inrush currents and this happens due to the transient nature of the motor. This causes a sudden voltage drop in line until the load (motor) starts to operate in steady state. This period could be few seconds but during this period other equipment that is connected to same node might not be able to operate. In an industrial site there may be lot of other motor as well that are connected to same node. In order to prevent motors operating under low voltage they can be equipped with under voltage relay protection which isolates the motor if such things happened.

Voltage Swells/Spikes

This is the opposite phenomena of voltage dips. Equipment can be damaged due to failure of insulation, destruction of sensitive electronic devices and electromagnetic interference caused by sudden change in voltage. Voltage spikes are often caused by lightning strikes and during switching operations of circuit breakers.

Voltage abnormalities (Image retrieved from:http://assets.tequipment.net)

Over voltages

Over voltages exceed the nominal voltage of a system over a period of time. They are mainly caused by malfunctioning voltage regulators on generators.

Voltage unbalance

Voltage unbalance occurs due to unbalance loading. Unbalance voltage supply creates zero sequence currents in wires which is responsible of heating the components. This is a serious problem in motor operating. Therefore it’s a common practice to use phase unbalanced relays to detect and unbalance cases and protect the motors against them.

Frequency variations

Frequency variations often happen in isolated networks due to faults and malfunctions of governors in power plants. Frequency variations can cause problems. Motor drives will not work properly, power generation may goes out of synchronism if the frequency has changed a lot from it’s nominal value. This may require isolating generators and reconnect them again. So having the nominal frequency value at all times over the whole network is critical.

Harmonics

A very common problem in power systems is effect of harmonics. The main cause for having harmonics in power systems are power electronic devices such as rectifiers, inverters, uninterruptable power suppliers (UPS) ,variable frequency drives(VFDs) and computers etc... In general any non-linear loads (loads where the voltage waveform is different than current waveform) are responsible for creating harmonics. The effect of harmonics is significant when loads are motors. Harmonics consists of negative sequence voltages which cancel the positive sequence voltages which creates the required flux to operate the torque of the motor. Since a negative sequence component creates a negative flux (which is responsible for creating negative torques) this will reduce the amount of positive torque available to operate the load. Besides negative sequence components, triplet harmonics (3^rd 9^th 15^th) which are responsible for creating zero sequence currents in neutral wire causes heating problems and eventually degrades the machine.   

I suggest the following links might be useful if you need more details.
Harmoincs
In depth details about harmonics and it's effects
Voltage abnormalities

Electrical power industry trends in Australia and the problems associated with it

Electrical energy is the main energy type we use in our almost every aspect of our lives. It is impossible to imagine a world without it. Although is a very basic need in modern society most people take it for  granted but generating electricity power and distributing it with a good efficiency to the places where it’s been used is a very complex and carefully designed process. I recently read a report by climate council of Australia (“Australia’s electricity sector: ageing inefficient and unprepared”) and as the title implies it focuses on the matters related to Australia electricity industry and propose the solutions for them .So I thought to share some key points I read in that report and share with you. I personally believe that it’s a responsibility of engineering community to educated general public about how science and technology is important for a better future and for a sustainable development. All the facts and data I go thorough in this article were extracted from that report and all the credit should go to the writer and the climate council of Australia.

Globally the energy sector accounts for the largest proportion of greenhouse gas emissions which is the main cause for climate change around the world as scientists speculate. In order to avoid the effects of climate changes countries have agreed to keep the temperature rise below 2 degrees and in order to achieve this goal every country has to reduce their greenhouse gas emissions it needs large scale changes to how countries produce their electricity and other energy methods.

Figure 1-Global energy related CO2 emission

The above graph shows how the energy sector is responsible for the emission of CO₂ one of the main component of greenhouse gases has increased over the time. As the graph shows it is an exponential growth. Coal is the main reason for this and as everybody knows coal is one of the main source for conventional electricity production. Australia places as 9th country in the top ten countries that emit greenhouse gases in electricity production. In 2011/2012 about 91% of Australia’s electricity was generated using fossil fuels.75% of the portion was from coal and 25% from natural gas. Australia’s per capita CO₂ emission is the world highest in 2011.So it is obvious that electricity generation in Australia is not ecofriendly as people might think. Australia produces 60% of emissions per MWh than USA. Even Australia is well behind China when reducing the emission of GHG (greenhouse gases) .Over the decade to 2012, China has reduced the GHG emission by electricity up to 16% but Australia was able to reduce that by less than 4%.As a result electricity sector responsible 33% of GHG emissions in Austalia.

Table 1-Global Emitters of CO2

The reason behind this is the way electricity is generated in Australia. As mentioned above coal and natural gas are the main sources used to generate electricity .Coal and gas power plants operates in Australia are getting old. Currently the average age of coal power plants in Australia is over 30 years where it is less than 2 years in the global scale and by 2030 over 65% of coal power plants will be over 40 years old. The age of the plant is an important parameter when it comes to it’s efficiency since an old plant operates on technology which is established over decades ago. The technology related with electricity generation is developing rapidly since these plants operates on low efficiency it becomes very expensive to produce electricity it’s and waste of natural resource. Some might see this is not a problem since we have time but building a megawatt scale power station will take at least a decade to plan, finance and build and operate in the required manner. Various much efficient technologies were introduced during the past decades in order to reduce the Carbon emission from plants but the old power plants are unable to adopt to these relatively new technologies since it’s not economically beneficial and those plants are constructed on old technology hence they are locked down to use that instead of moving to more efficient new technologies. One parameter to measure the emissions from a plant is cycle efficiency. It is determined by the steam temperature and the pressure conditions at the limits of a cycle. These efficiency are divided in to 3 categories, Subcritical, Super critical and ultra-critical. Most Australian coal power stations are subcritical hence they have become one of the least inefficient power stations in world.

Figure 2-Fuel usage for electricity generation in Australia 2012-13

Figure 3-Electricity production in Australia

As people realise the negative effects of using fossil fuel based electricity generation the world is slowly moving towards replacing fossil fuel with renewable energy sources like solar, wind, hydro and fuel cell (H₂) technologies. Hydro power is the leading renewable source in the world at the moment. AS it can be seen in above graphs it is the same trend in Australia. In Australia hydro power has supplied 8186MW (16.1% of total capacity) .New South Wales got the biggest portion of hydro power stations (55%). In global scale hydro power has supplied around 15% of electrical energy needs.

Table 2-Hydro power generation distribution worldwide

Wind power is also another emerging renewable source since it’s large reductions in costs associates with installation and equipment .Australia got one of the best wind fields in the world and Australia was able to generate over 6000 GWh in 2012/13 at an average capacity factor of 34%.(Capacity factor measures the ability to extract and store energy from wind by a turbine over a period of time).South Australia is the leading state is wind energy production. It is expected that there would be more 8900 MW of wind energy will be available by 2020 but still Australia is well behind most developed countries.

Solar power is another wildly used renewable energy source. It’s becoming more popular than wind because of it’s domestic use. While coal and other fossil fuels are expected to become expensive in future it’s obvious that solar energy will be less expensive than coal in near future because of the rising demand for solar PV. The advantage of solar is it can act as a distributed power source in houses and commercial premises or it can be scaled up to MW size and feed to the grid.

Over 1 157 000 Australian households had installed 3039 MW of solar PV by 2013. South Australia has the largest proportion of power generated by Solar. Although there is a decline in installing new solar PV in houses due to reductions in Feed in tariff rates it is expected that by 2020 the power generated by solar PV could be doubled. Although household solar PV is a popular alternative to grid power, Australia has only few field scale solar power stations. A 10 MW station at Greenough River in WA and 1MW project at Uterne in Northern territory. Besides these existing projects there are several new projects that are under construction. A 20 MW project at Royalla in ACT. A 102 MW project at Nyngan in NSW. A 3 MW project at Brokenhill.

The popularity of PV has caused them to reduce it’s price and more affordable to households. Furthermore many solar markets around the world getting closer to achieving ‘Socket parity’ point, where it is more economical to produce electricity using solar rather than buying it from the grid. Nowadays it’s more economically beneficial if people use electricity generated by solar for their needs rather than feeding into the grid. Because in that way they will reduce the amount of electricity they consume from grid. One of the major drawbacks of solar PV is it is unable to provide power when it is needed most. In generally the maximum solar insolation is received during noon time but the electrical peak arrives during evening times and most of the time solar power is unable to supply the demand. Therefore households have to use more grid generated power during this period. Therefore solution is to use battery system to store excess energy produced during day time and consume it during peak time. The price of the battery systems are relatively high but it’s becoming cheaper because of it’s popularity and with new battery designs like Tesla Power Wall it has become a worthy investment.    

There is no doubt that sooner or later the whole world has to change their way of producing energy therefore earlier is better. It could be cheaper to use fossil fuels at present but in about 20 years it won’t so nations should start planning and implementing renewable energy plants from now. It is a fact that Australia’s existing fossil fuelled plants can’t compete with renewable energy methods in long term because it’s obvious that price of fossil fuel is becoming higher while the cost of renewable energy solution becoming cheaper. According to the studies wind is the most suitable energy source for Australia along with field scale solar PV.

Table 3-Cost reduction potential for onshore wind systems

Table 4-Cost reduction potential for PV modules and systems

Although Solar, Wind, Hydro and other renewable energy sources are very promising technologies one problem associating with them is they are very sensitive to climate changes. It is very difficult to predict what would be the climate conditions in a region by 10 or 20 years. If the conditions changed dramatically in future the expected output will not be achieved by those plants and the investments done will be wasted. Therefore these aspects has to be taken into account when planning a large scale renewable plants .On the other hand when more renewable sources have been used , governments and other corresponding authorities will be forced to take environment protection more seriously. It will be helpful for a more sustainable and greener development. In conclusion, the fate of future generations in Australia and the rest of the world depends on how we address the energy crisis and take necessary steps to switch to renewable energy. 

References:

Australia’s Electricity Sector: Ageing, Inefficient and Unprepared        

esaa:data and statistics-energy in Australia