Embedded Generation

I. IntroductionWind energy is the most preferred form of renewable energy for electricity production. The power generated by wind turbines over time is very certain due to the uncertain nature of their source. This actually increases the problems inherent to the integration of large number of wind turbines into national grid, and making their share difficult to manage. Regulation of voltage and frequency, and successful operation of wind farms so that integration of wind power to national grid can be done efficiently with minimum losses. This technical essay presents the discussion from social and technical perspective, and presents some viable solutions for efficient energy utilisation.

II. Social PerspectiveThe electricity from the wind farm is beneficial to local communities as they are very near to the source. In addition to that, they will be using that energy for their own use and the surplus will go to national grid. The invariability of wind or solar PV allows different technologies to be introduced into the systems so that quality of power is very high for the consumers or sending electricity back to the grid. There are many social benefits in solving some of the difficult energy integration problems.

A. Creation of jobs for local peopleWind power companies need people that will be regulating the electricity at utility scale. In addition to that, wind turbines need maintenance to keep up with their performance. This creates opportunities for local communities. There are many different jobs that could be taken up most important and crucial are Safety Engineers, Protection Engineers, Energy Assessors, Line Engineers and Maintenance Engineers.

B. Educational visits, Research and DevelopmentThe wind farm can also be used as an educational resource. It can guide people for an eco-friendly environment to enhance the quality of their life and their local area [1]. Adding to that, knowledge of wind power will be improved and there will be more research into different areas. Integration and stabilizing of voltage and frequency is something that will be taken as a problem and there will be many new solutions proposed in research and development.

C. Allocation of LossesThere are losses in the power network and allocation of these losses must be considered. Embedded generation injects a significant amount of power into the network at off-peak times, this may increase total losses. On the other hand, during peak-on period, embedded generation may be compensated for reducing the losses in the system because its entire output is consumed locally. This reduces the need for centrally generated power and becomes beneficial for local community.

D. Awareness of Green Energy and Connection CostsAwareness of green energy can be beneficial to the society as a whole and to the environment. People will be able to live in a better environment with less CO2 going into the atmosphere from the electricity they are using and reduce global warming. Connection costs in embedded generation is another social aspect that needs consideration. It must be decided whether the new connected embedded generation should pay only the costs associated with making the new connection (shallow connection) or it should also pay the extra costs which are related with support of the system (deep connection) [3]. This it to make sure owners are compensated fairly and discouraged for over investment for a system.

III. Wind Turbine SystemsIf a wind farm is connected to large community or some person has installed a small wind turbine, there are usually three different types of systems used in remote areas:

Grid-connected

Stand-alone

Hybrid

A. Grid ConnectedIn grid-connected systems, small wind turbines can reduce the consumption of grid supplied electricity for different uses. If turbine can t deliver the amount of energy that is required, the grid makes up for the difference. When the wind turbine is generating more electricity than the required load, surplus is sent to the grid. There are still many requirements to make sure quality of electricity that is being supplied to the grid is very good and safe to use. There are usually 3 configurations for grid connected wind turbines:

1. In first configuration, turbine s output goes through synchronous generator is variable voltage, variable frequency alternating current. This power must be passed through an inverter before being fed to the home or grid to make sure grid has fixed voltage and fixed frequency AC [2]. Self-commutated inverters are used which need a reference from the utility grid to hold synchronization. When linked with a battery they can be used as part of UPS, which is important when there is no wind or in case of a blackout.

2. In second configuration, induction generators produce grid-quality (easy for both homes and utility) constant frequency AC power without any inverter. The output of induction generators is regulated by utility power, therefor if grid is not available, generator will not produce power.

3. In third configuration, turbine s output from a synchronous generator is variable voltage, variable frequency alternating current. Power must be made in fixed voltage and fixed frequency for consumption by locals or grid. Line commutated inverters are used which are actuated by utility-line power. Synchronous inverters (which are line-commutated) will stop functioning during a blackout.

B. Stand-aloneIn stand-alone system, wind turbines deliver energy production directly to local community and are not connected to national grid. Production of electricity is directly stored in battery banks or turbine runs in parallel with a diesel generator. From here, electricity will be converted so it can be used by various devices. Good control system and battery backup is needed in this configuration for efficient energy consumption and storage.

C. HybridHybrid system with battery backup for emergency power outages works well in remote areas where there are power cuts and grid is not available 24/7. Wind is strong in winter when less sunlight is available. Peak operating times of solar and wind occur at different time, so hybrid systems is a good solution. A good power inverter allows solar panels and wind turbine to charge a battery bank, even when the grid goes down. In addition to that, there is always a generator to make sure it will work when batteries run low on storage. Modern electronic controllers can operate and shift between these systems automatically.

IV. Technical PerspectiveFrom technical perspective, there are a few things to look and discuss when integrating wind power with national grid or local community is trying to use power for themselves. A. Voltage Regulation and Reactive PowerGrid must supply electricity to customers at a voltage within specific limits. If a generator at wind turbine is generating voltage, it will cause voltage level to rise. This rise depends on many factors:

Type and size of generator

Magnitude and direction of reactive power flow on the network.

Network impedance from infinite busbar to generator

Worst-case scenario is when customer load on network is at minimum and embedded generator is giving power. On other hand, if generator is used on-site, it doesn t affect network voltages.

Voltage drop ( |V|) in per unit is given as

|V| = (RP+XQ) / E EQ. (1)

So voltage rise may be limited by controlling the reactive power Q that is given by the generator in EQ. 1 [3]. In particular, when Q is negative (generator is taking reactive power), it is possible to reach |V| = 0. This method is good for circuits with high X R ratio, such as higher voltage overhead circuits. For LV cable distribution circuits with low X R ratio, this method does not work. So due to this, only very small embedded generation can be connected to LV networks. Tap-changing transformers and complete dynamic voltage control is needed to solve this problem [3].

B. Power QualityDue to different sources of electricity to grid, power quality is an important parameter to consider. Generators are obliged to produce adequate voltage waveform (with limited voltage harmonics content) while consumers are bound not to distort too much the current waveform (minor harmonics current content) [3].

Voltage flicker should be avoided as they represent presence of harmonics in voltage wave. Bad case for voltage flicker is when a fixed speed wind turbine is connected to a network with low fault levels. Power output of wind turbine varies rapidly due to wind turbulence, and on a system with low fault level, this can give voltage fluctuations. Systems with good fault levels will suffer less. In addition, a wind farm with several turbines will cause less flicker, as variations in power outputs of different turbines will cancel out.

Voltage and current at any point in distribution system should have a perfect sine, 50Hz waveform. This does not happen in reality, as there are various sources of harmonics connected to the networks. Inverter-coupled embedded generation also introduce harmonics into the network. All of these decrease the energy efficiency and must be solved.

C. Protection and StabilityInstallation of embedded generation must not affect distribution network and customers in network. So there are circuit breakers & isolators to isolate embedded generation from distribution network on both sides of the system. In addition to that, there are under and over voltage relays, under and over frequency relays, voltage presence sensors, over current and voltage controlled relays to give maximum protection and stability. Reverse power relay is added to avoid the generator to work as a motor [3].

V. Energy efficient solutionsSome balance of system and energy efficient solutions that are used in all embedded generation systems whether at utility scale or within homes are:

Power Compensators

Energy Storage

Voltage Regulators

HVDC

A. Variable Renewable Energy smoothing by Battery Energy StorageWind power and solar PV may quickly ramp down or increase abruptly. This can have negative effect for system s voltage levels in distribution network and system stability. Grid can t absorb this extra wind power produced. Batteries capture the wasted wind power, so operators can access the power that was unavailable to them in past. This system is being used for utility scale wind power and for local communities and does not give away any CO₂ emissions to the environment. Battery backup is a good way to save the energy that was otherwise going to waste because there was no load to consume it [5].

If batteries can compensate for changing power levels for even 20 minutes, this can give grid operator time to ramp up or turn down power from mainstream power plants. In this way, large amounts of renewable energy can be added [12]. Figure below shows how batteries are usually connected with wind power and solar PV panels. Intelligent battery management system (BMS) makes sure batteries are charged and discharged efficiently [7].

As renewables especially wind and solar power are sustainable, they are becoming a large source of electricity and in a few years, they will become a large part of generation to the grid. It still has unpredictability and variable contribution and this embedded generation should be integrated to grid as efficient as possible. From social perspective, efficient energy consumption will benefit the local communities and they will be encouraged to get themselves involved in more clean energy generation and displacing conventional generation with time. From technical perspective, power quality, voltage regulation, frequency regulation, protection and stability are the main areas of consideration when the grid is being integrated with renewable energy systems. Many solutions have been suggested and are in use in many countries. FACTs devices, HVDC transmission systems, energy storage, wide area monitoring systems and intelligent grid design [11] are all viable ways to improve energy efficiency.

STATCOMs are part of the flexible AC transmission systems (FACTS) family. They can solve grid operator s requirements for reactive power compensation, voltage control and quick fault currents resolution [6]. Placing a STATCOM at the point of interconnection will improve network reliability and protect renewable energy systems against network faults, voltage drops and other issues that result in wind turbines or solar arrays going offline [11]. From technical perspective, this is a great solution. From social and economic point of view, STATCOMs are expensive, are usually not replaced by all shunt inductors on transmission line, and are installed at the end of transmission line.Consider a transmission line in figure 2 that has two substations. Voltage at substation A is compensated by shunt inductors to maintain voltage across AC transmission line as close as possible to nominal voltage of the power network voltage. This has some drawbacks as it finds it difficult to tell other substations and perfectly compensating voltage across each segment of transmission line.On the second substation B, voltage is compensated by switch shut inductors and a STATCOM. As this substation is located at the end of transmission line, it is important that voltage at this substation to close to nominal voltage as much possible before distributing it to consumers. STATCOM can act faster and compensate for voltage fluctuation across the line as well as for voltage fluctuations caused by variations of the load. IntroductionWind energy is the most preferred form of renewable energy for electricity production. The power generated by wind turbines over time is very certain due to the uncertain nature of their source. This actually increases the problems inherent to the integration of large number of wind turbines into national grid, and making their share difficult to manage. Regulation of voltage and frequency, and successful operation of wind farms so that integration of wind power to national grid can be done efficiently with minimum losses. This technical essay presents the discussion from social and technical perspective, and presents some viable solutions for efficient energy utilisation.

II. Social PerspectiveThe electricity from the wind farm is beneficial to local communities as they are very near to the source. In addition to that, they will be using that energy for their own use and the surplus will go to national grid. The invariability of wind or solar PV allows different technologies to be introduced into the systems so that quality of power is very high for the consumers or sending electricity back to the grid. There are many social benefits in solving some of the difficult energy integration problems.

A. Creation of jobs for local peopleWind power companies need people that will be regulating the electricity at utility scale. In addition to that, wind turbines need maintenance to keep up with their performance. This creates opportunities for local communities. There are many different jobs that could be taken up most important and crucial are Safety Engineers, Protection Engineers, Energy Assessors, Line Engineers and Maintenance Engineers.

B. Educational visits, Research and DevelopmentThe wind farm can also be used as an educational resource. It can guide people for an eco-friendly environment to enhance the quality of their life and their local area [1]. Adding to that, knowledge of wind power will be improved and there will be more research into different areas. Integration and stabilizing of voltage and frequency is something that will be taken as a problem and there will be many new solutions proposed in research and development.

C. Allocation of LossesThere are losses in the power network and allocation of these losses must be considered. Embedded generation injects a significant amount of power into the network at off-peak times, this may increase total losses. On the other hand, during peak-on period, embedded generation may be compensated for reducing the losses in the system because its entire output is consumed locally. This reduces the need for centrally generated power and becomes beneficial for local community.

D. Awareness of Green Energy and Connection CostsAwareness of green energy can be beneficial to the society as a whole and to the environment. People will be able to live in a better environment with less CO2 going into the atmosphere from the electricity they are using and reduce global warming. Connection costs in embedded generation is another social aspect that needs consideration. It must be decided whether the new connected embedded generation should pay only the costs associated with making the new connection (shallow connection) or it should also pay the extra costs which are related with support of the system (deep connection) [3]. This it to make sure owners are compensated fairly and discouraged for over investment for a system.

III. Wind Turbine SystemsIf a wind farm is connected to large community or some person has installed a small wind turbine, there are usually three different types of systems usen integrating wind power with national grid or local community is trying to use power for themselves. &l