Introduction
In the recent years, some countries have been experiencing power shortages and coupled with the increasing cost of setting up and building large power plants such as hydro electric power plants, thermal and geothermal plants and nuclear plants in addition to building transmission lines has necessitated the need for smaller generation plants that can be set up near or at the consumer site to supplement the power obtained from centralized sources. This system of power generation brings along several issues, including power unreliability, security threats and pollution occurring due to release of sulfur dioxide, carbon monoxide, nitrogen into the air and waste heat. On the other hand, distributed generators (DGs) have fewer environmental emissions.
Despite this, distributed generators have an impact on the power system stability if their level of penetration on the grid surpasses a certain threshold. This dictates prior assessment of their impact on the power system stability both at the distribution and transmission levels.
This report focuses on distribution generation, its advantages and disadvantages, the positive and negative impact of the generators on the transient stability of the power system when they are interconnected into the system and when faults, such as short circuits, occur.
Distributed Generation
Definition
Distributed generation is power generation whereby individuals, companies or businesses self generates electricity for their use, and the excess is sold to the grid. Distributed generation can also be defined as generation with capacities ranging from 50 -100 MW (Purchala, et al., 2006). The generators are never dispatched and are mostly connected on the distribution lines with some being connected on the transmission lines. Most of this kind of generation is obtained from renewable-energy sources such as solar, biomass, hydro and wind.
Advantages of distributed generation
Good power quality and efficient network
When generating plants are set up near the load centers, the quality of power received is higher than when the plants are far. There is less demand from the customers on the transmission lines; consequently, the transmission lines transmit less power resulting in low losses and fewer voltage dips due to long lines. Moreover, the aging of the transmission lines plus the auxiliary equipment in substations such as circuit breakers and disconnectors (isolators) increase the losses.
When industries have their source of power, there are minimal interruptions (due to outages experienced in a normal scenario) and there is more production at low costs but the initial capital cost of setting up the generation plant is high. Purchala et al states:
High reliability level implies high investment and maintenance costs for the network and generation infrastructure. Due to the incentives for cost effectiveness that come from the introduction of competition in generation and the re-regulation of the network companies, it might be that reliability levels will decrease (Purchala, et al., 2006).
Moreover, it is stated:
Beyond efficiency, DG technologies may provide benefits in the form of reliable power for industries that require uninterrupted service. The Electric Power Research Institute reported that power outages and quality disturbances cost American businesses $119billion per year. In 2001, the International Energy Agency (2002) estimated that the average cost of a one-hour power outage was $6,480,000 for brokerage operations and $2,580,000 for credit card operations. The figures grow more impressively for the semiconductor industry, where a two hour power outage can cost close to $48,000,000. Given these numbers, it remains no mystery why several firms have already installed DG facilities to ensure consistent power supplies (Virginia Tech Consortium, 2007).
Environmental friendly generation
Environmental protection and maintenance are some of the major issues influencing the development of many distributed generation stations (Purchala, et al., 2006). Centralized generators such as thermal plants, nuclear plants and fossil-fuel plants emit dozens of waste materials and gases into the atmosphere and waters thus causing acid rain in some cases and pollution of the aquatic habitat (Virginia Tech Consortium, 2007). Most of the distributed generating plants produce electricity from renewable-energy sources such as solar, wind, biomass and small hydro plants. All of these sources cause minimal pollution or no pollution to the environment.
Affordable electricity prices
When generating plants are built far from the load centres, transmission lines have to be built. The cost of building transmission lines is high as compared to distribution lines. When customers are billed, the cost of setting up and maintaining the transmission grid is included. When the generators are close to the customer, the cost is less. Furthermore, power from renewable-energy sources is cheaper as compared to thermal or nuclear plants. Hence, the cost of electricity is affordable.
Energy security
Nuclear plants and natural gas plants are normally big targets for terrorists. When terrorists strike, and the power plant crumbles, the distribution generators hold up the system by ensuring continuous power supply.
Source of income
The individuals or companies that have the distribution generators sell the surplus electricity to the grid. This is a source of extra income for them.
Disadvantages of the distributed generators
The distributed generators play a major role in ensuring efficiency and reliability of power. However, when the generators are installed in large quantities, their operation affects the stability of the system leading to voltage fluctuations (Purchala, et al., 2006).
Some of the drawbacks of these generators include:
High financial cost
Most of the distributed generators are small scale. When computing the capital costs per KW, most of these generators have higher ones compared to centralized generators making them expensive. Purchala et al states:
Different DG technologies differ from each other in that respect quite significantly, ranging from 1000€/KW for combustion turbines to over 20,000€/KW for fossil fuel. If these numbers are compared with the capital costs of large centrally managed power plants, 750€/KW gas-fired, 1300€/KW Integrated Gasification Combined Cycle (IGCC) and 1600€/KW for nuclear, it can be noticed that the DG technology is quite expensive. Purchala et al. (2006)
Effect of DGs on power system quality and stability
Distributed generators greatly influence the power system transient stability when their penetration level is deep. Moreover, they influence the frequency of the system, since they lack the load-frequency control mechanism. In networks that are not heavily loaded, the voltage tends to rise especially in radial systems, and this may pose a problem when it comes to connecting to the grid.
Grid and DG interconnection issues
Most distributed generators are connected on the distribution network. When many machines are connected, there is a tendency of power flow from the low voltage to the higher voltage levels which eventually necessitates the installation of different protection schemes at the medium voltage level to accommodate the bi-directional flow of power thus increasing costs.
Power output unpredictability
Most distribution generating controllers are unable to accurately determine the output of their generators hence face penalties when they fail to balance between what they inject into the system and what they withdraw from the system.
Less choice between primary fuels
The increase in the number of distribution generators causes an increment in the various types of primary energy fuels used. The cost of supplying fuel for these generators is higher than that for centralized systems. Lately, the tendency of using gas as compared to coal is on the rise and could cause a negative outcome on the fuel prices.
Transient stability
Definition
Transient stability falls under rotor angle stability and is defined as the power system’s capability to uphold synchronism between its various equipments, including generators, transmission network and connected loads when there is severe disturbance or fault and to return to normalcy when the fault clears (Khan, Ali and Husein, 2011). Transient instability occurs when some generators experience an angular swing which leads to loss of synchronism with other generators and is associated with the transfer of real power. Sometimes, instability still occurs when the generators are synchronized. This is mostly due to voltage instability when there is lack of proper voltage control within the system, and is caused by systems which are weak and long transmission lines. This leads to the question: with so many upcoming distribution generators, how is transient stability maintained and what is their impact on the system?
Majority of the distribution generators are connected at the distribution level. When connected in small doses, they have a very negligible effect or no effect at all on the transient stability of the power system. However, when their system penetration level surpasses a certain limit, their impact goes beyond and affects the transmission network as well (Reza, et al., 2003).
The transient stability of a power system is greatly influenced by the distribution generator technology. Most of the centralized generators are of synchronous type and are easy to monitor and control and ensure synchronism. The distributed generators are of various types including:
- Uncontrolled synchronous generator
- Squirrel cage induction generator
- Synchronous generator comprising of grid voltage and frequency control
- Uncontrolled power electronic converter
- Power electronic converter comprising of grid voltage and frequency control.
Some factors are normally taken into account when evaluating the generators’ effect on transient stability. These are:
- The duration of oscillation of the rotor which is the time interval at which the rotor will remain within a specified bandwidth after a fault has occurred. The bandwidth is normally given as 10-4 p.u.
- The maximum deviation of the rotor during faults – defined as the full rotor speed during the fault. Under equilibrium conditions, the mechanical torque (input) equals the electromagnetic torque (output). During disturbances, the equality is distorted leading to the increment or decrement of the rotor speed.
- The critical clearing time (CCT) of the system – this is another factor used to evaluate the transient stability of the system. For stable systems, the critical clearing time of the generators is longer than the operation time of circuit breakers. Three authors state:
CCT value is based on the most severe failure with the influence on synchronous generator transient stability in the most unfavorable place of occurrence, i.e. the three-phase fault on the nearest bus bar of power substation, where the generator is connected (Eleschova, Smitkova and Belan, 2010)
Impact of squirrel cage generator on transient stability
The impact of generators on transient stability is felt when their penetration level is increased. This is done through increment of the generator’s active power. Effect of the squirrel cage induction generators on stability depends on their distance or location from synchronous generators. During faults, these generators hardly change the oscillation duration or the maximum rotor speed due to counterbalancing effect with the synchronous generators (centralized generators).
When in the locality of the centralized plants and a short circuit occurs, its stator frequency rises due to the increment of the speed of the centralized generator. Its slip frequency decreases and as a result the power decreases too. When this occurs, the speed of the centralized generators decrease. At a distance far from the centralized generator, the induction generator demands a lot of reactive power during a short circuit resulting in a lower voltage at the sending end (centralized generator), a decreased torque and increased rotor speed (Reza, et al., 2010).
Wind turbines use the induction generators and are of two types: fixed speed type (squirrel cage) or variable speed (doubly-fed). The variable speed type such as the doubly-fed induction generator has a longer critical clearing time (CCT) as compared to the fixed type. The critical clearing time is the maximum interval taken for fault clearance. This greatly enhances the transient stability of the system (Naimi and Bouktir, 2008).
Impact of synchronous generator (controlled and uncontrolled) on transient stability
Synchronous generators possess an excitation winding on the rotor. During faults, the excited generators supply a fault current which makes the terminal voltage be maintained at almost constant level or a higher value. As a result, the synchronizing torque remains high (Slootweg and Kling, 2003). Increasing their penetration level decreases their speed during faults. In the vicinity of centralized generators, an increase in the DG penetration level causes a decrease in the maximum rotor speed deviation of the centralized generators. For uncontrolled distribution generators, the oscillation duration increases during faults while for controlled generators, it decreases.
Impact of power electronic converter on transient stability
Power electronic has become an integral part of the power system. With more and more focus shifting towards distribution generation, a smart grid system will become an essential part. Harris states:
Presently less than 10 percent of power electronics are involved in the electric industry, from the generator to the use of the electricity. And in 20 years, nearly 90 percent of the power between the generator and its use will be all controlled by power electronics (Harris, 2011 pp. 6).
Despite the present less penetration of power electronics in the system, some distribution generators are coupled onto the grid by use of an electronic converter. The converters function to disconnect the generator from the grid when a severe fault occurs. Consequently, there is less generation due to disconnection and this causes a voltage drop at the terminal end. When generation decreases, the rotors decelerate and this in turn causes a reduction in the maximum speed deviation of the generators at the expense of voltage which may result in voltage instability (Slootweg and Kling, 2003).
Both the controlled and uncontrolled electronic converters are offline during faults. This implies that their maximum speed deviation is the same at that particular instance. When the generators coupled through power converters are many in the system, the possibility of causing a blackout is high since their disconnection may result in tripping of other types of generators and loads (Slootweg and Kling, 2003).
The oscillation duration varies due to the governor models used. The uncontrolled generators take a considerably longer time to attain the required bandwidth because the state of equilibrium has to be attained first. Controlled generators reach the state of equilibrium faster and thus the oscillation duration is minimal.
Positive impacts
Sections [3.2], [3.3] and [3.4] display how the different technologies of distribution generators behave when there is a fault in the system. To maintain transient stability, the two indicators: oscillation duration and maximum rotor speed must be kept at a minimum or should decrease.
- The squirrel cage induction generator has a positive impact on the transient stability since its presence in the system hardly affects the system’s stability. When the doubly-fed induction generator is used in wind turbines, the system’s stability is greatly enhanced since the system’s reactive power is easily controlled (Naimi and Bouktir, 2008).
- Generators coupled through the power electronic converters have both a positive and a negative impact. For controlled type, when they are few of them connected to the system, the transient stability of the system is easily maintained due to decreased oscillation duration and maximum speed deviation.
- Controlled synchronous generators impact positively on the transient stability as the maximum speed deviation as well as oscillation duration decrease during faults. The voltage stability of the system is also enhanced due to the constant voltage level that is experienced.
Negative impacts
- When many of controlled type generators coupled through power converters are connected, the transient stability as well as voltage stability of the system is affected.
- Uncontrolled synchronous generators decrease the transient stability due to prolonged oscillation duration.
Example – Integration of small hydro plant into the grid
This case study looks at the behavioral characteristics of a hydro power plant under steady state and dynamic conditions when the system is loaded and islanded and then load rejection is done.
A block diagram of a generator is shown in figure 1 below. It consists of a turbine, excitation system, control system (governor) and load.
The control system aids in speed and load management.
Under steady state conditions, the small hydro generator is let to run at full load and then gradually reduced to cause islanding and overloading whilst maintaining the same amount of load.
Under dynamic conditions, all components of a hydro generator including the excitation system, control system, turbine and prime mover are connected to the distribution grid. When a second small hydro plant is connected to the same distribution feeder or grid and the first hydro plant is islanded, there is loss of synchronism or tendency of the two generators to fall out of synchronism. To restore or prevent that from happening, the control system settings have to be changed or the generators disconnected and then reconnected. Thus, islanding a hydro plant causes a change in system response, which must be properly taken care of to ensure power reliability (Falleiros, et al., n.d).
The control settings can be changed by raising or lowering the settings, and this causes the speed response of the rotor to change causing deviations which either maintain transient stability or cause instability.
Conclusion
Distribution generators have become an important aspect of the power grid in many countries. It is professed as the technology which helps in curbing environmental pollution and enhancing global climatic change by the use of renewable sources of energy. In addition, the DGs provide many advantages including energy security in terms of less terrorist attacks, constant and quality reliable electricity, less voltage interruptions and voltage dips and help in regulating the electricity prices. However, several issues such as grid interconnection problems hinder its penetration in many areas. When the generators are set up with no proper planning, the problem of transient instability occurs. Therefore, careful and extensive study and planning of the power system network must be done before setting up the plants to ensure system stability is maintained.
This report also focused on the various types of technologies used for distribution generators. Each type of generator has been discussed and how it influences the transient stability. For induction type of generators, their impact on the transient stability is minimal. Controlled and uncontrolled synchronous generators impact differently with the uncontrolled generator having longer oscillation duration. Generators coupled through power electronic converters will also maintain transient stability when connected in small magnitudes.
References
Eleschova, Z., Smitkova, M., & Belan, A.,(2010). Evaluation of Power System Transient Stability and Definition of the Basic Criterion, International Journal of Energy, 1(4), pp 1.
Falleiros, J.A.B., Melo, M.A.A., Silva, M.A., Shinzato, P., Mauricio, W., Oliveria, Y.M.B.M., Mauricio, S.T., Vasconcelos, R.F.N., Holsbach, I.L, (n.d). Impact of Dispersed Generation in the Power System Performance. Web.
Harris, P., (2011). Distributed generation: Key part of our future. Web.
Khan, S., Ali, R.& Husein, S.(2002). Definition and classification of power system stability. Web.
Naimi, D & Bouktir, T., (2008).Impact of Wind Power on the angular stability of a Power System, Leonardo Electronic Journal of Practices and Technologies, (12), pp. 83-94.
Purchala, K., Belmans, R., Leuven, K.U., Exarchakos, L. & Hawkes, A.D,(2006). Distributed generation and the grid integration issues [online]. Web.
Reza, M., Slootweg, J.G., Schavemaker, P.H., Kling, W.L. & Van der Sluis, L. (2003). Investigating Impacts of Distributed Generation on Transmission System Stability. Web.
Slootweg, J.G & Kling, W.L,(2003). Impacts of Distributed Generation on Power System Transient Stability. Web.
Virginia Tech Consortium (2007). Benefits of Distributed Generation, What are the potential benefits of DG system. Web.