Operation and Control of Single Phase Micro- Sources in a Utility Connected Grid
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20-10-2010, 02:43 PM


Operation and Control of Single Phase Micro-
Sources in a Utility Connected Grid




Ritwik Majumder, Student Member, IEEE, Arindam Ghosh, Fellow, IEEE,
Gerard Ledwich, Senior Member, IEEE and Firuz Zare, Senior Member, IEEE



ABSTRACT: This paper proposes operation and control of
converter based single phase distributed generators (DG) in a
utility connected grid. A common utility practice is to distribute
the household single-phase loads evenly between the three phases.
The voltage unbalance between the phases remains within a reasonable
limit. However the voltage unbalance can be severe if
single-phase rooftop mounted PVs are distributed randomly between
the households. Moreover, there can also be single-phase
nonlinear loads present in the system. The cumulative effect of all
these will cause power quality problem at the utility side. The
problem can be macabre if three-phase active loads (e.g., induction
motors) are connected to the utility feeder. To counteract
this problem, we have proposed two different schemes. In this
first scheme a distribution static compensator (DSTATCOM) is
connected at the utility bus to improve the power quality. The
DSTATCOM only supplies reactive power and no real power. In
the second scheme, a larger three-phase converter controlled DG
is placed that not only supplies the reactive power but also provides
active power. The efficacies of the controllers have been
validated through simulation for various operating conditions
using PSCAD.
I. INTRODUCTION
S MORE countries are aiming at a reduction in greenhouse
gas emissions, the requirements for adding new
generation capacity can no longer be met by traditional power
generation methods of burning the primary fossil fuels such as
coal, oil, natural gas, etc. [1]. This is why distributed generators
(DG) have significant opportunity in the evolving power
system network. Both consumers and power utilities can benefit
from the widespread deployment of DG systems which
offer secure and diversified energy options, increase generation
and transmission efficiency, reduce greenhouse gas emissions,
improve power quality and system stability, cut energy
costs and capital expenditures, and alleviate the bottleneck
caused by distribution lines [2].
Properly sited DG can increase the feeder capacity limit, but
this does not necessarily produce an improvement in system
reliability or power quality, as quantified by standard indices
[3]. With improving reliability of the owner, the DG may reduce
the severity of voltage sags near the DG. The DG often
has a negative impact on reliability indices through sympathetic
tripping, required changes to utility overcurrent device settings,
and increased fuse blowing. The utility cannot assume
DG automatically improves system reliability, and action may
be required to ensure that reliability does not actually degrade
for other customers [3].


for more ::->

eprints.qut.edu.au/19288/1/c19288.pdf
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16-08-2011, 02:46 PM


.doc   19-OPERATION AND CONTROL OF SINGLE PHASE MICRO.doc (Size: 4.82 MB / Downloads: 220)
ABSTRACT
Today everyone is aiming at a reduction in greenhouse gas emissions, the requirements for adding new generation capacity can no longer be met by traditional power generation methods of burning the primary fossil fuels such as coal, oil, natural gas, etc. This is why distributed generators (DG) have significant opportunity in the evolving power system network. Both consumers and power utilities can benefit from the widespread deployment of DG systems which offer secure and diversified energy options, increase generation and transmission efficiency, reduce greenhouse gas emissions, improve power quality and system stability, cut energy costs and capital expenditures.
This paper proposes operation and control of converter based single phase distributed generators (DG) in a utility connected grid. A common utility practice is to distribute the household single-phase loads evenly between the three phases. The voltage unbalance between the phases remains within a reasonable limit. The single phase sources are operated to deliver available maximum power generated while the rest of the power demands in each of the phases are supplied by utility (and if available, three phase DG sources). However the voltage unbalance can be severe if single-phase rooftop mounted PVs are distributed randomly between the households. Moreover, there can also be single-phase nonlinear loads present in the system. The cumulative effect of all these will cause power quality problem at the utility side. The problem can be macabre if three-phase active loads (e.g., induction motors) are connected to the utility feeder.
A DSTATCOM can compensate for unbalances and nonlinearities, while providing reactive power support. The size of the dc capacitor determines how much reactive power support the DSTATCOM can provide without any drop in voltage. The choice of this capacitor is thus a trade-off between the reactive support and system response distribution static compensator (DSTATCOM) is connected at the utility bus to improve the power quality. The DSTATCOM only supplies reactive power and no real power. Alternatively a three phase DG-compensator can be connected at the PCC to share the real and reactive power with utility and to compensate for the unbalance and nonlinearities in the system.
The imbalance in three phase power is compensated two ways –either through a DSTATCOM or through a DG-compensator. With the proposed structure of distribution system, it is possible to operate single phase DG sources in a utility connected grid and this might become a useful tool as their penetration in distribution systems increases.
I. INTRODUCTION
More countries are aiming at a reduction in greenhouse gas emissions, the requirements for adding new generation capacity can no longer be met by traditional power generation methods of burning the primary fossil fuels such as coal, oil, natural gas, etc. [1]. This is why distributed generators (DG) have significant opportunity in the evolving power system network. Both consumers and power utilities can benefit from the widespread deployment of DG systems which offer secure and diversified energy options, increase generation and transmission efficiency, reduce greenhouse gas emissions, improve power quality and system stability, cut energy costs and capital expenditures, and alleviate the bottleneck caused by distribution lines [2]. Properly sited DG can increase the feeder capacity limit, but this does not necessarily produce an improvement in system reliability or power quality, as quantified by standard indices [3]. With improving reliability of the owner, the DG may reduce the severity of voltage sags near the DG. The DG often has a negative impact on reliability indices through sympathetic tripping, required changes to utility over current device settings, and increased fuse blowing. The utility cannot assume DG automatically improves system reliability, and action may be required to ensure that reliability does not actually degrade for other customers [3].
Application of single phase converter based DGs are very common in distribution level and with the increasing number of single phase micro sources in a utility connected grid has raised concern about power quality. For a microgrid, a common practice is to isolate the microgrid from the utility grid by an isolator if the voltage is seriously unbalanced [4]. However
when the voltages are not critically unbalanced, the isolator will remain closed, subjecting the microgrid to sustained unbalanced voltages at the point of common coupling (PCC), if no compensating action is taken. Unbalance voltages can cause abnormal operation particularly for sensitive loads and increase the losses in motor loads.
This paper proposes the operation and control of single phase micro-sources (DG) in a utility connected grid. While the DGs supply their maximum generated power, rest of the power demand of each phase is supplied by the utility and three phase DG connected at the PCC, if any. To counteract this problem, we have proposed two different schemes. In the first scheme, a distribution static compensator (DSTATCOM) is connected at the point of common coupling (PCC) to compensate the unbalance and nonlinear nature of the total load current and to provide the reactive power support. In the second scheme, a three phase DG, connected at the PCC, in place of the DSTATCOM to share both real and reactive power with the utility. The DG also compensates the system and makes the PCC voltage balanced. The efficacies of the controllers and improvement in power quality have been validated through simulation for various operating conditions using PSCAD.
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#3
07-01-2012, 05:25 PM

AUTOMATION IN POWER DISTRIBUTION
The demand for electrical energy is ever increasing. Today over 21% (theft apart!!) of the total electrical energy generated in India is lost in transmission (4-6%) and distribution (15-18%). The electrical power deficit in the country is currently about 18%. Clearly, reduction in distribution losses can reduce this deficit significantly. It is possible to bring down the distribution losses to a 6-8 % level in India with the help of newer technological options (including information technology) in the electrical power distribution sector which will enable better monitoring and control.
How does Power reach us?
Electric power is normally generated at 11-25kV in a power station. To transmit over long distances, it is then stepped-up to 400kV, 220kV or 132kV as necessary. Power is carried through a transmission network of high voltage lines. Usually, these lines run into hundreds of kilometres and deliver the power into a common power pool called the grid. The grid is connected to load centres (cities) through a sub-transmission network of normally 33kV (or sometimes 66kV) lines. These lines terminate into a 33kV (or 66kV) substation, where the voltage is stepped-down to 11kV for power distribution to load points through a distribution network of lines at 11kV and lower.
The power network, which generally concerns the common man, is the distribution network of 11kV lines or feeders downstream of the 33kV substation. Each 11kV feeder which emanates from the 33kV substation branches further into several subsidiary 11kV feeders to carry power close to the load points (localities, industrial areas, villages, etc.,). At these load points, a transformer further reduces the voltage from 11kV to 415V to provide the last-mile connection through 415V feeders (also called as Low Tension (LT) feeders) to individual customers, either at 240V (as single-phase supply) or at 415V (as three-phase supply). A feeder could be either an overhead line or an underground cable. In urban areas, owing to the density of customers, the length of an 11kV feeder is generally up to 3 km. On the other hand, in rural areas, the feeder length is much larger (up to 20 km). A 415V feeder should normally be restricted to about 0.5-1.0 km. Unduly long feeders lead to low voltage at the consumer end.

Bottlenecks in Ensuring Reliable Power
Lack of information at the base station (33kV sub-station) on the loading and health status of the 11kV/415V transformer and associated feeders is one primary cause of inefficient power distribution. Due to absence of monitoring, overloading occurs, which results in low voltage at the customer end and increases the risk of frequent breakdowns of transformers and feeders. In fact, the transformer breakdown rate in India is as high as around 20%, in contrast to less than 2% in some advanced countries.
In the absence of switches at different points in the distribution network, it is not possible to isolate certain loads for load shedding as and when required. The only option available in the present distribution network is the circuit breaker (one each for every main 11kV feeder) at the 33kV substation. However, these circuit breakers are actually provided as a means of protection to completely isolate the downstream network in the event of a fault. Using this as a tool for load management is not desirable, as it disconnects the power supply to a very large segment of consumers. Clearly, there is a need to put in place a system that can achieve a finer resolution in load management.
In the event of a fault on any feeder section downstream, the circuit breaker at the 33kV substation trips (opens). As a result, there is a blackout over a large section of the distribution network. If the faulty feeder segment could be precisely identified, it would be possible to substantially reduce the blackout area, by re-routing the power to the healthy feeder segments through the operation of switches (of the same type as those for load management) placed at strategic locations in various feeder segments.


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