International Journal of Innovative and Emerging Research in Engineering
Volume 2, Issue 3, 2015
Available online at www.ijiere.com
International Journal of Innovative and Emerging
Research in Engineering
e-ISSN: 2394 - 3343
p-ISSN: 2394 - 5494
Grid Interfacing Technologies for Distributed Generation
and Power Quality Issues - A Review
Seema Jadhava, Ruchi Harchandania
a
Department of Electrical Engineering, Fr. C. Rodrigues Institute of Technology, Vashi, Navi Mumbai, India.
ABSTRACT:
Small scale energy generation is the need of today’s energy scenario. Renewable energy based Distributed
Generation (DG) provides the best option. Interconnection of DG to grid through power electronic converter
enables fast and smooth control of power flow, synchronous generator maintains voltage profile whereas
induction generator provides better integration in varying power output conditions. In addition to reliable, clean
and efficient power, DG introduces problems like poor power quality, interference with protection scheme and
instability in power system. This paper compares and discusses interfacing techniques, their suitability with type
of DG and power quality issues for different sources.
Keywords: Distributed Generation, Grid interface, Power electronic converter, Power quality, Synchronous
Generator, Induction Generator.
I. INTRODUCTION
Due to increase in power demand and depletion of conventional resources, there is a gap between demand and supply
of electric power [1]. Also due to increasing concern over greenhouse gas emission, there is a need for environment friendly
resources to generate power. The integration of renewable energy based small and medium scale power generating units
called Distributed Generation (DG), into the power distribution system has given a solution to this power crisis and
environmental issue. Recently, DGs have gained significant importance due to reduced transmission and distribution losses,
reliability due to backup generation, relief to the utility against uncertain growth of power demand and reduced cost of grid
expansion [2].
The distributed generation covers a wide range of schemes for local power generation from renewable energy sources
of in an environmentally sustainable way. These schemes are mainly based on solar energy, wind energy, fuel cells and
micro-turbine engines. The bi-directional power flow between the grid and the distributed generation results in grid capacity
enhancement, virtually uninterrupted power supply and optimum energy cost due to the possibility of use/buy/sell options
[3]. The impact of DG on the operation of distribution network depends on the grid interfacing technology. These
technologies include synchronous generator, induction generator and power electronic converter [4]. The sources like wind
and small hydro are interfaced using induction generator. The micro-turbines are interconnected to grid using synchronous
generator whereas solar PV, fuel cell etc. are integrated using power electronic converters.
The conventional power systems are operated radially. However, with the interconnection of small DG in the
distribution system, the operation of power system is altered. Hence for the safe and stable operation of interconnected
power system, it must satisfy the technical requirements as per IEEE standards [5, 6]. The major issue from utility
perspective is power quality like voltage fluctuations, harmonic voltage distortion and voltage sag whereas from end user
perspective it can be per unit cost and maintenance [4]. There are many other challenges such as islanding detection,
stability, protection co-ordination, re-synchronization etc. [7].
This paper describes the various interfacing techniques for grid connected DG, power quality issues and their mitigation
techniques. The rest of this paper is arranged as follows: Section II explains grid interconnected DG system, Section III
describes DG interfacing techniques, Section IV presents power quality issues and subsequently Section V concludes about
the techniques and the issues. The operation of DG in grid connected mode is explained briefly in the next section. This
document is a template. An electronic copy can be downloaded from the Journal website. For questions on paper
guidelines, please contact the journal publications committee as indicated on the journal website. Information about final
paper submission is available from the conference website.
II. GRID INTERCONNECTED DG SYSTEM
The power capacity of DG is small (less than 10 MW) [8]. The single line diagram of grid connected DG is shown in
Fig 1. The DG generates electric power using either conventional sources or renewable sources. It is interfaced with grid
through suitable technique and a coupling transformer. DGs are distributed throughout the power system and closer to the
loads. There are two basic modes of operation of DG [9]:
i)
Grid connected mode
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ii)
Islanded mode
Fig 1. Structure of Grid interconnected DG System
The DGs normally operate in grid connected mode. In this mode, they are used to exchange power during peak shortages
and fulfil demand on the power system. Whereas during abnormal conditions in main grid, the DG is disconnected from
grid and works in islanded mode as a micro-grid [10]. A micro-grid contains a number of DG units with local loads
operating in grid connected mode or autonomous mode. The international standards specify the guidelines for
disconnection time [5, 6, 11, 12]. The interfacing technologies for the grid connected DGs are discussed in next section.
III. INTERFACING TECHNOLOGIES
Some technologies like induction generator based wind or small hydro can be directly connected to the grid. However,
due to power quality issues and starting transients they require power converters for interfacing with grid. Solar
photovoltaic (PV) needs only power electronic converters like DC to DC and DC to AC for interconnection. There are
three basic interfacing technologies for DGs [13].
A. Synchronous Generator:
Synchronous generators are conventional electric generators which convert mechanical power into electric power. They
generate both active and reactive power. Since their capacity is not sufficient to regulate the voltage of main grid, the
interconnected synchronous generators are generally operated with unity power factor and they supply only active power
[4]. They can withstand step load changes and can operate in islanded condition due to inherent inertia. There is possibility
that they interfere with overcurrent protection of grid [14]. Also the voltage waveform produced is distorted and contains
third harmonic component. During islanded mode, the fault current produced by these DGs may not be sufficient to trip
the breaker. The block diagram of synchronous generator interfaced DG system is shown in Fig 2.
Fig 2. Block diagram of Synchronous Generator interfaced DG system
The output power from small hydro turbines or micro-turbines cannot be directly connected to the grid due to power
quality issues and transients. Hence they are interfaced using power electronic converters, where the output power is first
rectified and then converted to AC using an inverter. They are used to provide backup generation.
B. Induction Generator:
Induction generators are induction machines that convert mechanical power into electrical power when rotated at speeds
greater than synchronous speed. They are mainly used with wind turbines and some low-head hydro applications. The
major advantages of the cage-rotor induction generators are that they are relatively less expensive, they require very less
maintenance and these motors are robust compared to a synchronous generators [15]. But they require reactive power
(VARs) to operate which can be supplied either from the electric power system, capacitor banks or PE-based VAR
generators. During under voltage situations, the induction generator will further decrease the system voltage and cause
voltage stability problems [4]. They cannot be started directly on line as it can cause transients due to inrush currents. It is
not possible to operate induction generators in islanded mode as they need another source to provide excitation. If isolated
with capacitor banks, it can cause serious voltage quality issues [4]. The block diagram of induction generator interfaced
DG system is shown in Fig 3. The power generated using wind or small hydro is of pulsating nature and affects the grid
operation. Hence the output power is fed into the grid using power electronic converters.
Fig 3. Block diagram of Induction Generator interfaced DG System
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C. Power Electronics Converter:
DGs with renewable energy source at input generally use power electronic converter for interfacing with AC grid. The
sources like fuel cells, photovoltaic cells and battery storage generate DC power. This power is then converted to AC power
of desired voltage and frequency using Voltage Source Converters (VSC) and fed into the AC grid through filter as shown
in Fig 4.
Fig 4. Block diagram of Converter interfaced DG System
. The DG system consists of a renewable energy source, a DC/DC converter and an inverter. The VSCs use insulated
gate bipolar transistor (IGBT) switches that are controlled by pulse width modulation (PWM). PWM technique provides
better control and minimizes power quality issues [16]. The most common PWM techniques are Hysteresis Band PWM
and Sine PWM. The advantage of using inverters is that they respond very quickly to the load changes and in the events of
faults. This has enabled the application of islanding operation in micro-grids. However, due to no inertia it cannot provide
energy buffer during step load changes [17].
Table I summarizes the interfacing technologies and their performance. Based on the comparison it can be concluded
that the trade-off needs to be made between the performance and the cost of installation the DG interfacing technology
[13]. The power quality issues caused by integration of DG are discussed in the next section.
Interfacing Technology
Synchronous Generator
Induction Generator
Power Electronic Converters
Table I. Interfacing Technologies
Type of Source
Performance
Micro-turbines and Highly robust,
small hydro
easy control and
better efficiency
Wind and small
Economic and
hydro
better efficiency
Fuel cells,
Fast control,
photovoltaics
highly efficient and
wind, biomass,
most Economic
battery,
microturbines and
small hydro
Major issues
Less economic
Requires reactive power and
less robust
No-inertia and requires
energy storage device
IV. POWER QUALITY ISSUES
Power quality issues is a general term used to describe the influence of power electronic converters on power and voltage
quality of grid. Interconnection of DG causes power quality issues like instability of voltage profile, distortion of voltage
waveform, voltage fluctuation and unbalance in distribution network. These issues further lead to mal-operation of
protection devices, faulty metering and operation of loads, heating and aging of equipment connected in power system like
transformers, cables and motors, interference with communication system etc. Harmonics generated by DG will lead to
reduced power factor, and reduces system capacity. Sensitive loads should be protected from adverse effect of voltage
fluctuations [4].
A. Voltage regulation:
The voltage regulation of grid is possible with exchange of reactive power between grid and DG. The synchronous
generators are capable of both generating and absorbing reactive power. The overexcited synchronous generators will
produce reactive power [18]. However, they are not allowed to regulate the grid voltage as per utility standards because
they can interfere with the main grid voltage regulation scheme [4, 5, 6]. There is possibility of voltage hunting between
DG and capacitor banks if DG is supplying reactive power.
The real power flow from DG can also cause the voltage rise of feeder at DG location. If Induction generator is used
for interfacing, it will absorb reactive power but may not fully counteract the voltage rise of feeder. If synchronous
generator is used, it can be made to supply or absorb reactive power but voltage of feeder may not be controlled to the
required level [19]. Voltage regulating transformer tap setting becomes difficult with the increased and decreased
penetration of DG hence can cause under and over voltage issues.
B. Voltage sag and swell:
The voltage level at DG interconnection depends on network configuration, load conditions and the instantaneous power
produced by the DG system [13]. The voltage sag and swell is the consequence of connection or disconnection of DG into
the power system. Voltage swell can occur on feeder due to reconnection of DG during lightly loaded conditions and large
power production situations. Similarly voltage sag can occur due to disconnection of DG under heavy load conditions.
After a short voltage dip the synchronous generator recovers the voltage near to the voltage before the disturbance. Whereas
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due to the lack of reactive power support, induction generator does not recover the voltage [17, 20]. Fuel cell and some
micro turbines are not capable of meeting the demand of step load change therefore result into voltage sag. Hence these
must be supplemented with battery or flywheel storage to achieve the improved reliability expected from standby power
applications.
C. Harmonics:
Most of the DGs are interfaced using power electronic inverters and these inverters are main sources of harmonics in
the power system. Harmonic filters can be installed at output of DG to attenuate the harmonics generated by the switching
operation. The Total Harmonic Distortion limit at Point of Common Coupling (PCC) is 5 percent based on the requirements
of IEEE standard 519-1992 [21]. Harmonics generated by PWM (pulse width modulation) inverter can also cause resonance
with power system, which further increases the voltage distortion level and harmonics. The PWM switching inverters
produce a much lower harmonic current content than the line-commutated, thyristor-based inverters hence PWM inverters
are preferred than thyristor-based inverters [4].
D. Voltage fluctuation and unbalance:
Fluctuating wind speeds cause voltage fluctuations and flicker. This voltage fluctuations can persist for minutes to hours
depending on the power output of DG, characteristics of feeder and load. It results into flickering of lighting devices. Also
starting of induction generator causes voltage dip up to 40 % in the system due to inrush of magnetizing current which can
last for several seconds [22]. Hence a soft-start circuit is required for large connected induction DG as per IEC 61400-21
standards [23]. In PV based DG, active power injection depends on irradiance. Due to variation in irradiance, active power
varies and voltage fluctuates. In large PV system, voltage fluctuation becomes more severe.
Interconnection of single phase DG can result into unbalance in three phase voltages. This causes unbalanced phase
currents and may lead to excessive values of neutral currents. Hence causes tripping of the feeders in distribution system.
Three phase unbalance also affects motors and other devices connected in the system Non-linearity in power system makes
it difficult to deal with unbalance [15].
V. CONCLUSION
DGs provide clean, reliable and economic power. The choice of source and integrating technology has major impact on
distribution system. To achieve maximum benefits, understanding the issues related to the integration of DG on power
system is required. Due to inherent inertia, synchronous generator can withstand load variation and can operate in islanded
condition. Induction generator has lower cost compared to a synchronous generator but require reactive power for their
startup. Response of power electronic inverters to load changes and faults is fast hence they are the most popular for
interfacing. However, they need energy storage devices for sudden load changes. Other issues like protection, islanding
and stability etc. should be taken care while integration. Standards need to be followed for optimal penetration of DG
power.
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