Grid connected Renewable Energy Systems

European Association for the Development of Renewable Energies, Environment and Power Quality (EA4EPQ)

Abstract.

This paper deals with the technical review of power quality problems associated with the renewable based distributed generation systems and how the custom power devices like STATCOM, DVR and UPQC play important role in power quality improvement. IEEE and IEC standard for the grid connected renewable energy systems are one of the critical point of interest for the selection of custom power devices. Special attention has been given on the compensation of reactive power, harmonic current and voltage fluctuation during the interconnected as well as islanding mode of operation. As the solar and wind are the most abundant resources therefore our point of interest are limited to PV and Wind energy systems only.

Keywords

Renewable Energy Systems, Grid integration, Power Quality, Custom Power Devices, Distributed Generation

1. Introduction

Centralized power generation systems are facing shortage of fossil fuel and also polluting the environment. Long transmission lines are one of the main causes for electrical power losses. Therefore attention has been grown on distributed generation (DG) network systems by the integration of renewable energy system into the grid. With the increase of the renewable energy penetration to the grid, power quality (PQ) of the medium to low voltage power transmission system is becoming a major area of interest. Now a day, several studies are accomplished for the integration of renewable energy systems to the Grid using power electronics converters. The main purpose of the power electronic converters is to convert the energy from one stage into another and integrate to the grid with the highest possible efficiency, the lowest cost and to keep a superior performance. But as the inverters (dc to ac converter) are in general very sensitive to voltage sags / harmonic currents which have a negative effect on the lifetime of the end user devices and on the network, therefore Concerning DG, a voltage / current disturbance can cause the disconnection of a significant amount of energy that could have a major impact on the network operation.

On the other hand, Custom Power Devices like STATCOM (Shunt Active Power Filter), DVR (Series Active Power Filter) and UPQC (Combination of series and shunt Active Power Filter) are the latest development of interfacing devices between distribution supply (grid) and consumer appliances to overcome the any kind of voltage / current disturbances and improve the power quality by compensating the reactive and harmonic power generated or absorbed by the load.

Now a day, Renewable Energy like solar and wind are the most prominently using as DG sources and day by day penetrating level into the grid is also increasing. And it has a significant impact on the flow of power and voltage conditions at the customers and utility equipment. Though DG impacts include voltage support, diversification of power sources, reduction in transmission and distribution losses, improve reliability [1], but on the contrary, awareness on power quality problems is also growing up. Therefore this paper deals here with a technical survey on the research and development in the renewable energy based power system that are connected to the grid, classify the PQ problems related to solar and wind energy integrated to the grid, impact of poor PQ, indentify the probable connection topologies of custom power devices into the system to overcome the PQ problems and propose a custom power park for the future grid connected distributed generation system.

2. Power Quality Issues (DG)

Though one of the most common problems related to power quality is wiring and grounding, approximately 70 to 80% of all power quality related problems can be attributed to faulty connections and/or wiring [2], power frequency disturbances, electro magnetic interference, transient, harmonics, power factor are the other categories (shown in table 1) that are related with the source of supply and types of load [3].

Table 1 – Categories of PQ problems

Power Freq Disturbance

Electro Magnetic Interferences

Power System Transient

Power System Harmonics

Electrostatic Discharge

Power Factor

* Low Freq phenomena

* Produce Voltage sag / swell

* High freq phenomena

* interaction between electric and magnetic field

* Fast, short-duration event

* Produce distortion like notch, impulse

* Low frequency phenomena

* Produce waveform distortion

* Current flow with different potentials

* Caused by direct current or induced electrostatic field

* Low power factor causes equipment damage

Within these phenomena, harmonics are the most dominating one. Regardless of the load category, the same fundamental theory can be used to study the power quality problems associated with harmonics. And the effects of Harmonics on power quality specially described in [4]. According to the IEEE standard, harmonics in the power system should be limited by two different methods; one is the limit of harmonic current that a user can inject into the utility system at the point of common coupling (PCC) and the other is the limit of harmonic voltage that the utility can supply to any customer at the PCC. Details of these limits can be found in [5]. Again, when the DG will be connected to the grid then for the stability of the connection some standard on voltage, current as well as frequency limit should be followed. These interconnection standard details are given in [6].

3. Grid integration of Renewable Energy Systems - Power Quality Issues

A Solar Photovoltaic Systems:

Though the output of PV panel depends on the solar intensity and cloud cover, the PQ problems are not only depends on irradiation but also based on the overall performance of solar photovoltaic system including PV modules, inverter, filters controlling mechanism etc. Studies, presented in [7], shows that the short fluctuation of irradiance and cloud cover play an important role for the low-voltage distribution grids with high penetration of PV. Therefore special attention should be paid to the voltage profile and the power flow on the line. It also suggests that Voltage and power mitigation can be done using super capacitor which increases around one fifth of the PV system cost. Voltage swell may also occur when heavy load is removed from the connection. Concerning DG, any kind of voltage disturbance causes the disconnection of inverter from the grid and therefore it results losses of energy. Also long term performance of grid connected PV system shows a remarkable degradation of efficiency due to the variation of source and performance of inverter [8].

The general block diagram of grid connected PV system is shown in 2 and the system can be a single-phase or three phase depending on the grid connection requirements. The PV array can be a single or a string of PV panels either in series or parallel mode connection. Centralized or decentralized mode of PV systems can also be used and the overview of these PV-Inverter-Grid connection topologies along with their advantages and disadvantages are d out in [9].

These power electronics converters along with the operation of non-linear appliances inject harmonics to the grid. On the over hand voltage fluctuation due to the irradiation, cloud cover or shading effect could make the PV system instable in term of grid connection. Therefore, special control design is required in the controlling part of the inverter [10-11].

In general, grid-connected PV inverter is not able to control the reactive and harmonic currents drawn from the non-linear loads. An interesting controlling mechanism has been presented in [12] where PV system is used as an active filter to compensate the reactive and harmonic current as well as injecting power to the grid. This system can also operate in stand alone mode. But the overall control circuit becomes somewhat more complex. Present research [13] also shows that a remarkable achievement has been done on improvement of Inverter control to provide the reactive power compensation and harmonic suppression as ancillary services. A multifunctional PV Inverter for grid connected system (3) has been developed recently and presented in [14] shows the reliability through UPS functionality, harmonic compensation, reactive power compensation capability along with the connection capability during the voltage sag condition. But result shows that PQ improvement is still out of IEEE range.

B. Wind Energy System:

A simplified diagram representing some of the common types of wind energy systems are shown in 4. From the design perspective view it is found that some generators are directly connected to the grid through a dedicated transformer while others incorporate power electronics. Many designs, however, include some level of power electronics to improve controllability. Whatever the design or connection configuration, each turbine itself has effect on the power quality of the transmission system. Recent analysis and study [15] shows that Comparing the results of the tower shadow and vertical wind shear to the results of the horizontal wind shear and yaw error, it can be concluded that the impact of the yaw error and horizontal wind shear on the power (torque) and voltage oscillations is more severe than effects due to the tower shadow and vertical wind shear.

A literature survey [16] of the new grid codes adapted for wind power integration has identified the problems of integrated large amounts of wind energy to the electric grid. It suggests that new wind farms must be able to provide voltage and reactive control, frequency control and fault ride-through capability in order to maintain the electric system stability. Whereas, for the existing wind farms with variable speed doubly fed induction generators and synchronous generators, frequency response into the turbine control system can be incorporated by the software upgradation. Wind farms with fixed speed generators have to be phased out because they cannot offer voltage or frequency control. An overview of the developed controllers for the converter of grid connected system has also been discussed in [17] and showed that the doubly fed induction generator (DFIG) has now the most efficient design for the regulation of reactive power and the adjustment of angular velocity to maximize the output power efficiency. These generators can also support the system during voltage sags. However, the drawbacks of converter-based systems are harmonic distortions injected into the system. Being a single-stage buck-boost inverter, recently proposed Z-source inverter (ZSI) can be a good candidate to mitigate the PQ problems for future DG systems connected to the grid [18] (5).

Anti-islanding is one of the important issues for grid connected DG system. A major challenge for the islanding operation and control schemes is the protection coordination of distribution systems with bidirectional flows of fault current. This is unlike the conventional over-current protection for radial systems with unidirectional flow of fault current. Therefore extensive research in being carried out and an overview of the existing protection techniques with islanding operation and control, for preventing disconnection of DGs during loss of grid, has been discussed in [19].

4. Impact of Power Quality Problems

The impacts of power quality are usually divided into three broad categories: direct, indirect and social. A detail of these impacts has been described in [20].

A recent survey based on interviews and web based submission, conducted over a 2-year period in 8 European countries, has been reported in [21]. Survey reported PQ costs due to the effect of Voltage dips and swells, Short interruptions, Long interruptions, Harmonics, Surges and transients, Flicker, unbalance, earthing and electromagnetic compatibility (EMC) problems. It is found that the cost of wastage caused by poor PQ for EU-25 according to this analysis exceeds €150bn where Industry accounts for over 90% of this wastage. Dips and short interruptions account for almost 60% of the overall cost to industry and 57% for the total sample. The study also shows that the economic impact of inadequate PQ costs industry some 4% of turnover and services some 0,15%. 6 shows the PQ costs for the EU-25 countries in sector-wise. At the same time it is necessary to consider the impact of DG in terms of the cost of power quality. In [22], a method to evaluate the dip and interruption costs due to DG into the grid has been proposed. Based on the operating hours, the frequencies of PQ events occur and cost of PQ evens indicates the positive or negative impact of DG.

5. Mitigation of PQ problems

There are two ways to mitigate the power quality problems - either from the customer side or from the utility side. First approach is called load conditioning, which ensures that the equipment is less sensitive to power disturbances, allowing the operation even under significant voltage distortion. The other solution is to install line conditioning systems that suppress or counteracts the power system disturbances. There are several devices like Flywheels, Supercapacitors, Energy storage systems, Constant Voltage Transformers, Noise Filters, Isolation Transformers, Transient Voltage Surge Suppressors, Harmonic Filters etc are used for the mitigation of specific PQ problems. Custom power devices (CPD) like DSTATCOM, DVR and UPQC are recently added in the mitigation of any kind of PQ problems associated with utility distribution and the type of end user appliances. Here our special concern is on CPD and how they play important role in mitigating PQ problems due to the renewable energy systems, especially solar and wind, integrated with the grid.

6. Role of Custom Power Devices

Custom Power (CP) concept was first introduced by N.G. Hingorani in 1995 [23]. Custom Power embraces a family of power electronic devices, or a toolbox, which is applicable to distribution systems to provide power quality solutions. This technology has been made possible due to the widespread availability of cost effective high power semiconductor devices such as GTOs and IGBTs, low cost microprocessors and techniques developed in the area of power electronics.

DSTATCOM is a shunt-connected custom power device which primary aims are power factor correction, current harmonics filtering, load DC offset cancellation and load balancing. It can also be used for voltage regulation at a distribution bus [24]. It is often referred as shunt or parallel active power filter. It can be consist of voltage or current source PWM converter depending on the storage element as a dc capacitor or inductor, Fig. 8. It operates as a current source and compensates current harmonics by injecting the harmonic components generated by the load but phase shifted by 180 degrees. Moreover, with an appropriate control scheme, the shunt active power filter can also compensate the load power factor.

DVR is a series-connected custom power device to protect sensitive loads from supply side disturbances except outages. It can also act as a series active filter, isolating the source from harmonics generated by loads. It consists of a voltage-source PWM converter equipped with a dc capacitor and connected in series with the utility supply voltage through a low pass filter (LPF) and a coupling transformer [25], 9. This device injects a set of controllable ac voltages in series and synchronism with the distribution feeder voltages such that the load-side voltage is restored to the desired amplitude and waveform even when the source voltage is unbalanced or distorted.

UPQC is the integration of series and shunt active filters, connected back-to-back on the dc side and sharing a common DC capacitor [26], 10. The series component of the UPQC is responsible for mitigation of the supply side disturbances: voltage sags/swells, flicker, voltage unbalance and harmonics. It inserts voltages so as to maintain the load voltages at a desired level; balanced and distortion free. The shunt component is responsible for mitigating the power quality problems caused by the consumer: poor power factor, load harmonic currents, load unbalance, DC offset. It injects currents in the ac system such that the source currents become balanced sinusoids and in phase with the source voltages.

STATCOM is already reported for wind power applications such as islanding performance, stability enhancement, transient, flicker mitigation of wind generator, etc. [27-28]. As the traditional STATCOM works only in leading and lagging operating mode, its application is therefore limited to reactive power support only. On the other hand, wind is intermittent and stochastic in nature. Therefore the fluctuating power cannot be smoothed by using STATCOM, because it has no active power control ability. To overcome this problem, Battery Energy Storage System (BESS) has been incorporated with STATCOM (STATCOM/BESS) [29], which has both real and reactive power control ability (11).

Very recent research reports that a significant research and development has been done on UPQC implementation to the grid connected PV or Wind energy systems [31-32]. As the UPQC has almost all the mitigation properties of existing PQ problems in the transmission and distribution grid along with the compensating harmonics and reactive power generated by the load therefore placement of UPQC in distributed generation network is very important.

A structure has been proposed in [31], 13, where PV is connected to DC link in UPQC as an energy source. It works both in interconnected and islanded mode. UPQC has the ability to inject power using PV to sensitive load during source voltage interruption. Advantage of this system is voltage interruption compensation and active power injection to grid in addition to the other normal UPQC abilities. It has the higher efficiency and functioning ability in compare with other common grid-connected PV systems and also causes a reduction of total system cost. But the system's functionality may fall down if enough sun is not available during the voltage interruption condition.

Similarly the application of a unified power quality conditioner (UPQC) to overcome the grid integration problems of the FSIG is investigated in [32], 14. As the ability of wind generator connected to the grid influenced by the in the event of system faults and dynamic reactive power compensation and the wind driven, fixed-speed induction generator (FSIG) on its own fails to fulfil these requirements therefore The role of the UPQC in enhancing the fault ride-through capability of the generator has been investigated under both full and partial terminal voltage restoration. Result shows UPQC as a prime contestant for the application of PQ mitigation in grid connected wind energy system.

Another interesting idea was proposed in [33 - 34] to improve the power quality in a power part using the custom power devices and it has been extended more by using supervisory control technique to coordinate the custom power devices by proving the pre-specified quality of power [35], 18.

7. Conclusion

Recent trends in the power generation and distribution system shows that penetration level of DG into the grid increasing rapidly. End user appliances are becoming more sophisticated to the power quality condition. Extensive Research on controlling of the Custom power devices for the mitigation of PQ problems are also carrying out. Therefore now more concentration should be given on the placement of CPDs in between conventional grid, GD and user appliances for the PQ improvement of overall distribution networks.

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