RECENT TRENDS IN DISTRIBUTED GENERATION - TECHNOLOGY, GRID INTEGRATION, SYSTEM OPERATION Tatsumi Ichikawa
Christian Rehtanz
Komae Research Laboratory CRIEPI Tokyo, Japan
[email protected]
ABB Switzerland Ltd Corporate Research Baden-Dättwil, Switzerland
[email protected]
Abstract – The paper discusses the recent trends in distributed generation (DG). Technological trends like fuel cells, microturbines and wind power are summarized. Guidelines for grid integration are analyzed as a basic need for the wide area installation of DG. The operational requirements are discussed beyond the trend of increasing numbers of DGs in particular areas which leads to microgrid operations. Protection of DG is summarized as well. In addition to these general trends a couple of recent achievements of research and development in the sector of DG are presented. This contains integration of DG, stability of DG-control and operational aspects.
tion. The old, centralized model of transporting electricity made extensive use of long-distance high-voltage transmission. This usually meant building unsightly overhead power lines. And although there have been continual improvements in long-distance transmission efficiency, a significant proportion of the power transmitted is still lost. DG allows us to place power generation facilities much closer to end-users, reducing transmission losses. If DG-units are rare, the system operator can tolerate them. If their number is growing they need to be coordinated with the system's control center. The aggregation of several DG-units to a microgrid opens up a large field of customer oriented and optimized operation. This paper points out the recent trends in technology, grid integration and operation of DG. Section 2 underlines some technological trends, which are supporting the wide area use of DG now and in the near future. Section 3 summarizes actual numbers of DG installations in Europe and Japan. Ongoing topics are guidelines for the network integration of DG. The actual status is presented in section 4. Beside the network integration, the operation of DG is a fast developing trend. The increasing number of DG in a particular area of a power system leads to new solutions for operation and protection. The requirements and basic trends are presented in section 5. Section 6 comprises recent research and development results on several specific areas of DG and outlines some recent activities in detail.
Keywords: Distributed Generation, Grid Integration of DG, System Operation of DG, Protection of DG, Control of DG, Fuel Cell, Microturbine, Wind Energy, Microgrid.
1 INTRODUCTION Thomas Edison was a man of great foresight. When he set up his first heat-and-electricity plant near Wall Street in 1882, he imagined a world of micropower. Edison thought the best way to meet customers' needs would be with networks of decentralized power plants in or near homes and offices. After a century with power stations getting bigger and transmission grids needed to transmit wider, the idea of local generation for local consumption is back. There are several reasons for this. One is market liberalization. Small, local power plants offer a cheap way into market and cause only low investments. They do not suffer huge transmission losses. The surplus heat they generate can be employed for useful purposes. Therefore local power becomes economically competitive. Another reason is the demand for reliable, uninterrupted power. The situation in California is the best motivation for distributed power generation. The distributed generation (DG) framework is mo ving away from traditional large-scale power generation plants (100 MW's to GW's) located near the natural resource converted to electricity, to small power generators (kW's to MW's) sited directly at the loads. Enabling this concept is more of a regulatory than a technological issue. However, there are still a couple of technical hurdles facing it. The gradual shift from centralized to distributed generation means changes not only in the kinds of power plants used, but in the way electricity is transported from the point of production to the point of consump-
2 TECHNOLOGICAL TRENDS DG comprises several technologies. Internal combustion engines are old technologies, which have many moving parts, are maintenance intensive and are not environmental friendly. Today, Photovoltaic is marketable only for small-scale applications. On the other hand there are recent technological developments, which are marketable today or in the near future. Microturbines and Fuel Cells are economically feasible due to combined heat and power production and small-scale applicability. The well-established wind power is going in the direction of larger scales with onshore and offshore wind-farms providing higher economical efficiency. 2.1 Microturbine Microturbines are one of the most promising DG products currently on the market. The key technical features are: • High efficiency (as high as 85%)
• •
Low emission (< 15ppm NOx and CO) Fuel flexibility: oil, diesel, natural gas, biogas, methanol, hydrogen, etc. • Low maintenance cost (high speed single shaft engine and static power electronics converter) • Remote control of power production available Typical applications for microturbines are industrial, commercial and public buildings, large residential buildings, fun parks etc. Microturbines are currently sold on the market. They always require a power electronics interface to connect them to the distribution grid. Most attractive applications for microturbines make use of the thermal energy as well for heating or cooling. For example, Figure 1 shows the microturbine providing electricity and heat to the ABB Corporate Research Center in Switzerland.
converters are allowing to fulfill all guidelines for the grid connection in a very flexible way. An additional trend is a higher output voltage up to 4 kV, which e.g. reduces the energy transmission losses and allows simplified setups of wind farms. 2.3.2 Wind farms The second trend concerns wind farm configurations with low cost profile and high efficiency. There are several solutions existing in AC and/or DC technology. Due to the existing power electronic converters within the today's windmills, there are several opportunities of combining AC and DC parts within a farm setup. For example the intermediate DC-circuits of the units can be coupled directly before converting to the grid frequency. In [6] and [7] an overview is given about the actual state of the art as well as recent trends. In general this is an ongoing research topic on power system, control as well as material side. 3 PRESENT STATUS OF DG INSTALLATION The installation of DG is growing worldwide. The trends in the two regions Japan and Europe are discussed in the following, which are driven by the Kyoto Conference.
Figure 1 : ABB MT100 microturbine.
2.2 Fuel Cell A bright vision in the area of DG is the wide area introduction of fuel cells for private households. Fuel cells are providing simultaneously electricity and heating and shall be a replacement for nowadays gas heating in many countries. This application is of special interest because the production is as close as possible to the demand. A couple of activities from several big companies are ongoing for immobile as well as mobile use. Different technologies in the power ranges up to 5 kW [1], up to 250 kW [2] or as backup power [3] are available as pilot installations. 2.3 Wind-Power There are two recent trends concerning wind power. The first one is the introduction of permanent magnetized direct driven machines and the second one is wind farm with DC interconnections. 2.3.1 Permanent magnetized direct driven generators Direct driven generators are on the market since several years. Without the need of a gearbox the design has less moving parts and is more environmental friendly because of less oil, which is especially important for offshore usage and less noise. Usually electrically excited machines are used. With the recent introduction of permanent magnetized generators the construction can be further simplified while reducing the rotor losses [4][5]. This reduces the vulnerability, operation and maintenance costs. The nowadays power electronic
3.1 Japan In Japan, the Ministry of Economy, Trade and Industry (METI) set the target of introduction capacity for new types of renewable energy generations and high efficiency co-generation systems using natural gas in the year 2010 as shown in Table 1 [8]. Table 1 : Target of introducing generation capacity for new types of energy in Japan [8] Record in 1999 (MW)
Target in 2010 (MW)
Photovoltaic
209
4,820
Wind Power
83
3,000
Solid Waste
900
4,170
Biomass
Generation Systems
Renewable Energy Sources
Demand Side Energy Utilization
80
330
Natural Gas Cogeneration System
1,520
4,640
Fuel Cell
12
2,200
Actual trend of introduction for photovoltaic power generations and wind power generations up to 2000 is shown in Figure 2 and of co-generation system using combustion engines is shown in Figure 3.
Cumulative Capacity (MW)
350 300 250
PV
200
WP
150 100 50 0
1992 1993 1994 1995 1996 1997 1998 1999 2000 Year
Figure 2 : Trend of introduction of photovoltaic and wind power generation in Japan
3.2 Europe The member states of the EU are required to take measures to ensure that the share of renewable energy sources develops according to the limits accepted at Kyoto in order to reduce the emission of greenhouse gases (GHG). The EU is recognizing that fuel-efficient generating techniques such as combined heat and power can reduce GHG emissions. Figure 4 shows the contributions of renewable electricity within the EUcountries and the targets for 2020 [9]. Combined heat and power generation is not included. The major parts are large hydropower generation, which does not belong to the distributed generation. Altogether the part without large hydro is 3.2% of the EU's electricity production with a target of 12.5 % in 2010.
5000 4000
Commercial
3000
Industry
2000
2000
1999
1998
1997
1996
1995
1993 1994
1992
1991
1990
1989
1988
0
1986 1987
1000 1985
Cumulative Capacity (MW)
6000
Year
Figure 3 : Trend of introduction of co-generation systems in Japan
To accomplish the target, METI applied the program for subsidizing the introduction of distributed generations especially for the renewable power generation systems. Electric utility companies in Japan cooperate to the introduction of DGs by purchasing electricity from distributed generations voluntary.
4 REGULATIONS AND TECHNICAL REQUIREMENTS FOR GRID CONNECTION Clear guidelines for the network connections must be worked out as well as contracts for the use of the network and for taking over the generated power. Technical standards for grid interaction and certification of connections are needed. 4.1 Typical Procedure of Connection Certification The interconnection of DG to the grid is a question of physical network parameters as well as network operation. The operation and management of the DG must be adapted to the requirements as well as the network operation management system. Dependent on kind and number of DG a decentralized energy management system is necessary (see also section "microgrid" below). The distribution grid operator has to provide: • network data and distribution grid capacity, • determination of spectral impedances, • definition of technical limits, • network alteration or reinforcement, • protection adaptation or alteration. The operator of the DG has to provide and fulfil: • operational schedules, • technical data (power converter data, etc.), • protection data, • fulfillment of technical requirements.
Targets for 2010
European Union (targets 2010 in brackets) renewable electr. contribution: 13.9% (22.1%) excluding large hydro: 3.2% (12.5%)
Figure 4 : Renewable electricity contribution within the European Union in % [9]
4.2 Worldwide The following standards concerning DG are currently discussed in the different working groups [8]: • IEEE P1547 draft 08 aims at providing functional, technology neutral technical requirements [11]. • IEEE 929-2000 collects major technical interconnection requirements for photovoltaics [12]. • IEC 1000-3-3 specifies the limits of voltage fluctuation and flicker in low voltage power supply systems for equipment with rated current up to 16A [13]. • IEC 1000-3-5 corresponding standard for > 75A [14]. • IEEE 519 is also applied for DPG, defining harmonics, unbalance and sags [15]. A lot of regulatory and scientific focus has been on protection and metering. Generally grid connection without reverse power flow has no impact on protection. When feeding back, protection concepts have to be
adapted. The metering (net meting or power flow dependent metering) is related to country specific payments or incentives for distributed energy. 4.3 Japan The Japanese technical grid-interconnection guidelines distinguish the requirements by the type of interconnection (inverter interfacing connection or direct connection of AC generator), operating condition (with reverse power flow or without reverse power flow) and the interconnected utility system class (low voltage distribution line, middle voltage distribution line, high voltage transmission line and so on, depending on the capacity of generators) [16]. Table 2 shows the classification of applicable DG systems for each utility system with the history of revision of guideline. It was established first as the guideline for interconnection of AC rotating generator without reverse power flow to middle voltage distribution line in 1986. Table 2 : History of Japanese technical guideline of gridinterconnection of distributed generators
Utility System
HVT
MVD
LVD
Capacity per Customer or Generator
Genera-tor Type
Reverse Power Flow
Not Allowed
Allowed
More than 2,000 kW
ACG
Aug., 1986
IIG
June 1990
Less than 2,000 kW
ACG
Aug., 1986
IIG
June, 1990
Less than 50 kW
ACG
Mar., 1991
Not Determined
IIG
Mar., 1998
Mar., 1993
Mar., 1993
HVT: High Voltage Transmission System, MVD: Middle Voltage Distribution System (6.6 kV), LVD: Low Voltage Distribution System (100/200 V) ACG: AC Generator, IIG: Inverter Interfacing Generator
A remaining issue for guideline is the technical requirements for rotating AC generator interconnection with reverse power flow into the low voltage distribution line. The main issue is the prevention of islanding and the evaluation of islanding detection methods. This is now ongoing by NEDO through the research project of promo ting distributed generations. 4.4 Europe Despite the pressure for harmonization the policies for DG are still differing within the European Union (EU) [8]. They differ in terms of • openness to DG, • the tax and price incentives offered, • regulatory arrangements. The incentives are influencing the metering. Net metering for example is allowed e.g. in Germany, Denmark, Spain, Switzerland. In Italy and Portugal two different meters are used, one for measuring the supplied and
another for measuring the consumed energy from the utility grid. Usually the DG operator is responsible for the protection, but in most of the countries a main circuit breaker must be available to grid operator staff or remotely controlled by the grid operator. Islanding is usually not allowed due to security reasons. The European Committee for Electrotechnical Standardizations, CENELEC, has drawn up the standard EN 50160 [17], regulating the quality of power supplied to the customer. It describes the voltage characteristic of electricity supplied by public dis tribution systems, indication conditions and limits. EN 50160 is not applied in a number of countries, despite these limits refer to the public distribution network. Generally, utilities are setting their own limits for the voltage quality that DG-units feed into the grid. 5
TECHNICAL REQUIREMENTS FOR DG OPERATION AND PROTECTION
5.1 From Single Units to Microgrids Whereas single DG unit is usually easy to tolerate by the grid, a lot of technical challenges are coming up with an increasing density of DG installations. The transmission grid will develop itself in a balancing or equalizing network. The control centers have to be adapted. Maintenance concepts have to be coord inated. The combination of heat and power with e.g. fuel cells or microturbines on the low voltage level are leading to new requirements for the operation and energy management. One major recent development is the microgrid or virtual utility combining several DG units with a coordinated and optimized operation. A typical microgrid or distributed power station might contain about 10 MW of aggregated generating capacity produced in a number of small distributed generation resources. These might include any mix of combined heat and power plants, wind turbines, microturbines, fuel cells and photovoltaic generators. The structure of a microgrid is shown in Figure 5. The microgrid is operated by a control center, which optimizes the use of the different distributed generators (DG) and distributed storages (DS). Business center DS
DS DG DG DG
DG
DG
Control center
DG
DG DG DG DG DG DG
DG
DS DG
DG
DG
Figure 5 : Mircrogrid or Virtual Utility Concept (DG=Distributed Generation, DS=Distributed Storage)
In the control center the demand for and supply of energy is monitored, and power switched from the main grid to the microgrid, as it is needed. This control center is connected to a business center, which acts in the energy market to trade with the capacity of the microgrid or to buy complementary energy for the consumers in the microgrid. This business center takes care of administration, energy trading and sales on behalf of the operator, while another provides dedicated maintenance and services. The advantages of microgrids are: • Synergies for personnel resources, primary energy purchase, maintenance • high availability • energy mix • modularized operation planning • extended service • resource optimization • economic efficiency Many different types of companies and organizations can benefit from a microgrid system. Residential complexes are obvious candidates, as are rural communities beyond the reach of existing power grids. Others include hotel and tourist complexes, hospitals, shopping malls, industrial parks and large industries with many dispersed sites. Utility companies themselves also have a regular need for additional sources of power. They may have to fuel new growth beyond their current grid, for instance, or require temporary or permanent power support to enable the rehabilitation of old infrastructure. Rural utilities with distributed loads, island communities, energy service providers and power providers in developing countries that lack infrastructure could also benefit from microgrids. 5.2 Protection Beside the operation the protection of the grid in presence of DG is an important task. DG affects the protection of the grid in several ways, dependent on the voltage level (LV or MV) where the DG is connected [18]. • Settings in distance relays (impedance measurement) are affected by the infeed effect. The determined impedance is changed due to current input along the protected line. This may shift the selective tripping schedule and selectivity may not be achieved. A resetting of the distance relay parameters is necessary whenever a significant power is introduced into the MV-network. • The selectivity of short circuit current indicators may be compromised. In case of a fault, the indicators may not only trip from the feeder side, but also from the dead end side of the line. This may happen if there is a significant short circuit current contribution by DG (possible with synchronous generators, but not usually with inverter based systems). • The fusing scheme on the LV level may have to be adjusted to prevent overloading of lines. If a DG is introduced along a line, the line may be overloaded between the DG and a load on the same line without
blowing the fuses at the end of the line. In this case additional fuses are needed. • Reverse power relays (these relays are located at each transformer in a meshed LV grid and trip in case of a fault in the corresponding MV line) in meshed systems may have to be replaced, as they may trip under normal working conditions when DG on the low voltage level are delivering power to the medium voltage level. An individual analysis has to be made to determine, if the nominal current differs sufficiently from the fault current to achieve selectivity. • Additional protection devices are needed for DG interconnection, as required from regulations (e.g. loss of mains detection). It can be concluded that selectivity can be compromised both on LV and MV level. Accordingly, measures have to be taken (adjustment of protection devices, changes in protection schemes). Both meshed and non-meshed networks have their advantages for the placing of DG. • Meshed distribution networks have a high short circuit power. Their advantage is a relatively balanced voltage profile and high reliability through redundancy. These networks can usually handle more aggregated DG power, if the short circuit power stays within allowable limits. • Non-meshed networks have a low short circuit power, are relatively simple to design, but the voltage profile is more vulnerable to load steps. The introduction of DG usually has a higher impact on the voltage profile in these configurations (Depending on the location in the network). Non-meshed networks do not need reverse power relays and therefore do not need any alternative solution for these relays when introducing DG. A meshed grid can usually be converted to a nonmeshed grid by opening the disconnectors in the according cable distribution cabinets. This could be necessary, when the short circuit current in a meshed system increases above allowable values (according to the installed equipment) by introducing DG. The splitting of a meshed network reduces the short circuit power in any given point in the grid. Furthermore, the splitting of the network removes the need for reverse power relays and can improve the selectivity of short circuit current indicators. Conclusively, there are no non-solvable problems concerning protection of the grid even with high numbers of DG. 6
RECENT RESEARCH AND DEVELOPMENT RESULTS
6.1 Study on Grid Connection Techniques for Dispersed PV Systems under High- Density Massive Connection [19] 6.1.1 Project Objectives Installation of photovoltaic (PV) generation has shown steadily increase in these years in Japan. From a viewpoint of power system planning and operation, since PV systems are usually connected to a distribution line, increase of PV systems causes considerable con-
cerns on their impact on power quality, stability and safety in distribution system. The purpose of the project is as follows: 1) To clarify problems caused by the high-density massive interconnection of PV systems 2) To develop countermeasures for solving the problems 3) To study the technique for enhancing the utilization factor of PV power generation This project was 4-year project completed in the fiscal year 2000 under entrustment of The New Energy and Industrial Technology Development Organization (NEDO). 6.1.2 Major Results (1) Clarification of problems caused by the high-density massive interconnection of PV systems a. Influence on the power quality of distribution line First, an analytical investigation was conducted on influence on the power quality, such as voltage distortion due to harmonics and voltage rise caused by the high-density massive interconnection of PV systems. As a result, it was clarified that the possible installation volume of PV systems in residential area was mainly restricted by the voltage rise of distribution line caused by the reverse power flow from PV systems. For analysis more in detail, a dynamic simulation program including models for high- and low-voltage distribution lines was developed. b. Stability of PV system for line service works on power distribution line Instantaneous voltage drop and power interruption tests were conducted at the Rokko Test Site in order to clarify the stability of PV inverters for service works and system switching of power distribution line. As a result, it was found that operation of most inverters is momentarily interrupted, but no case of fault or damage was observed and no particular problem was raised. c. Safety for faults on power distribution line To clarify the ability of islanding protection of PV inverters on the market, single unit operation tests were conducted at the Akagi Test Center of CRIEPI (Central Research Institute of Electric Power Industry), as it is important for the safety aspect of distribution line. In succession, low voltage single phase, parallel operation tests of several units (3 ~ 5 units) and high voltage three-phase, parallel operation tests of many units (84 units) were conducted at the Rokko Test Site. As a result, there was no problem with the single unit operation. However, the continuing islanding phenomenon was observed in some cases when several units of the same product were put in parallel operation. On the other hand, an interesting result was given that the islanding phenomenon is effectively prevented even by a mixture operation of several tens units employing different types of islanding protection system. To clarify the reliability of various islanding protection, simulation models were formed for analyzing some typical algorithms employed by PV inverters on the market, and the detailed EMTP analysis was conducted
for them. As a result, it was found that combinations of detection of a faint fluctuation of frequency or phase angle of line voltage as a passive way and the frequency shift or reactive power injection as an active way are relatively effective. (2) Research and development on countermeasures against the problems a. Development and evaluation of distributed control type voltage stabilization system To suppress the voltage rise on high voltage power distribution line caused by reverse power flow from PV systems, a new distributed control type voltage stabilization method was proposed, and demonstration system was built at the Akagi Test Site, which has two control layers, one is the reactive power control by middle-scale PV systems in the high voltage feeder, and the other is the active power control by residential PV systems in the low voltage branch feeders. As a result of the field test, it was proved that the voltage rise is effectively suppressed by the reactive power control at the similar level to a total active power output of PV systems. It was also found that the voltage regulation system of small residential PV system might be simplified only as active power control. b. Development of highly reliable islanding protection system A new islanding protection system was developed, which is based on combination of the highly sensitive passive method for detecting rapid change in harmonics and frequency and the active method of reactive power control. Then it was affirmed that the islanding phenomenon is detected perfectly in the indoor single unit test at the Akagi Test Site, and also by parallel operation test using 18 units in the high- voltage feeder at the Rokko Test Site. (3) Study on the technique to improve the utilization factor of PV power generation Multi-function PV inverter that performs not only the basic DC to AC power conversion, but various valuable additional functions such as smoothing the fluctuation on PV output power and customer’s load, compensation of harmonics and reactive power of loads was developed for value added PV application. In the indoor performance test at the Akagi Test Site, a long-term field test using 5 units was conducted in the low voltage branch feeder at the Rokko Test Site. It was confirmed that the PV power output fluctuation, load power variation, harmonics and reactive power from customer’s load are reduced to around 1/10 of that obtained by the test with conventional inverter. 6.2 Study on smoothing effects of wind power and its influence on power system [20] 6.2.1 Project Objectives Wind power generation has shown remarkable increase in recent years in Japan. From a viewpoint of power system planning and operation, increase of wind power generators causes considerable concerns on their
bad influence on power quality such as frequency and voltage deviation. The purpose of the project is as follows: 1) Wind condition across Hokkaido, where development of wind power is rapidly proceeding, is to be observed throughout a year and wind characteristics in the area is to be examined in the light of meteorological conditions, correlation of wind at different sites, and other factors. Wake of a wind turbine, which could play a significant role in a wind farm, is also to be studied. In addition, feasibility of short-term wind prediction is to be investigated. 2) A smoothing effect of output fluctuation of distributed wind turbines is to be examined by using output power estimated from measured wind data. Output fluctuation in case of massive penetration of wind power is to be analyzed. Impacts of output fluctuation of wind turbines on a power system are to be studied from viewpoints of system frequency as a system-wide problem and of system voltage as a local problem. This project was done in the fiscal year 2000 and 2001 under entrustment of NEDO in Japan. 6.2.2 Major Results (1) Analysis of wind characteristics a. Observation of Wind Wind conditions at the heights of 20m and 40m are observed at about thirty points (sixteen sites) across Hokkaido from December 2000 to November 2001 (one year). b. Analysis of Wind Conditions It is confirmed that wind conditions in Hokkaido tend to be strong in winter and weak in summer. It should be noted that wind conditions significantly varies from site to site due to local topography and other factors. Power spectra of wind velocity have peaks at the period of a few minutes and a few days: the former corresponds to effects of a local terrain and vegetation while the latter depends on meteorological disturbance. Statistical analyses on fluctuation of wind velocity and direction show that maximum fluctuation of wind velocity and direction reaches 15m/s and 180 degrees in ten minutes, respectively Coherences between wind velocity at two points both on a wind farm scale and on a broader scale is quantitatively examined: it was found that they decrease as the distance between the points increases. Wind velocity at two points in a hypothetical wind farm (i.e., area within about 2km) shows considerable correlation for the fluctuation with period of 10 minutes while wind velocity at the two sites varies almost independently for the fluctuation with period of less than 1 minute. Coherence tends to be smaller on a complex terrain than on a flat terrain; it also gets smaller in case of wind direction perpendicular to arrays of observation masts. c. Short-term wind prediction method The following methods are examined for improving accuracy of a prediction: e.g., usage of a local meteorological model with a smaller mesh size, revision of forecast with MOS (Model Output Statistics) – regression
formula obtained with actual data and forecast over a long period --, successive revision of forecast with actual values. The results show that, while successive revision with actual data is effective for the forecast up to 3 to 6 hours, MOS is mostly advantageous for the longer period. In order to reduce the forecast error in case of passage of depression, man-assisted revision of a forecast could be helpful. (2) Estimation of wind power output and its influence on power system Output fluctuation characteristics of wind power are examined using wind power output converted from the observed wind data with the aerodynamic simulation and with a power curve (characteristics between wind velocity and wind power). The former method is applied to the selected cases with distinctive features while the latter covers all observed data. a. Seasonal changes of output fluctuation Fluctuation of total output of distributed wind turbines is larger in winter than in summer; this is partly due to differences in capacity factor among seasons. However, seasonal differences in output fluctuation during a day are not so large; the fluctuation reaches almost their installed capacity throughout a year. b. Output fluctuation in a wind farm Output of wind turbines at two sites in a wind farm shows considerable correlation for the fluctuation with the period of about 10 minutes while output power of two turbines in a wind farm varies almost independently for the fluctuation with the period of less than 1 minute. This implicates poor smoothing effect in a wind farm for a period of some ten minutes. c. Spatial correlation of wind fluctuation in a wider area Spatial correlation of wind fluctuation in a wider area considerably differs from day to day: it tends to be larger when an atmospheric disturbance such as depression is observed. In case of high correlation, coherence between outputs at two points can be fairly high for the period of about 100 minutes even if distance between the two points exceeds some hundred kilometers. On the other hand, correlation is mostly negligible for the distance less than some ten kilometers (3) Impacts of high penetration of wind turbines on a power system Case studies are carried out on impacts of massive penetration of wind turbines on a power system – i.e., impacts on system frequency and on system voltage. a. Impact on system frequency Digital simulation studies on system frequency considering load frequency control are executed while varying the followings as parameters: system capacity, capacity and type of regulating power, and output fluctuation pattern of wind power. The study reveals: - Impacts of wind power on system frequency depend on relative magnitude of output fluctuation of wind power with demand fluctuation.
- If regulation power is almost exhausted for compensating for demand fluctuation, impacts of output fluctuation of wind turbines on system frequency can be very sensitive. - Gradual change of wind power output can degrade system frequency if the magnitude of the change is considerably large.
removing short-cycle component (on the order of a few seconds to a few hours) from the input signal. The output of the filter is used as a target signal of power output from the combined system. Tests were carried out to verify the smoothing effect by charging or discharging the battery system. Figure 6 shows an example of a result from the test at the testing site using the redox-flow battery.
The last result implicates the importance of absorbing output fluctuation of wind power with economic load dispatching control and manual operation of a generation system. Fluctuation of system frequency depends on the amount of obtained regulating power while the extent of output fluctuation varies with geographical allocation of wind turbines. It should be noted that studies with the input data corresponding to a specific power system are indispensable to obtain accurate results. b. Impact on system voltage Simulation studies on system voltage on a transmission line (66-77kV) are carried out while varying the followings as parameters: capacity of a wind farm, type of a wind turbine (fixed or variable speed machines), line impedance, and short circuit capacity at a substation, and others on a model system. The study reveals that line impedance between a wind turbine and substation and system load at a bus nearby a wind farm has a considerable influence on voltage deviation. 6.3 Verification of Smoothing Performance of Wind Power Fluctuation with Storage Battery [21] 6.3.1 Project Objectives Wind power generation output varies every moment due to the effects of wind, geographical conditions and other factors. In order to promote introduction of much more wind power plants, therefore, evaluating it and also establishing means of reducing fluctuations in wind power output are imperative. The purpose of the project is as follows: 1) The effect of smoothing fluctuations in power output from wind power generation system combined with battery storage is to be verified. As for type of battery storage, sodium-sulfur batteries, lead-acid batteries, and redox-flow batteries are to be tested. Three existing wind power generation facilities were selected and temporarily combined with one of the above battery storage systems. 2) In order to evaluate the effectiveness of the systems with batteries and flywheels (hereinafter referred to as “batteries and others”) in detail, digital simulation analyses are to be conducted to look into the effect of power fluctuation control, and costs and operations were analyzed. This project was done in the fiscal year 2000 and 2001 under entrustment of NEDO in Japan. 6.3.2 Major Results (1) Verification Tests a. Power output fluctuation smoothing tests The wind power output signal is applied to a low pass filter with a time constant (smoothing time constant) for
Figure 6 : An Example of test results for wind power output smoothing tests
The tests have confirmed that the system is able to smooth fluctuations in the power output of wind turbines. It is also confirmed that the cycle of power output fluctuations to be smoothed (or a smoothing time constant to be set) affects not only the smoothing effect but also the specifications and operation of battery systems. Sodium-sulfur and redox-flow batteries installed to the verification test systems were primarily designed for smoothing loads for the load-leveling. It is guaranteed that the optimum efficiency could be obtained when the battery operates at rated output. Accordingly, efficiency was slightly affected by the power consumption of an auxiliary unit for the battery operation when the system operated to charge or discharge repeatedly while changing power output as in the case of operations of smoothing wind power output fluctuations. b. Pattern operation tests Tests were carried out on a pattern of charging during small power demand and discharging during large power demand in the battery system for the purpose of the more effective use of unstable wind power generation. Pattern operation tests have confirmed that success in the patterned operation of the wind power generation system with battery energy storage system depends greatly in wind conditions during operation, the rated kW output and rated kWh capacity of batteries. (2) Digital Simulation Analyses of Wind Power Generation Systems with Batteries and Others Digital simulation analyses were conducted with a model representing the characteristics of batteries and with data derived from the above verification tests. Through these simulation analyses, a system configuration, operation control techniques, and the power output fluctuation effects were analyzed, and thereby the effectiveness of wind power generation systems with batteries and others was assessed.
The installed capacity of the battery system required for smoothing wind power output fluctuations depends mainly on a cycle of fluctuations in the output of wind power generation systems. In order to smooth power output fluctuations on a long cycle, a large smoothing time constant of a control circuit must be set. In this case, batteries with large capacity are required. The results suggest that when a wind power generator with a rated capacity of 300 kW, with batteries of 200 kW and 400 kWh capacities, is operated to smooth power output fluctuations with a smoothing time constant of one hour or so, the amplitude of a 10-or-less minute cycle component, and that of a 30-minute-or-less cycle component, contained in the output of wind power, can be reduced by approximately 70 to 90% and 40 to 80%, respectively. When power output fluctuations to be smoothed occur on a shorter cycle, the kWh capacity can be decreased almost in proportion to that cycle. The kW capacity (the capacity of an inverter) can also be lowered, if only a little. There is a possibility of the kWh capacity being further reduced through charge and discharge control improvements. It was found that newly developed analytical tools could be used effectively in designing systems of this kind. 6.4 Stability Analysis of Low Voltage Grid Connection of DG One basic requirement for vendors and manufacturers of DG is an easy network connection for almost all of the application cases without redesigning or engineering major parts of the devices. Most of the DPG sources are connected to the distribution grid via power electronic converters. In this case the controller of one type of DGunit shall remain the same independent from the number of units used in a particular grid area or even at one common connection point. Examples are wind parks with equal converters or several microturbines in a housing complex. Various authors have noted [22][23] that mass introduction of power electronic devices to a distribution network could result in stability and harmonics difficulties. The following section investigates the effect of connecting concentrations of DG-inverters to a low voltage bus-bar (A specific 100kW inverter is chosen) [18]. In this study, the unit is locally controlled and no coordinating control is allowed. Similar models can be developed for different DGs, with the appropriate power electronics interface. With the exception of the converter, all elements shown in Figure 7 are modeled as linear time invariant systems in a rotating dq0 coordinate system. By assuming ideal switching, and no energy loss or storage in the converter a linearized model can be derived [24].
Prime Mover
Rectifier
Converter
~
=
Grid EMC filter
~ =
~
Generator
DC link
Figure 7 : Inverter interconnection scheme
This results in a voltage controlled current source model:
[∆I g,dq ] = [H ]⋅ [∆V g ,dq ]
(1)
The model gives an indication of the inverter currents produced when grid voltage fluctuations occur. The equation is a general description of various DGs while values of the matrix [H] depend on the specific system. Figure 8 illustrates a low voltage network with a connected DG. Note that the figure represents a single bus (node Vl) connected via a transmission line (Zl1) to a strong grid. By adjusting the transmission line characteristics a weak grid condition can be simulated. Vs
Transmission line
DG-Inverter 1
Il
Vl
Il = f (Vl )
Z l1
~ Stiff generator
Zl2
DG-Inverter n
Il
Il = f (Vl )
Figure 8 : LV equivalent network with DGs
The question under investigation is how many DGs can be connected to Vl without causing stability problems. Zl2 is zero for this case. For our purposes, the setup is considered in a control theoretic framework as shown in the next figure. Vs
∆ Vl
+ -
e
H
+
∑ Il
+
Zl
Figure 9 : Negative feedback of the grid impedance Zl = Zl1
In this case, delta is a scalar (representing the no. of additional DG-units) and the stability question reduces to identifying the bounds within which delta can vary without resulting in an unstable system. One approach is to use the robust stability result from [25] which gives an upper bound on the size of delta that guarantees loop stability. The models and analysis techniques presented above were tested on a typical scenario of the small town in Germany. The permissible concentrations of DG units at various nodes in the low voltage network were calculated under the constraints of only one DG model (100 kW) used, all DGs connected to one bus, no disturbance loads at this bus, transmission lines Zl1 modeled with
equivalent grid values calculated at specific nodes using the data from the small town case study.
LPC
Demand & Supply Interface
Operation control subsystem
Sectional switch with protection relays
5 Customer
4
Customer
Communication line
3 Units
Substation (A)
2
Distribution line
1 0 0
75
Substation (B)
150
285
385
Customer
475
Distance from MV/LV transformer (m)
Figure 10 : Number of units in safe operation at increased distance from the MV node.
Figure 10 shows the effect of the line inductance on the number of DG in safe operation. It is evident that an increased inductance (both transformer and cables) leads to a reduced stability of the system. These numbers depend strongly on the DG-unit control parameters and the filter values (L, R and C). They apply only for this specific inverter. Additional DG via transmissions lines Zl2 can be added to the node of interest in order to make the simple configuration more realistic. The results obtained are similar in case of short lines (< 1 km), while for lines longer than 1 km the interaction of the new unit can be neglected.
Central operation control system
Customer
Customer
Autonomous control area to cope with line failure
Figure 11 : A typical configuration of Demand Area Power System [27]
6.5 New Technologies for Distributing and Managing Power in Power Systems with a Large Number of DG A large number of DG systems will be penetrated on the distribution lines in the century. This may influence the operation of the radial distribution system in power quality, safety, and so on. To avoid the problems and utilize the DG effectively in the future, several new power system concepts using advanced power electronics and communication technologies are proposed and investigated.
‘Demand & Supply Interface’ (D&S interface) is installed in each customer for dealing with a balance of energy supply and demand in order to meet the mutual needs of customers and power providers by fully utilizing advanced communication networks. The interface autonomously controls DG and loads taking account of energy saving, minimizing energy cost, stable power supply, and safety, based on several information such as energy price, operation status of DG, load, and utility system etc. Each D&S interface is operated in linkage with the ‘Central Operation Control System’ that manages the entire system. In addition, ‘Operation Control Subsystem’ is installed in each section of the system. The subsystem takes charge of controlling DG and sectional switches in its section autonomously and rapidly in the event of line failure. The research and development on the key comp onent and software techniques such as Loop Controller, D&S interface, and operating method of the entire system is now being carried on under the support of utility companies in Japan.
6.5.1 Demand Area Power System for Realizing Free Access of DG
6.5.2 FRIENDS (Flexible, Reliable and Intelligent Electrical Energy Delivery System)
For the purpose of realizing free access and effective utilization of DG in future, a concept of ‘Demand Area Power System’ is proposed and investigated by CRIEPI in Japan [26][27]. A typical configuration of the system is shown in Figure 11. The system is defined as the segment that includes middle and low voltage distribution system. Its facility formation is required to flexibly deal with power flow congestion due to the interconnection of multiple DG, and to maintain stable power supply reliability for a longer period. To meet these requirements with low cost, the loop formation contributing to enhance power supply capability and restrain voltage fluctuation is adopted to conventional radial distribution line. The use of ‘Loop Power Controller’ (LPC) enabling flexible power flow control including fault current between loops is considered to be effective for upgrading system reliability.
As a new concept for future power delivery system, FRIENDS is proposed in 1992 and investigated by cooperative research project of some universities in Japan [28]. The concept of the FRIENDS intends to attain the following functions by using DG & energy storage system (DGS), power electronics technologies, communication technologies and intelligent facilities, etc. Namely, (1) Flexibility in reconfiguration of the system in normal and fault states, (2) High reliability in power supply, (3) Multi-menu services or customized power quality services to allow consumers to select the quality of electric power and the supplier, (4) Load leveling and energy conservation, (5) Enhancement of information services to customers, (6) Efficient demand side management, etc.
The major characteristic of FRIENDS is to introduce so-called power quality control center (QCC) as an enabler of connecting DGs to power distribution system. With QCC, the power system can be operated without interrupting power supply by flexibly changing the system configurations after occurrence of a fault. Further, each consumer can select the quality of electrical power independently through QCC. More specifically, multiple power quality services named "customized power quality services" for each consumer can be provided. The concept of customized or unbundled power quality services is shown in Figure 12. Consumers can select qualities of power independently according to his necessity. Commodity Power Quality Monitoring Services
Quality Control Center Commodity Commodity Power Power
Constant Constant Voltage Voltage
Nonharmonics harmonics
NonNon interruptible interruptible
Direct Direct Current Current
Combination AnyAny Combination
Power Consumer
Figure 12 : Concept of customized power quality services in FRIENDS
In addition, Since DGSs are allocated on the demand side of the FRIENDS, energy conserving measures due to the demand side management (DSM) can be also expected. To operate the FRIENDS efficiently, such electronics technologies play important roles as static protection scheme, micro-computers (work stations) and data communication lines which connect computers and supply various types of information to each customer. 6.5.3 Premium Power Park The term “premium Power” connotes providing a customer with level of electrical power enhanced beyond what is typically received in exchange for a premium over existing rates. A Premium Power Park provides that enhanced electrical power arrangement to a group of customers in a commercial or industrial development. To demonstrate this concept, American Electric Power [AEP], EPRI, and Siemens have teamed together to develop a Premium Power Park at an existing industrial park site in Ohio, USA. This project is intended to demo nstrate the ability to coordinate several power quality devices as a retrofit to the established distribution system to provide Premium Power to multiple tenants in the park [29]. 7 CONCLUSIONS The trend for the use of DG is supported by recent technological developments, like fuel cells, microturbines, wind farm constructions as well as advanced storage technologies. The integration has to solve several topics starting from guidelines up to energy management in the liberalized markets. Conclusively the integration of DG needs at first a couple of rules and guidelines:
• • • •
contracts for grid usage and interconnection incentive and price contracts connection certification technical standards
The decentralized energy management needs load models forecast and optimization methods for combined heat and power • power flow characteristics • storage capabilities • •
For the wide area application of DG with an increasing number several additional topics have to be considered: • coordinated protection • adapted transmission grid to be balancing grid • adapted energy and distribution management systems • such enabler for connecting DGs to power delivery system as the Demand Area Power System, FRIENDS, microgrid or virtual utility concepts • maintenance concepts • use of DG for ancillary services (frequency, voltage control, reserve power) All in all the, authors believe that the trend in the direction of DG will increase rapidly. It is worth to support this direction for a long term environmental friendly and sustainable energy supply. 8 ACKNOWLEDGEMENTS The authors acknowledge the support of the following colleagues from ABB Switzerland Ltd - Corporate Research: Alec Stothert for section 6.4, Christoph Häderli for section 5.2 and Marco Suter, Alice Piazzesi, Nicola Celanovic for additional information and support. The authors also would like to thank the support from Mr. K. Sekinuma and N. Goto of NEDO, and the cooperation of Prof. K.Nara of Ibaraki University, Mr. K. Fujita of IAE, and Dr. T. Nanahara, Dr. H. Kobayashi and Mr. T. Ishikawa of CRIEPI. 9
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