Electric Vehicle Charging Station Location using Fuzzy Optimization

Electric vehicle charging station location is a basic problem in integrating electric vehicles in electric power systems. An electric vehicle plugged into the electric distribution system may absorb or produce active and/or reactive power [1-4] depending on the need of the electric power system, Table 1 below.

Electric vehicle charger operating modes [4].

When finding the location of EV charging stations in order to support the electric distribution systems, the cost of EV charging, the impact on distribution system losses and voltage profile of the system are parameters needed to be considered. These variables are to be looked into when plugged in EV is either acting as a generator or a load given a system demand level.

Recent studies have solved this EV charging station location problem. In [5], a mixed integer programming solution was developed with site accessibility, local jobs and population densities and trip attributes as main constraints. A genetic programming approach is utilized in [6] for simulation of electric vehicles on a real map of a European city where the optimal solution of the charging infrastructure is derived based on mean trip times of electric vehicles. A two step procedure is proposed in [7] where the authors included environmental factors and service radius of EV charging stations in the screening first step and built a modified primal-dual interior point algorithm (MPDIPA) for optimal sizing of EV charging stations with the minimization of total cost associated with EV charging stations to be planned as the objective function with losses and voltage profile included in the problem. Reference [8] introduces an optimization process for sizing and siting of EV charging stations, modeling the charging demand and the structure of road network to where the solution approach was graph theory. Level 1 and level 2 charging stations are discussed in [9] and how to allocate them for residential EV users using simulation-optimization strategy.

Recent studies do not consider uncertainties and imprecision which can be captured using fuzzy optimization. Fuzzy set theory can provide a simpler yet powerful solution for allocating EV charging stations in electric distribution systems. The Civanlar test system [11] will be utilized for the study and assuming that capital investment of the EV charging station is the same for all distribution system candidate nodes while considering time of use (TOU) electricity tariff, distribution system losses and voltage profiles.

References

[1] Chenye Wu, Hamed Mohsenian-Rad, and Jianwei Huang, “PEV-based Reactive Power Compensation for Wind DG Units: A Stackelberg Game Approach”, in Proc. of the IEEE Conference on Smart Grid Communications (SmartGridComm’12), Tainan City, Taiwan, October 2012.

[2]   Chenye Wu, Hamed Mohsenian-Rad, Jianwei Huang, Juri Jatskevich, “PEV-Based Combined Frequency and Voltage Regulation for Smart Grid”, the 3rd IEEE Innovative Smart Grid Technologies Conference, Washington DC, Jan 2012.

[3]    M. Kisacikoglu, B. Ozpineci, L. M. Tolbert, "V2G Reactive Power Compensation Using a PHEV Bidirectional Charger Interface Rated at Level 1, 2, and 3 Charging Standards," IEEE Energy Conversion Congress and Exposition, Atlanta, Georgia, Sept. 12-16, 2010.

[4]     M. Kisacikoglu, B. Ozpineci, L. M. Tolbert, "Examination of a PHEV Bidirectional Charger System for V2G Reactive Power Compensation," IEEE Applied Power Electronics Conference, Palm Springs, California, Feb. 21-25, 2010, pp. 458-465.

[5]    Chen, T. D., et al, “The Electric Vehicle Charging Station Location Problem: A Parking-Based Assignment Method for Seattle”, on-line: http://www.caee.utexas.edu/prof/kockelman/public_html/TRB13EVparking.pdf

[6]    Hess, A. Et al, “Optimal Deployment of Charging Stations for Electric Vehicular Networks”, on-line:http://conferences.sigcomm.org/co-next/2012/eproceedings/urbane/p1.pdf

[7]    Liu, Zhipeng, Wen, F. and  Ledwich, G. F. , “Optimal Planning of Electric-Vehicle Charging Stations in Distribution Systems”, IEEE Transactions on Power Delivery, Jan. 2013, Vol. 28 , Issue 1.

[8]    Jia, L., Hu, Z., Song, Y., Luo, Z., “Optimal siting and sizing of electric vehicle charging stations”, 2012 IEEE International Electric Vehicle Conference (IEVC), 4-8 March 2012

[9]    Xi, X., et al, “Simulation-Optimization Model for Location of a Public Electric Vehicle Charging Infrastructure”, on-line:http://www.ise.osu.edu/ISEFaculty/sioshansi/papers/charge_infra.pdf

[10]“Siting Electric Vehicle Charging Stations”, on-line: http://www.sustainabletransportationstrategies.com/wp-content/uploads/2012/05/Siting-EV-Charging-Stations-Version-1.0.pdf

[11]On-line: http://ee.sharif.ir/~e_hajipour/index_files/Test_Cases_Data.pdf

Demand Side Management in Visayas: Making It Work

As Visayas is having power problems, demand side management brings new opportunities to alleviate need for investments in new generation.

Inquirer reports...Ten major commercial establishments in Cebu let go of at least 174,847 kilowatt hours (kwH) of power from the local power grid over the past two weeks, allowing more electricity to be used by residences.

The “deloading” from the power grid was made by firms which relied on their own backup generators during periods of peak demand for electricity in Cebu.

See link for full report: Firms start power deload

Reliability of Philippine Power Systems

Reliability of power systems define the utility's capability to minimize outage frequency and duration. For regulatory purposes, power system reliability indices are evaluated for performance based regulation (PBR). In the Philippines three performance indicators on system reliability are among the indices evaluated to assess transmission and distribution electric systems for regulatory years involved. System Average Interruption Frequency Index (SAIFI), System Average Interruption Duration Index (SAIDI) and Customer Average Interruption Duration Index (CAIDI) are the indices required by Energy Regulatory Commission (ERC) from the utilities and whose formulas are given below. The data presented herein are acquired from the ERC website and from the National Electrification Administration (NEA) website.

The transmission electric system reliability performance are provided below in the following two figures. The first five years show separated indices for the three regions while the next five years show combined transmission performance for the whole nation. It is revealed from the figures that transmission system SAIFI and SAIDI are lower for the last five years which indicate that outage frequency and duration were minimized in the bulk power system.

The next three figures illustrate the reliability performance of four Distribution Utilities (DUs) in Mindanao. The SAIDI presented here are said to be "Planned" SAIDI, which I assume incurred during scheduled maintenance outages of the DUs. For SAIFI and SAIDI, Cotabato Light and Power Company, Inc. (CLPC) performs better than the other DUs. For CAIDI, Mactan Electric Company, Incorporated (MECO) and Cagayan Electric Power and Light Co., Inc. (CEPALCO) provides minimum customer outage duration. What is interesting is that MECO and CEPALCO reported the same CAIDI values for different four year intervals. The CAIDI formula above does not apply to the CAIDI values as the SAIDI are "Planned". The DU reliability assessment here does not consider the GWh sales and number of customers served of the DU which I believe has an impact to the DU reliability performance. This kind of evaluation is slated in the upcoming articles.

NEA reports a reliability criteria for Electric Cooperatives (ECs). The highest score for this aspect is 5.0 as defined by NEA, which is not reported in their website. Figure below shows that at least four ECs perform up to the par of the NEA reliability criteria.

Power Quality Problems Always Exist

The Philippine Distribution Code (PDC) has Power Quality (PQ) performance regulation for Distribution Utilities. In Section 3.2 of the PDC, it provides generalities and specific limitations for quality regulation. I have copied section 3.2.1.2 from the PDC for the purpose of this article, see below.

In this section, the PDC points to the identification of a PQ problem in the distribution system by classifying presence of quality parameters. Going beyond this section are specified limits of the parameters stated above which clearly identifies the existence of a PQ problem. It is interesting that this PDC section identifies PQ problems where some of the classifications always exist in operating a distribution system. My rationale are the following:

• Power system frequency is seldom at its nominal value of 60 Hz. The continuous variation of loads impact the maintenance of frequency at 60 HZ. Item (a) of the said section implies that a PQ problem due to frequency deviation always exists.
• Harmonic frequencies always exist in distribution systems. The issue if these harmonics are acceptable or not is another facet. But voltage harmonics and current harmonics are always present in a power system due to the popular usage of electronic-based devices and equipment. Item (c) of the said section above implies that a PQ problem exist due to the presence of harmonic frequencies, whether tolerable or not.
• Items (c) and (d) of the said section are combined definitions for voltage unbalance. In a practical distribution system, voltage unbalance exist at all nodes due to the two-phase and single-phase laterals in the three-phase distribution feeder and unequal loading of the three-phase distribution lines. If this unbalance in voltage is acceptable of not is about the magnitude of the unbalance. If we follow the thesis of the said items, then a PQ problem exist even at acceptable levels of voltage unbalance.

The PDC definition for the existence of a PQ problem, with respect with the section above, gives us an understanding that a PQ problem exist not withstanding the magnitude of frequency deviations, harmonic frequencies and voltage unbalance magnitudes.

Conflict in Quality Regulation of Distribution Utilities

The Philippine Distribution Code (PDC) requires all User connection points that all Power Quality (PQ) requirements under PDC Section 3.2 be complied with. The PDC section 3.2.3.4 stresses this policy for voltage variations, see below. There are other similar statements for other PQ indices to be complied along section 3.2 of the PDC.

For new distribution system interconnection, the PDC points to the same section 3.2 that new connection points must have PQ within limitations specified in Section 3.2. In this case, the Distribution Utility (DU) must ensure that these PQ limits are complied with. See section 5.2 and further below.

The Energy Regulatory Commission (ERC) under its Work in Progress provides a seemingly conflicting objective with the PDC, as the Section 4 of the Guideline for Monitoring of PQ standards for DUs allows the DU to have 95% of connection points along a given distribution feeder to comply with section 3.2 of the PDC, see figure below. This means that 5% of the connection point can be accepted not to comply with the said PQ performance requirements. Practically, if the DU feeder has 100 connection points, 5 of those connection points can be operated not following the PQ limits specified in the PDC section 3.2.

The conflicting objective in these regulatory documents is clear. One document, the PDC, specifies that the DUs must comply with PQ limitations per section 3.2 in all connection points. On the other hand, the Guideline points that only 95% of those connection points must be ensured by the DU to have PQ level in accordance with the PDC section 3.2.

The ERC and DUs must adhere the PDC since this is the “code” of operating, financing and planning distribution systems. The Guideline must be reviewed as this is a Work in Progress. If the Guideline supersedes the PDC, then the PDC states that a PQ problem exists in the distribution system as section 3.2.1.2 below stresses this point.

Technical Performance of an Electric Cooperative supplied with Wind Power

Wind power supply has further implications on system reliability because of its variability and can impact power quality in its nature of power production since it uses power electronics for power control that can produce harmonics and with its variability which can result to unacceptable voltage variations and voltage flickers. Ilocos Norte Electric Cooperative (INEC) source its power supply at specific times from the NorthWind Power Development Corporation.

INEC sources its power requirement to supply its franchise demand from National Power Corporation (NPC), NorthWind and Wholesale Electricity Spot Market (WESM). This is an economic operations strategy by INEC. INEC is fed by all these power suppliers from the grid via bulk power transformers to its distribution franchise.

National Electrification Administration (NEA) reports Electric Cooperatives (EC) scorecard for the year 2006. From this report, INEC performs very well technically as shown in the figure below. Aside from reliability and project implementation, INEC performed up to the expected par with all the other technical operations performance requirements. The uncoupling of power supply, voltage transformation thru transformers which go together with INEC’s own technical intervention within their distribution franchise, from the wind power to the INEC distribution system aids in delivering power with such a high quality of supply and technical performance. Overall, the analysis shows that an electric distribution system fed by a wind power plant can be operated up to the level of technical criteria required by the NEA or by Energy Regulatory Commission (ERC).

Benefits of System Loss Reduction

Aside from impacting or not impacting electricity rates, system loss reduction has the following benefits:

• Reduction of fuel emissions due to lesser use of fossil-fuel generating plants - this has societal impact as it cover environmental concerns.
• Utility system capacity savings - decrease in losses provides released extra capacity for the distribution lines and transformers.
• Promotion of Energy Efficiency - it will be noted that the Distribution Utility (DU) is an energy-efficient electric company as it tries to decrease its system loss.
• Improvement of system voltage profile - the utility is regulated to supplying a range of voltage level and reduction of losses will produce a marginal system voltage quality that may be acceptable. This will also provide good power quality at the convenience outlets of consumers allowing their electric equipment/appliances to operate without mis-operation or loss of life.
• Increase Utility Commercial Appeal - a DU aiming at system loss reduction gets an added commercial appeal in the restructured power industry. This is important in the changing environment of the power industry, have you seen MERALCO TV commercials?

There is an optimum level of system loss reduction unique for all Distribution Utilities (DUs) where further reduction of losses will not result to further reduction of operation cost but more investment expenses. On the other hand, as demand rises in a distribution service franchise, technical losses and non-technical losses (utility own use) will at some point come with the demand increase. The equation of generation equals losses plus demand holds true. This is revealed by Philippine Power Statistics as shown in the figures below (the values include transmission and distribution losses, but this does not negate the point) :

Finding ways to reducing system loss must be a continuing program since it always provide benefits to the DUs and electric consumers.

Comments on the Enforcement of Reliability and Power Quality Standards, and Quality Reliability Index (QRI) for Distribution Utilities

In 2006, I made significant technical comments on the issues of reliability and quality of electric power supply being delivered by Distribution Utilities in the Philippines. Energy Regulatory Commission (ERC) provides the link Work in Progress for the said issues.

The complete documents of comments from various entities can be accessed here - ERC Reliability and Power Quality Comments.

Analysis of Voltage Unbalance Regulation

Abstract

This paper presents an analysis of the present voltage unbalance regulation in the deregulated power industry in the Philippines. The regulation of voltage unbalance was examined in the light of various standards pertaining to voltage unbalance. The response of three-phase electrical equipment to voltage unbalance is evaluated as per limits of Philippine Distribution Code (PDC). The voltage unbalance limits for transmission and distribution were investigated if it were practical using a numerical simulation of an electric power system. Recommendations and conclusions were drawn as per the analytical outline for voltage unbalance regulation.

See Full Paper - Voltage Unbalance Regulation

Line Losses, Voltage Drop and Voltage Unbalance on Parallel Overhead Distribution Lines

Abstract

It has been a general practice by distribution utilities to parallel overhead distribution lines/feeders in a common right of way and the same pole as they terminate from a common substation. This paper studies the impact of modeling parallel overhead distribution lines. The modeling of this kind of circuits in the regulated status of electric distribution business is important as the modeling can impact line losses, voltage drop and voltage unbalance in the lines. A sample case was set-up and is simulated in two cases; parallel lines modeling and taking the parallel lines individually. The study utilized load flow calculations for analyzing line losses, voltage drops and voltage unbalance. The results showed that line losses are not affected by the kind of modeling applied to parallel overhead distribution lines but voltage drop and voltage unbalance yielded different results in the two cases simulated.

See full paper - Parallel Distribution Lines

Static Voltage Stability Analysis for Electric Subtransmission Systems

The study of voltage stability as indicated in the Philippine Grid Code (PGC) is the center of this paper. Standards and industry practice for voltage stability problem-solving are cited and were referred when provided an example simulation. Voltage stability is a must when looking at a power system if it can handle load growth and at the same time maintaining acceptable voltage levels at all system nodes pre and post-contingency. The static voltage stability simulations utilized practical solutions for voltage instability which are discussed and evaluated using Power-Voltage (PV) and Voltage-MVAR (VQ) curves. This report serves as a tutorial for practicing engineers on the important topic of voltage stability.

See paper by clicking this - Static Voltage Stability Analysis for Electric Subtransmission Systems

Philippine Electric Cooperative Data

It is interesting to study the electric cooperatives’ data publicly posted in websites. The following websites provides these data:

• Energy Regulatory Commission (ERC)
• National Electrification Administration (NEA)

Primarily, the ERC data has the following columns:

1. Distribution line length in circuit-kilometers (ckm) – this item refers to the existing overhead lines owned by each electric cooperative. The data must include single-phase, two-phase and three-phase distribution circuits operated in the cooperative’s franchise area, irregardless of voltage level.

2. Number of customers – the total sum of all electric consumers in each electric cooperative service area. This may be the total of residential plus commercial plus industrial plus barangay electrification. Normally this data is the number of electric meters being maintained by the cooperative.

3. Sales volume – this data is in MWh unit. The total sales of each electric cooperative in year 2006 which was supplied to the total number of customers and which was procured from the transmission operator or other electric power supply entities.

4. Consumption per customer – this data is obtained from dividing data number 3 to data number 4. Though looking at average, this is an indicator of how much energy is utilized by each facility.

5. Structure – is the ratio of the mix of residential sales plus barangay electrification to the total energy sales. This shows on how “rural” is the service area of the electric cooperative. A higher value of structure tells us that that the service area is that the demand comprises much of residential and barangay electrification which is more rural, while a lower structure value gives an indication that the service area has commercial and industrial bulk of energy users.

6. Density - this data is MWh sales divided by the ckm of distribution lines. This reveals how much dense is the service area of the electric cooperative in terms of loading in distribution lines.

7. Distribution Supply Metering (DSM) charge – this data is presented in peso per kwh is reflected in the electric bill.

NEA has many available data but in this article we will focus only on two:

1. Reliability score – ERC has promulgated reliability guidelines for electric distribution companies using standard indices. Reliability is the capability of the electric cooperative to minimize frequency and duration of unforced outage in their service area. In this data, NEA may have summarized the electric cooperative data into one single score.
2. Power Quality (PQ) score – same with reliability, PQ is a performance indicator for electric distribution companies, but in this case ERC has devised their own indices. Also, NEA may have summarized the electric cooperative data into one PQ single score.

In the coming articles, I will try to analyze these data and extract technical and objective evaluation thereof.

Integrating Power Quality in Power System Planning

Power Quality (PQ) has become a major concern in electric power systems with the increased proliferation of computer electronic loads and power electronic devices in the power system, and more consumer interest in power delivery issues. Understanding PQ requires taking on the viewpoints of the electric utilities, consumers and operators of electrical equipment. Regulatory codes require compliance with stated standards of power system performance measured in terms of quality and reliability. Yet notwithstanding all the quantitative and qualitative technical aspects of PQ, it has heretofore remained primarily a consequential characteristic rather than a planned objective for power systems. Perhaps it is time to consider integrating PQ objectives in the planning process. As such, PQ must fit in with all the other objectives of power system planning – thermal capacity adequacy, voltage security, stability, etc. The challenge for the planner is to take into account the already complicated planning process and integrate PQ.

Different conventional planning tools are utilized for power system planning: power flow studies, short circuit calculations, transient stability analysis and electromagnetic transient simulations, among others. The different planning time horizons utilized in these tools can be utilized for analyzing PQ issues arising from the simulations since PQ concerns account for time ranges.

Case studies show how PQ concepts can be applied to power system planning. Overall, there are opportunities to integrate PQ analysis in conventional power system planning studies. The above discussion of integrating PQ analysis in various studies can be useful in providing a PQ viewpoint in the planning of electric power systems. The result is a power system planned and operated not only for economics and reliability but also for power quality.

Please see the complete article - http://www.pterra.com/Pterra%20Tech%20Blog%2026%20-%20PQ%20Planning.pdf

Harmonic Penetration in Electric Transmission Systems

When the physicians of the power system (planners and operators) treat for resource inadequacy, congestion, instability and all the modern-day maladies of competitive power markets, their regimen may come with an increasingly common side effect – harmonics. The utilization of static var compensators (SVC), induction generators, source converters, underground and submarine cables, direct current converters, to name a few, to provide solutions to power system problems can lead to increasing harmonic penetration in the power system. Harmonic generating equipment coupled with system resonance conditions effects are cumulative and can be detrimental to system operations if not mitigated.

See the complete article on this link - http://www.pterra.com/Pterra%20Tech%20Blog%2024%20-%20Harmonics.pdf

Concept for Distribution System Planning

Rationale - Since distribution utilities (DUs) are regulated with their operational power quality (PQ) and system reliability, which accounts for System Average Interruption Frequency Index (SAIFI) and System Average Interruption Duration Index (SAIDI), it may be considered to utilize the historical data for DU planning in the context of probabilistic evaluation.

Concept Methodology-

1. Gather PQ and DU system reliability data for a certain time range, maybe five years.
2. Establish level of PQ, SAIFI and SAIDI to be utilized for planning purposes by applying 95% probability acceptance.
2. Identify planning alternatives to solve problems of PQ and system reliability.
3. Compute and simulate levels of PQ, SAIFI and SAIDI per planning alternative.
3.1. This may utilize DU system reliability software/spreadsheet and PQ program simulation.
4. Compute relative system unreliability cost and cost incurred for poor PQ for each planning alternative.
4.1. System Unreliability Cost = SAIDI x Cost of electricity (P/kWH)
4.2. Poor PQ Cost = Energy Losses (due to Poor PQ) x Cost of electricity (P/kWH)
5. Compute total cost of each alternative.
5.1. Total Cost = Invesment Cost + Operational Cost + System Unreliability Cost + Poor PQ Cost
6. Two Methods for Planning Altenratives Comparison
6.1. Compare Total Costs of Planning Alternatives.
6.2. Benefit/Cost (B/C) Method, where [(Base Reliability + Base PQ) - (Alternative Reliability + Alternative PQ)]/ Total Cost of Alternative
7. Choosing appropriate planning alternative.
7.1. Choose planning alternative with Lowest Total Cost.
7.2. Choose planning alternative with Highest B/C.

Voltage Security in Philippine Power Systems

Voltage security in power system planning and operations is an important factor of grid and distribution codes in the Philippines. The power system, system components and customer devices respond to the magnitude of voltage in a manner that would impact its operation. Power system interruption cascades when voltage collapse is experienced. Generators and other system elements tend to be saturated when keeping the voltage secured at certain limits. Customer devices might misoperate or may expel from its connection point when voltage is not appropriately within the devices’ operating regions. These are some of the reasons why it is important for a power system to be planned and operated in terms of voltage security. With areas where generation deficit is a problem, voltage security is hard to achieve since generators provide and assist in keeping the voltage secured in terms of reactive power management.

Philippine transmission systems must be operated in normal conditions within voltages of 0.95 per-unit up to 1.05 per-unit while distribution systems are expected to have voltage levels from 0.90 per-unit to 1.10 per-unit. These voltage magnitudes should be kept in steady-state conditions. Scheduled maintenance of lines and other components must be conducted making sure that these voltage performance standards must be kept. When a fault or a transient event occurs, the voltage levels can not deviate from 0.90 per-unit to 1.10 per-unit as long as 0.00833 seconds to 60 seconds. In this case, voltages magnitudes are classified as voltage sags or swells. Above 60 seconds, and the voltage levels are still deviating from the said limitations, these voltages are called long duration voltage variations.

Voltage security can be planned and operated by reactive power management and predicting the voltage profile in contingency analysis. Reactive power supervision entails updated reactive power capability curves of existing generators and list of static and dynamic reactive power devices installed in the power systems. When running a post-contingency voltage assessment, the reactive power capabilities of generators, reactors, capacitors and static VAR devices (SVDs), and operating points of tap-changing transformers, must be considered since these options can be enable in running appropriate post-contingency power flow solution. The area/zone/buses in study must be identified by the power system engineer(s) for proper scenario and conditions setting. The voltage ranges limits and voltage deviation limits must be defined for the area or zone or buses in study for analysis purposes. Together with the identification of the area in study, possible system contingency scenarios, assigning of voltage limits at the area in study the power flow solution or transient simulation can be automatically configured to report all the violations as a result of each probable contingency. Usually, in power flow computations, these can be presented in tables or graphs while for transient simulations, whether in phase domain or time domain, these voltages are analyzed using time versus voltage plots.

Philippine power systems just like any other power systems must be operated in voltage security. Voltage must be operated and planned accordingly for the power system and its components to operate securely. Codes define limitations and thus appropriate prediction and prevention of unsecured voltage can be analyzed. Knowledge of existing reactive power devices in the power system and proper coordination of these devices will enhance voltage security together with applying computer solutions that can automatically provide flexible programming of studying areas of voltage violations.