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Abstract - The Chilean electricity market, formally established in 1982, has a classical Pool type structure, with auditable costs and centralized operations. During its history, the Chilean electricity system has faced high growth rates in energy demands, transmission congestions, this along with a system expansion based on probabilistic criteria incorporated in its economic assessments. In this context, economic opportunities have been created in order to introduce levels of flexibility, by means of special protection schemes (SPS). This paper presents the development of these SPSs, distinguishing economic motivations and technical design criteria. A classification of the different types of SPSs is proposed. Also, the advantages and disadvantages of the different solutions are being presented through application examples. Finally, the future challenges in this field are presented.

Index Terms— Special Protection Systems, System Integrity Protection Schemes, Remedial Action Schemes, electricity markets.

I. INTRODUCTION

n 1982, the new institutional framework that defines the Chilean electricity market was formally created. This is a classical Pool type structure, with auditable costs and a

centralized operation coordinated by an Economic Load Dispatch Center (EDC). In Chile, this institution combines both the Independent System Operator (ISO) and the Market Operator (MO). In this market; self-dispatch capability, purchases, and sale offers, are not allowed.

Likewise, throughout its history, the Chilean electricity system has also faced high growth rates in energy demand [1]. In contrast, the transmission infrastructure has had structural problems and has not been dynamic, thus increasing the amount of congestion and its susceptibility to system instability. This has caused situations of price decoupling for different zones within the system, increasing the probability unserved energy. In this context, opportunities and economic contributions have been created to introduce flexibility levels, using different operating and investment strategies of the power system [2].

Special Protection Schemes (SPS) are a response to the new needs of flexibility observed in a power system, representing a viable planning alternative to extending transmission system capability [3, 4]. It is noteworthy that these types of resources are highly technical in origin, in order to comply with design and operation standards for electrical systems as contained in

the regulation. However, an economic analysis by different market players has generated an interest for developing these technologies, aiming to meet specific requirements. This can also be interpreted in the light of recent developments regarding smart grids [5]. In this paper the Chilean experience of this technology and its future projections along with a classification of the SPS development opportunities are presented.

This paper is organized into four sections. Section II aims to classify the SPS´s, distinguishing some major control actions that are feasible for implementing together with economic incentives. Based on this classification, Section III summarizes specific SPS examples done in Chile. Finally, Section IV presents conclusions and future work.

II. CLASSIFICATION OF SPS FOR THE CHILEAN CONTEXT

An SPS, also called Remedial Action Scheme (RAS), is an automatic protection system designed to detect abnormal or predetermined system conditions, and take corrective actions other than and/or in addition to the isolation of faulted components to maintain system reliability [6]. Such action may include changes in demand, generation (MW and MVAr), or system configuration to maintain system stability, acceptable voltage, or power flows. As defined in [6], an SPS does not include (a) under-frequency or under-voltage load shedding or (b) fault conditions that must be isolated or (c) out-of-step relaying (not designed as an integral part of an SPS). From the definition of NERC and other references on this subject, control resources of a system are varied and can be summarized in the following categories [3, 6]:

• Supply side: Generator Rejection, Turbine Valve Control, Power System Stabilizer Control, Discrete Excitation, Dynamic Braking, Generator Runback, AGC Actions.

• Demand side: Load Curtailment.

• Network side: Advance Warning Scheme Angular Stability, Advance Warning Scheme Overload Mitigation, Congestion Mitigation, Shunt Capacitor Switching, Tap-Changer Control, SVC/STATCOM Control, HVDC Controls, Bypassing Series, Busbar Splitting.

In all of these cases a highly technical approach to the phenomenon that originated and defined the control resources (Traditional Scheme) can be observed. This contrasts with how SPS evolved in Chile, from when the first system was

Challenges for Special Protection Systems in the Chilean Electricity Market

A. De La Quintana, Member IEEE, R. Palma-Behnke, Senior Member IEEE

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installed in the country in 19961. In the latter case, control actions are defined in terms of the need for economical operation of the electrical system from a systemic or market player point of view. The figure below summarizes the contrasting view between a traditional scheme and the one applied in Chile.

TraditionalScheme

Standards(security  issues)

Main  ControlResources

Chilean  observedScheme

SPS  definition

Economicalmotivation

SPS  definition

Standards(security  issues)constraints

driverdriver

•Load  curtailment•Generator rejection•Generator run back  •Local  generation•Topology change•MVAr compensation•FACTS  (SVC,  STATCOM)

Fig.1. SPS drivers: traditional vs. Chilean scheme

While operating standards, which come from security analysis of an electrical system; are present in both schemes, Chile’s case represents technical constraints that must be accomplished. In other words, within the framework of the feasible operating space determined by these constraints, SPS solutions can be developed with different economical motivations for specific market players, whether they are for their own benefit or for the system as a whole.

Indeed, the Chilean regulatory framework, operated with minimum overall operating cost criteria, recognizes the feasibility of developing SPSs for specific contingencies (Title VI-10) [7]. These schemes, proposed by most market players, require the approval of the ECD, which verifies that the security standards and the quality of the electrical supply are not being violated [7]. A precedent in this regard was set by the Experts Panel [2], which ruled in favor of SPS schemes of economical origin in a dispute arising from the application in the northern grid.

From this precedent, the following four economic drivers for defining SPS that developed in the Chilean market can be defined as:

Driver A: Release congestion for an economical dispatch

Given the longitudinal structure of the Chilean electricity network, there often are congested areas. In the event that the affected sector corresponds to a consumption area or an expensive generation supply cost, then it is economically convenient to relax the N-1 criteria in order to take advantage of the thermal capacity of the corridor. In this way rationing or the high local costs of generating power can be avoided. However, in order to maintain N-1 security standards, in the event of an N-1 contingency in the corridor, SPS control actions are needed. In fact, in the consumption area, following the contingency there shall be a load shedding equivalent to the overload that is being experienced by the circuits in the corridor. Additionally, until the fault has not been cleared; it is feasible to supply the local consumption by expensive local generation. Nevertheless, this solution is cheaper compared with the corresponding unserved energy costs.

1 SPS: Generator Rejection/Runback, ENDESA in TalTal Power Station.

Driver B: Time shifting between generation/transmission

The permit application process for the execution of transmission expansion projects has gotten more complex in the last decade [1]. Consequently, there is a high probability of experiencing a temporary mismatch of generation-transmission expansion projects, thus preventing power generating sources to dispatch the available energy at full capacity. Therefore, the economic incentive is created to operate the existing network above the N-1 criteria, implementing an SPS that, in case of contingency, maintains the security and quality standards of the electrical system supply. This translates into SPS generation rejection/run back and/or load curtailment control actions, respecting regulation margins that the system may have.

Driver C: Maximizing availability of generation assets

When facing extreme contingencies the protection schemes defined by the EDC come into play, which supposedly is optimal according to previous systemic studies. However, additional SPS equipment investment, not contemplated by the EDC, can allow for maximizing the availability of power generation by a market player. Thus, an economical benefit for the market player and also for the whole system can be achieved, preventing the cascading effects that may come from dragging out any and avoidable additional power generation units. This may be supplemented by reactive power compensation actions being activated by the SPS, which help maintain the system’s transient stability.

Driver D: Avoid rationing risk by relieving congestion

The hydrothermal condition of the Chilean system makes it vulnerable from an energy adequacy level point of view, particularly in dry years and in congested areas. Thus, water resources that supply reservoirs in congested areas acquire great strategic value. Consequently, when congestion is present, in order to avoid future rationing risk resulting from a dry season, the economic incentive to use more expensive energy from other areas appears, even when in the short-term there are local water resources available. If importing this energy, which is considered "safe", forces the operation of the transmission system above the N-1 criteria, the development of an SPS that will maintain security standards in case of a contingency is necessary.

The following diagram summarizes typical control resources used in each of the four aforementioned economic drivers:

Economical driver

Control  resource

Driver A Driver B Driver C Driver D

Load  curtailment

Generator  rejection

Generator  run  back  

Local  generation

Topology  change

MVAr compensation

FACTS  (SVC,  STATCOM)

Fig. 2.Control resources allocation by economical driver

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III. CASE STUDIES

Starting from the classification presented in the previous section, this section presents case studies that show the Chilean experience.

A. Release congestion for an economic dispatch

Fig. 3. EDAC North Zone Case

According to Figure 3, under normal operating conditions, the load supply located north of the Maitencillo Sub-station mainly depends on the power transfer capacity of the Maitencillo-Cardones 220 kV and Maitencillo-Cardones 110 kV lines, and the local inexpensive power generated at the Guacolda, Taltal, Huasco, and Diego de Almagro Power Stations (DDA). The need for local power generation has been resolved with the inexpensive supply for the Taltal power plant with natural gas. However, restrictions on the natural gas supply coming from Argentina had a direct effect on the availability of this supply, considering that only one of its units could operate with diesel oil (subsequently another unit was enabled).

Taltal power plant operation using diesel oil turned out to be more complex than normal operations running on natural gas, since the original design of these generating units was to operate on natural gas, so operating these plants for extended periods of time with diesel oil increased the risk of failure for these units.

In the event that during normal operations all generating units located north of the Maitencillo Sub-station, i.e. DDA, Huasco, Taltal (with diesel oil) and eventually Las Cenizas, are on service; in the occurrence of a total or partial blackout of the CIS (Central Interconnected System), the Recovery Plan Service (PRS), in the northern part of the SIC, will be delayed by the length of time it takes to stop the power generating units. This doesn’t take into consideration that some of these power generating units might be out of service for a longer period as a result of the blackout.

On the other hand, taking into account the expected growth in demand north of the Cardones Sub-station and the available transmission system capacity, it was determined that for the years of 2008 and 2009, energy supply from the DDA, Huasco, Taltal (run on diesel oil) and eventually the Las Cenizas Power Stations would be required for much of the day in order to supply the consumption to the north of the Quillota Sub-station.

So, if one of the power generating units at the Guacolda or Taltal Power Stations were out of service, whether it is for maintenance, failure or a deficit in the fuel supply, there is a risk that during the hours of high demand the north zone will operate without the security "N-1" criteria and/or the EDC

may find the need to reduce consumption in order to maintain security.

In connection with the previous point, and in order to increase the transmission capacity of the Quillota-Los Vilos 2x220 kV and Vilos-Pan de Azucar 2x220 kV lines, it was determined that there is a need to implement a Generator Rejection/Runback type SPS that acts on non-critical consumption located in the north area of the SIC, so that, in case of a contingency; any of the circuits of the 220 kV Quillota-Los Vilos or Los Vilos-Pan de Azucar lines consumption will be disconnected automatically so that the power supply capacity is not exceeded for the circuit remaining in service.

This SPS was in operation for four months, during which additional 180 MVA were transferred, with cheaper energy coming from the south area.

B. Time shifting between generation/transmission

Fig. 4. Tinguiririca Case

The Confluencia and Higuera Power Plants are run-of-river hydroelectric plants able to supply160 MW. The transport of this energy to the main transmission system is through the Tinguiririca Sub-station, especially built for this purpose. In order to allow the evacuation of this energy, the expansion of the Tinguiririca - Punta Cortes transmission line is necessary. The expansion project involves a voltage upgrade of the corridor from 154 kV to 220 kV.

The construction and commissioning of these plants were planned in coordination with the upgrade of the transmission network. However, due to delays in awarding the tendering process of these upgrades, the generation units were ready to go into operation long before the network expansion was complete.

To allow for the evacuation of energy in the Confluencia and La Higuera Power Plants, it was necessary to design a Generator Rejection/Runback type SPS, in order to overcome transfers under the "N-1" criteria for the Tinguiririca-Punta Cortés section. Thus in the case of a contingency in the corridor, the overload in the remaining circuits can be overcome by releasing or reducing power generation in Confluencia and Higuera power plants, until a sufficiently low enough level that would reduce the overload. This SPS

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currently is in operation and it will remain active until the transmission network upgrade is finished.

C. Maximizing availability of generation assets

Fig. 5. EDAG Taltal Case.

As shown in Figure 5, the Taltal Power Plant is located on the north end of the CIS and has two machines that can run on gas or diesel oil. Energy is carried through a 220 kV double circuit line that is 180 km long. Often the 52JT3 and 52J1 switches will inconveniently open up and cause a significant load shedding for the Diego de Almagro sector. This load curtailment causes the activation of a protection protocol for both machines, completely removing them from operation.

Putting these machines back into service is delayed by a slow start up, a typical characteristic for this type of machine, generating high energy unavailability in the Diego de Almagro sector.

To avoid this, a SPS that detects the opening of these switches and executes the command to release or reduce power generation for one or both generating machines was developed. The operation of the scheme is governed according to the contingencies that may occur in the transmission circuits and contingencies that may occur at the Diego de Almagro Sub-station. Specifically, the contingency is verified by the Carrera Pinto line and the circuit related with Diego de Almagro’s power consumption.

When a contingency as those mentioned occurs, and depending on the magnitude of the loadshedding , the protection scheme operates by releasing one of the machines and reducing power generation in the other one, while maintaining at least one of the machines in operation, even under the most extreme contingency conditions. With the previous action, we can avoid total unavailability of the plant by having at least one machine in operation, significantly reducing the unavailability of energy in the Diego de Almagro sector and increasing the energy load supplied by the Taltal Power Station.

This protection scheme has been in operation since 1996 and it is currently in operation.

D. Avoid rationing risk by relieving congestion

In this case, as presented in Figure 6, the SPS requirement is generated by the need to increase power transfers for the double-circuit Navia Polpaico line beyond the "N-1" criteria levels when the power transfer flows in the Polpaico-Navia direction.

Fig. 6. SPS–Polpaico/Navia

The need to increase power transfer is due to a highly probable situation of lower rainfall rates in the southern area (a current situation at the date of implementation of this scheme). This scenario forces the system to supply power to the central area (the capital) with any available power generated in the northern area. Without increasing this power transfer, the central area may be exposed to power rationing risk. Transfer can be increased beyond the N-1 criteria levels, but, in case one of the circuit switches opens in this operating condition, this situation would lead to overloading of the circuits remaining in service. The resulting power flow cannot be reduced by manual operation, because the protection scheme requires executing complex simultaneous control actions involving releasing energy, reducing/increasing power generating and topological changes to the electrical network in the SPS influence area.

In turn, an SPS like the one that was implemented, may act automatically, safely and efficiently carrying out these operations in real time.

The implemented SPS is an automatic load shedding scheme, for a reduction or shedding of power generation and/or automatic maneuvers within the transmission system and it is only activated in cases where transfer exceeds the limits of the N-1 criteria, for which, logic controls have programmed additionally a capacity curve of said line as a function of temperature, thus maximizing transfer rates in the line.

In economic terms, profit achieved by using this SPS is 23 million US dollars. This value corresponds to the difference in the costs of fuel spent between having and not having the SPS during the two years of operation that it was in service.2

IV. CONCLUSIONS AND FUTURE DEVELOPMENTS

This paper presents the development context of SPSs in Chile, this came into being due to an economic motivation and technical design criteria for specific situations. Based on this experience, a classification of SPSs is proposed. Also, through

2 Dept. of Operations Planning, DFD/Memo DPO N°18/2009, CDEC-SIC

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application examples, the advantages and disadvantages of the different solutions can be identified.

Figure 7 summarizes the current and projected participation for the different types of SPSs with economic drivers in Chile. Changes can be explained because in the future, system congestions will persist or increase, due to the high load growth rates and the rising legal bureaucracy for transmission projects. From this we can say a leading SPS would be the Driver A type, which would free congestion for cheaper power transmission. Also, the future challenges in this area will focus on the progressive integration of renewable energy, which will require the appropriate SPS solutions.

Driver  A

Driver  B

Driver  C

Driver  D

Current Future Fig. 7. Current and projected Market Share

The Chilean SPS experience can provide a basis for similar developments in other countries in order to discuss the services being offered and related regulations.

V. REFERENCES [1] Energy Ministry, “The National Energy Strategy is a road map that

provides policy guidelines that will power the country over the next few years”, http://www.minenergia.cl/documentos/estudios/national-energy-strategy-2012-2030.html, 2012.

[2] Dictamen 16 de 2008 del Panel de expertos, “Aplicación del criterio N-1 en el tramo Maitencillo-Cardones 220 kV”,www.panelexpertos.cl, 2008.

[3] Moreno, R., Pudjianto, D., and Strbac, G., “Integrated Reliability and Cost-Benefit-Based Standards for Transmission Network Operation”, Journal of Risk and Reliability, Vol 226, No 1, pp 75-87, Feb 2012.

[4] IEEE PSRC Working Group C4 Report, “Global Industry Experiences With System Integrity Protection Schemes (SIPS)”, October, 2009.

[5] McCalley J., Oluwaseyi O., Krishnan V., Dai R., Singh C., Jiang K. “System Protection Schemes: Limitations, Risks, and Management”, PSERC , Final Project Report, 2010.

[6] Glossary of Terms Used in NERC Reliability Standards NERC: North American Electricity Reliability Corporation Adopted by NERC Board of Trustees: February 12, 2008.

[7] National Energy Commission, “Chilean Grid-Code: Norma técnica de Calidad y Seguridad de Servicio”, www.cne.cl, 2010.

VI. ACKNOWLEDGMENT

This paper has been partially supported by Fondecyt Grant # 1120317, and the Faculty of Physics Science and Mathematics of Universidad de Chile (CMM, DIE, ISCI), and CONECTA company.

VII. BIOGRAPHIES

Alfredo De la Quintana (M’00) was born in Santiago, Chile. received the Electrical Engineer diploma (1984) from the Universidad de Santiago de Chile, Santiago, Chile. From 1991 he has worked at CONECTA as R&D manager of the company leading the development of SPS solutions, core business of the company.

R. Palma-Behnke (SM’04) was born in Antofagasta, Chile. He received his B.Sc. and M.Sc. in Electrical Engineering from the Pontificia Universidad Católica de Chile and the Dr. Ing. Degree from the University of Dortmund, Germany. He is currently Associate Professor in the Electrical Engineering Department at the University of Chile, Santiago. His research field is the planning and operation of electrical systems in competitive power markets and new technologies.