VOLTAGE DIPS: VERIFICATION, VALIDATION AND CERTIFICATION PROCEDURE FOR PV INSTALLATIONS A. Linares(2), J. Fernández(1), M. Friend(1), E. Llarena(1), C. Montes (1), G. Moncho(1), N. Losada(1), O. González(1), D. Molina(1), A. Pío(1) & M. Cendagorta(1) (1) Instituto Tecnológico y de Energías Renovables, S. A. (ITER) Pol. Industrial de Granadilla, s/n, E 38600 Granadilla de Abona, España Tlf. +34 922 747 700, Fax +34 922 747 701, E-mail:
[email protected] (2) Agencia Insular de Energía de Tenerife Pol. Industrial de Granadilla, s/n, E 38600 Granadilla de Abona, España Tlf. +34 922 747 700, Fax +34 922 747 701, E-mail:
[email protected]
ABSTRACT: In Spain, since October 2011 and according to Royal Decree 1565/2010 from the 29th of November, for all the PV energy production facilities, whether single or grouped, that have an installed power above 2 MW, it is compulsory that they meet the requirements for response to voltage dips, as per the Operational Procedure P.O. 12.3, established by the Transmission System Operator and approved in the Resolution dated 4 October 2006 of the Ministry of Industry Tourism and Trade's General Secretariat of Energy. Such response consists on, for these facilities, to remain linked to the electricity grid, without becoming disconnected because of falls in voltage directly associated with correctly offset short circuits that may arise in the electricity system. Also, in order to become compliant with P.O. 12.3, a specific Verification, Validation and Certification Procedure (PVVC9) was established, with the purpose of obtaining the corresponding credential, via an accredited body such as the Spanish Association for Standardization and Certification, AENOR. The purpose of the present paper is to present the procedure followed in order to obtain the P.O. 12.3 comply certification, highlighting the technical problems that had to be solved, as well as the associated costs. Keywords: Photovoltaic, Large-Grid Connected PV Systems, Grid Management, Grid Stability, Voltage Stabilisation, Cost Reduction, Electrical Properties, Inverters.
1
INTRODUTION 2
Energy is a key element in any activity, as it is necessary both for industrial processes and for ancillary activities, while constituting a major cost factor. Responsible production and the efficient use of energy by organisations are key factors in achieving sustainability. Currently, there is therefore serious concern with saving energy and with the environmental impact of its generation. According to International Energy Agency (IEA) analysis, over the period 2000-2011, solar photovoltaic was the fastest-growing renewable power technology worldwide. Cumulative installed capacity of solar PV reached roughly 65 GW at the end of 2011, up from only 1.5 GW in 2000. In 2011, Germany and Italy accounted for over half the global cumulative capacity, followed by Japan, Spain, the United States and China. As mentioned before, during recent years in Spain there has been a significant growth in the number of electric producers with Renewable Energies. This is a very dynamic and technologically fast pacing sector. Also, due to the structural characteristics of the Spanish electric system, it has been considered necessary to establish a set of additional technical requirements to such sector, in order to guarantee their correct functioning as well as enabling their growth. The Royal Decree 661/2007 from the 25th of May, which regulates the production of energy by Renewable means, establishes the specific technical requirements that are mandatory. Among these requirements, the way the system need to response to voltage dips is established, so that it must remain linked to the electricity grid, without becoming disconnected because of falls in voltage directly associated with correctly offset short circuits that may arise in the electricity system.
PV INTEGRATION IN THE GRID NETWORK
Red Eléctrica de España (REE) runs the power system, both on the Spanish mainland, as well as on the insular and extra-peninsular systems. In doing so, it ensures a safe, continuous power flow from the power generators to the centres of consumption. Electricity cannot be stored in large amounts and therefore production must always keep abreast of consumption in a precise, instantaneous way, and this requires a constant balance. The role of REE as system operator consists in maintaining the balance and, for this purpose, it forecasts consumption and operates and oversees the generating and transmission installations in real time, thus ensuring that production planned at the power stations coincides at all times with the actual consumer demand. Should a difference arise between the two, it sends the appropriate instructions to the power stations to increase or reduce their output. REE likewise handles the so-called adjustment services, the purpose of which is to bring the production programs resulting from the daily and intra-daily power markets in line with the quality, reliability and safety requirements of the power system. Adjustment services or adjustment markets involve the overcoming of technical restrictions, the assigning of complementary services and deviation handling. Therefore, such organization is responsible for guaranteeing the continuity and security of the power supply, co-ordinates the energy production-transport binomial system, maintaining the required quality parameters, controls the electric exchange with foreign grids, facilitates the access of third parties to the electric transmission grid, and ensures the development and expansion of the transmission grid by establishing
uniform and consistent criteria. The electric system is very much a key to fulfill the ambitious national and European objectives in relation to the development of the Renewable Energies. The management of such electric system, prioritizing the integration of Renewable Energies, is one of the main objectives. Particularly, by publishing several decrees and rules, it has been established the obligation to be ascribed to Control Centres of Renewable Energes [6], which operates by acting as the system operator interlocutor for certain facilities, and the obligation to comply with certain requirements in order to respond to voltage dips. Compliance with these requirements is essential in order to enable a proper operation of the system under safety conditions and, consequently, makes possible to maximize the energy generation by Renewable Energies in the electric pull system. 3
3.4 Operational Procedure PO 12.3 provides that the corresponding facility must be capable of withstanding (operates without disconnecting) voltage dips at the grid connection point, caused by three-phase, two-phase or one-phase to ground short-circuits, with the profiles of magnitude and duration shown in Figure1. That is, there will be no disconnection of the installation due to voltage dips in the grid connection point included in the shaded area in Figure1. V (%)
Fault start
100 95 80
Fault cleared
VOLTAGE DIPS
3.1 Definition. A voltage dip is a sharp attenuation of the voltage, followed by its recovery after a short lapse of time, that is, it involves a temporary reduction of the voltage at a point of the power grid, below the threshold of the dip (IEC 61000-4-30). Typically, the dip is related to the occurrence and termination of a short circuit in the grid. It is characterized by two parameters, the voltage and the duration of the dip. Also, a failure can produce a phase jump in the voltage waveform. 3.2 Difficulties from the system operator point of view. The different installations and technologies that make use of Renewable Energies, cogeneration or waste materials as their energy source, must meet several conditions that allow withstanding voltage dips without disconnecting, avoiding cascading disconnections that could affect the continuity of the electric supply. If there is a delay in the dynamic control of the voltage, there is a risk of extending both, in time and depth, the voltage dips, affecting all connected agents and endangering the system transient stability. The importance of withstand voltage dips is that, when there is a breakdown or “fault” at a point in the grid, leading the protections to act, there is time enough for isolating the problematic zone, so there is no cascading disconnections on the energy generation components and, therefore, causing a general power outage. 3.3 Normative With the approval of the Royal Decree 1565/2010, of 19th of November, which regulates and amends certain aspects related to energy generation activities under the special regime, PV facilities forming part of power groupings of over 2 MW are obligated to comply with the corresponding requirements in the Operational Procedure P.O. 12.3. “Requisitos de respuesta frente a huecos de tensión de las instalaciones de producción de Régimen Especial”, approved in the Resolution dated 4th October 2006 of the Ministry of Industry Tourism and Trade's General Secretariat of Energy, as a necessary condition to receive the tariff.
20
0 0,15 Zone 1
0,5 Zone 2
0,65
1
15
Time (s)
Zone 3
Figure 1: Voltage versus time curve that defines the area of voltage dip in the grid connection point that must be withstood by the installation. It is necessary to bear in mind that, during the duration of both, the maintenance period of the “fault” and the voltage recovery after its clearance one, cannot exist consumption of reactive and active power by the facility in de connection point to the grid, except during the 150 ms immediately subsequent to the beginning of the dip, and the 150 ms immediately subsequent to its recovery.
Figure 2: Permissible operating area during periods of voltage dip and recovery, considering the voltage at the grid connection point. Both during the period of dip and the recovery, the facility must provide the maximum current possible to the electrical system (Itotal). Such input current must take place so that the operating point for the facilities is located within the shaded area depicted in Figure 2, earlier than 150 ms from the beginning of the fault or its clearance. Thus, for voltages lower that 85% at the grid connection point, the facility must generate reactive
power, while for voltages between the 85% and the lowest voltage admissible for the normal operation of the electric grid, the facility must produce reactive power. Because of how complex it is to verify that the requirements are met, the PV sector has developed, together with the rest of the involved agents, the Appendix III “Photovoltaic Installations” of the Procedure for Verification, Validation and Certification of the PO 12.3 on the response of wind farms and photovoltaic plants in the event of voltage dips (hereinafter PVVC9). According to such procedure, the required tests that have to be performed for three-phase inverters are summarized in the following table. Table I: Voltage dips characteristics for three-phase PVECU tests. Voltage – Time
Faults
Vres500ms
3-phase
Vres500ms
2-phase (isolated)
Power before dip Pout > 80% 10%
