In this paper the challenges and requirements related to the connection of the photovoltaic (PV) generating stations to ...
Plants. Industry Academia Collaboration (IAC) Conference, 2015, Energy and sustainable development Track, Apr. 6 – 8, 2015, Cairo, Egypt. http://www.iacconf.com/
Overview of Grid Code and Operational Requirements of Grid-connected Solar PV Power Plants H. Khairy1, M. EL-Shimy*2, G. Hashem2 M.Sc researcher – Ain Shams Unniversity, cairo, Egypt Electric Power and Machines Department, Ain Shams University Ain Shams Faculty of Enginnering, Cairo, Egypt *Corresponding author:
[email protected]; 00201005639589 #
2
Abstract— In this paper the challenges and requirements related to the connection of the photovoltaic (PV) generating stations to the utility grid are investigated considering various grid codes, international standards, and international practices. The impact of the connection voltage level and PV generator capacity is also considered in the survey. In addition, operational and control requirements during normal operation and abnormal conditions are presented. The SCADA systems applications in the PV power plants are also presented. The information presented in the paper may be considered a key factor in planning, operation, and control of PV power plants connected to the grid at various voltage levels. In addition, the overview of various international standards shows the discrepancies between them. Consequently, a careful selection of a standard or adaptation of a specific standard to the conditions of a considered power grid is necessary. This is of special importance in developing countries such as Egypt. Index Terms—PV power plants, grid connection, grid codes, international standards, SCADA
I. INTRODUCTION Currently Egypt and many other developing countries are facing a major power and energy problems. These problems are attributed to many factors. These factors include insufficient power generation, low network reliability and insufficient maintenance, insufficient fossil fuels, and bad economy [1]. Due to their availability and competitive prices, renewable energy can contribute in the national energy security. The whole world is expanding in using the renewable energy, especially German, China and USA [2]. The nuclear power can be considered as a feasible alternative to renewable energy. With nuclear power, huge amounts of energy can be generated using small amounts of nuclear. In addition, the variability, intermittency, and dispatchability of nuclear power sources are significantly better in comparison with most
renewable energy sources; however, there are these two major disadvantages related to the nuclear power. These are the nuclear waste and the safety. Although the waste is very small, it is more dangerous and hazardous in comparison to fossil fuels; the nuclear waste needs to be buried deep down to earth for thousands of years so the radioactivity can diminish. It should also be kept safe from earthquakes, floods and terrorist attack. The nuclear power is although reliable but maintaining the safety of the plant is very expensive. In case of any accident, the nuclear power station can result into a catastrophe. Another operational issue related to large nuclear plants is the MW/sec rate limit capability. This rate limit is not large enough to accommodate the variations of the system load, especially during its peaking. Therefore, nuclear power plants may be considered as baseloaders. On the other side, the rate limit of renewable energy sources such as wind and solar power plants is much higher than that of the nuclear plants. Therefore, renewable energy sources show a good fit to large load variations. Problems such as the inherent intermittency and variability of renewable energy sources are currently given high R&D focus [1, 2]. Connecting a renewable energy plant to the utility grid is facing a lot of operation and control challenges, also the renewable energy sources have to comply with the grid code of the relevant country. Most grid codes mandate that the operational capabilities of renewable energy sources be as close as possible to conventional power plants. According to SANDIA report [3] PV installations are typically separated into three categories: residential, non-residential, and large or utility scale. Nonresidential PV would include installations at government buildings and retail stores ranging from tens of kilowatts (kW) to several MW, while residential installation would be installed in the homeowner's premises, typically less than 10 kW. These types of installations are typically on the customer’s side of the meter and the
energy produced is used predominantly on site. Customer-side generation is under state jurisdiction, and their interconnection is conducted pursuant to state-specific interconnection procedures. The PV plants may be connected to either low voltage or medium voltage power grid depending on the plant size and the available output power. The grid regulation required for connecting the PV plant to the medium voltage power grid may vary than that required for connecting the PV plant to the low voltage grid. Both German grid code [4] and SANDIA report [3] deals with the technical requirements for connecting photovoltaic plant to the medium voltage power grid, while the IEEE paper [5] deals mainly with the interconnection regulation of connecting PV plants to the low voltage power grid and the regulation modification when the PV plant is connected to the medium voltage power grid. In this paper the challenges and requirements related to the connection of the photovoltaic generating stations to the utility grid will be investigated considering various grid codes and international standards. The impact of the connection voltage level is also considered in the survey. In addition, operational and control requirements during normal operation and abnormal conditions are presented. The SCADA systems applications in the PV power plants are also presented. II. GRID
CODE AND STANDARD REQUIREMENTS
In this section grid code requirements for connecting PV plants to either the LV or MV power grid will be investigated. The grid code can be divided into two main categories; normal operation and under grid disturbance requirements. Both of these categories will be investigated in this section. A. Normal operation requirements The normal operation requirements can be divided to frequency deviation, voltage deviation, active power control,+ and reactive power control. A.1. Frequency deviation According to IEEE 929-2000 [5], a small PV system connected to the LV grid side has to operate properly within a frequency range of 59.3 Hz (98.83%) - 60.5 Hz (100.83%) based on nominal frequency of 60 Hz. This means that the PV plant has to trip when the frequency drops to 59.2 Hz (98.66%) or increased to 60.6 Hz (101%). When the frequency lies outside the allowable limits the inverter should cease to energize the utility lines within 6 cycles. On the
other hand, the IEC 61727 [6] stated that the frequency range is 49 Hz (98%) to 51 Hz (102%) based on a system frequency of 50 Hz, when the system frequency lies outside these limits the PV system must be disconnected within 0.2 sec (10 cycles). It is clear that the international standards provide discrepancies in the connection requirements. Therefore, the proper settings should be re-determined according to the considered system operational practices and characteristics. When the PV system is connected to the MV grid side, the frequency deviation is required to meet the requirements in Table I as stated in Germany, France and Spain grid codes [7]. TABLE I Frequency tolerance (in Hz) according to German, France, and Spain grid codes for PV plants connected to the MV grid Country Frequency tolerance
Germany
France
Spain
47.5 < f < 51.5
48 < f < 51
47.5 < f < 52
10 cycles
n/a
n/a
10 cycles
n/a
n/a
Over frequency trip time (sec.) Underfrequency trip time(sec.)
According to SANDIA [3] the grid connected PV generator must meet off-nominal frequency (ONF) tolerance requirements. For example, large-scale PV plants connected in the Western Electricity Coordinating Council (WECC) footprint may need to comply with the existing WECC ONF requirement. In addition, the proposed NERC PRC-024-1 [8] requirement addresses a generator frequency tolerance curve. The details of both the WECC ONF and the NERC PRC-024-1 frequency ride through requirements are shown in Fig. 1 and Table II.
No trip Zone
Fig. 1: NERC PRC-024 frequency ride-through curves (60 Hz system) TABLE II WECC frequency ride-through requirements (60 Hz system) WECC frequency ride-through requirement Under-frequency limit > 59.4
Over-frequency limit 60 to < 60.6
Minimum time N/A (continuous)
< 59.4
> 60.6
3 min
< 58.4
> 61.6
30 sec
< 57.8
-
7.5 sec
< 57.3
-
45 cycle
< 57
> 61.7
Instantaneous
A.2. Voltage deviation According to IEEE 929-2000 [5], an PV system connected to the LV grid side must be able to operate healthy within the voltage window of 106 - 132V at the PCC, that is 88% to 110% of nominal voltage which is 120 V. That means the system will trip when the voltage becomes outside these limits with disconnection time as shown in Table III. For system with line voltage greater than 120 V, the same ratio of 88% 110% of nominal voltage is applied. It has to be noticed that if the system line voltage differ than 120 V, the percentage value of nominal voltage in Table III should be followed. According to IEC 61727 [6, 7] the voltage limit for LV grid connected PV plants is from 85% to 110% for nominal voltage with disconnection times as shown in Table IV when the voltage becomes outside these allowable limits. In countries with increased PV penetration, in normal conditions the voltage limits specified by the LV GCs should not exceed the limits expresses in Table V. The maximum allowed voltage rise caused by the PV systems should be less than 3% and is estimated in terms of short circuit power of PCC and the apparent power of the PV system [7]. TABLE III Response to voltage deviations for PV systems connected to the LV grid according to IEEE 929-2000 Maximum trip Voltage at PCC (120 V base) time (Cycles) V < 50% 6 50% < V < 88.33%
120
88.33% < V < 110%
Normal operation
110% < V < 137.5%
120
V 137.5 %
2
TABLE IV Response to voltage deviations for PV systems connected to the LV grid according to IEC61627 Voltage range
Disconnection time (cycles)
V < 50%
5
50% < V < 85%
100
85% < V < 110%
Normal operation
110% < V < 135%
100
V > 135%
2.5
TABLE V Voltage limits for high PV penetration countries Germany
Spain
France
80% < V < 110%
85% < V < 110%
90% < V < 110%
Another point that attention has to be paid for is voltage flickers i.e. the frequent variation of voltage which could lead to modulations of light intensity of incandescent. Even more flickers can affect the operation of some apparatus like computers, instrumentation and communication equipment. Any voltage flicker resulting from the connection of the inverter to the utility system must not exceed the permissible limits. The voltage flicker produced by the connection of a PV plant to the power grid must not exceed the border line of irritation in Fig. 2 according to [5, 9]. This is in order to minimize the noxious effects to other customers on the utility system.
Fig. 2: Voltage flickers limit [5]
A.3. Active power control Two modes of active power control are required when connecting large PV plant to a MV grid. The first one is when the plant is intended to operate at constant output power while the second mode is when the plant is required to participate in frequency control of the grid.The PV plant has to control the output power by reducing it in steps of 10% of the rated power. A set point given by the utility grid operator has to be reachable at any operation point of the plant; usually set points of 100%, 60%, 30% and 0% of the plant rated power are used [4, 7]. When the system frequency is above or below the nominal value, it means that there is too much or too less generated power in the system compared to the load. For example, as in grid connected wind generators the system operator prefers to have a plant capable of offering the so called “spinning reserve” for security issues [10], it means that the plant should be available and ready to respond to the frequency changes by reducing or increasing their output automatically as shown in Fig. 3. This
requirement is also applicable to PV power plants. Fig. 4 shows how the plant output power has to be controlled with respect to frequency; it is shown that the plant output power has to be reduced with a gradient of 40%/Hz when frequency increased than 100.4%. The plant output power is permitted to increase again when the frequency is below 100.1%. When the frequency is above 103% or below 95% the plant has to be disconnected from the grid [4, 7].
Usually the plant has to be able to inject the required reactive power within a few minutes and as often as required.
Fig. 5: Power factor – active power dependency requirements for PV plants connected to the MV grid
Fig. 3: Active power and frequency control in - wind power plants
Fig. 4:
During a symmetrical fault or heavy load steady changes, the plant has to support the utility voltage by injecting more reactive current to the network, voltage control should comply to Fig. 6. In Germany, if the voltage drops more than 10% of nominal voltage the control must inject reactive current at the low voltage side of the plant transformer with a contribution of at least of 2% of the rated current per percent of voltage drop [4, 7]. The plant must be capable of injecting the required reactive current within 20 ms into the grid. If required, the plant has to be able to supply reactive current at least 100% of the rated current. In case of an unsymmetrical fault, the reactive current must not exceed values that cause voltages higher than 110% of nominal voltage in the non-faulty phases.
Fig. 4: Active power and frequency control - PV power plants
A.4. Reactive power control PV plants connected to the MV power grid has to be able to supply reactive power to the grid at any point of operation to achieve a power factor of between 0.95 lag and 0.95 lead [3, 4, 11] to support grid voltage stability under normal operation. The reactive power has to be supplied during the feed-in operation, which means there is no need to supply reactive power during night. The reactive power set point can be one of the following operational modes [4, 7]: A fixed power factor. A variable power factor depending on the delivered active power (Fig. 5). A fixed reactive power in MVAR. A variable reactive power depending on the voltage (Fig. 6 and 7).
Fig. 6: Required injected reactive current with respect to voltage drop, according to German grid code for the MV grid
In Spain, the amount of injected reactive current during voltage drop is defined by the
polygonal curve ABCD. In overvoltage conditions the reactive current is defined by the curve D’-C’ which is the mirror of the polygonal ABCD. Above voltage level of 1.3 p.u of nominal voltage, the PV plant must be disconnected by the means of protective relay [16] as shown in Fig. 7.
Each individual current harmonic should be limited to the percentages listed in Table VI when the voltage at the PCC is ranging between 120V and 69KV. The limits in this table are a percentage of the fundamental frequency current at full system output. Even harmonics in these ranges should be less than 25% of the odd harmonic limits listed.
These requirements are for six-pulse converters and general distortion situations. IEEE standard [9] gives a conversion formula for converters with pulse numbers greater than six, also gives different current harmonic limits for different voltage levels at the PCC. The voltage harmonic distortions are limited to the values in Table VII depending on the voltage level. The values in this table are in percentage (%) of fundamental frequency voltage and for conditions lasting for more than one hour. For shorter periods the values can be exceeded by 50% [9]. Fig. 7: Reactive current requirement during faults according to Spain grid code for MV grid
Finally, for PV plant connected to the LV power grid, the PV system should operate at a power factor higher than 0.85 (lagging or leading) when output active power is higher than 10% of the rating. Most PV inverters designed for utility-interconnected service operates close to unity power factor. Specially designed systems that provide reactive power compensation may operate outside of this limit with utility approval [5]. A.5. Short circuit limits The short circuit current may exceed the utility grid limit at the point of connection of the PV plant. The short circuit current of a synchronous generator is typically eight times the rated current. For a PV plant, the short circuit current is typically the same as the rated current [4]. Therefore, there is no need to limit the PV plant short circuit current by external current limiter. A.6. Harmonics The harmonic component of the injected current has to be in accordance with clause 10 of IEEE Std. 519-1992 [9]. This is to avoid the adverse effect of harmonics on the other equipments connected to the grid. The harmonic limits mentioned [5, 9] are summarized as follows: Total harmonic current distortion at the PCC should be less than 5% of the fundamental frequency current at the rated inverter output.
TABLE VI Current harmonic limits recommended by IEEE 929-1992 for six-pulse converter Odd harmonics
Distortion limit
3rd – 9th
< 4.0%
th
th
< 2.0%
th
th
< 1.5%
11 - 15 17 - 21 rd
23 - 33
rd
Above the 33rd
< 0.6% < 0.3%
TABLE VII Voltage harmonic limits recommended by IEEE 519-1992 Individual Total voltage voltage Voltage at the PCC distortion distortion (%) (THD %) 69 KV and below 3.0 5.0 69.001 KV up to 1.5 2.5 161 KV More than 161.001 1.0 1.5 KV
B. Under grid disturbance requirements The main goals are to ride through momentary network faults and at the same time to provide grid support which is called fault ride through capability (FRT). If a large PV plant is immediately disconnected instead of helping the system to regain a steady state operating point, the electrical grid stability will be even more negatively affected. These requirements apply to large PV plant connected to the MV power grid.
Table VIII The term fault ride through (FRT) is related FRT requirements for wind generator according to different to how the plant has to act in the case of utility grid codes voltage drop because of faults to maintain grid Fault Fault duration Min voltage Voltage Grid code stability, reliability and operational security. The duration based on 50Hz level restoration (ms) system (cycles) (% of Vnom) (s) Germany general form of FRT requirements is depicted in 150 7.5 0 1.5 Fig. 8 [12]; above the solid line the plant must(E.On) Denmark 100 5 25 10 not be disconnected from the utility grid while underneath the line there are no requirementsSpain 500 25 20 1 to stay connected. However, each code can add more constraints on the connection and the disconnection of the plant. Four main parameters can define the FRT requirements which are the minimum acceptable voltage during the fault (Vmin), fault duration, voltage restoration time and steady state voltage (Vss). These FRT requirements are applicable for both wind and PV plants. For wind plant the FRT parameters according to some different grid codes are shown in Table VIII, more information and international coverage of these values in can be found in [12]. While in PV grid connected systems, Fig. 9 represent the FRT limits in the German E.ON Netz grid code [4, 7, 11]. The plant must not be Fig. 9: FRT limits according to German grid code disconnected above borderline 1, which means it must not disconnect when voltage drops to According to SANDIA [3], in the US the FRT 0% of the utility nominal voltage with a duration requirements for wind plants were first not more than 150 ms (7.5 cycles based on standardized in the Federal Energy Regulatory 50Hz system). The voltage restoration time Commission (FERC) Order 661A [13]. This must not exceed 1500 ms (75 cycles on 50Hz requirement often applies to transmissionsystem) with minimum acceptable steady state connected PV plants, even though the standard voltage equals to 90% of the utility nominal states that it applies only to wind plants. FERC’s voltage. Underneath border line 3 there are no FRT requirement mandates that a generator requirements to remain connected to the grid. shall withstand zero voltage at the point of In the area above borderline 2 and beneath common coupling (PCC) -typically the primary borderline 1, the following options are available side of the station transformer- for up to 0.15 according to an agreement with the utility seconds (7.5 cycles based on frequency of operator: feed-in of a short circuit current, or 50Hz) and the ensuing voltage recovery period. short-time disconnection up to 2 seconds, or The FERC requirement is not specific about the depending on the concept of grid connection requirement to ride-through during the voltage borderline 2 can be moved. Below borderline 2 recovery period. a short-time disconnection can be carried out in The North American Electric Reliability any cases, also a longer disconnection time are Corporation (NERC) proposed PRC-024-1 possible. standard [8] addresses voltage tolerance for all generators. If approved, NERC’s voltage ridethrough (VRT) standard will have to be reconciled with FERC Order 661A and other LVRT regional standards that may exist. Fig. 10 shows the FRT curve contained in the proposed NERC PRC-024-1 requirement.
Fig. 8: General form of FRT requirements
Fig. 10: FRT limit according to (NERC) PRC-024-1
C. SCADA Integration Requirements FERC Order 661A also contains Supervisory Control and Data Acquisition (SCADA) requirements for wind plants. As mentioned in the previous discussion, SCADA requirements contained in FERC Order 661A are sometimes applied to large-scale PV plants. The purpose of the requirement was for the plant owner to be able to transmit data and receive instructions from the transmission provider in order to protect system reliability. SCADA data to be shared are based on needs for Real-time monitoring, operations and control (line switching, generation dispatch, etc.), State estimation (to determine real-time stability), Remedial action schemes (planned response to contingencies), Remote communication, and Safety issues (confirming energized/deenergized components). Further details on SCADA for power system applications can be found in [14 - 16]. Local SCADA system in PVplants is composed of data acquisition unit, RTU, and a communications unit. The SCADA system could measure and collect PV array temperature, solar irradiance, DC output voltage and current, inverter output AC voltage and current relay switch state and so on. Data acquisition unit consisted for current transformer (DCT and ACT), voltage transformer (PT) and communication unit such as RS485 or Ethernet ah shown in Fig.11. Further information about the usage of SCADA systems in the PV power plants can be found in [19].
Fig. 11: SCADA in grid connected PV plant [16]. III. CONCLUSIONS This paper presented a detailed analysis of the requirements of connecting PV plants to the power grid. Various voltage limits and capacities of the PV plants are carefully considered in the specification of various requirements. In addition, the survey covers many international standards, grid codes, and practices. The normal operation as well as under grid disturbance requirements in details. In addition, the SCADA system requirements and its potential capabilities are demonstrated. The salient finding is the discrepancies between various standards. This is may be attributed to the significant dependency between a proper set of requirements from one side and the characteristics, operational capabilities, control options, stiffness of the considered power systems from the other side. Therefore, integration of new power generation facilities to a power grid should be carefully made according to a carefully selected standard or according to a revised form of a carefully selected standard. This requires elaborated analysis and simulation of the considered grid. Due to their special operational and control characteristics, the proper construction of grid code and standardized connection requirements is essential even if the characteristics of the considered system are well known. REFERENCES [1] M. EL-Shimy, “Wind Energy Conversion Systems: Reliability Prospective,” Encyclopedia of Energy Engineering and Technology, vol. 2, pp. 2184 - 2206, 2014. [2] REN21 (2014). "Renewables 2014: Global Status Report". pp. 15, 16. Available at http://www.webcitation.org/6SKF06GAX
[3] A. Ellis, et al., "Utility-Scale Photovoltaic Procedures and Interconnection Requirements," Sandia report SAND2012- 2090, February 2012. [4] E. Troester, "New German grid codes for connecting PV systems to the medium voltage power grid.," presented at the." 2nd International workshop on concentrating photovoltaic power plants: optical design, production, grid connection, 2009. [5] "IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems," IEEE Std 929-2000 , vol., no., pp.i,, 2000 [6] International Electrotechnical Commission, "IEC 61727 Photovoltaic (PV) systems – Characteristics of the utility interface", 2004. [7] B.-I. Crăciun, et al., "Overview of Recent Grid Codes for PV Power Integration," 13th International Conference on Optimization of Electrical and Electronic Equipment (OPTIM), pp. 959-965, 2012. [8] Std, N.E.R.C. "PRC-024-1/Draft 6." Draft on Generator Frequency and Voltage Protective Relay Settings (2013). [9] "IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems," IEEE Std 519-1992, pp.1, 112, April 1993 [10] M. EL-Shimy. “Renewable energy: Grid Codes and Integration”. Lecture notes available at: http://shimymb.tripod.com/id12.html [11] Y. Yang, et al., Advanced Control of Photovoltaic and Wind Turbines Power Systems. In Advanced and Intelligent Control in Power Electronics and Drives Springer International Publishing, 2014. [12] M. EL-Shimy, and N. Ghaly, “Grid-Connected Wind Energy Conversion Systems: Transient Response,” Encyclopedia of Energy Engineering and Technology, vol. 2, pp. 2162 - 2183, 2014. [13] Order 661 - Interconnection with Wind Energy, issued by Federal Energy Regulatory Commission (FERC) of United States, June 2, 2005. [14] "IEEE Guide for Monitoring, Information Exchange, and Control of Distributed Resources Interconnected With Electric Power Systems," IEEE Std 1547.3-2007, pp.1158, 2007 [15] Std, N.E.R.C. "PRC-024-1/Draft 6." Draft on Generator Frequency and Voltage Protective Relay Settings (2013). [16] H. Guozhen, et al., "Solutions for SCADA system Communication Reliability in Photovoltaic Power Plants," presented at the Power Electronics and Motion Control Conference (IPEMC), 2009.
