Interfacing Solar Energy to Electric Power Grid
Interfacing Solar Energy to Electric Power Grid Bingsen Wang Dept. of Electrical Engineering
Arizona Workshop on Renewable Energy 10/17/2008
List of Topics • • • • •
Introduction Configurations of grid interface Basic inverter topologies Operational constraints Summary
11/21/2008
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Worldwide PV Installation Capacity
The existing PV installation capacity up to the end of 2007. (Source: Renewables 2007 Global Status Report, www.ren21.net.)
11/21/2008
3
Overview • Grid‐connected PV systems always have a connection to the public electricity grid via a suitable inverter. because a PV module delivers only dc power.
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4
Three Concepts of Grid Connection • Grid‐connected PV systems can be subdivided into three kinds: – decentralized grid‐connected PV systems, – quasi‐centralized grid‐connected PV systems, – centralized grid‐connected PV systems.
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Decentralized System • For so‐called decentralized systems, that most commonly have the photovoltaic module installed on house roofs, relatively small photovoltaic generators of only a few kW are connected to the mains via an inverter adapted to the photovoltaic generator capacity. • They most commonly feed into the low voltage grid. • The difference between photovoltaic generator energy provision and the current energy demand of the respective household is balanced by the grid.
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Public Grid 11/21/2008
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Quasi‐centralized System • "Quasi centralized" systems are a very rare mixture of small scale systems and large scale photovoltaic power plants. • The individual solar generators are combined to larger units on the direct current (DC) side with an electrical capacity ranging between some 100 kW up to several MW. • As the electric energy is fed into the medium voltage power grid, a transformer is needed. • Quasi‐centralized systems have not yet been put into practice on a large scale.
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Public Grid 11/21/2008
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Centralized System • Centralized systems with several 100 kW or few MW are typically mounted on the ground or on very large roofs. • The energy generated by photovoltaics is fed into the low or medium voltage grid by means of one or several inverters and a transformer. • Photovoltaic plants of this type show currently electric capacities between some 100 kW and up to 5 MW. However, even higher capacities are achievable from a technical point of view without any problems. 11/21/2008
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~
Public Grid
8
Inside PV Power Plant • PV modules may be connected to the grid with module inverters, string inverters or central inverters
11/21/2008
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Inverter‐PV Configurations • Module inverters with small power ratings are fixed on the back side of every module. They can adjust an optimal MPP per device that results in a high total energy yield of the PV system. This decentral concept necessitates high effort if a monitoring system should be applied. • String inverters convert the DC power of a whole module string. Compared to the module inverter, the MPP control is less optimal if the incident light is unevenly distributed or shading arises on some modules. However, a monitoring system is easier to implement. • Central inverters offer the best monitoring possibility because only one data interface and one processing unit are necessary. However, no individual MPP tracking is possible. 11/21/2008
10
Inverter Topologies • Type I: Inverters with a 50 Hz transformer: simple topology, high reliability, high volume and weight, maximum efficiency of 95%; • Type II: Inverters with a high frequency transformer: costly concept, low volume and weight, maximum efficiency of 91%; • Type III: Transformerless inverter: low weight, voltage transfer ratio up to 1:3, maximum efficiency of 95%; • Cuk‐ or Zeta‐inverter: transfer ratio up to 1:5, maximum efficiency of 91%; • Resonance inverter: complex control, maximum efficiency of 95%.
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Type I: Low Frequency Isolation • PV inverter with self commutated full bridge and line frequency transformer
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Type II: High Frequency Isolation • PV inverter with high‐frequency transformer
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Type III: Without Transformers • Transformerless PV inverter with a boost converter stage.
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Operational Constraints Operation of grid connected PV inverter is subject to certain standards. One of the standards is the IEEE Std 929‐2000: IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems • Power quality (PQ): – The quality of power provided by the PV system for the on‐site ac loads and for delivery to the interconnected utility is governed by practices and standards addressing voltage, flicker, frequency, and distortion.
• Safety and protection functions: – Proper and safe operation of the PV systems
11/21/2008
15
PQ‐Normal Voltage Operating Range Utility‐interconnected PV systems do not regulate voltage, they inject current into the utility. Therefore, the voltage operating range for PV inverters is selected as a protection function that responds to abnormal utility conditions, not as a voltage regulation function. •Small system (≤ 10 kW) The operating window for these small PV systems is 106‐ 132 V on a 120 V base, that is, 88‐110% of nominal voltage. This range results in trip points at 105 V and at 133 V. •Intermediate (>10kW, ≤500kW) and large (>500 kW) systems Utilities may have specific operating voltage ranges. If not, operating between 88% and 110% of the appropriate should be followed 11/21/2008
16
PQ‐Voltage Flicker Any voltage flicker resulting from the connection of the inverter to the utility system at the PCC should not exceed the limits defined by the maximum border line of irritation curve identified in IEEE Std 519.
Border Line of Irritation
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PQ‐Frequency • The utility controls system frequency, and the PV system shall operate in synchronism with the utility. – Small PV systems installed in North America should have a fixed operating frequency range of 59.3‐60.5 Hz. – Utilities may require adjustable operating frequency settings for intermediate and large systems.
11/21/2008
18
PQ‐Waveform Distortions • The PV system output should have low current‐distortion levels to ensure that no adverse effects are caused to other equipment connected to the utility system. – Total harmonic current distortion shall be less than 5% of the fundamental frequency current at rated inverter output. – Each individual harmonic shall be limited to the percentages listed in Table. Even harmonics in these ranges shall be 0.85 (lagging or leading) when output is > 10% of rating. • Most PV inverters designed for utility‐interconnected service operate close to unity power factor. • Specially designed systems that provide reactive power compensation may operate outside of this limit with utility approval.
11/21/2008
20
Response to Voltage Disturbances • For nominal 120 V base, the inverter should sense abnormal voltage and respond as follows.
“Trip time” refers to the time between the abnormal condition being applied and the inverter ceasing to energize the utility line. The inverter will actually remain connected to the utility to allow the inverter to sense utility electrical conditions for the “reconnect” feature. 11/21/2008
21
Response to Frequency Disturbances • When the utility frequency is outside the range of 59.3 ‐ 60.5 Hz, the inverter should cease to energize the utility line within six cycles. • The purpose of the allowed time delay is to ride through short‐term disturbances to avoid excessive nuisance tripping.
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Response to Islanding • Islanding: – A condition in which a portion of the utility system that contains both load and distributed resources remains energized while isolated from the remainder of the utility system.
• Anti‐islanding features required of the PV inverter to ensure that the inverter ceases to energize the utility line when the inverter is subjected to islanding conditions.
11/21/2008
23
Islanding Detection • PV systems are protected against the vast majority of potential islanding situations by voltage and frequency detection schemes. • However, it is possible that circumstances may exist on a line section that has been isolated from the utility and contains a balance of load and PV generation that would allow continued operation of PV systems.
11/21/2008
24
Reconnect after a Utility Disturbance • Following an out‐of‐bounds utility event that has caused the PV system to cease to energize the utility line, line energization should remain disabled until continuous normal voltage and frequency have been maintained by the utility for a minimum of 5 min, at which time the inverter is allowed to automatically reconnect the PV system to the utility.
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25
DC Current Injection • The PV system should not inject dc current > 0.5% of rated inverter output current into the ac interface under either normal or abnormal operating conditions. • Two methods to prevent DC injection – One method is to incorporate an ac output isolation transformer in the inverter. – The other method, which uses a shunt or dc‐current sensor, initiates inverter shutdown when the dc component of the current exceeds the specified threshold.
11/21/2008
26
Other Requirement • Grounding: – The PV system and interface equipment should be grounded in accordance with applicable codes.
• Utility‐interface disconnect switch Two situations exist where utilities may choose not to require a utility‐interface disconnect switch: – If a utility has operating procedures that do not require such a switch for PV systems. – When certified nonislanding inverters are used.
11/21/2008
27
Summary • • • •
Grid connected PV systems are predominant Interface configurations Inverter topologies Operational constraints:
11/21/2008
28
Questions?
11/21/2008
29
Arizona Workshop on Renewable Energy 10/17/2008
List of Topics • • • • •
Introduction Configurations of grid interface Basic inverter topologies Operational constraints Summary
11/21/2008
2
Worldwide PV Installation Capacity
The existing PV installation capacity up to the end of 2007. (Source: Renewables 2007 Global Status Report, www.ren21.net.)
11/21/2008
3
Overview • Grid‐connected PV systems always have a connection to the public electricity grid via a suitable inverter. because a PV module delivers only dc power.
11/21/2008
4
Three Concepts of Grid Connection • Grid‐connected PV systems can be subdivided into three kinds: – decentralized grid‐connected PV systems, – quasi‐centralized grid‐connected PV systems, – centralized grid‐connected PV systems.
11/21/2008
5
Decentralized System • For so‐called decentralized systems, that most commonly have the photovoltaic module installed on house roofs, relatively small photovoltaic generators of only a few kW are connected to the mains via an inverter adapted to the photovoltaic generator capacity. • They most commonly feed into the low voltage grid. • The difference between photovoltaic generator energy provision and the current energy demand of the respective household is balanced by the grid.
=
=
=
~
~
~
Public Grid 11/21/2008
6
Quasi‐centralized System • "Quasi centralized" systems are a very rare mixture of small scale systems and large scale photovoltaic power plants. • The individual solar generators are combined to larger units on the direct current (DC) side with an electrical capacity ranging between some 100 kW up to several MW. • As the electric energy is fed into the medium voltage power grid, a transformer is needed. • Quasi‐centralized systems have not yet been put into practice on a large scale.
=
~
Public Grid 11/21/2008
7
Centralized System • Centralized systems with several 100 kW or few MW are typically mounted on the ground or on very large roofs. • The energy generated by photovoltaics is fed into the low or medium voltage grid by means of one or several inverters and a transformer. • Photovoltaic plants of this type show currently electric capacities between some 100 kW and up to 5 MW. However, even higher capacities are achievable from a technical point of view without any problems. 11/21/2008
=
~
Public Grid
8
Inside PV Power Plant • PV modules may be connected to the grid with module inverters, string inverters or central inverters
11/21/2008
9
Inverter‐PV Configurations • Module inverters with small power ratings are fixed on the back side of every module. They can adjust an optimal MPP per device that results in a high total energy yield of the PV system. This decentral concept necessitates high effort if a monitoring system should be applied. • String inverters convert the DC power of a whole module string. Compared to the module inverter, the MPP control is less optimal if the incident light is unevenly distributed or shading arises on some modules. However, a monitoring system is easier to implement. • Central inverters offer the best monitoring possibility because only one data interface and one processing unit are necessary. However, no individual MPP tracking is possible. 11/21/2008
10
Inverter Topologies • Type I: Inverters with a 50 Hz transformer: simple topology, high reliability, high volume and weight, maximum efficiency of 95%; • Type II: Inverters with a high frequency transformer: costly concept, low volume and weight, maximum efficiency of 91%; • Type III: Transformerless inverter: low weight, voltage transfer ratio up to 1:3, maximum efficiency of 95%; • Cuk‐ or Zeta‐inverter: transfer ratio up to 1:5, maximum efficiency of 91%; • Resonance inverter: complex control, maximum efficiency of 95%.
11/21/2008
11
Type I: Low Frequency Isolation • PV inverter with self commutated full bridge and line frequency transformer
11/21/2008
12
Type II: High Frequency Isolation • PV inverter with high‐frequency transformer
11/21/2008
13
Type III: Without Transformers • Transformerless PV inverter with a boost converter stage.
11/21/2008
14
Operational Constraints Operation of grid connected PV inverter is subject to certain standards. One of the standards is the IEEE Std 929‐2000: IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems • Power quality (PQ): – The quality of power provided by the PV system for the on‐site ac loads and for delivery to the interconnected utility is governed by practices and standards addressing voltage, flicker, frequency, and distortion.
• Safety and protection functions: – Proper and safe operation of the PV systems
11/21/2008
15
PQ‐Normal Voltage Operating Range Utility‐interconnected PV systems do not regulate voltage, they inject current into the utility. Therefore, the voltage operating range for PV inverters is selected as a protection function that responds to abnormal utility conditions, not as a voltage regulation function. •Small system (≤ 10 kW) The operating window for these small PV systems is 106‐ 132 V on a 120 V base, that is, 88‐110% of nominal voltage. This range results in trip points at 105 V and at 133 V. •Intermediate (>10kW, ≤500kW) and large (>500 kW) systems Utilities may have specific operating voltage ranges. If not, operating between 88% and 110% of the appropriate should be followed 11/21/2008
16
PQ‐Voltage Flicker Any voltage flicker resulting from the connection of the inverter to the utility system at the PCC should not exceed the limits defined by the maximum border line of irritation curve identified in IEEE Std 519.
Border Line of Irritation
11/21/2008
17
PQ‐Frequency • The utility controls system frequency, and the PV system shall operate in synchronism with the utility. – Small PV systems installed in North America should have a fixed operating frequency range of 59.3‐60.5 Hz. – Utilities may require adjustable operating frequency settings for intermediate and large systems.
11/21/2008
18
PQ‐Waveform Distortions • The PV system output should have low current‐distortion levels to ensure that no adverse effects are caused to other equipment connected to the utility system. – Total harmonic current distortion shall be less than 5% of the fundamental frequency current at rated inverter output. – Each individual harmonic shall be limited to the percentages listed in Table. Even harmonics in these ranges shall be 0.85 (lagging or leading) when output is > 10% of rating. • Most PV inverters designed for utility‐interconnected service operate close to unity power factor. • Specially designed systems that provide reactive power compensation may operate outside of this limit with utility approval.
11/21/2008
20
Response to Voltage Disturbances • For nominal 120 V base, the inverter should sense abnormal voltage and respond as follows.
“Trip time” refers to the time between the abnormal condition being applied and the inverter ceasing to energize the utility line. The inverter will actually remain connected to the utility to allow the inverter to sense utility electrical conditions for the “reconnect” feature. 11/21/2008
21
Response to Frequency Disturbances • When the utility frequency is outside the range of 59.3 ‐ 60.5 Hz, the inverter should cease to energize the utility line within six cycles. • The purpose of the allowed time delay is to ride through short‐term disturbances to avoid excessive nuisance tripping.
11/21/2008
22
Response to Islanding • Islanding: – A condition in which a portion of the utility system that contains both load and distributed resources remains energized while isolated from the remainder of the utility system.
• Anti‐islanding features required of the PV inverter to ensure that the inverter ceases to energize the utility line when the inverter is subjected to islanding conditions.
11/21/2008
23
Islanding Detection • PV systems are protected against the vast majority of potential islanding situations by voltage and frequency detection schemes. • However, it is possible that circumstances may exist on a line section that has been isolated from the utility and contains a balance of load and PV generation that would allow continued operation of PV systems.
11/21/2008
24
Reconnect after a Utility Disturbance • Following an out‐of‐bounds utility event that has caused the PV system to cease to energize the utility line, line energization should remain disabled until continuous normal voltage and frequency have been maintained by the utility for a minimum of 5 min, at which time the inverter is allowed to automatically reconnect the PV system to the utility.
11/21/2008
25
DC Current Injection • The PV system should not inject dc current > 0.5% of rated inverter output current into the ac interface under either normal or abnormal operating conditions. • Two methods to prevent DC injection – One method is to incorporate an ac output isolation transformer in the inverter. – The other method, which uses a shunt or dc‐current sensor, initiates inverter shutdown when the dc component of the current exceeds the specified threshold.
11/21/2008
26
Other Requirement • Grounding: – The PV system and interface equipment should be grounded in accordance with applicable codes.
• Utility‐interface disconnect switch Two situations exist where utilities may choose not to require a utility‐interface disconnect switch: – If a utility has operating procedures that do not require such a switch for PV systems. – When certified nonislanding inverters are used.
11/21/2008
27
Summary • • • •
Grid connected PV systems are predominant Interface configurations Inverter topologies Operational constraints:
11/21/2008
28
Questions?
11/21/2008
29
