Puerto Rico Electric Power Authority’s Minimum Technical Renewables Interconnection Requirements

Author: National Renewable Energy Laboratory^[1]

The Puerto Rico Electric Power Authority (PREPA) has established (MTR) for interconnection of wind turbine generation and photovoltaic (PV) power plants. During a stakeholder workshop conducted by the U.S. Department of Energy (DOE), the National Renewable Energy Laboratory (NREL), and the Government of Puerto Rico in May 2012, barriers to renewable energy deployment were discussed. The stakeholders, including PREPA, the Puerto Rico Energy Affairs Administration, and representatives from the Governor’s Office, requested DOE and NREL assistance in a technical review of the MTRs for variable renewable energy generation.

NREL has conducted a review of these requirements based on generic technical aspects and electrical characteristics of wind and PV power plants, and on existing requirements from other utilities in both the United States and Europe. The purpose of this review is to analyze each aspect of the PREPA MTRs, comparing and contrasting with interconnection requirements from similar power systems, identifying areas of concern, and generating recommendations and suggestions for improvements or additional study.

Over the course of the collaboration on the review of the MTRs (2012-2013), PREPA staff reviewed the draft recommendations and incorporated some of the recommendations into the 2012 revisions of the MTRs. In addition, PREPA engineers responded to the NREL recommendations by providing a technical rational based on their operational experience or previous modeling results for not modifying some of the requirements. For the sake of convenience, in this report we shall refer to the June 14, 2012, version of the PREPA MTR document as the “original” MTR, and to the August 15, 2012, version as the “modified” MTR.

Minimum Technical Requirements of Interconnection of PV Facilities (ver. June 14, 2012)

The proponent shall comply with the following minimum technical requirements:

Voltage Right-Through

a. PREPA’s Low Voltage Ride-Through (LVRT) Requirements:

1. From Figure 1, PREPA requires all generation to remain online and be able to ride-through three phase and single phase faults down to 0.0 per-unit (measured at the point of interconnection), for up to 600 ms.
2. All generation remains online and operating during and after normally cleared faults on the point of interconnection.
3. All generation remains online and operating during backup-cleared faults on the point of interconnection.
4. During the fault conditions, the PV facility shall operate on maximum reactive current injection mode.

b. PREPA’s Overvoltage Ride-Through (OVRT) Requirements:

1. PREPA requires all generation to remain online and able to ride-through symmetrical and asymmetrical overvoltage conditions specified by the following values:

Overvoltage (pu)	Minimum time to remain online (seconds)
1.4 – 1.25	1
1.25 – 1.15	3
1.15 or lower	indefinitely

Voltage Regulation System (VRS)

Constant voltage control shall be required. Photovoltaic System technologies in combination with Static Var Controls, such as Static Var Compensators (SVCs), STATCOMs, DSTATCOMs are acceptable options to comply with this requirement. A complete and detailed description of the VRS control strategy shall be submitted for evaluation.

a) Photovoltaic Facilities (PVF) must have a continuously-variable, continuously-acting, closed loop control VRS; i.e. an equivalent to the Automatic Voltage Regulator in conventional machines.
b) The VRS set-point shall be adjustable between 95% to 105% of rated voltage at the POI. The VRS set-point must also be adjustable by PREPA’s Energy Control Center via SCADA.
c) The VRS shall operate only in a voltage set point control mode. Controllers such as Power Factor or constant VAR are not permitted.
d) The VRS controller regulation strategy shall be based on proportional plus integral (PI) control actions with parallel reactive droop compensation. The VRS Droop shall be adjustable from 0 to 10% and must also be adjustable by PREPA’s Energy Control Center via SCADA.
e) At zero percent (0%) droop, the VRS shall achieve a steady-state voltage regulation accuracy of +/-0.5% of the controlled voltage at the POI.
f) The VRS shall be calibrated such that a change in reactive power will achieve 95% of its final value no later than 1 second following a step change in voltage. The change in reactive power should not cause excessive voltage excursions or overshoot.
g) The generator facility VRS must be in service at any time the PVF is electrically connected to the grid regardless of MW output from the PVF.

Reactive Power Capability and Minimum Power Factor Requirements

The total power factor range shall be from 0.85 lagging to 0.85 leading at the point of interconnection (POI). The reactive power requirements provide flexibility for many types of technologies at the Renewable Energy Facility. The intent is that a PVF can ramp the reactive power from 0.85 lagging to 0.85 leading in a smooth continuous fashion at the POI.

The +/-0.90 power factor range should be dynamic and continuous at the point of interconnection (POI). This means that the PVF has to be able to respond to power system voltage fluctuations by continuously varying the reactive output of the plant within the specified limits. The previously established power factor dynamic range could be expanded if studies indicate that additional continuous, dynamic compensation is required. It is required that the PVF reactive capability meets +/-0.85 Power Factor (PF) range based on the PVF Aggregated MW Output, which is the maximum MVAr capability corresponding to maximum MW Output. It is understood that positive (+) PF is where the PVF is producing MVAr and negative (-) PF is where the PVF is absorbing MVAr.

This requirement of MVAr capability at maximum output shall be sustained throughout the complete range of operation of the PVF as established by Figure 2.

Short Circuit Ration (SCR) Requirements

Short Circuit Ratio values (System Short Circuit MVA at POI/PV Facility MVA Capacity) under 5 shall not be permitted. The constructor shall be responsible for the installation of additional equipment, such as synchronous condensers, and controls necessary to comply with PREPA’s minimum short circuit requirements.

Frequency Ride Through (FTR)

57.5 -61.5 Hz	No tripping (continuous)
61.5 -62.5 Hz	30 sec
56.5 -57.5 Hz	10 sec
< 56.5 or > 62.5 Hz	Instantaneous trip

Frequency Response/Regulation

PV facility shall provide an immediate real power primary frequency response, proportional to frequency deviations from scheduled frequency, similar to governor response. The rate of real power response to frequency deviations shall be similar to or more responsive than the droop characteristic of 5% used by conventional generators. PV facility shall have controls that provide both for down-regulation and up-regulation. PV technologies, in combination with energy storage systems such as, but not limited to BESS, flywheels, hybrid systems are acceptable options to comply with PREPA’s frequency response and regulation requirements.

For large frequency deviations (for example in excess of 0.3 Hz), the PV facility shall provide an immediate real power primary frequency response of at least 10% of the maximum AC active power capacity (established in the contract) for a time period not less than 10 minutes. The time response (full 10% frequency response) shall be less than 1 second. During disturbances or situations that provoke the system frequency to stay below 59.7 Hz for 10 minutes or more, after the ninth minute the real power primary frequency response shall not decrease at a ramp rate higher than 10% of the maximum AC active power capacity per minute. For smaller frequency deviations (for example less than 0.3 Hz), the PV facility response shall be proportional to the frequency deviation, based on the specified 5% droop characteristic. The operational range of the frequency response and regulation system shall be from 10% to 100% of the maximum AC active power capacity (established in the contract). The PV facility power output at the POI shall never exceed the maximum AC active power (established in the contract).

Ramp Rate Control

Ramp Rate Control is required to smoothly transition from one output level to another. The PV facility shall be able to control the rate of change of power output during some circumstances, including but not limited to: (1) rate of increase of power, (2) rate of decrease of power, (3) rate of increase of power when a curtailment of power output is released; (4) rate of decrease in power when curtailment limit is engaged. A 10 % per minute rate (based on AC contracted capacity) limitation shall be enforced. This ramp rate limit applies both to the increase and decrease of power output and is independent of meteorological conditions.

Power Quality Requirements

The developer shall address, in the design of their facilities potential sources and mitigation of power quality degradation prior to interconnection. Design considerations should include applicable standards including, but not limited to IEEE Standards 142, 519, 1100, 1159, and ANSI C84.1. Typical forms of power quality degradation include, but are not limited to voltage regulation, voltage unbalance, harmonic distortion, flicker, voltage sags/interruptions and transients.

Special Protection Schemes

PV facility shall provide adequate technology and implement special protection schemes as established by PREPA in coordination with power management requirements.

General Interconnection Substation Configuration

An interconnecting generation producer must interconnect at an existing PREPA switchyard. The configuration requirements of the interconnection depend on where the physical interconnection is to occur and the performance of the system with the proposed interconnection. The interconnection must conform, at a minimum, to the original designed configuration of the switchyard. PREPA, at its sole discretion, may consider different configurations due to physical limitations at the site.

Modeling and Validation

The Contractor shall submit to PREPA a Siemens -PTI certified PSS/E mathematical model and data related to the proposed PV facility. When referred to the PV facility model, this shall include but is not limited to PV inverters, transformers, collector systems, plant controllers, control systems and any other equipment necessary to properly model the PV facility for both steady-state and dynamic simulation modules. It is required that the Contractor submits both an aggregate and detailed version of the PV facility model. At a later stage in the process, it is also required that the Contractor submits as-built PSS/E mathematical models of the PV Facility.

The Contractor shall be required to submit user manuals for both the PV inverter and the PV facility models including a complete and detailed description of the voltage regulation system (VRS) and frequency regulation system model implementation. The mathematical models shall be fully compatible with the latest and future versions of PSS/E. It is preferred that the models are PSS/E standard models. In the case that the Contractor submits user written models, the Contractor shall be required to keep these models current with the future versions of the PSS/E program until such time that PSS/E has implemented a standard model. The Contractor shall submit to PREPA an official report from Siemens -PTI that validates and certifies the required mathematical models, including subsequent revisions. The data and PSS/E model shall also be updated and officially certified according to PREPA requirements when final field adjustments and parameters measurements and field tests are performed to the facility by the contractor. The mathematical model (either PSS/E standard or user written model) of the PV facility shall be officially certified by Siemens -PTI before a specific and validated PSS/E mathematical model of the complete PV facility be submitted to PREPA. The Contractor shall be responsible of submitting the official reports and certifications from Siemens – PTI, otherwise the mathematical model shall not be considered valid.

The Contractor shall be responsible to submit Siemens – PTI certified PSSE mathematical models of any kind of compensation devices (ie. SVC, STATCOMs, DSTATCOMs, BESS, etc.) used on the PV facility. It is preferred that the models are standard models provided with PSS/E. In the case that the Contractor submits user written models, the PV facility Contractor shall be required to keep these models current with the future versions of the PSS/E program until such time that PSS/E has implemented a standard model. In its final form, the mathematical model shall be able to simulate each of the required control and operational modes available for the compensation device and shall be compatible with the latest and future versions of PSSE. Final adjustments and parameters settings related with the control system commissioning process shall be incorporated to the PSSE mathematical model and tested accordingly by the PV facility Contractor and PREPA system study groups. The Contractor shall also perform on-site field tests for the identification, development, and validation of the dynamic mathematical models and parameters required by PREPA for any kind of compensation devices used at the PV facility. The mathematical models of the PV facility and its required compensation devices shall be officially certified by Siemens PTI before a specific and validated PSS/E mathematical model of the complete PV facility be submitted to PREPA. The Contractor shall be responsible of submitting the official reports and certifications from Siemens – PTI, otherwise the mathematical models shall not be considered valid.

PV facility Owners that provide user written model(s) shall provide compiled code of the model and are responsible to maintain the user written model compatible with current and new releases of PSS/E until such time a standard model is provided. PREPA must be permitted by the PV facility Owner to make available PV Facility models if required to external consultants with an NDA in place.

Transient Mathematical Model

The Contractor shall be responsible of providing a detailed transient model of the PV facility and to show that it is capable of complying with PREPA’s transient Minimum Technical Requirements.

Dynamic System Monitoring Equipment

The developer of the PV facility shall be required to provide and install a dynamic system monitoring equipment that conforms to PREPA’s specifications.

Minimum Technical Requirements of Interconnection of Wind Facilities (ver. June 14, 2012)

The proponent shall comply with the following minimum technical requirements:

Voltage Right-Through

a. PREPA’s Low Voltage Ride-Through (LVRT) Requirements:

1. From Figure 1, PREPA requires all generation to remain online and be able to ride-through three phase and single phase faults down to 0.0 per-unit (measured on the point of interconnection), for up to 600 ms.
2. All generation remains online and operating during and after normally cleared faults on the point of interconnection.
3. All generation remains online and operating during backup-cleared faults on the point of interconnection.
4. During the fault conditions, the wind generation facility shall operate on maximum reactive current injection mode.

b. PREPA’s Overvoltage Ride-Through (OVRT) Requirements:

1. PREPA requires all generation to remain online and able to ride-through symmetrical and asymmetrical overvoltage conditions specified by the following values:

Overvoltage (pu)	Minimum time to remain online (seconds)
1.4 – 1.25	1
1.25 – 1.15	3
1.15 or lower	indefinitely

Voltage Regulation System (VRS)

Constant voltage control shall be required. Wind Turbine Generation (WTG) technologies in combination with Static Var Controls, such as Static Var Compensators (SVC), STATCOMs, DSTATCOMs are acceptable options to comply with this requirement. A complete description of the VRS control strategy shall be submitted for evaluation.

a) Wind Generation Facilities (WGF) must have a continuously-variable, continuously-acting, closed loop control VRS; i.e. an equivalent to the Automatic Voltage Regulator in conventional machines.
b) The VRS set-point shall be adjustable between 95% to 105% of rated voltage at the POI. The VRS set-point must also be adjustable by PREPA’s Energy Control Center via SCADA.
c) The VRS shall operate only in a voltage set point control mode. Controllers such as Power Factor or constant VAR are not permitted.
d) The VRS controller regulation strategy shall be based on proportional plus integral (PI) control actions with parallel reactive droop compensation. The VRS Droop shall be adjustable from 0 to 10% and must also be adjustable by PREPA’s Energy Control Center via SCADA.
e) At zero percent (0%) droop, the VRS shall achieve a steady-state voltage regulation accuracy of +/-0.5% of the controlled voltage at the POI.
f) The VRS shall be calibrated such that a change in reactive power will achieve 95% of its final value no later than 1 second following a step change in voltage. The change in reactive power should not cause excessive voltage excursions or overshoot.
g) The generator facility VRS must be in service at any time the WGF is electrically connected to the grid regardless of MW output from the WGF.

Reactive Power Capability and Minimum Power Factor Requirements

The total power factor range shall be from 0.85 lagging to 0.85 leading at the point of interconnection (POI). The reactive power requirements provide flexibility for many types of technologies at the Renewable Energy Facility. The intent is that a WGF can ramp the reactive power from 0.85 lagging to 0.85 leading in a smooth continuous fashion at the POI.

The +/-0.90 power factor range should be dynamic and continuous at the point of interconnection (POI). This means that the WGF has to be able to respond to power system voltage fluctuations by continuously varying the reactive output of the plant within the specified limits. The previously established power factor dynamic range could be expanded if studies indicate that additional continuous, dynamic compensation is required. It is required that the WGF reactive capability meets +/-0.85 Power Factor (PF) range based on the WGF Aggregated MW Output, which is the maximum MVAr capability corresponding to maximum MW Output. It is understood that positive (+) PF is where the WGF is producing MVAr and negative (-) PF is where the WGF is absorbing MVAr.

This requirement of MVAr capability at maximum output shall be sustained throughout the complete range of operation of the WGF as established by Figure 6.

Short Circuit Ration (SCR) Requirements

Short Circuit Ratio values (System Short Circuit MVA at POI/WGF MVA Capacity) under 5 shall not be permitted. The constructor shall be responsible for the installation of additional equipment, such as synchronous condensers, and controls necessary to comply with PREPA’s minimum short circuit requirements.

Frequency Ride Through (FTR)

57.5 – 61.5 Hz	No tripping (continuous)
61.5 – 62.5 Hz	30 sec
56.5 – 57.5 Hz	10 sec
< 56.5 or > 62.5 Hz	Instantaneous trip

Frequency Response/Regulation and Inertial Response

WTG facility shall provide an immediate real power primary frequency response, proportional to frequency deviations from scheduled frequency, similar to governor response. The rate of real power response to frequency deviations shall be similar to or more responsive than the droop characteristic of 5% used by conventional generators. WTG facility shall have controls that provide both down-regulation and up-regulation. Wind turbine technologies, in combination with energy storage systems such as BESS, flywheels, hybrid systems are acceptable options to comply with PREPA’s frequency regulation requirements.

For large frequency deviations (for example in excess of 0.3 Hz), the WGF shall provide an immediate real power primary frequency response of at least 10% of the maximum AC active power capacity (established in the contract) for a time period not less than 10 minutes. The time response (full 10% frequency response) shall be less than 1 second. During disturbances or situations that provoke the system frequency to stay below 59.7 Hz for 10 minutes or more, after the ninth minute the real power primary frequency response shall not decrease at a ramp rate higher than 10% of the maximum AC active power capacity per minute. For smaller frequency deviations (for example less than 0.3 Hz), the WGF response shall be proportional to the frequency deviation, based on the specified 5% droop characteristic. The operational range of the frequency response and regulation system shall be from 10% to 100% of the maximum AC active power capacity (established in the contract). The WGF power output at the POI shall never exceed the maximum AC active power (established in the contract).

The WTG controls shall have the capability to provide inertial response.

Ramp Rate Control

Ramp Rate Control is required to smoothly transition from one output level to another. The WTG facility shall be able to control the rate of change of power output during some circumstances, including but not limited to: (1) rate of increase of power (2) rate of decrease of power, (3) rate of increase of power when a curtailment of power output is released; (4) rate of decrease in power when curtailment limit is engaged. A 10 % per minute rate (based on nameplate capacity) limitation shall be enforced. This limit applies both to the increase and decrease of power output and is independent of meteorological conditions.

Power Quality

The developer shall address, in the design of their facilities potential sources and mitigation of power quality degradation prior to interconnection. Design considerations should include applicable standards including, but not limited to IEEE Standards 142, 519, 1100, 1159, ANSI C84.1, IEC 61400-21, IEC 61000-3-7 and IEC 61000-3-6. Typical forms of power quality degradation include, but are not limited to voltage regulation, voltage unbalance, harmonic distortion, flicker, voltage sags/interruptions and transients.

The developer shall submit the Power Quality Tests Result Report of the wind turbines as described in the IEC 61400-21 standard. This report includes: general wind turbine data, wind turbine rated data at terminals, voltage fluctuations coefficients (flicker coefficients), current harmonics components, current interharmonics components, current high frequency components, response to voltage drops, active power data, reactive power data, grid protection data and reconnection time. The wind turbines shall not exceed the flicker emission limits established by the IEC 61000-3-7 standard and the harmonics emission limits of IEC 61000-3-6.

Wind Power Management

WTG facility shall provide adequate technology (communicating technology and the corresponding control equipment) and implement wind power management requirements (ramp rate limits, output limits, curtailment) as established by PREPA.

Special Protection Schemes

WTG facility shall provide adequate technology and implement special protection schemes as established by PREPA in coordination with wind power management requirements.

Wind Generation Forecasting System

WTG facility shall provide adequate technology to support wind generation forecasting systems (short term and day-ahead) as established by PREPA. Individual turbine’s availability shall be included.

General Interconnection Substation Configuration

An interconnecting generation producer must interconnect at an existing PREPA switchyard. The configuration requirements of the interconnection depend on where the physical interconnection is to occur and the performance of the system with the proposed interconnection. The interconnection must conform, at a minimum, to the original designed configuration of the switchyard. PREPA, at its sole discretion, may consider different configurations due to physical limitations at the site.

Modeling and Validation

The Contractor shall submit to PREPA a Siemens -PTI certified PSS/E mathematical model and data related to the proposed WTG facility. When referred to the WTG facility model, this shall include but is not limited to wind generator, transformers, collector systems, plant controllers, control systems and any other equipment necessary to properly model the WTG facility for both steady-state and dynamic simulation modules. It is required that the Contractor submits both an aggregate and detailed model of the WTG facility model. At a later stage in the process, it is also required that the Contractor submits as-built PSS/E mathematical models of the WTG facility.

The Contractor shall be required to submit user manuals for both the Wind Turbine Generator and WTG Facility models including a complete and detailed description of the voltage regulation system (VRS) and frequency regulation system model implementation. The mathematical models shall be fully compatible with the latest and future versions of PSS/E. It is preferred that the models are PSS/E standard models. In the case that the Contractor submits user written models, the Contractor shall be required to keep these models, as well as its corresponding user manual, current with the future versions of the PSS/E program until such time that PSS/E has implemented a standard model. The Contractor shall submit to PREPA an official report from Siemens -PTI that validates and certifies the required mathematical models, including subsequent revisions. The data and PSS/E model shall also be updated and officially certified according to PREPA requirements when final field adjustments and parameters measurements and field tests are performed to the facility by the contractor. The mathematical model (either PSS/E standard or user written model) of the WTG facility shall be officially certified by Siemens -PTI before a specific and validated PSS/E mathematical model of the complete WTG facility be submitted to PREPA. The Contractor shall be responsible of submitting the official reports and certifications from Siemens – PTI, otherwise the mathematical model shall not be considered valid.

The Contractor shall be responsible to submit Siemens – PTI certified PSSE mathematical models of any kind of compensation devices (ie. SVC, STATCOMs, DSTATCOMs, BESS, etc.) used on the WTG facility. It is preferred that the models are standard models provided with PSS/E. In the case that the Contractor submits user written models, the WTG facility Contractor shall be required to keep these models current with the future versions of the PSS/E program until such time that PSS/E has implemented a standard model. In its final form, the mathematical model shall be able to simulate each of the required control and operational modes available for the compensation device and shall be compatible with the latest and future versions of PSSE. Final adjustments and parameters settings related with the control system commissioning process shall be incorporated to the PSSE mathematical model and tested accordingly by the WTG facility Contractor and PREPA system study groups. The Contractor shall also perform on-site field tests for the identification, development, and validation of the dynamic mathematical models and parameters required by PREPA for any kind of compensation devices used at the WTG facility. The mathematical models of the WTG facility and its required compensation devices shall be officially certified by Siemens -PTI before a specific and validated PSS/E mathematical model of the complete WTG facility be submitted to PREPA. The Contractor shall be responsible of submitting the official reports and certifications from Siemens – PTI, otherwise the mathematical models shall not be considered valid.

WTG facility Owners that provide user written model(s) shall provide compiled code of the model and are responsible to maintain the user written model compatible with current and new releases of PSS/E until such time a standard model is provided. PREPA must be permitted by the WGF Owner to make available WGF models if required to external consultants with an NDA in place.

Transient Mathematical Model

The contractor shall be responsible of providing a detailed transient model of the WTG facility and to show that it is capable of complying with PREPA’s transient Minimum Technical Requirements.

Dynamic System Monitoring Equipment

The developer of the Renewable Energy Facility shall be required to provide and install a dynamic system monitoring equipment that conforms to PREPA’s specifications.

Minimum Technical Requirements of Interconnection of PV Facilities (ver. Aug. 15, 2012)

The proponent shall comply with the following minimum technical requirements:

Voltage Right-Through

a. PREPA’s Low Voltage Ride-Through (LVRT) Requirements:

1. From Figure 1, PREPA requires all generation to remain online and be able to ride-through three phase and single phase faults down to 0.0 per-unit (measured at the point of interconnection), for up to 600 ms.
2. All generation remains online and operating during and after normally cleared faults on the point of interconnection.
3. All generation remains online and operating during backup-cleared faults on the point of interconnection.
4. During the low voltage fault conditions, the PV facility shall operate on reactive current injection mode. This mode of operation shall be implemented with a reactive current droop characteristic which shall have an adjustable slope from 1 to 5%. A dead band of 15 % is required.

b. PREPA’s Overvoltage Ride-Through (OVRT) Requirements:

1. PREPA requires all generation to remain online and able to ride-through symmetrical and asymmetrical overvoltage conditions specified by the following values illustrated in Figure 1:

Overvoltage (pu)	Minimum time to remain online (seconds)
1.4 – 1.3	150 ms
1.3 – 1.25	1 s
1.25 – 1.15	3 s
1.15 or lower	indefinitely

Voltage Regulation System (VRS)

Constant voltage control shall be required. Photovoltaic System technologies in combination with Static Var Controls, such as Static Var Compensators (SVCs), STATCOMs and DSTATCOMs are acceptable options to comply with this requirement. A complete and detailed description of the VRS control strategy shall be submitted for evaluation.

a) Photovoltaic Facilities (PVF) must have a continuously-variable, continuously-acting, closed loop control VRS; i.e. an equivalent to the Automatic Voltage Regulator in conventional machines.
b) The VRS set-point shall be adjustable between 95% to 105% of rated voltage at the POI. The VRS set-point must also be adjustable by PREPA’s Energy Control Center via SCADA.
c) The VRS shall operate only in a voltage set point control mode. Controllers such as Power Factor or constant VAR are not permitted.
d) The VRS controller regulation strategy shall be based on proportional plus integral (PI) control actions with parallel reactive droop compensation. The VRS Droop shall be adjustable from 0 to 10%.
e) At zero percent (0%) droop, the VRS shall achieve a steady-state voltage regulation accuracy of +/-0.5% of the controlled voltage at the POI.
f) The VRS shall be calibrated such that a change in reactive power will achieve 95% of its final value no later than 1 second following a step change in voltage. The change in reactive power should not cause excessive voltage excursions or overshoot.
g) The generator facility VRS must be in service at any time the PVF is electrically connected to the grid regardless of MW output from the PVF.
h) The VRS dead band shall not exceed 0.1%.

Reactive Power Capability and Minimum Power Factor Requirements

The total power factor range shall be from 0.85 lagging to 0.85 leading at the point of interconnection (POI). The reactive power requirements provide flexibility for many types of technologies at the Renewable Energy Facility. The intent is that a PVF can ramp the reactive power from 0.85 lagging to 0.85 leading in a smooth continuous fashion at the POI.

The +/-0.90 power factor range should be dynamic and continuous at the point of interconnection (POI). This means that the PVF has to be able to respond to power system voltage fluctuations by continuously varying the reactive output of the plant within the specified limits. The previously established power factor dynamic range could be expanded if studies indicate that additional continuous, dynamic compensation is required. It is required that the PVF reactive capability meets +/-0.85 Power Factor (PF) range based on the PVF Aggregated MW Output, which is the maximum MVAr capability corresponding to maximum MW Output. It is understood that positive (+) PF is where the PVF is producing MVAr and negative (-) PF is where the PVF is absorbing MVAr.

This requirement of MVAr capability at maximum output shall be sustained throughout the complete range of operation of the PVF as established by Figure 2.

Short Circuit Ration (SCR) Requirements

Short Circuit Ratio values (System Short Circuit MVA at POI/PV Facility MVA Capacity) under 5 shall not be permitted. The constructor shall be responsible for the installation of additional equipment, such as synchronous condensers, and controls necessary to comply with PREPA’s minimum short circuit requirements.

Frequency Ride Through (FTR)

57.5 – 61.5 Hz	No tripping (continuous)
61.5 – 62.5 Hz	30 sec
56.5 – 57.5 Hz	10 sec
< 56.5 or > 62.5 Hz	Instantaneous trip

Frequency Response/Regulation

PV facility shall provide an immediate real power primary frequency response, proportional to frequency deviations from scheduled frequency, similar to governor response. The rate of real power response to frequency deviations shall be similar to or more responsive than the droop characteristic of 5% used by conventional generators. PV facility shall have controls that provide both for down-regulation and up-regulation. PV technologies, in combination with energy storage systems such as, but not limited to BESS, flywheels and hybrid systems are acceptable options to comply with PREPA’s frequency response and regulation requirements.

For small frequency deviations (for example less than 0.3 Hz), the PV facility response shall be proportional to the frequency deviation, based on the specified 5% droop characteristic. The frequency response dead band shall not exceed 0.02%. For large frequency deviations (for example in excess of 0.3 Hz), the PV facility shall provide an immediate real power primary frequency response of at least 10% of the maximum AC active power capacity (established in the contract). The time response (full 10% frequency response) shall be less than 1 second.

If energy storage systems are utilized to comply with the frequency regulation requirements, and during a disturbance the system frequency stays below 59.7 Hz, the facility frequency response shall be maintained for at least 9 minutes. After the ninth minute the real power primary frequency response shall not decrease at a ramp rate higher than 10% of the maximum AC active power capacity per minute.

The operational range of the frequency response and regulation system shall be from 10% to 100% of the maximum AC active power capacity (established in the contract). The PV facility power output at the POI shall never exceed the maximum AC active power (established in the contract).

Ramp Rate Control

Ramp Rate Control is required to smoothly transition from one output level to another. The PV facility shall be able to control the rate of change of power output during some circumstances, including but not limited to: (1) rate of increase of power, (2) rate of decrease of power, (3) rate of increase of power when a curtailment of power output is released; (4) rate of decrease in power when curtailment limit is engaged. A 10 % per minute rate (based on AC contracted capacity) limitation shall be enforced. This ramp rate limit applies both to the increase and decrease of power output and is independent of meteorological conditions. The ramp rate control tolerance shall be +10%.

Power Quality Requirements

The developer shall address, in the design of their facilities potential sources and mitigation of power quality degradation prior to interconnection. Design considerations should include applicable standards including, but not limited to IEEE Standards 142, 519, 1100, 1159, and ANSI C84.1. Typical forms of power quality degradation include, but are not limited to voltage regulation, voltage unbalance, harmonic distortion, flicker, voltage sags/interruptions and transients.

Special Protection Schemes

PV facility shall provide adequate technology and implement special protection schemes as established by PREPA in coordination with power management requirements.

General Interconnection Substation Configuration

An interconnecting generation producer must interconnect at an existing PREPA switchyard. The configuration requirements of the interconnection depend on where the physical interconnection is to occur and the performance of the system with the proposed interconnection. The interconnection must conform, at a minimum, to the original designed configuration of the switchyard. PREPA, at its sole discretion, may consider different configurations due to physical limitations at the site.

Modeling and Validation

The Contractor shall submit to PREPA a Siemens -PTI certified PSS/E mathematical model and data related to the proposed PV facility. When referred to the PV facility model, this shall include but is not limited to PV inverters, transformers, collector systems, plant controllers, control systems and any other equipment necessary to properly model the PV facility for both steady-state and dynamic simulation modules. It is required that the Contractor submits both an aggregate and detailed version of the PV facility model. At a later stage in the process, it is also required that the Contractor submits as-built PSS/E mathematical models of the PV Facility.

The Contractor shall be required to submit user manuals for both the PV inverter and the PV facility models including a complete and detailed description of the voltage regulation system (VRS) and frequency regulation system model implementation. The mathematical models shall be fully compatible with the latest and future versions of PSS/E. It is preferred that the models are PSS/E standard models. In the case that the Contractor submits user written models, the Contractor shall be required to keep these models current with the future versions of the PSS/E program until such time that PSS/E has implemented a standard model. The Contractor shall submit to PREPA an official report from Siemens -PTI that validates and certifies the required mathematical models, including subsequent revisions. The data and PSS/E model shall also be updated and officially certified according to PREPA requirements when final field adjustments and parameters measurements and field tests are performed to the facility by the contractor. The mathematical model (either PSS/E standard or user written model) of the PV facility shall be officially certified by Siemens -PTI before a specific and validated PSS/E mathematical model of the complete PV facility be submitted to PREPA. The Contractor shall be responsible of submitting the official reports and certifications from Siemens – PTI, otherwise the mathematical model shall not be considered valid.

The Contractor shall be responsible to submit Siemens – PTI certified PSSE mathematical models of any kind of compensation devices (ie. SVC, STATCOMs, DSTATCOMs, BESS, etc.) used on the PV facility. It is preferred that the models are standard models provided with PSS/E. In the case that the Contractor submits user written models, the PV facility Contractor shall be required to keep these models current with the future versions of the PSS/E program until such time that PSS/E has implemented a standard model. In its final form, the mathematical model shall be able to simulate each of the required control and operational modes available for the compensation device and shall be compatible with the latest and future versions of PSSE. Final adjustments and parameters settings related with the control system commissioning process shall be incorporated to the PSSE mathematical model and tested accordingly by the PV facility Contractor and PREPA system study groups. The Contractor shall also perform on-site field tests for the identification, development, and validation of the dynamic mathematical models and parameters required by PREPA for any kind of compensation devices used at the PV facility. The mathematical models of the PV facility and its required compensation devices shall be officially certified by Siemens PTI before a specific and validated PSS/E mathematical model of the complete PV facility be submitted to PREPA. The Contractor shall be responsible of submitting the official reports and certifications from Siemens – PTI, otherwise the mathematical models shall not be considered valid.

PV facility Owners that provide user written model(s) shall provide compiled code of the model and are responsible to maintain the user written model compatible with current and new releases of PSS/E until such time a standard model is provided. PREPA must be permitted by the PV facility Owner to make available PV Facility models if required to external consultants with an NDA in place.

Transient Mathematical Model

The Contractor shall be responsible of providing a detailed transient model of the PV facility and to show that it is capable of complying with PREPA’s transient Minimum Technical Requirements.

Dynamic System Monitoring Equipment

The developer of the PV facility shall be required to provide and install a dynamic system monitoring equipment that conforms to PREPA’s specifications.

Minimum Technical Requirements of Interconnection of Wind Facilities (ver. Aug. 15, 2012)

The proponent shall comply with the following minimum technical requirements:

Voltage Right-Through

a. PREPA’s Low Voltage Ride-Through (LVRT) Requirements:

1. From Figure 1, PREPA requires all generation to remain online and be able to ride-through three phase and single phase faults down to 0.0 per-unit (measured on the point of interconnection), for up to 600 ms.
2. All generation remains online and operating during and after normally cleared faults on the point of interconnection.
3. All generation remains online and operating during backup-cleared faults on the point of interconnection.
4. During the low voltage fault conditions, the wind generation facility shall operate on reactive current injection mode. This mode of operation shall be implemented with a reactive current droop characteristic which shall have an adjustable slope from 1 to 5%. A dead band of 15 % is required.

b. PREPA’s Overvoltage Ride-Through (OVRT) Requirements:

1. PREPA requires all generation to remain online and able to ride-through symmetrical and asymmetrical overvoltage conditions specified by the following values illustrated in Figure 1:

Overvoltage (pu)	Minimum time to remain online (seconds)
1.4 – 1.3	150 ms
1.3 – 1.25	1 s
1.25 – 1.15	3 s
1.15 or lower	indefinitely

Voltage Regulation System (VRS)

Constant voltage control shall be required. Wind Turbine Generation (WTG) technologies in combination with Static Var Controls, such as Static Var Compensators (SVC), STATCOMs and DSTATCOMs are acceptable options to comply with this requirement. A complete description of the VRS control strategy shall be submitted for evaluation.

a) Wind Generation Facilities (WGF) must have a continuously-variable, continuously-acting, closed loop control VRS; i.e. an equivalent to the Automatic Voltage Regulator in conventional machines.
b) The VRS set-point shall be adjustable between 95% to 105% of rated voltage at the POI. The VRS set-point must also be adjustable by PREPA’s Energy Control Center via SCADA.
c) The VRS shall operate only in a voltage set point control mode. Controllers such as Power Factor or constant VAR are not permitted.
d) The VRS controller regulation strategy shall be based on proportional plus integral (PI) control actions with parallel reactive droop compensation. The VRS Droop shall be adjustable from 0 to 10%.
e) At zero percent (0%) droop, the VRS shall achieve a steady-state voltage regulation accuracy of +/-0.5% of the controlled voltage at the POI.
f) The VRS shall be calibrated such that a change in reactive power will achieve 95% of its final value no later than 1 second following a step change in voltage. The change in reactive power should not cause excessive voltage excursions or overshoot.
g) The generator facility VRS must be in service at any time the WGF is electrically connected to the grid regardless of MW output from the WGF.
h) The VRS dead band shall not exceed 0.1%.

Reactive Power Capability and Minimum Power Factor Requirements

The total power factor range shall be from 0.85 lagging to 0.85 leading at the point of interconnection (POI). The reactive power requirements provide flexibility for many types of technologies at the Renewable Energy Facility. The intent is that a WGF can ramp the reactive power from 0.85 lagging to 0.85 leading in a smooth continuous fashion at the POI.

The +/-0.90 power factor range should be dynamic and continuous at the point of interconnection (POI). This means that the WGF has to be able to respond to power system voltage fluctuations by continuously varying the reactive output of the plant within the specified limits. The previously established power factor dynamic range could be expanded if studies indicate that additional continuous, dynamic compensation is required. It is required that the WGF reactive capability meets +/-0.85 Power Factor (PF) range based on the WGF Aggregated MW Output, which is the maximum MVAr capability corresponding to maximum MW Output. It is understood that positive (+) PF is where the WGF is producing MVAr and negative (-) PF is where the WGF is absorbing MVAr.

This requirement of MVAr capability at maximum output shall be sustained throughout the complete range of operation of the WGF as established by figure 6.

Short Circuit Ration (SCR) Requirements

Short Circuit Ratio values (System Short Circuit MVA at POI/WGF MVA Capacity) under 5 shall not be permitted. The constructor shall be responsible for the installation of additional equipment, such as synchronous condensers, and controls necessary to comply with PREPA’s minimum short circuit requirements.

Frequency Ride Through (FTR)

57.5 – 61.5 Hz	No tripping (continuous)
61.5 – 62.5 Hz	30 sec
56.5 – 57.5 Hz	10 sec
< 56.5 or > 62.5 Hz	Instantaneous trip

Frequency Response/Regulation and Inertial Response

WTG facility shall provide an immediate real power primary frequency response, proportional to frequency deviations from scheduled frequency, similar to governor response. The rate of real power response to frequency deviations shall be similar to or more responsive than the droop characteristic of 5% used by conventional generators. WTG facility shall have controls that provide both down-regulation and up-regulation. Wind turbine technologies, in combination with energy storage systems such as BESS, flywheels and hybrid systems are acceptable options to comply with PREPA’s frequency regulation requirements.

For small frequency deviations (for example less than 0.3 Hz), the WGF response shall be proportional to the frequency deviation, based on the specified 5% droop characteristic. The frequency response dead band shall not exceed 0.02%. For large frequency deviations (for example in excess of 0.3 Hz), the WGF shall provide an immediate real power primary frequency response of at least 10% of the maximum AC active power capacity (established in the contract). The time response (full 10% frequency response) shall be less than 1 second.

If energy storage systems are utilized to comply with the frequency regulation requirements, and during a disturbance the system frequency stays below 59.7 Hz, the facility frequency response shall be maintained for at least 9 minutes. After the ninth minute the real power primary frequency response shall not decrease at a ramp rate higher than 10% of the maximum AC active power capacity per minute.

The operational range of the frequency response and regulation system shall be from 10% to 100% of the maximum AC active power capacity (established in the contract). The WGF power output at the POI shall never exceed the maximum AC active power (established in the contract).

The WTG controls shall have the capability to provide inertial response.

Ramp Rate Control

Ramp Rate Control is required to smoothly transition from one output level to another. The WTG facility shall be able to control the rate of change of power output during some circumstances, including but not limited to: (1) rate of increase of power (2) rate of decrease of power, (3) rate of increase of power when a curtailment of power output is released; (4) rate of decrease in power when curtailment limit is engaged. A 10 % per minute rate (based on nameplate capacity) limitation shall be enforced. This limit applies both to the increase and decrease of power output and is independent of meteorological conditions. The ramp rate control tolerance shall be +10%.

Power Quality

The developer shall address, in the design of their facilities potential sources and mitigation of power quality degradation prior to interconnection. Design considerations should include applicable standards including, but not limited to IEEE Standards 142, 519, 1100, 1159, ANSI C84.1, IEC 61400-21, IEC 61000-3-7 and IEC 61000-3-6. Typical forms of power quality degradation include, but are not limited to voltage regulation, voltage unbalance, harmonic distortion, flicker, voltage sags/interruptions and transients.

The developer shall submit the Power Quality Tests Result Report of the wind turbines as described in the IEC 61400-21 standard. This report includes: general wind turbine data, wind turbine rated data at terminals, voltage fluctuations coefficients (flicker coefficients), current harmonics components, current interharmonics components, current high frequency components, response to voltage drops, active power data, reactive power data, grid protection data and reconnection time. The wind turbines shall not exceed the flicker emission limits established by the IEC 61000-3-7 standard and the harmonics emission limits of IEC 61000-3-6.

Wind Power Management

WTG facility shall provide adequate technology (communicating technology and the corresponding control equipment) and implement wind power management requirements (ramp rate limits, output limits, curtailment) as established by PREPA.

Special Protection Schemes

WTG facility shall provide adequate technology and implement special protection schemes as established by PREPA in coordination with wind power management requirements.

Wind Generation Forecasting System

WTG facility shall provide adequate technology to support wind generation forecasting systems (short term and day-ahead) as established by PREPA. Individual turbine’s availability shall be included.

General Interconnection Substation Configuration

An interconnecting generation producer must interconnect at an existing PREPA switchyard. The configuration requirements of the interconnection depend on where the physical interconnection is to occur and the performance of the system with the proposed interconnection. The interconnection must conform, at a minimum, to the original designed configuration of the switchyard. PREPA, at its sole discretion, may consider different configurations due to physical limitations at the site.

Modeling and Validation

The Contractor shall submit to PREPA a Siemens -PTI certified PSS/E mathematical model and data related to the proposed WTG facility. When referred to the WTG facility model, this shall include but is not limited to wind generator, transformers, collector systems, plant controllers, control systems and any other equipment necessary to properly model the WTG facility for both steady-state and dynamic simulation modules. It is required that the Contractor submits both an aggregate and detailed model of the WTG facility model. At a later stage in the process, it is also required that the Contractor submits as-built PSS/E mathematical models of the WTG facility.

The Contractor shall be required to submit user manuals for both the Wind Turbine Generator and WTG Facility models including a complete and detailed description of the voltage regulation system (VRS) and frequency regulation system model implementation. The mathematical models shall be fully compatible with the latest and future versions of PSS/E. It is preferred that the models are PSS/E standard models. In the case that the Contractor submits user written models, the Contractor shall be required to keep these models, as well as its corresponding user manual, current with the future versions of the PSS/E program until such time that PSS/E has implemented a standard model. The Contractor shall submit to PREPA an official report from Siemens -PTI that validates and certifies the required mathematical models, including subsequent revisions. The data and PSS/E model shall also be updated and officially certified according to PREPA requirements when final field adjustments and parameters measurements and field tests are performed to the facility by the contractor. The mathematical model (either PSS/E standard or user written model) of the WTG facility shall be officially certified by Siemens -PTI before a specific and validated PSS/E mathematical model of the complete WTG facility be submitted to PREPA. The Contractor shall be responsible of submitting the official reports and certifications from Siemens – PTI, otherwise the mathematical model shall not be considered valid.

The Contractor shall be responsible to submit Siemens – PTI certified PSSE mathematical models of any kind of compensation devices (ie. SVC, STATCOMs, DSTATCOMs, BESS, etc.) used on the WTG facility. It is preferred that the models are standard models provided with PSS/E. In the case that the Contractor submits user written models, the WTG facility Contractor shall be required to keep these models current with the future versions of the PSS/E program until such time that PSS/E has implemented a standard model. In its final form, the mathematical model shall be able to simulate each of the required control and operational modes available for the compensation device and shall be compatible with the latest and future versions of PSSE. Final adjustments and parameters settings related with the control system commissioning process shall be incorporated to the PSSE mathematical model and tested accordingly by the WTG facility Contractor and PREPA system study groups. The Contractor shall also perform on-site field tests for the identification, development, and validation of the dynamic mathematical models and parameters required by PREPA for any kind of compensation devices used at the WTG facility. The mathematical models of the WTG facility and its required compensation devices shall be officially certified by Siemens -PTI before a specific and validated PSS/E mathematical model of the complete WTG facility be submitted to PREPA. The Contractor shall be responsible of submitting the official reports and certifications from Siemens – PTI, otherwise the mathematical models shall not be considered valid.

WTG facility Owners that provide user written model(s) shall provide compiled code of the model and are responsible to maintain the user written model compatible with current and new releases of PSS/E until such time a standard model is provided. PREPA must be permitted by the WGF Owner to make available WGF models if required to external consultants with an NDA in place.

Transient Mathematical Model

The contractor shall be responsible of providing a detailed transient model of the WTG facility and to show that it is capable of complying with PREPA’s transient Minimum Technical Requirements.

Dynamic System Monitoring Equipment

The developer of the Renewable Energy Facility shall be required to provide and install a dynamic system monitoring equipment that conforms to PREPA’s specifications.

References