PPL Electric Utilities
Grid-Tied Solar Inverter Requirements
Richard Michael Jr. – Distribution Standards
1
NOTICE: Although this manual outlines requirements for solar
inverters connecting to PPL Electric Utility’s grid, inverters must
still undergo testing before they are connected. Please direct
questions concerning required testing to PPL Electric Utilities.
2
Contents
Introduction .................................................................................................................................................. 4
Grid-Tied Inverter Requirements Summary.................................................................................................. 5
Grid Interconnection ..................................................................................................................................... 7
The Photovoltaic System............................................................................................................................... 9
Inverter Configurations ............................................................................................................................... 11
String Inverters........................................................................................................................................ 11
Centralized Inverters ............................................................................................................................... 11
Micro-Inverters ....................................................................................................................................... 11
General Specifications and PPL Inverter Requirements ............................................................................. 12
IEEE 1547 and UL 1741 Compliance........................................................................................................ 12
External Protective Devices .................................................................................................................... 13
Enclosure Type ........................................................................................................................................ 14
Smart Features and PPL Inverter Requirements......................................................................................... 15
Level 1 Smart Requirements ................................................................................................................... 15
Fault Ride Through .............................................................................................................................. 15
Level 2 Smart Requirements ................................................................................................................... 17
Reactive Power Support...................................................................................................................... 17
Active Power Curtailment ................................................................................................................... 19
Ramp Rates / Slow Ramp (Soft Start) ................................................................................................. 20
Level 3 Smart Requirements ................................................................................................................... 21
Communication ................................................................................................................................... 21
Additional Information................................................................................................................................ 22
Micro-Inverters ....................................................................................................................................... 22
Inverter Temperature Ranges ................................................................................................................. 23
Inverter Warranties................................................................................................................................. 24
Glossary ....................................................................................................................................................... 25
Contact PPL ................................................................................................................................................. 27
3
Introduction
With worldwide increases in solar penetration raising concerns for power system stability and
control, manufacturers are enhancing their solar inverters by adding various “smart” capabilities to help
alleviate these adverse effects. Through the exploitation of these capabilities, inverters will help manage
the power grid to enhance reliability as well as increase the quality of power delivered. Not only does this
benefit the electric utility, but also the photovoltaic (PV) system owner. Inverters are designed to
disconnect from the utility power grid upon the occurrence of an abnormal condition for the protection of
equipment. When the inverter disconnects, the solar installation is no longer generating revenue. By
helping to manage the power quality on the grid, the inverter minimizes the number of situations
requiring disconnection, thereby maximizing return on investment.
Selecting an inverter can prove rather challenging, as modern smart inverters contain a wealth of
features and specifications. This document aims to significantly simplify this task. It is important to note
that the intention of this document is strictly to assist with the selection of smart grid-tied inverters. If an
inverter will have no interconnection with the power system, PPL has no regulations or concerns
regarding that inverter. PPL’s REMSI website provides a list of accepted inverter models which can be
connected to the PPL system with little complication. These models were selected based on smart
features, technical ratings, warranties, and brand reputation. The accepted models are all very high quality
inverters which meet PPL specifications.
Customers are free to shop for inverters which are not included within the accepted model list,
but will need to ensure that the model they choose meets the PPL requirements. To assist with meeting
these requirements, this document outlines the purpose of each smart feature and provides PPL
requirements for each. Furthermore, customers will also find information and requirements concerning
general inverter specifications which are not necessarily considered “smart”. Customers are encouraged to
use this as a resource when selecting an inverter to help choose a model which meets PPL requirements.
If there are any questions or concerns regarding the choice of an inverter, please contact PPL
Electric Utilities to verify compliance prior to making a purchase.
4
Grid-Tied Inverter Requirements Summary
General Capabilities
Solar inverters in operation at all solar installations connected to PPL Electric
Utility’s power system shall meet general inverter requirements.
•
•
•
•
•
IEEE-1.1: Solar inverters connecting to PPL Electric Utility’s power system shall be rated as
IEEE 1547 compliant with the allowance of smart capabilities extended by IEEE 1547a.
IEEE-1.2: Solar inverters connecting to PPL Electric Utility’s power system shall be rated as UL
1741 compliant with the allowance of smart capabilities extended by IEEE 1547a.
IEEE-2.1: Solar inverters connecting to PPL Electric Utility’s power system shall undergo the
testing required by IEEE 1547 and UL 1741 before connecting.
EPD-1.1: Solar installations connecting to PPL Electric Utility’s power system with a generating
capacity greater than 1 MW shall have a recloser installed at the point of connection. Installations
with less generating capacity may be required to install a disconnecting device with remote
capabilities upon evaluation by PPL Electric Utilities.
ET-1.1: Solar inverters connecting to PPL Electric Utility’s power system which are placed
outdoors shall be protected by an outdoor enclosure which meets one of the ratings outlined in the
enclosure types section.
Smart Capabilities
Level 1
Solar inverters in operation at all solar installations connected to PPL Electric
Utility’s power system shall meet level 1 smart inverter requirements.
•
•
•
•
•
•
•
•
FRT-1.1: Solar inverters connecting to PPL Electric Utility’s power system shall have an
absolute upper set point for voltage.
FRT-1.2: Solar inverters connecting to PPL Electric Utility’s power system shall have an
absolute lower set point for voltage.
FRT-2.1: Solar inverters connecting to PPL Electric Utility’s power system shall have an absolute
upper set point for frequency.
FRT-2.2: Solar inverters connecting to PPL Electric Utility’s power system shall have an absolute
lower set point for frequency.
FRT-3.1: Solar inverters connecting to PPL Electric Utility’s power system shall have at
minimum one upper intermediate set point for voltage.
FRT-3.2: Solar inverters connecting to PPL Electric Utility’s power system shall have at
minimum one lower intermediate set point for voltage.
FRT-4.1: Solar inverters connecting to PPL Electric Utility’s power system shall have at
minimum one upper intermediate set point for frequency.
FRT-4.2: Solar inverters connecting to PPL Electric Utility’s power system shall have at
minimum one lower intermediate set point for frequency.
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Level 2
Solar inverters in operation at all solar installations with generating capacity
greater than or equal to 5kW connected to PPL Electric Utility’s power system
shall meet level 2 smart inverter requirements.
•
•
•
•
•
RPS-1.1: Solar inverters connecting to PPL Electric Utility’s power system shall have a 0.9
leading adjustable power factor rating or less.
RPS-1.2: Solar inverters connecting to PPL Electric Utility’s power system shall have a 0.9
lagging adjustable power factor rating or less.
RPS-1.3: Solar inverters connecting to PPL Electric Utility’s power system shall possess the
ability to autonomously manipulate the power factor in response to the power system voltage
level.
APC-1.1: Solar inverters connecting to PPL Electric Utility’s power system shall possess active
power curtailment capabilities with the ability to vary power output in increments of 5% or less.
RR-1.1: Solar inverters connecting to PPL Electric Utility’s power system shall possess slow
ramp (soft start) capabilities.
Level 3
Solar inverters in operation at all solar installations with generating capacity
greater than or equal to 100kW connected to PPL Electric Utility’s power system
shall meet level 3 smart inverter requirements.
•
•
•
•
•
COM-1.1: Solar inverters connecting to PPL Electric Utility’s power system shall communicate
using either the MODBUS or DNP3.0 application protocol.
COM-1.2: Solar inverters connecting to PPL Electric Utility’s power system shall communicate
using either the TCP/IP or UDP transport protocol.
COM-2.1: Solar inverters connecting to PPL Electric Utility’s power system shall possess either
an RJ45 Ethernet or an RS-485 serial communication port.
COM-2.2: Solar inverters connecting to PPL Electric Utility’s power system which only have an
RS-485 serial communication port will require a controller to convert the RS-485 serial signals to
RJ45 Ethernet signals.
COM-2.3: Solar inverters connecting to PPL Electric Utility’s power system shall communicate
over our network via an external 900 MHz radio or CalAmp cellular modem.
**Note: Inverters must meet the requirements for each smart capability level which the solar
installation’s generating capacity falls within. For example, inverters at a 150kW installation
must meet level 3, level 2 and level 1 requirements. Similarly, a 60kW installation must have
inverters which meet level 2 and level 1 requirements.
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Grid Interconnection
The first decision a customer must make before even beginning to research inverter models is
whether the PV system will be connected to the electrical power system, or whether the system will act as
the only power supply for the facility (household or industrial complex). These two configurations differ
in the method by which excess energy is utilized during periods of insufficient sunlight.
It should come as no surprise that the solar installation will produce its maximum power during
the day. In many cases, if the PV system was sized appropriately, the amount of power generated will
exceed what is required by the facility. Instead of simply discarding this power, it would be far more
beneficial to be able to store it and use it later. Grid-tied inverters allow customers to sell this excess
power to the utility, offsetting the cost of power
consumed from the grid in the absence of sunlight.
Installations lacking grid-tie capabilities store this
excess energy in batteries, which can be used in
Net Metering
place of the solar panels in the absence of sunlight.
Net metering is a system under which excess power provided to the
A facility which is intended to be
grid compensates for the power the facility consumes from the grid.
completely self-sufficient and not sell excess power
That is, the total power a customer pays for is equal to the total power
consumed by the facility from the grid, minus the power the solar
produced would not install a grid-tied inverter and
installation exports to the grid. Net metering is the system which
would have no connection to the electric grid
makes grid-tied PV generation worthwhile. If the solar panels can
whatsoever. While providing the facility with
generate more electricity during peak sunlight hours than the facility
power, the solar panels would also charge batteries
requires, the excess power can be exported to the power system and
sold to the utility, which will compensate for the grid power consumed
which will act as the power source in the absence
in the absence of sunlight.
of sunlight. Due to the inadequate storage capacity
of modern batteries, this option is not typically
feasible as most batteries are unable to sustain
facility loading for long durations. In addition, on a
cloudy day, the solar installation may not provide
the necessary amount of power to the facility, let
alone enough to charge the batteries. A more
practical option for solar generation is a grid-tied
scheme.
Grid-tied solar installations allow the
facility to accept power from the grid during times
of inadequate solar generation and export power to
the grid in instances of excess generation. On a
sunny day, instead of storing the power in batteries
to use later, excess power can be sold to the utility
to compensate for power consumed from the grid.
Example: Suppose a customer’s house consumes 5,000kWh of
electricity from the grid over the course of a month. This is electricity
The customer will be credited for excess power
which could not be supplied to the household by the solar installation
sold to the utility during peak production periods,
perhaps at night, or during a cloudy day. From excess power generated
and this credit will be used towards power
on sunny days, the solar installation exports 3,000kWh to the grid.
consumed from the grid when the solar production
When the customer receives their monthly bill, they will only be
is insufficient (see net metering insert). On a
charged for 2,000kWh, since 3,000kWh of the 5,000kWh consumed
by the grid has been compensated for by the PV installation. In some
cloudy day, the solar installation will provide the
circumstances, a solar installation may generate more power than
facility as much power as possible, but any
consumed by the facility, which will result in a net profit for the
owner.
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additional power required will be taken from the power grid. At night, the facility will be powered
completely from the grid. Batteries may also be used for grid-tied systems to allow the installation to
supply active and reactive power to the facility and grid for a short amount of time in the absence of
sunlight.
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The Photovoltaic System
A PV system is far more than arrays of solar panels. In addition to the panels, several other
important components must be present which are essential to converting electricity into a form which can
be used in a household or on the power grid. The main supplementary components are metering
equipment, transformers, disconnect switches, batteries, and inverters. When considering PV generation,
one should be knowledgeable on the function of each.
The most prominent feature of a PV installation is the solar panels. The main purpose of the solar
panels is to simply absorb sunlight and convert it into electricity. The electrical current generated by the
solar panels is direct current (DC), which is incompatible with household devices, industrial equipment,
and the power grid. Before this energy can be used by the facility or be exported to the power grid, it must
be converted into alternating current (AC). This is the role of the inverter.
Solar inverters are responsible for many tasks, the most important being the conversion of DC
power provided by solar panels into the AC power transmitted on the power grids and utilized in
facilities. In addition to their primary function, inverters essentially act as the brains of the system. The
inverter does not simply convert DC electricity to any AC waveform, but instead ensures that the AC
waveform produced adheres to tight frequency and magnitude tolerances. In the case of a grid-tied solar
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inverter, these parameters will be matched to those of the power grid. Inverters are responsible for
initiating and ceasing generation, and are the point at which the user communicates with the installation.
Furthermore, inverters possess the ability to sense a fault on the electrical grid and disconnect the PV
system to prevent harm to its equipment. Smart inverters also have the capability to distinguish between
faults and momentary discrepancies on the grid and remain connected during the latter. The numerous
other important features of solar inverters will be discussed in the smart features section of the document.
A host of other components are necessary for the proper functioning of a PV system. Meters are
necessary to measure power flow between the solar installation, facility, and the electrical grid.
Disconnect switches act as a secondary method of isolating the inverter from the grid or the facility.
Transformers are responsible for taking the output voltage of the inverter (usually household voltage i.e.
120V or 240V) and stepping it up to the distribution system voltage (12kV on PPL’s distribution system).
With these components, a solar installation can function properly and significantly reduce the cost of a
customer’s electric bill.
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Inverter Configurations
There are three prominent configurations for PV installations and corresponding inverters exist for each.
These three inverter types are discussed below.
String Inverters
As their name suggests, string inverters are designed to manage a single string of PV panels; therefore,
each string of panels requires its own inverter. This is by far the most popular configuration for PV
installations, due in part to the long lifetimes of the string inverters as well as the increased efficiency
compared to centralized configurations. In a string configuration, each inverter is responsible for
maximum power point tracking (MPPT) on a single string rather than the whole installation, allowing
each string to operate at maximum efficiency.
Centralized Inverters
Centralized inverters are intended to manage multiple strings in an installation. This means that an entire
distributed energy resource (DER) will require only one inverter. Centralized inverters are very popular
for larger installations due to their increased lifetimes as well as the simplicity arising from having only
one inverter.
Micro-Inverters
Micro-inverters are intended to manage a single solar panel. Thus, each panel in the installation will
require its own inverter. Many claim that with an inverter managing the MPPT of each panel individually,
the installation as a whole will see a drastic increase in efficiency. For reasons of complexity,
inconclusive data, and lack of smart capabilities, PPL generally recommends against the use of microinverters. For more information regarding this type of inverter, see the micro-inverter information within
the smart capabilities section of the document.
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General Specifications and PPL Inverter Requirements
Solar inverters in operation at all solar installations connected to PPL Electric Utility’s power system
shall meet general inverter requirements.
IEEE 1547 and UL 1741 Compliance
Purpose:
IEEE 1547 is the Institute of Electrical and Electronics Engineers (IEEE) Standard for
Interconnecting Distributed Resources with Electric Power Systems. It acts as a universal standard
intended to ensure DER operate safely and provide power to the grid reliably. UL 1741 is the
Underwriters Laboratories (UL) Standard for Static Inverters and Charge Controllers for Use in
Photovoltaic Power Systems. This is a national standard which references IEEE 1547 for many
interconnection requirements and adds additional requirements for safety and testing. These standards
outline both technical requirements as well as required testing procedures which must be performed by
PPL before an inverter is placed into service.
Neither standard currently allows inverters to utilize any smart capabilities. A patch to IEEE
1547 termed IEEE 1547a has been released to resolve this issue and IEEE and UL are in the process of
creating an update to standards 1547 and 1741 respectively to permit important smart inverter
functionalities. Although the standards do not currently allow an inverter to use its smart capabilities,
many smart inverters are still rated as IEEE 1547 and UL 1741 compliant. In such situations, this
compliance refers to the interconnection and safety requirements of the inverter.
An important feature of nearly all grid-tied solar inverters which is defined in IEEE 1547, but
typically further emphasized in inverter datasheets, is anti-islanding protection. Islanding is the condition
under which the main generator for a section of utility line is disconnected (due to protective/
sectionalizing device operation), but the section continues to be energized by a distributed generator (DG)
such as a customer PV installation. This situation can be dangerous to both utility workers and civilians
who believe wires are de-energized. Furthermore, equipment may potentially be damaged due to the
significant deviation of the electrical system from standard operating parameters. Every grid-tied inverter
contains anti-islanding protection which allows the inverter to detect that it is energizing an “island” and
disconnect immediately. Most datasheets will reemphasize this in addition to stating compliance with
IEEE 1547.
PPL Inverter Requirement:
IEEE-1.1:
Solar inverters connecting to PPL Electric Utility’s power system shall be rated as IEEE
1547 compliant with the allowance of smart capabilities extended by IEEE 1547a.
IEEE-1.2:
Solar inverters connecting to PPL Electric Utility’s power system shall be rated as UL
1741 compliant with the allowance of smart capabilities extended by IEEE 1547a.
IEEE-2.1:
Solar inverters connecting to PPL Electric Utility’s power system shall undergo the
testing required by IEEE 1547 and UL 1741 before connecting.
Note:
For more information regarding required testing, please contact PPL Electric Utilities.
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External Protective Devices
Purpose:
In the event of an inability to communicate with a solar installation’s inverter(s), PPL should still
be capable of remotely disconnecting the installation from the power grid with intentions of protecting
both the installation and the utility’s equipment. All solar installations larger than 1 MW connected to
PPL Electric Utility’s grid are required to have a backup sectionalizing device called a recloser installed
at the point at which the DG connects to the grid. The recloser allows the utility to remotely disconnect
the large amount of power produced by the installation in an emergency situation where communication
with the inverter(s) is lost. Based on an evaluation by PPL Electric Utilities, customers may be required to
install a disconnecting device with remote disconnection capabilities for installations less than 1MW as
well. This determination will be based on the generation capacity of the installation and safety concerns
for the particular section of line the DER connects to.
PPL Inverter Requirement:
EPD-1.1:
Solar installations connecting to PPL Electric Utility’s power system with a generating
capacity greater than 1 MW shall have a recloser installed at the point of connection.
Installations with less generating capacity may be required to install a disconnecting
device with remote disconnection capabilities upon evaluation by PPL Electric Utilities.
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Enclosure Type
Purpose:
The purpose of enclosures is to ensure inverters are protected properly from their environment.
The protection level of enclosures is defined in terms of National Electrical Manufacturers Association
(NEMA) and International Protection Marking (IP) standards.
PPL Inverter Requirement:
ET-1.1:
Solar inverters connecting to PPL Electric Utility’s power system which are placed
outdoors shall be protected by an outdoor enclosure rated as one of the following:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
NEMA 3
NEMA 3R
NEMA 3S
NEMA 3X
NEMA 3RX
NEMA 3SX
NEMA 4
NEMA 4X
NEMA 6
NEMA 6P
IP 54
IP 56
IP52
IP 67
IP 52
IP 54
IP 56
IP 65
IP 67
IP 68
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Smart Features and PPL Inverter Requirements
Level 1 Smart Requirements
Solar inverters in operation at all solar installations connected to PPL Electric Utility’s power system
shall meet level 1 smart inverter requirements.
Fault Ride Through
Purpose:
Contrary to what the name suggests, fault ride through does not allow the solar inverter to remain in
service during a fault, but instead prevents disconnections for slight system deviations which may be
incorrectly interpreted as a fault. There are four different components to fault ride through. These
components are:
1.
2.
3.
4.
Low Voltage Ride Through (LVRT)
High Voltage Ride Through (HVRT)
Low Frequency Ride Through (LFRT)
High Frequency Ride Through (HFRT)
Fault ride through establishes multiple set
points to mandate the amount of time the inverter
is permitted to remain in service at different levels
of voltage or frequency deviations. By
manipulating these set points, “must remain
connected” and “must disconnect” regions will be
created on a voltage (or frequency) vs. time curve.
The picture to the right shows an example of such
a curve.
At exactly 100% or slight deviations from
100% of the nominal values of frequency and
voltage, the inverter will remain connected. If
these operating points diverge excessively from
their nominal values, the inverter will disconnect. As deviations from the nominal values increase, the
time at which the inverter will remain connected will decrease. In the event of an actual fault, the voltage
or frequency is expected to change drastically from the nominal value, causing the inverter to disconnect
almost instantaneously.
There are many examples which demonstrate the importance of this feature both for the utility and the
customer. One common example is the interconnection of a large load to the power system. Upon
connection, the power system may be stressed in attempting to provide the necessary amount of power to
the new load in addition to supporting its previous loading. This strain may cause a slight decrease in
voltage and frequency. An inverter without fault ride through capabilities may interpret this as a fault, and
trip offline. If there are several solar inverters connected to that section of line and all disconnect, the line
will experience appreciable power quality issues.
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In this situation, inverters with fault ride through capabilities will remain online to assist the power
grid. In addition to the power quality assistance for the utility, the benefits of this feature for the inverter
owner are quite obvious. If the inverter trips offline for every power line deviation from normal operating
parameters, this will significantly affect the revenue generated by the solar installation. With fault ride
through capabilities, the inverter will only cease generation of power and resultant revenue when
absolutely necessary.
Many inverter manufacturers will not explicitly advertise whether their inverters have voltage or
frequency ride through capabilities; however, a simple search through the inverter’s operating manual can
determine if these capabilities are present. The operating manual will provide detailed explanations of all
settings that can be changed on the inverter. Within the manual, you will find settings concerning tripping
set points for frequency and voltage. For each set point, there will be two settings. One of these holds the
voltage or frequency level and the other holds the time for which the inverter is required to remain
connected at that level. There will typically be an absolute upper level and an absolute lower level, above
and below which respectively, the inverter trips offline as fast as possible. Most inverters only have one
intermediate set point on both the upper and lower ends. Below is an example from of an inverter
manual’s setting definitions for the intermediate under voltage set point.
Parameter Name
Definition
Default Value
Adjustable Range
SET U<
Indicates percentage of nominal system
voltage for intermediate under voltage set
point.
Indicates the amount of time for which the
inverter is to remain connected between the
intermediate under voltage value and the
absolute under voltage value.
88%
55% - 88%
2 sec
0.16 sec to 5 sec
SET TIME U<
PPL Inverter Requirement:
FRT-1.1:
Solar inverters connecting to PPL Electric Utility’s power system shall have an absolute
upper set point for voltage.
FRT-1.2:
Solar inverters connecting to PPL Electric Utility’s power system shall have an absolute
lower set point for voltage.
FRT-2.1:
Solar inverters connecting to PPL Electric Utility’s power system shall have an absolute
upper set point for frequency.
FRT-2.2:
Solar inverters connecting to PPL Electric Utility’s power system shall have an absolute
lower set point for frequency.
FRT-3.1:
Solar inverters connecting to PPL Electric Utility’s power system shall have at minimum
one upper intermediate set point for voltage.
FRT-3.2:
Solar inverters connecting to PPL Electric Utility’s power system shall have at minimum
one lower intermediate set point for voltage.
FRT-4.1:
Solar inverters connecting to PPL Electric Utility’s power system shall have at minimum
one upper intermediate set point for frequency.
FRT-4.2:
Solar inverters connecting to PPL Electric Utility’s power system shall have at minimum
one lower intermediate set point for frequency.
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Level 2 Smart Requirements
Solar inverters in operation at all solar installations with generating capacity greater than or equal to
5kW connected to PPL Electric Utility’s power system shall meet level 2 smart inverter requirements.
Reactive Power Support
Purpose:
Nearly all electrical devices consume active power when in use. When an electric bill is issued,
the price paid is for the active power consumed. This, however, is not the only form of power that
manifests itself on electrical lines. A component of power called reactive power is also present due to
certain electrical components connected to the grid which do not consume power, but instead temporarily
store and discharge it. Reactive power simply circulates on the power lines and is never consumed by
devices. The advantages and disadvantages of this component of power are beyond the scope of this
document, but one should know that overall power quality can be improved through the management of
reactive power.
The relationship between active and reactive power is described by the power factor, which can
take on values between 0 and 1. When the power factor is 1, the only power which exists on the line is
active power. On the other hand, when the power factor is 0, only reactive power exists on the line.
Determining whether an inverter has reactive power support capabilities involves finding a power factor
adjustment rating within the inverter datasheet. This rating defines the range of power factors over which
the inverter is able to provide power. The numbers provided for the rating represent the endpoints for this
range. For a power factor adjustable rating of 0.9 leading to 0.9 lagging, the inverter can provide power at
power factors between 0.9 leading and 1, and also at power factors between 0.9 lagging and 1. The
minimum adjustable power factor range required for solar inverters connecting to PPL’s power system is
0.9 leading ... 0.9 lagging (0.9-1 ind. / cap.; +/- 0.9). Ranges greater than or equal to this, which lie in the
green areas of the semicircle diagram below are acceptable. Ranges less than this, which lie in the red
area of the semicircle diagram below are unacceptable.
PPL Inverter Requirement:
RPS-1.1:
Solar inverters connecting to PPL Electric Utility’s power system shall have a 0.9 leading
adjustable power factor rating or less.
RPS-1.2:
Solar inverters connecting to PPL Electric Utility’s power system shall have a 0.9 lagging
adjustable power factor rating or less.
RPS-1.3:
Solar inverters connecting to PPL Electric Utility’s power system shall possess the ability
to autonomously manipulate the power factor in response to the power system voltage
level.
17
Examples:
Power Factor Adjustable: 0.8 leading ... 0.8 lagging; 0.8-1 ind. / cap.; +/- 0.8 – Acceptable
Power Factor Adjustable: 0.85 leading ... 0.85 lagging; 0.85-1 ind. / cap.; +/- 0.85 - Acceptable
Power Factor Adjustable: 0.9 leading ... 0.9 lagging; 0.9-1 ind. / cap.; +/- 0.9 – Acceptable
Power Factor Adjustable: 0.95 leading ... 0.95 lagging; 0.95-1 ind. / cap.; +/- 0.95 – Unacceptable
18
Active Power Curtailment
Purpose:
The purpose of active power curtailment is to provide the PV system owner as well as the electric
utility the ability to manage the amount of active power injected into the power system. If more power is
provided to the power system than is required, generation will need to be reduced as this situation places
utility and customer equipment at high risk for damage. Older solar inverters can either operate at full
output power, or remain off. Therefore, in this situation these inverters would be shut off. This means the
solar installation would be restricted from even supplying the amount of power necessary for the facility
at which it resides. Smart solar inverters with active power curtailment capabilities allow the inverters to
generate a fraction of their total rated output power. With this capability it is guaranteed that the solar
installation will produce the maximum amount of power possible without being tripped offline, providing
the installation owner with the maximum amount of revenue.
PPL Inverter Requirement:
APC-1.1:
Solar inverters connecting to PPL Electric Utility’s power system shall possess active
power curtailment capabilities with the ability to vary power output in increments of 5%
or less.
19
Ramp Rates / Slow Ramp (Soft Start)
Purpose:
The change in output power level of inverters is referred to as ramping. Ramp rate controls allow
solar inverters to transition smoothly between output power levels at a predetermined pace. The situation
in which this is most beneficial occurs during the initial powering up of the inverter. As the sun rises, the
amount of irradiance absorbed by solar panels slowly increases, thereby increasing the output power of
the panels at the same rate. When the panel output power reaches a certain value, the inverter turns on. In
older style inverters, this turn on resulted in an abrupt increase in output inverter power. This rapid
change can be dangerous for the power system and can result in large scale customer disconnections. For
this reason, new smart inverters have the capability to slowly increase their output power level to avoid a
damaging start up transient. This capability is referred to as slow ramp or soft start.
PPL Inverter Requirement:
RR-1.1:
Solar inverters connecting to PPL Electric Utility’s power system shall possess slow
ramp (soft start) capabilities.
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Level 3 Smart Requirements
Solar inverters in operation at all solar installations with generating capacity greater than or equal to
100kW connected to PPL Electric Utility’s power system shall meet level 3 smart inverter requirements.
Communication
Purpose:
Utility communication with solar inverters has significant benefits for both the utility and the
installation owner. Communication capabilities will allow the utility to remotely disconnect inverters
during situations which could cause damage to power system equipment, customer facilities, and
customer solar installations. Furthermore, these capabilities will allow the inverters to share their
operating measurements (i.e. output power, output current, line voltage, etc.) and operating parameters
(i.e. set points for active power, reactive power, fault ride through). Using the operating measurements,
the utility can vary the operating parameters in an attempt to improve the reliability of the circuit,
ultimately increasing the amount of time inverters stay online and generate revenue.
PPL Inverter Requirement:
COM-1.1:
Solar inverters connecting to PPL Electric Utility’s power system shall communicate
using either the MODBUS or DNP3.0 application protocol.
COM-1.2:
Solar inverters connecting to PPL Electric Utility’s power system shall communicate
using either the TCP/IP or UDP transport protocol.
COM-2.1:
Solar inverters connecting to PPL Electric Utility’s power system shall possess either an
RJ45 Ethernet or an RS-485 serial communication port.
COM-2.2:
Solar inverters connecting to PPL Electric Utility’s power system which only have an
RS-485 serial communication port will require a controller to convert the RS-485 serial
signals to RJ45 Ethernet signals.
COM-2.3:
Solar inverters connecting to PPL Electric Utility’s power system shall communicate
over our network via an external 900 MHz radio or CalAmp cellular modem. The
decision on which method of communication will be implemented will be made by PPL.
**Customers should contact PPL to determine whether their installation will communicate using a
900MHz radio or a CalAmp cellular modem. This decision will be based on the geographical location of
the installation.
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Additional Information
Micro-Inverters
A micro-inverter is not necessarily a “smart capability,” but rather a “smart technology” in the
world of inverters. As the name suggests, micro-inverters are very small inverters which are designed to
manage only one panel. This means that under this configuration, every panel in an installation will
require a micro-inverter. Some argue that since each micro-inverter manages only one solar panel and
performs MPPT on each individually, the solar installation as a whole will be more efficient. Studies
concerning this claim are inconclusive.
Micro-inverters are not a new technology, as past attempts to introduce them into the world of PV
technology have been made. These attempts were unsuccessful due to the significantly low reliability and
high maintenance costs of those generations of micro-inverters. The newer generation has not been in
existence long enough to determine whether improvements have been made; however, manufacturers
claim that the lifetime and maintenance costs are comparable to larger string and central inverters.
Furthermore, very few micro-inverters possess the smart capabilities discussed in earlier sections of this
document.
The argument of increased efficiency stems from the ability of micro-inverters to perform MPPT
on each individual panel as mentioned earlier. Solar panel strings can be thought of as similar to a string
of Christmas lights whereas if there is one bad panel in the string, the rest of the string is restricted by that
panel. The more solar energy a panel absorbs,
the more electrical current it can produce. If
one panel in a string is covered by a cloud, it
will produce significantly less current than the
other panels could potentially produce;
however, since all of the current in the string
must pass through that shaded panel, the total
current in the string is limited. The diagrams to
the right illustrate this argument. With a string
configuration, the string inverter would attempt
to extract the maximum power from the string
as a whole, which is limited by the shaded
panel. Micro-inverters would extract the
maximum power from each panel individually
meaning the shaded panel will not inhibit the
power production of adjacent panels.
For the reasons detailed above, PPL does not recommend using micro-inverters for a solar
installation unless the installation is subject to complex shading conditions. No standards in addition to
those provided in this document, however, are enforced for the use of micro-inverters.
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Inverter Temperature Ranges
All inverters have an operable temperature range rating, but the implications of this rating can
prove rather elusive. There are a range of possible scenarios which could occur upon exiting this range
including damage to the inverter, production of poor quality power affecting the customer facility and
other facilities connected to the power system, and partial or complete curtailment of output power.
Thankfully, the third scenario will occur for all modern inverter designs.
Due to the occurrence of the third scenario, PPL does not enforce any regulations for temperature
range. Inverters are designed to maintain a certain degree of power quality within their rated temperature
range. If an inverter is unable to produce the required quality of power, it will reduce output or cease
production entirely. Since the power system and customer facilities are unaffected in these situations,
there is no concern from the utility standpoint. However, customer revenue is at risk of being affected by
an inverter with too narrow of a temperature range.
An understanding of the true meaning of the temperature rating is vital to this discussion. The
rating is not the ambient temperature as one may expect, but instead is the temperature of the inverter.
This is typically measured on the inverter heat sink. For this reason, careful placement of the inverter is
recommended. For example, on a day with a considerably high ambient temperature, an inverter which is
operating at its maximum output and is completely exposed to the sun may operate dangerously close to
its upper temperature limit. Under these circumstances, the inverter will likely curtail its output power
resulting in a loss of revenue for the customer. Placing the inverter in a shaded location may help to
alleviate this concern.
Several scenarios may occur when the inverter infringes upon the boundaries of its temperature
rating. On the low end, there is not much concern for power curtailment as electronics typically are
unaffected by low temperatures. If the ambient temperature is below the lower boundary, oftentimes
inverters will not turn on, but if the inverter is already operating and the temperature approaches or
crosses the lower boundary, output power will likely be unaltered. The high end of the boundary is where
deviations in operating procedure are probable. Most inverters will reduce output power as they near the
upper boundary, and shut off entirely when that level is reached. Different combinations of power
curtailment and shut off schemes are possible depending on the inverter manufacturer, but it can be
assured that high operating temperatures result in high probability of decreased customer revenue.
Most inverters have temperature ranges of -13⁰F to 140⁰F or greater. These ranges will far exceed
ambient temperatures typical of Pennsylvania’s climate. Other inverters have temperature ranges which
are slightly less, such as -4⁰F to 120⁰F. Ambient temperatures outside of this range are highly unlikely but
not impossible. Inverters may exist which have a smaller temperature range than -4⁰F to 120⁰F, but to
avoid considerable losses in revenue during times of extreme temperatures, PPL does not recommend
purchasing an inverter with such a limited range. As mentioned, the temperature monitored by the
inverter is the operating temperature, but ambient temperature plays a large role.
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Inverter Warranties
Solar inverters are designed to operate for many years with minimal maintenance, which is
evident in the rather long warranties of inverters. Examining the warranty is a good way to determine the
quality of the inverter. Nearly every inverter in the accepted model list found on the REMSI page offers a
standard warranty of 10 years. Some lower powered inverters have a standard warranty of 5 years, which
is still acceptable for lower powered devices. Most, including those with only a 5 year standard warranty,
have extended warranties of 15 to 20 years. When shopping for solar inverters, one will quickly find that
these lengthy warranties are very common amongst all reputable inverter brands. PPL recommends
avoiding inverters whose warranties are less than those discussed in this section.
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Glossary
Active Power (Real Power) – power consumed by resistive elements of a circuit which performs useful
work.
Alternating Current (AC) – Flow of electric charge which periodically reverses direction. This is the type
of electricity which flows on power lines since it can be easily transmitted over long distances.
Current – The flow of charge.
Direct Current (DC) – Unidirectional flow of electric charge. Solar panels will produce direct current
electricity which must be converted to alternating current by an inverter in order to use in a household or
on the electrical grid.
Distributed Energy Resource(s) (DER) – Smaller power sources located near points of electrical power
consumption (i.e. not connected to the bulk power system).
Distributed Generation/ Generator (DG) – Electricity generated by a distributed energy resource.
Fault – An abnormal electric current. On the electrical power system, this is usually caused by an
electrical path between wires or to ground.
Generating Capacity – The maximum electrical output a photovoltaic installation is able to produce.
Grid-Tie Inverter – An inverter which possesses the ability to output power to an electrical power grid. A
Grid-Tie inverter meets standards for safely connecting to or disconnecting from the power grid based on
monitored grid conditions, as well as ensuring that the power generated by the solar installation is
compatible with the grid power in terms of voltage level, frequency, and phase.
Inverter – Electronic device used to change direct current to alternating current.
Island – A section of the electrical distribution line which has been disconnected from the main generator
(i.e. larger nuclear or coal facility) due to operation of a protection or sectionalizing device, and is being
energized by distributed energy resources.
Islanding – The act of distributed energy resources creating an island.
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Maximum Power Point Tracking (MPPT) – ability of the inverter to track and extract the maximum
output power of a collection of solar panels for a given condition.
Parallel – Wiring such that current can divides into multiple paths. (Ex. For a central inverter, each string
of panels is connected in parallel)
Photovoltaic Effect – Ability of a semiconductor to generate direct current electricity when exposed to
light.
Photovoltaics (PV) – Method of generating direct current electricity by exploiting photovoltaic effect of
some semiconductor materials.
Reactive Power – power which is stored and discharged in inductive and capacitive circuit elements and
performs no useful work.
Series – sequentially; one following another. If you start with one solar panel, wire the output of that
panel to the input of another panel and continue to do this with several panels, you have wired these
panels in series. The output of each panel passes through all of the panels past it in the series. (Ex. For a
string inverter, all of the panels attached to the inverter are connected in series.)
String – group of solar panels wired in series.
Voltage – The driving force in an electrical circuit.
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Contact PPL
For any general questions or concerns pertaining to the solar inverter requirements, please submit
an online REMSI contact form which can be found at https://www.pplelectric.com/at-yourservice/electric-rates-and-rules/remsi/contact-us.aspx. Submissions will be directed to the appropriate
PPL employee who will address the issue promptly.
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