Post on 24-Mar-2020
Interconnection Standard for
Distributed Resources
Yahia Baghzouz
UNLV
Overview
IEEE Std 1547 & 1547a
◦ Voltage limitations
◦ Frequency limitations
◦ Harmonic limitations
Anti-islanding
◦ Passive methods
◦ Active methods
Some tests on local inverters
Expansion of IEEE Std 1547
Publication Year: 2003
IEEE Std. 1547 scope and purpose
Scope: The standard establishes criteria and
requirements for interconnection of distributed
resources (DR) with electric power systems (EPS).
Purpose : The document provides a uniform
standard for interconnection of distributed
resources with electric power systems. It provides
requirements relevant to the performance,
operation, testing, safety considerations, and
maintenance of the interconnection.
Interconnection System Response to
Abnormal Voltages (1547)
Interconnection System Response to
Abnormal Voltages (1547a)
Interconnection System Response to
Abnormal Frequencies (1547)
Interconnection System Response to
Abnormal Frequencies (1547a)
Maximum Harmonic Current Distortion
Synchronization Parameter Limits for
Synchronous Interconnection
Maximum Voltage Distortion for Synchronous
Machines
Grid-Tied PV Inverters
Grid-tied inverters
• monitor the PV array and track the maximum power
and operate at that point,
• sense the presence of the grid, synchronize to it, and
inject a current in phase with the voltage,
• monitor the grid and disconnect in case of trouble (e.g.,
large swings in voltage or frequency).
INVERTER
Functions of Grid-Tied PV Inverters
Maximum Power Tracking Grid synchronization Grid Monitoring - Disconnect
Centralized PV inverter configuration
Lower cost, less reliable (single point of failure), more
losses due to string diodes.
PV panels in strings with individual inverters
Increased cost, higher reliability, no additional losses from string diodes.
Panels with individual inverters
Most costly, highest reliability, easy expansion.
Single-phase single-stage inverter
The capacitor on the DC side is used as a buffer and to limit current distortion.
The full-bridge converter converts the DC voltage to AC by PWM. A PLL is used to synchronize with the utility voltage.
The LC filter reduces the harmonic content of the inverter output signals
The PV array is connected to the utility grid through an electrical isolation transformer. A bulky low frequency transformer is required in this situation.
Single-phase multi-stage inverter
The inverter converts DC to high-frequency AC, then back
to low-frequency AC.
The high-frequency transformer is more efficient and much
lighter that the low-frequency transformer.
Maximum Power Point Tracking (MPPT)
Tracking the maximum power point of a PV array is an essential
task of the inverter. Due to its simplicity, the perturb-and-observe
(P&O) algorithm is perhaps the most commonly used in practice
for MPP tracking.
◦ It is based on the fact that when the DC voltage is perturbed by a
small increment dV, a change in dP takes place due to the nonlinear I-
V characteristic of a photovoltaic array.
◦ Based on the sign of dP, the voltage is then perturbed up or down
and the process is repeated until maximum power is reached.
Perturbations are repeated on a periodic basis.
Maximum Power Point Tracking (MPPT)
Incremental Conductance Method
Expected inverter response to utility
voltage and frequency disturbances
PV inverter manufacturers market inverters that are
expected to meet current interconnection standards (i.e.,
IEEE Std. 1547): They are expected to
◦ disconnect within 10 cycles if the voltage drops below 50% or rises
above 120% of its nominal value.
◦ disconnect within 10 cycles if the supply frequency drops below
59.3 Hz or rises above 60.5 Hz.
◦ disconnect within 2 seconds cycles If the voltage drops to a value
between 50%-88%,
◦ disconnect within 1 second if the voltage rises to a value between
110%-120% of the nominal value.
Inverter islanding detection
Standard protection of grid-connected PV systems
consists of four relays that will prevent islanding under
most circumstances.
◦ over-voltage relay,
◦ under-voltage relay,
◦ over-frequency relay,
◦ under-frequency relay.
However, if the local load closely matches the power
produced by the inverter, the voltage and/or frequency
deviations after a power outage may be too small to
detect, i.e., fall within the non-detection zone.
In this case, additional schemes are required to minimize
the probability of an island to occur.
Voltage and frequency deviations
RVPD /)1( 2 LVQD /)1( 2
VV
1
1'
1
1'
After utility disconnect,
Let the ratio of PS/PD = α, and QS/QD = β.
Before disconnect,
Possible Cases
Case A: PS > 0 and QS > 0: The voltage decreases. The frequency
depends on the values of α and β.
Case B: PS > 0 and QS < 0: Both the voltage and frequency
decrease.
Case C: PS < 0 and QS > 0: Both the voltage and frequency
increase.
Case D: PS < 0 and QS < 0: The voltage increases. The frequency
depends on the values of α and β.
Case E: PS = 0 and QS ≠ 0: The voltage remains constant, while
the frequency changes (decreases if QS < 0 or increases if QS > 0).
Case F: PS ≠ 0 and QS = 0: The frequency remains constant, while
the voltages changes (increases PS < 0 or decreases if PS > 0).
Case G: PS = 0 and QS = 0: Both the voltage and frequency
remain constant.
Common Active Anti-Islanding Techniques
Voltage harmonic monitoring: inverter monitors voltage total harmonic distortion and shuts down if this parameter exceeds some threshold.
Phase jump detection: phase between inverter's terminal voltage and its output current is monitored for sudden jumps.
Slide-mode frequency shift: the voltage-current phase angle of inverter is made a function of system frequency.
Impedance measurement: perturbation periodically applied to inverter current. This will force a detectable change in voltage if the utility voltage is disconnected.
Active frequency drift: inverter uses a slightly distorted output current to cause the frequency of the voltage to drift up or down when utility is disconnected.
Islanding Test on18 kW PV System
Inverter Manufacturer: Trace Technologies Inc.,
Rating: 30 kVA, 120/208V, 3-Phase.
Anti-islanding technique for critical case: unknown
Schematic diagram of Test Circuit
Test Procedure and Apparatus
Connect the transient recorder, load bank, and meters for reading current or power flow into the load and utility grid as shown in Figure.
Adjust the load bank to the desired fraction of load relating to generated power.
Open the utility disconnect while recording the voltage and current waveshapes.
Repeat the two steps above for different generation-load power mismatch levels.
PV System A Switching Events
Event
No.
Case PS
(kW)
PD
(kW)
QS= -QD
(kVAR)
A.1 D -9.8 14.8 -0.8
A.2 B 4.9 15.1 -0.9
A.3 E 15 14.9 -0.8
Case B: PS > 0 and QS < 0: the voltage decreases and frequency → 0.
Case D: PS < 0 and QS < 0: The voltage increases, and frequency → 0.
Case E: PS = 0 and QS < 0: The voltage remains constant, and frequency → 0.
Event A.1 (α = -0.66, β = -1)
Event A.2 (α = +0.32, β = -1)
Event A.3 (α ≈ 0, β = -1)
TEST ON 2 KW PV SYSTEM
DC-side measurement
AC-side measurement
Response to an overvoltage ( V < 1.2 pu)
Response to a large overvoltage (V > 1.2 pu)
PV Array Size: 2 kW (peak)
DC-SIDE VOLTAGE AND CURRENT
The inverter utilizes the Perturb-and-Observe method for
MPPT. The voltage is perturbed by nearly 4 V, or 1.5% of
the nominal value every 2 seconds.
AC-SIDE VOLTAGE AND CURRENT
The AC current THD measures nearly 4% (the limit is 5%).
INVERTER RESPONSE TO 14% OVERVOLTAGE
The inverter shut down after 56 cycles. The inverter is
in compliance with IEEE Std. 1457 which allows up to a
maximum of 60 cycles for 1.1 < V < 1.2 p.u.
INVERTER RESPONSE TO 33% OVERVOLTAGE
The inverter shut down within 8 cycles. The inverter is in
compliance with IEEE Std. 1457 which calls for a maximum of
10 cycles for V > 1.2 p.u.
Variability and Uncertainty of PV Power
Photovoltaic resources differ from conventional and fossil-fired resources in a fundamental way: their fuel (sunlight) cannot presently be controlled or stored.
There are two major attributes of variable generation that notably impact the bulk power system planning and operations:
◦ Variability: The output of variable generation changes according to the availability of sunlight, resulting in fluctuations in the plant output on all time scales.
◦ Uncertainty: The magnitude and timing of variable generation output is less predictable than for conventional generation.
PV Power Variability
Display of power generated by a small PV array on two
different days in February.
• Red curve: large power fluctuations on a cloudy day,
• Black curve: smooth power production on a clear day.
PV Power variability of local 14 MW plant
Due to lack of inertia, PV power can change by up
50% in 0.5-1.0 minute time frame, and by up to 70% in
2-10 minute time frame, many times per day!
Relation between solar irradiance and power produced
by a small PV system on a cloudy day
In a small PV system, global solar
irradiance and power production
are stronly correlated (Correlation
Coefficient ≈ 1)
≈ 4 hours
1 second sampling rate
The power is proportional to the spatial average irradiance of
several sensors, rather than the reading of a single sensor.
Source: Kuszmaul, S., A. Ellis, J. Stein, L. Johnson, Lanai High-Density Irradiance Sensor Network for Characterizing Solar Resource Variability of MW-Scale PV System, 35th IEEE PVSC, Honolulu, HI, 2010
Relation between solar irradiance and power
produced by a 6 MW PV system on cloudy day
Example of clouds over DeSoto PV Site (Central FL)
System Type: Single-Axis Tracking
System Rating: 25 MW
Solar variability is influenced by cloud type, size and speed
Solar variability is influenced by cloud type,
size, and speed
Variability and Uncertainty of PV Power
Power system planners and operators are already familiar with
designing a system which can be operated reliably while
containing a certain amount of variability and uncertainty,
particularly as it relates to system demand.
However, large scale integration of variable generation can
significantly alter familiar system conditions due to unfamiliar
and increased supply variability and uncertainty.
Time Time
without PV with large scale PV
Call for Expansion of IEEE Std. 1547
Recommended Practice for Establishing Methods and Procedures that
Provide Supplemental Support for Implementation Strategies for Expanded
Use of IEEE Standard 1547
1547.8 Pending
2011
2012
2011
2013
2014