Renewable Interconnection
Standard & Experimental Tests
Yahia Baghzouz
UNLV
Las Vegas, NV, USA
Overview
IEEE Std 1547
◦ Voltage limitations
◦ Frequency limitations
◦ Harmonic limitations
Expansion of IEEE Std 1547
◦ 1547-1 through 1547-8
Anti-islanding
◦ Passive methods
◦ Active methods
Islanding tests on local inverters
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
Interconnection System Response to
Abnormal Frequencies
Maximum Harmonic Current Distortion
Synchronization Parameter Limits for
Synchronous Interconnection
Maximum Voltage Distortion for Synchronous
Machines
Expansion of IEEE Std. 1547 (http://grouper.ieee.org/groups/scc21)
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
Medium Voltage Standard in Germany (10 kV-110 kV)
Medium Voltage Standard in Germany (10 kV-110 kV)
Medium Voltage Standard in Germany (10 kV-110 kV)
Inverter Response to Power Outage - Test
Islanding occurs when the disconnected part of the
power network is sustained by the connected PV
systems for a significant period of time.
Islanding is not desirable for several reasons:
◦ Creation of a hazard for utility line workers by
touching a line that is supposed to be de-energized,
◦ lack of control over voltage and frequency in the
island.
◦ Interference with restoration of normal service.
Expected inverter response
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.
PV System A: 18 kW Fixed Array (installed in 1999)
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)
PV System B: 25 kW
2-Axis Tracking (installed in 2003)
Inverter Manufacturer: Advanced Energy Systems, LTD
Inverter Rating: 30 kVA, 120/208V, 3-Phase.
Anti-islanding technique for critical case: unknown
PV System B Switching Events
Event
No.
Case PS
(kW)
PD
(kW)
QS = -QD
(kVAR)
B.1 D -4.7 20.3 -1.2
B.2 B 4.7 20.3 -1.3
B.3 E 0.1 20.4 -1.2
Event B.1 (α = -0.26, β = -1 )
Event B.2 (α = +0.26, β = -1)
Event B.3 (α ≈ 0, β = -1)
Test Summary
Both systems shut down in less than 5 cycles after the
utility outage – in compliance with the Interconnection
standard that allows up to 10 cycles.
◦ The over-voltage and under-voltages relays isolated the inverter
when there was mismatch between the PV power and local load
power.
◦ The under-frequency relay isolated the inverter when there was a
match between the PV power and local load power.
◦ Although small, the reactive power generated by both inverters will
ultimately drive the frequency of the islanded system to zero, thus
triggering the under-frequency relay under all resistive load
conditions.
◦ An adjustable reactive load (in addition to the resistive load) would
be needed to match both real and reactive powers and test for
islanding under zero deviation in both voltage and frequency.
SIMPLE 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.
Break!