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!