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