*Eastern Interconnection Phasor Project*

Performance Requirements Task Team (PRTT)

Performance Requirements

**Phase Angle Installation Issues**

*Task Team Leader:*

Damir Novosel, KEMA

*Prepared by:*

Virgilio Centeno, Virginia Tech

*Contributors:*

Gerard Stenbakken, NIST

Henry Huang, PNNL, CERTS

# Table of Content {#table-of-content .unnumbered}

#  {#section .unnumbered}

[1. Glossary of Terms 3](#glossary-of-terms)

[2. Introduction 4](#introduction)

[2.1 Brief overview of the problem 4](#brief-overview-of-the-problem)

[2.2 Organization of this document 4](#organization-of-this-document)

[3. Installation Phase Errors: 4](#installation-phase-errors)

[3.1 Incorrect voltage or current type
4](#incorrect-voltage-or-current-type)

[3.2 Incorrect phase sequence 5](#incorrect-phase-sequence)

[3.3 Incorrect phase 'a' determination
5](#incorrect-phase-a-determination)

[3.4 Incorrect Polarity: 6](#incorrect-polarity)

[3.5 Delta-Wye transformation in the instrument transformers:
6](#delta-wye-transformation-in-the-instrument-transformers)

[4. System phase shifts 7](#system-phase-shifts)

# Glossary of Terms

This section provides some useful definitions pertinent to
GPS-synchronized devices, communication protocols and communications
media.

**GPS** -- Global Positioning System. A satellite based system for
providing position and time. The accuracy of GPS based clocks can be
better than 1 microsecond.

**Negative Sequence:** A three phase sequence in which phase a lags
phase 'b' by 120 degrees and phase 'c' lags phase 'b" by 120 degrees.

**PMU** -- Phasor Measurement Unit. A device that samples analog voltage
and current data in synchronism with a GPS-clock. The samples are used
to compute the corresponding phasors. Phasors are computed based on an
absolute time reference (UTC), typically derived from a built-in GPS
receiver.

**Positive Sequence:** A three phase sequence in which phase 'a' leads
phase 'b' by 120 degrees and phase 'b' leads 'c' by 120 degrees.

**Reference Angle:** A measured or estimated angle which is subtracted
from all angles to obtain a common reference. In a case of two angle
measurement the remote angle is assumed to be the reference angle.

**Referenced Angle:** Angle difference obtained when the reference angle
is subtracted from a given PMU angle.

**UTC** -- Coordinated Universal Time (initials order based on French).
UTC represents the time-of-day at the Earth\'s prime meridian (0°
longitude).

# Introduction

## Brief overview of the problem

When installing a phasor measurement unit, there are several possible
sources of large phase and magnitude errors due to incorrect wiring of
the input signals, different phase 'a' definition across utility
boundaries, different input phases to the PMU, and different phasor
quantities reported. Local information can be used to detect some of
these errors but most errors require the use of a reference angle or
additional information obtained from other properly wired devices. In
this document it is assumed that error checking is performed at the
control center and that the person installing the unit is in contact
with the control center. Expected angle differences and magnitudes are
obtained from the state estimator.

## Organization of this document

This document describes some of the more common individual installation
errors that may result in the measurement of incorrect phase angle
differences. For each individual error symptom, explanations and
corrective actions of the error are given. Compound errors are possible
but to reduce confusion, the user is advised to consider the possibility
of compound errors but to correct one individual error at a time and
then check for additional errors.

#  Installation Phase Errors:

## Incorrect voltage or current type

**Symptoms:**

a\) Voltage magnitudes 70% larger than expected

b\) Referenced voltage angles appear to be displaced by +30 degrees

c\) Current magnitudes 40% smaller than expected

d\) Referenced current angles appear to be displaced by +30 degrees

**Corrective Action:**

-   Voltage inputs should be wired as line to neutral values and any
    line to line voltage should be rewire as line to neutral. Current
    inputs should be wired as line currents and any delta current should
    be re-wired as a line current.

**Explanation:**

All voltage inputs to the PMU should be line to neutral voltages and all
current inputs should be line currents. A line to line voltage has a
magnitude equal to 1.73 times its corresponding line to neutral voltage.
A delta current has a magnitude equal to 0.577 times its corresponding
line current. More important, line to line voltages and the delta
currents have their angle displaced by +30 degrees with respect to their
corresponding line to neutral voltage and line current values.

## Incorrect phase sequence

**Symptom:**

Zero or close to zero magnitude of the positive sequence phasor while
the magnitude of the individual phases is not zero.

**Corrective Action:**

-   At substation, identify phase \'b\' and \'c\' and swap their
    connection.

-   Check for correct phase 'a' determination.

**Explanation:**

PMUs are set to compute positive sequence phasors out of three phase
signals normally labeled \'a\', \'b\' and \'c\'. The positive sequence
is obtained by rotating the phase \'b\' phasor by 120 positive degrees,
rotating the phase \'c\' phasor by 120 negative degrees, adding the two
results to the phase \'a\' phasor, and dividing the result by three,
Figure 1.

![](media/image1.wmf){width="4.2243055555555555in" height="1.58125in"}In
a perfectly balanced system and for a correctly wired unit the result of
these operations is a phasor equal to the phase 'a' phasor. Any error in
the identification of the system phases will cause two of the phases to
be connected incorrectly. If this happens the three phase system will be
wired as a negative sequence to the PMU and the signal will be filtered
by the positive sequence algorithm resulting in a zero or close to zero
magnitude.

This installation error does not need remote information for its
diagnosis. Most PMUs provide information on the magnitude of the
positive sequence phasors and the magnitude of the individual phase
signals. If the magnitude of the positive sequence phasor given by the
PMU display is zero but the magnitude of the individual phases is not
zero, an incorrect sequence wiring is the most likely cause.

## Incorrect phase 'a' determination

**Symptoms:**

a\) Referenced voltage phase angle is close to ± 120 degrees from the
expected angle.

b\) Disagreement between the real power flows and the measured angle
differences. Power always flows from the high angle to the low angle.

**Corrective action:**

-   For a positive angle difference, input signal should be re-wire to
    rotate phase \'c\' to \'a\', \'a\' to \'b\' and \'b\' to \'c\'.

-   For a negative angle difference, input signals should be re-wire to
    rotate phase \'a\' to \'c\', \'b\' to \'a\', and \'c\' to \'b\'.

**Explanation:**

Phase \'a\' is usually known at the substation and it should be properly
wired. If for any reason phase \'a\' is mistaken for one of the other
phases the positive sequence algorithm will be computed with respect to
the incorrect phase \'a\'. If phase \'b\' is wired as phase \'a\', with
the correct phase sequence, a phasor with an error of negative 120
degrees will be computed. If phase \'c\' is wired as phase \'a\', with
the correct sequence, the computed phasor will have a positive 120-phase
shift.

## Incorrect Polarity:

**Symptoms:**

a\) Referenced Angles appear to be 180 degrees out of phase

b\) Incorrect agreement of locally referenced current angles with
measured real and reactive power flows. (Real and reactive flows not
measured by the same unit)

**Corrective Action:**

-   Polarity of all connection should be checked and re-wire if an error
    is found or suspected.

**Explanation:**

In the unlikely case that a signal is wired with the wrong polarity, the
magnitudes of the phasors will show the correct values but the
referenced angles will be 180 degrees out of phase. Any real or reactive
flow computed with an incorrectly wired voltage or incorrectly wired
current will give real and reactive flows in opposite direction to the
actual flows.

## Delta-Wye transformation in the instrument transformers:

**Symptoms:**

a\) Phase angle degrees of 30 or -30 degrees.

b\) Phasor magnitudes 73% higher than expected.

c\) Phasor magnitudes of 58% smaller than expected.

**Corrective Actions:**

-   The primary wiring of the instrument transformers should be compared
    with the wiring of the secondary input to the PMU. Primary and
    secondary windings should be wired in the same type of connection,
    delta-delta or wye-wye.

-   A 73% higher than expected magnitude indicates a wye primary and a
    delta secondary connection which will also result in a positive
    30-degree phase shift.

-   A 57% smaller than expected magnitude should be accompanied by a
    negative 30-degree phase shift and indicates a delta connected
    primary and a wye connected secondary.

**Explanation:**

In the rare case of a delta-wye transformation in three phase instrument
transformers, a phase shift of 30 degrees and a magnitude error factor
of square of root of three (1.73) will be observed in the measured three
phase signal. Delta-Wye transformation are used to obtained specific
signals for relaying operations but are not needed for PMU measurements.
The PMUs assume phase to neutral voltages and line currents as input
signals.

# Non Wiring Errors

## System phase shifts

**Symptoms:**

a\) Phase angle degrees of 30 or -30 degrees.

b\) Phasor magnitudes 73% higher than expected.

c\) Phasor magnitudes of 58% smaller than expected.

**Action:**

This error should be documented and corrected only at the data
concentrator or application program.

**Explanation**

Transformers at the substation may be wired with a delta-wye or
wye-delta transformation. This transformation will result in an error
factor of square root of three in the magnitude and the 30-degree shift
on the angle between primary and secondary signals. For PMU signals all
voltage inputs should be phase to neutral voltages and all currents
should be line currents.

## Unbalanced System

**Symptoms:**

a)  Small magnitude errors

b)  Small angle oscillations

**Action:**

This error should be documented but no action should be taken.

**Explanation**

Magnitude unbalance results in magnitude errors and negative sequence
components in the signal. A 10% unbalance in one phase results in a 3.3%
magnitude error in the computed Phasor. State estimator values are
computed with single phase measurements and may not be affected by the
unbalance. Unbalance input also results in a small negative sequence
component that shows as a small oscillation in the angle.

# Quick Reference Chart for Wiring Errors

+-----------------------------------+-----------------------------------+
| **Symptom**                       | **Probable Cause**                |
+-----------------------------------+-----------------------------------+
|                                   |                                   |
+-----------------------------------+-----------------------------------+
| **Incorrect Phase angle**         |                                   |
|                                   |                                   |
| **Apparently correct Magnitude**  |                                   |
+-----------------------------------+-----------------------------------+
| ± 30 degree phase shift           | Incorrect Signal type             |
|                                   |                                   |
|                                   | Delta Wye Transformation          |
+-----------------------------------+-----------------------------------+
| ± 120 degree phase shift          | Incorrect Phase "a" determination |
+-----------------------------------+-----------------------------------+
| 180 degree phase shift            | Incorrect Polarity                |
+-----------------------------------+-----------------------------------+
| **Incorrect Phase angle and**     |                                   |
|                                   |                                   |
| **Incorrect Magnitude**           |                                   |
+-----------------------------------+-----------------------------------+
| ± 30 degree phase shift           | Incorrect Signal type             |
|                                   |                                   |
|                                   | Delta Wye Transformation          |
+-----------------------------------+-----------------------------------+
| Close to Zero or Zero Magnitude   | Incorrect Phase Sequence          |
|                                   |                                   |
| of Positive Sequence phasor       |                                   |
+-----------------------------------+-----------------------------------+