Hydro One characterizes power quality performance in accordance with measurement and assessment protocols that reflect current industry consensus as articulated in relevant international standards.


Power quality characteristics can be classified into two basic categories:

1)   Steady-state (continuous) — This refers to the averaged characteristic of the relevant electrical parameter, typically voltage, over the longer term. How much can the voltage magnitude vary from the nominal value? How distorted is the voltage waveform? What is the imbalance among the three-phase voltages? Standards define protocols for quantifying each of these aspects of Power Quality.

2)   Disturbances (including reliability) —  This refers to momentary variations that can occur at random intervals, including sustained interruptions (reliability), momentary interruptions, voltage sags, voltage swells, and transients. All of these disturbances can impact a facility to an extent that depends on the sensitivity of equipment and/or processes at the facility.

Each of these basic categories has indices associated with it that are quantified in accordance with accepted industry protocols such that they provide a foundation for characterizing delivered power quality in a consistent and unambiguous manner. The indices can then be used to establish baseline performance levels as a function of system characteristics or particular aspects of individual delivery points. Table 1 below provides a brief description of these indices along with applicable industry standard(s) that Hydro One relies upon to govern their characterization. Typical planning and compliance targets (or thresholds) are also identified to indicate performance that can be expected in practice, according to Hydro One's internal policies, governing regulations and industry norms.


​Table 1

PQ Parameter Definitions and Reference Levels


​PQ Parameter
Planning / Compliance Target(s)​
Reference Regulation / Industry Standard(s)​

Power Frequency Variation​

​Frequency range of supplied voltage waveform, quantified as the mean value over 10 s intervals

< +/- 0.5 Hz ​

NPCC Directory 12​

Steady-state long duration (> 1 min) voltage variation​

​Range of rms value of supplied voltage, quantified as the mean value over a prescribed time interval and expressed as a percentage of nominal

TSC Appendix 2:
In accordance with IESO Market Rules Appendix 4.1

+/-6% of nominal voltage (for voltage levels < 50 kV)

Table 2 below (for voltage levels < 1 kV) ​

TSC Appendix 2

IESO market rules Appendix 4.1

CSA Standard CAN3-C235-83 (reaffirmed in 2015).

Voltage Unbalance​

A condition in a 3-phase system in which the rms values of the line-to-line voltages (fundamental frequency component), or the phase angles between consecutive line voltages, are not all equal. The degree of inequality is usually expressed as the ratio of the negative sequence components to the positive sequence component.

​Voltage unbalance is considered in relation to long term effects, i.e. for durations of 10 min or longer

TxTSC Appendix 2:

Up to 2% - 95% of the time over minimum 1 week interval

According to CAN/CSA E 1000 2-2-97

Up to 3% (per IEC for LV and MV networks)


​NOP-32 (internal HONI policy)
(for 1-35 kV Systems)

(for voltage levels < 1 kV)

​Voltage Harmonics

​A measure of the distortion present in the normally 60 Hz sinusoidal nature of the waveform. This is usually expressed in terms of the relative magnitude of individual harmonic orders (frequency components) or as Total Harmonic Distortion (THD) reflecting aggregated contribution of multiple harmonic components.

Table 4 below

- Long term effects (arise from harmonics levels that are sustained for 10 min or more):

Table 3 below for compatibility levels
Long term THD limit = 8%

- Short term effects

Table 3 below multiplied by a scale factor K (given in CAN/CSA-C61000-2-2:04) for compatibility levels

Short term THD limit = 11%

​IEEE 519 2014

IEEE 519 2014

CAN/CSA-C61000-2-2:04 (re-affirmed 2014)

Voltage Flicker​

​Repetitive or sudden fluctuations of voltage magnitude that results in variations in light intensity from electric luminaires, particularly incandescent lighting, perceptible to the human eye as “flicker”. These fluctuations may be produced by fluctuating loads, operation of transformer tap changers and other operational adjustments of the supply system or equipment connected to it.

In normal circumstances the value of rapid voltage changes is limited to 3% of nominal supply voltage. However step voltage changes up to 4% can occur infrequently (for example due to capacitor switching operations in the supplying network)

Voltage fluctuations occurring in low voltage networks are termed “flicker”. Flicker severity is calculated with respect to both short term (< 3 s) and long term (> 10 min) effects.

​Planning Level:

Short-term flicker Index (Pst):

HV: <= 0.8
MV: <= 0.9

Long-term flicker Index (Plt)HV: <= 0.7
MV: <= 0.6

Compatibility Level for LV

Pst <= 1.0
Plt <= 0.8


CAN/CSA - C61000-2-2 Flicker severity is measured in accordance with CAN/CSA 61000-4-15.

Voltage Sag​

A momentary reduction of voltage magnitude relative to nominal or pre-event voltage magnitude, characterized in terms of magnitude and duration. This can occur in any combination of phases.​

0.1 pu - 0.9 pu for ≤ 1min​

​IEEE 1159-2009

IEC 61000-2-8

​Voltage Swell

A momentary increase of voltage magnitude relative to the nominal or pre-event voltage magnitude, characterized in terms of magnitude and duration. This can occur in any combination of phases.​

>1.1pu for ≤ 1min​

​IEEE 1159-2009

IEC 61000-2-8


​A long-duration reduction of voltage magnitude relative to nominal or pre-event voltage, characterized in terms of magnitude and duration.

This can occur in any combination of phases.

​0.1 pu - 0.9 pu for > 1min

IEEE 1159-2009

IEC 61000-2-8​


​A long-duration rise of voltage magnitude relative to nominal or pre-event voltage, characterized in terms of magnitude and duration.

This can occur in any combination of phases.

​ > 1.1pu for > 1min

IEEE 1159-2009

IEC 61000-2-8

Voltage Transient​

A very brief (< 1 cycle) fluctuation in the magnitude of voltage. The primary causes are switching events and lightning, either direct strike or induced current. Depending on their severity (magnitude and associated energy discharge), such events may cause equipment damage.

TSC Appendix 2 specifies that “All Equipment shall be able to withstand capacitor switching surges that transiently increase voltage to twice normal levels. Sustained voltage changes shall be limited to 4% for capacitor switching events”.​

At present there is no agreed Standard on characterization of voltage transients.

Voltage Interruption​

​A sudden and sustained reduction of the voltage on all phases at a particular point on an electricity supply system below a threshold.

​Short term:
≤ 10% of nominal for ≤ 1min

Long term:
0% for > 1min

​IEEE 1159-2009

IEC 61000-2-8

​Table 2

Recommended Voltage Variation Limits Applicable to Circuits up to 1000 V, at Service Entrances
Nominal System Voltage(V)​

​Normal Operating Conditions
Extreme Operating Conditions​

Min (V)

Max (V)

Min (V)

Max (V)

​Single Phase System











​Three Phase System (4-Wire)
[3-ph Grounded Y]











Three Phase System (3-Wire)











Source: CSA Standard CAN3-C235-83 (reaffirmed in 2015)

​Table 3

Compatibility levels for individual harmonic voltages in low-voltage networks (rms values as percent of rms value of fundamental component)
​Odd harmonics
non-multiple of 3*
Odd harmonics
multiple of 3*
​Even harmonics

​Harmonic order

Harmonic voltage

Harmonic order

Harmonic voltage

Harmonic order

Harmonic voltage

























17 < h < 49​

​2,27 x (17/h) - 0,27

21 < h45​



​0,25 x (10/h) + 0,25

* The levels given for odd harmonics that are multiples of three apply to zero sequence harmonics. Also, on a three-phase network without a neutral conductor or without load connected between line and ground, the values of the 3rd and 9th harmonics may be much lower than the compatibility levels, depending on the unbalance of the system.
Note: The Canadian deviation consists of revised compatibility levels for the 3rd, 9th, 15th, and 21st harmonics orders; other values remain unchanged.

​ ​

Source: CAN/CSA-C61000-2-2:04 Electromagnetic Compatibility (EMC) - Part 2-2: Environment - Compatibility levels for low-frequency conducted disturbances and signalling in public low-voltage power supply systems (re-affirmed 2014)

​Table 4

​Transmission Level Voltage Distortion Limits
​Bus Voltage V at PCC
Individual harmonic (%)​
​Total harmonic distortion THD (%)

​V < 1.0 kV



​1 kV < V < 69 kV



​69 kV < V < 161 kV



161 kV < V​



*High-voltage systems can have up to 2.0% THD where the cause  is an HVDC terminal whose effects will have attenuated at points in the network where future users may be connected.

Source: IEEE Std 519-2014 Recommended Practices and Requirements for Hamonic Control in Electrical Power Systems



PQ Compatibility Guidelines & Standards

It needs to be recognized that targets and limits prescribed by industry standards are thresholds that, according to the associated assessment protocols, may be violated on occasion, either as a consequence of events occurring beyond a utility's ability to control (e.g. naturally caused events or catastrophic equipment failures) or due to certain switching operations inherent to normal power system operations.  Compliance with prescribed performance targets is judged on the basis of temporal and spatial probability distributions, requiring for instance that the levels be respected over more than 95% (or 99% in some instances) of a specified monitoring period (temporal), and covering more than 95% (or 99%) of the Delivery Points across the entire network.  Accordingly a delivery point's compliance with PQ performance standards does not eliminate the possibility of a particular customer facility experiencing a PQ disturbance.  This means that care is needed by customer's, when selecting or specifying equipment essential to the proper functioning of their processes, to ensure that their facility exhibits a desired level of tolerance to expected PQ disturbances.

There is industry wide evidence indicating that a large majority of costly process disruptions experienced by industrial processes are a result of the equipment or its controls responding undesirably to voltage sags – some resulting from events occurring on the utility delivery system and others originating inside the facility itself.  For instance, surveys conducted by EPRI indicate that a facility is 8-20 times more likely to experience a voltage sag than an interruption (depending on the supply being directly from Transmission, Sub-transmission, or Distribution network.  Accordingly, several industry trade groups have developed equipment specification guidelines and/or standards aimed at reducing the likelihood of equipment or sensitive processes being disrupted adversely due to voltage sag events.  Reference is made in this respect to the following resources:

a)   CBEMA / ITIC Curve:  Developed in 1970's by the Computer and Business Equipment Manufacturer's Association (CBEMA) as a guideline for the organization's members, aimed at defining a prescribed tolerance to voltage magnitude variations for specified durations.  The industry group subsequently changed its name to Information Technology Industry Council (ITIC).  The specified tolerance curve (see below) maps out a susceptibility profile with the horizontal axis denoting the duration of an event and the vertical axis indicating the percent of nominal voltage.  The curve has been applied to general power quality evaluation even though it was primarily developed for 120 V computer and business equipment.

a)   SEMI F47-0706: Specification for semiconductor processing equipment voltage sag immunity, originally published in 2000 and updated in 2006.  The figure below depicts the required voltage sag ride-through capability curve that semiconductor processing, metrology, and automated test equipment must be designed and built to conform.  The equipment must, as a minimum, be able to continuously operate without interruption at voltage levels associated with the "Unaffected Region". 

Diagram: SEMI F47-0706

a)   IEC 61000-4-11 & -34:  These standards define the setup, equipment requirements and test methods for characterizing equipment immunity to PQ disturbances such as voltage sags, momentary interruptions and voltage variations. The standards also describe four categories for characterizing the immunity of the tested equipment to such disturbances:

  • Performance Criteria A:  "Performance within specification limits" as defined by user or equipment vendor

  • Performance Criteria B: "Temporary degradation which is self-recoverable"

  • Performance Criteria C: "Temporary degradation which requires operator intervention"

  • Performance Criteria D: "Loss of function which is not recoverable"

While the above test standards do not enforce a performance criteria on equipment, additional EMC standards list the necessary test levels and criteria that must be met for particular applications, as follows:

  • IEC 60601-1-2:  EMC for Medical Devices

  • CISPR 24:  Immunity for ITE equipment

  • IEC 61326:  Electrical equipment for measurement, control and laboratory use

  • IEC 60601-2-24: Particular requirements for the safety of infusion pumps

  • IEC 60601-2-12:  Particular requirements for the safety of lung ventilators

  • IEC 61326-2-1:  Test and operational conditions and performance criteria for sensitive test and measurement equipment

  • IEC 61000-6-1:  Immunity for residential, commercial and light-industrial environments

  • IEC 61000-6-2:  Immunity for industrial environments

b)   IEEE Std 1668-2017:  This is a non-industry specific recommended practice for voltage ride-through performance and compliance testing for all electrical equipment connected to low-voltage power systems than can experience malfunction or shutdown as a result of reductions in supply voltage lasting less than 1 minute.  It clearly defines test methods for determining the sensitivity of equipment to voltage sags.  Analysis of real-world sags provides the foundation for both test methods and performance criteria, aligning themselves as closely as possible to the end user's electrical environment.  Voltage sags are classified as three general types that can occur at the terminals of sensitive equipment, distinguished in terms of the experienced sag impacting primarily a phase to ground voltage (Type I), a phase-to-phase voltage (Type II), or all three phases simultaneously (Type III).  This forms the basis of prescribing voltage sag immunity test levels as depicted in Figures below. 

Diagram: SEMI F47-0706

Recommended Type l and Type ll voltage sag immunity test levels (IEEE Std 1668-2017)

Diagram: SEMI F47-0706

Recommended Type lll voltage sag immunity test levels
(IEEE Std 1668-2017)