Power Quality Information

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 3

Compatibility levels for individual harmonic voltages in low-voltage networks (rms values as percent of rms value of fundamental component)

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". 

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.