Transmission System Power Quality Standards Chapter 1: Introducton
38
Chapter 1: Introduction
1.1 Power Quality Definition and Requirements
1.1.1 In this Standard, Power Quality is defined as the degree to which the Voltage at the point of connection to the User of the Transmission System is maintained to be Sinusoidal at declared rated Voltage and Frequency within the stipulated limits. 1.1.2 Major Power Quality Problems that are associated with the Transmission System
is Voltage Sag or Voltage Dip. Voltage sag would occur in the System during the short-circuit fault. The most severe Voltage Sag would occur at the fault location and the Voltage Sag effects would be propagated throughout the Systems with the magnitude of the Voltage Dip attenuated.
1.1.3 Voltage Sag could also be caused by short circuit faults in User’s System and propagated to other Users’ Systems through the Transmission Systems or through the Distribution System as the case may be. Majority of problems associated with harmonics are caused by non-linear loads that draw non-sinusoidal current from the system and thus causing the supply voltage to be distorted. Frequent switching of large load such as motors is also another source of power quality problems that result in voltage flicker to be experienced by other Users.
1.1.4 The approaches to resolving power quality problems depend on the types of the power quality problems. Both utility and Customers have their individual roles and responsibilities in resolving power quality problems. Individual Customer shall ensure that any non-linear load to be connected to the Transmission System does not produce harmonic currents that would result in voltage at Point of Common Coupling to be distorted beyond the limits as set out in section 2.4 of this Standard. Likewise the utility shall continue to monitor and ensure that Harmonic limits as specified in section 2.4 are complied with.
1.1.5 Voltage sag could affect sensitive loads usually connected at low voltage network of the Customer’s installation. Not withstanding the presence of several voltage transformations from transmission voltage to low voltage, voltage sag caused by Transmission faults can affect Customers voltage sensitive loads and the effect is more pronounced during severe three-phase faults on the Transmission System. It shall be the responsibility of the utility to provide appropriate information on voltage sag and for the individual customer to ensure sufficient ride-through capability of the voltage sensitive equipment.
Transmission System Power Quality Standards Chapter 1: Introducton
39
1.1.6 The Standards contained in this document specify the Power Quality requirements of the bulk supply point that delivers electricity supply to the Distribution System and other bulk supply Users in terms of stable voltage and frequency within specific limits so that Generator, TNB Transmission equipment or Consumer’s equipment directly connected to the Transmission System can operate safely within its design performance without suffering undue damage or breakdown. 1.1.7 The requirements and limits specified in this Standard shall also be complied with
by all Users connected or intending to be connected to the Transmission System. 1.1.8 Consequently these Standards also determine the investment required in provision
and/or procurement of Supplementary Services such as Reactive Power, Active Power response, hot standby generation, fast start generation, demand control and black start by TNB Transmission. They also indicate the investment required both in the Transmission and Distribution Systems for containing harmonics, voltage unbalance as well as the permitted types and levels of special Loads adversely affecting the power quality and requirements for compensation plant to limit their adverse effects. Compliance with these Standards is the responsibility of TNB Transmission, Generators, Distributors, Network Operators and Non-Embedded Customers.
1.1.9 In order to achieve the required Transmission Power Quality these Standards will be used by TNB Transmission in planning, developing, maintaining and operating the Transmission System as well as in connecting generation and demand to the System. In turn the equipment utilised by the Generators, TNB Transmission, Distributors and Customers also need to comply with the provisions of this Standard. Compliance with these Standards is therefore not optional and where a connecting party’s plant or equipment is likely to be non-compliant at the planning stage then such plant and/or equipment will not be energised until appropriate remedial measures are put in place and are fully functional. In addition, TNB Transmission shall put in place appropriate monitoring facilities to ensure compliance with the provisions of this Standard as part of its Licence. 1.1.10 In cases where, the nature and operation of the new types of plant and equipment
to be connected to the Transmission System is perceived to be likely to cause problems to other customers and user’s of the System, but not fully covered by this Standard, appropriate remedial measures shall be put in place based on expert advice within a mutually agreed time period.
Transmission System Power Quality Standards Chapter 1: Introducton
40
1.2 Scope
1.2.1 This Standard covers most of the power quality related phenomena generated by various types of plant and equipment connected to the Transmission System. In each particular case the Transmission Power Quality that should be maintained is indicated together with the remedial approach and responsibilities of parties.
Transmission System Power Quality Standards Chapter 2: Transmission Power Quality Standards
41
Chapter 2: Transmission Power Quality Standards
2.1 Voltage Sag or Voltage Dip
2.1.1 Voltage “sag”, “dip” or “swell” is the transient phenomena which can affect the voltage level and is usually experienced during System faults and the subsequent recovery period. These phenomena can adversely affect some customer equipment that are sensitive to such changes. This Standard does not specifically cover voltage “sag”, “dip” or “swell”.
2.1.2 The voltage sags are caused by faults on the Transmission System due to short circuits incidents which are cleared by System Protection. The nature and technical detail of such events is a function of the location of the connection point on the System, the distance of the short circuit from the connection point and the short-circuit level at the connection point, the severity (magnitude and duration) of the short circuit. Voltage Sag may also be caused by short-circuit fault in one User’s System and propagated to other Users’ Systems through the Transmission System.
2.1.3 In most cases, the Transmission System faults were cleared within the primary protection fault clearing time. Statistics of monitored events of voltage sags indicates that not all faults on the Transmission System affect voltage sensitive customer demand. However for certain severe fault, the voltage sag may be felt by voltage sensitive customer up to (three hundred) 300km away from the fault location.
2.1.4 There is no single Standard for voltage sag but statistical information is available from TNB Transmission in terms of past records of voltage sag events. If Users or prospective Users have equipment which are sensitive to voltage sags, they can request such statistical information from TNB at the application stage to enable compatibility assessment to be carried out, if necessary. Whenever necessary, remedial actions may be rendered by TNB and/or the affected Customer through appropriate Agreement.
Transmission System Power Quality Standards Chapter 2: Transmission Power Quality Standards
42
2.2 Voltage Step Change
2.2.1 Under System operational conditions, elements of the Transmission System need to be switched to optimise the operation of the System, to facilitate maintenance and to isolate faulty equipment for repair. Under such operational activities, switching of any element or equipment shall not cause unacceptable Voltage Step Changes. The voltage change is deemed unacceptable if it changes by more than the limits set out in Sectıon 4.2.4 of the Transmission System Reliability Standards.
2.2.2 Limits of voltage changes due to Load, frequent and infrequent operational switching of Load both by TNB and the User are defined in Sectıon 4.2.5 of the Transmission System Reliability Standards.
2.3 Voltage Fluctuations and Flicker
2.3.1 The limits of Flicker that are acceptable on the Transmission System are in accordance with the “Engineering Recommendation P28, Issued by The Electricity Council of UK in 1989 entitled Planning Limits for Voltage Fluctuation Caused by Industrial, Commercial and Domestic Equipment in the United Kingdom” (ER P28). TNB Transmission Division uses the procedures contained in this document to plan the connection of Fluctuating Loads and applies the limits therein in measuring and monitoring the levels of Flicker at such points of connection.
2.3.2 In accordance with ER P28, voltage fluctuations at a Point of Common Coupling with a fluctuating Load directly connected to the Transmission System shall not exceed:
(1) 1% of the voltage level for step changes, which may occur repetitively. Any large voltage excursions other than step changes or less frequent step changes may be allowed up to a level of 3% provided that this does not constitute a risk to the Transmission System or, in TNB’s view, any other party connected to the System.
(2) The planning limits for the Short and Long Term Flicker Severity applicable for Fluctuating Loads connected to the Transmission System are as set out in the table below (see table 2.1).
Transmission System Power Quality Standards Chapter 2: Transmission Power Quality Standards
43
Table 2.1: Maximum allowable flicker severity Transmission System
Voltage Level at which the Fluctuating Load is Connected Absolute Short Term Flicker Severity (Pst) Absolute Long Term Flicker Severity (Plt) 500, 275 and 132kV 0.8 0.6 Less than 132kV 1.0 0.8
2.3.3 In connecting a Fluctuating Load at a particular point or Point of Common Coupling it is necessary to assess the total Flicker due to the Fluctuating Load itself and the background Flicker measured at that point. In assessing the total Flicker TNB shall use the so called “RSS Rule”. This assessment is carried out by the RSS Rule by obtaining the sum of the squares of the Flicker from the Fluctuating Load and the background Flicker, with the square root of this sum yielding the total assessed Flicker at that Point of Common Coupling due to the specific Fluctuating Load and the background Flicker at that point.
2.3.4 Connection of more than one Fluctuating Load at the same Point of Common Coupling or addition of further Fluctuating Load at the same location is carried out on a “first come first served” basis on the Transmission System. This means that whilst the first Fluctuating Load may be within the allowable Flicker limits and not require any Flicker mitigation or compensation equipment, the second Fluctuating Load which may cause violation of Flicker limits will be obliged to invest in appropriate Flicker mitigation or compensation equipment. At the planning stage, in carrying out the assessment of the total Flicker due to more than one Fluctuating Load connecting at a particular Points of Common Coupling TNB shall use the methodology described in P28.
2.3.5 In assessing level of Flicker due to a Fluctuating Load at the planning stage the assessment will be carried out at a realistic Weak System operating condition giving the minimum number of connections with which that point on the System can be operated within normal voltage limits with maintenance outages or following Secured Contingency Events. This condition shall be considered as representative of the type of System conditions under which the Flicker due to a Fluctuating Load will be at its highest value as the voltage fluctuations due to the Fluctuating Load will be at their highest level under such conditions.
2.3.6 If under the operating conditions described in 2.3.5 the Flicker due to a Fluctuating Load exceeds the limits then Flicker compensation which will reduce the Flicker to an acceptable level shall be installed. For the avoidance of doubt the acceptance Criteria for Flicker is the 95% values of Short and Long Term Flicker Severity i.e., Pst and Plt will be below the limits specified in the above table. Here the 95% values mean the 95th percentile of the cumulative probability function of
Transmission System Power Quality Standards Chapter 2: Transmission Power Quality Standards
44
the values of Flicker Severity measured and observed during the total observation period. The System overall performance yardstick for the level of Flicker reduction to be achieved shall be based upon enabling connection of other similar Fluctuating Load and/or permitting for future extension and development of the location with further Fluctuating Load installations. The Flicker compensation method and equipment to be used shall be of a type that does not cause resonance problems with the existing and intended future developments of the Transmission System.
2.3.7 If the permissible Flicker limits cannot be met even with installation of Flicker compensation at a particular Points of Common Coupling at a particular System voltage level then connection of the Fluctuating Load at a higher voltage level or elsewhere in the System shall be considered. The connection point of choice will need to have to experience lower voltage fluctuations than the first connection point considered under the operational conditions in 2.3.5.
2.4 Harmonics
2.4.1 The limits of Harmonics that are acceptable on the Transmission System are in accordance with the IEC Standard 61000-3-6 “Assessment of emission limits for distorting loads in MV and HV power systems”. TNB Transmission uses the procedures contained in this document to plan the connection of Loads producing Harmonics and applies the limits therein in measuring and monitoring the levels of harmonics at such points of connection.
2.4.2 All plant and equipment connected to the Transmission System, and that part of the Transmission System at each connection site, should be capable of withstanding the following distortions of the voltage waveform in respect of harmonic content. The maximum total levels of Harmonic Distortion at any connection point on the Transmission System from all sources under both planned outage and Secured Contingency Event conditions, unless abnormal conditions prevail, shall not exceed:
(1) at 500kV, 275kV and 132kV, a maximum planning limit for Total Harmonic Distortion of 3% with no individual Harmonic Voltage greater than that shown in the table 2.2;
(2) infrequent short duration peaks may be permitted by TNB to exceed the above levels for harmonic distortion;
(3) TNB will make an assessment for the connection of each Load to the Transmission System, which may result in harmonic emission limits being specified for these Loads in the relevant Agreement. The assessment will take
Transmission System Power Quality Standards Chapter 2: Transmission Power Quality Standards
45
into account the position of existing and prospective Users’ Plant and Apparatus in relation to harmonic emissions. Users must ensure that connection of distorting Loads to their User Systems do not cause any harmonic emission limits specified in the Connection Agreement to be exceeded; and
(4) Users shall ensure that the Immunity Level of their plant and Apparatus is compatible with the electromagnetic disturbances present on the System including harmonics.
2.4.3 In assessing level of Harmonic Distortion and Individual Harmonic Levels at the planning stage the assessment will be carried out at a realistic Weak System operating condition giving the minimum number of connections with which that point on the System can be operated within normal voltage limits with maintenance outages or following Secured Contingency Events. This condition shall be considered as representative of the type of System conditions under which the Harmonic Distortion and Individual Harmonic Levels will be at their highest value.
2.4.4 If under the operating conditions described in 2.4.3 the Harmonic Distortion and Individual Harmonic Level due to the Load exceed the limits then harmonic filters which will reduce the harmonics to an acceptable level shall be installed to bring the levels within the limits. The System overall performance yardstick for the level of harmonic reduction to be achieved shall be based upon enabling connection of other similar harmonic producing Load and/or permitting for future extension and development of the location with further harmonic producing Load installations. The harmonic filters to be installed shall be of a type that does not cause resonance problems with the existing and intended future developments of the Transmission System.
Transmission System Power Quality Standards Chapter 2: Transmission Power Quality Standards
46
Table 2.2: Indicative Values of Planning Levels for Harmonic Voltage in HV (35 < kV < 230) and EHV (kV >230) Odd Harmonics (Non-multiple of 3) Odd Harmonics (Multiple of 3) Even Harmonics Order “h” Harmonic Voltage (%) Order “h” Harmonic Voltage (%) Order “h” Harmonic Voltage (%) 5 7 11 13 17 19 23 25 >25 2.0 2.0 1.5 1.5 1.0 1.0 0.7 0.7 0.2+0.5(25/h) 3 9 15 21 >21 2.0 1.0 0.3 0.2 0.2 2 4 6 8 10 12 >12 1.5 1.0 0.5 0.4 0.4 0.2 0.2
The total harmonic distortion level is 3%
2.5 Phase Unbalance and Traction Load
2.5.1 In planning the limits on voltage unbalance, TNB makes reference to “Engineering Recommendation (E/R) P24, issued by the Electricity Council of UK in 1984 entitled “AC Traction Supplies to British Rail” and its successor document P 29 issued in 1990 entitled “Planning Limits for Voltage unbalance in the United Kingdom”.
2.5.2 In all power Systems including the Transmission System, the impedances of each phase of the transmission lines are unequal due to differences in their physical geometry, and therefore create a voltage unbalance at the termination points of the lines. As a general rule, the longer the line the higher this unbalance will be. In addition the demands connected to each phase at a Demand Supply Point are not precisely equal as customers connected to individual phases utilise their equipment at varying times. Also there are large single-phase connected demands such as railway traction Loads connected at various points. Together these factors create a level of unbalance between the phases of the transmission lines within the Transmission System causing the so called negative phase sequence currents to circulate in the Transmission System which can be harmful to plant equipment if they exceed certain levels.
2.5.3 To prevent the circulation of excessive negative sequence currents conductor transpositions are applied over the length of line to reduce the level of inequality between the phase impedances of the transmission lines in the Transmission
Transmission System Power Quality Standards Chapter 2: Transmission Power Quality Standards
47
System. This process requires a change in the physical position of each phase conductor over the length of the line. Thus a full transposition would involve each phase conductor to physically occupy a different position at each third of the line length. This undertaking by itself is sometimes not sufficient to reduce the circulating negative phase sequence currents due to the connected single-phase and unbalanced three-phase Loads. In such cases it is necessary to install compensation equipment at the point of connection of the specific Load to achieve balanced voltages within specified limits given below.
2.5.4 All plant and equipment connected to the Transmission System, and that part of the Transmission System at each connection site, should be capable of withstanding the following distortions of the voltage waveform in respect of Phase Unbalance at the planning stage.
1) Under planned outage conditions, the maximum negative phase sequence component of the phase voltage on the Transmission System should remain below 1% unless abnormal conditions prevail (P29 limit is not exceeding 2% for 1 minute).
2) Under the planned outage conditions, infrequent short duration peaks with a maximum value of 2% are permitted for Phase Unbalance, subject to the prior agreement of TNB under the connection agreement. TNB will only agree