There are various organizations on the national and international levels working in concert with engineers, equipment manufacturers, and research organizations to come up with standards governing guidelines, recommended practices, and harmonic limits.
The primary objective of the standards is to provide a common ground for all involved parties to work together to ensure compatibility between end-use equipment and the system equipment is applied. An example of compatibility (or lack of compatibility) between end-use equipment and the system equipment is the fast-clock problem. The end-use equipment is the clock with voltage zero-crossing detection technology, while the system yields a voltage distorted with harmonics between 30th and 35th. This illustrates a mismatch of compatibility that causes misoperation of the end-use equipment.
This article focuses on standards governing harmonic limits, including IEEE 519-1992, IEC 61000-2-2, IEC 61000-3-2, IEC 61000-3-4, IEC 61000-3-6, NRS 048-02 and EN 50160.
Standard # 1. IEEE 519-1992:
It should be emphasized that the philosophy behind this standard seeks to limit the harmonic injection from individual customers so that they do not create unacceptable voltage distortion under normal system characteristics and to limit the overall harmonic distortion in the voltage supplied by the utility.
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The voltage and current distortion limits should be used as system design values for the worst case of normal operating conditions lasting more than 1 h. For shorter periods, such as during start-ups, the limits may be exceeded by 50 percent. This standard divides the responsibility for limiting harmonics between both end users and the utility. End users will be responsible for limiting the harmonic current injections, while the utility will be primarily responsible for limiting voltage distortion in the supply system.
The harmonic current and voltage limits are applied at the PCC. This is the point where other customers share the same bus or where new customers may be connected in the future. The standard seeks a fair approach to allocating a harmonic limit quota for each customer.
The standard allocates current injection limits based on the size of the load with respect to the size of the power system, which is defined by its short-circuit capacity. The short-circuit ratio is defined as the ratio of the maximum short-circuit current at the PCC to the maximum demand load current (fundamental frequency component) at the PCC as well. The basis for limiting harmonic injections from individual customers is to avoid unacceptable levels of voltage distortions. Thus the current limits are developed so that the total harmonic injections from an individual customer do not exceed the maximum voltage distortion shown in Table 7.3.
Table 7.3 shows harmonic current limits for various system voltages. Smaller loads (typically larger short-circuit ratio values) are allowed a higher percentage of harmonic currents than larger loads with smaller short-circuit ratio values. Larger loads have to meet more stringent limits since they occupy a larger portion of system load capacity. The current limits take into account the diversity of harmonic currents in which some harmonics tend to cancel out while others are additive.
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The harmonic current limits at the PCC are developed to limit individual voltage distortion and voltage THD to the values. Since voltage distortion is dependent on the system impedance, the key to controlling voltage distortion is to control the impedance. The two main conditions that result in high impedance are when the system is too weak to supply the load adequately or the system is in resonance.
The latter is more common. Therefore, keeping the voltage distortion low usually means keeping the system out of resonance. Occasionally, new transformers and lines will have to be added to increase the system strength. IEEE Standard 519-1992 represents a consensus of guidelines and recommended practices by the utilities and their customers in minimizing and controlling the impact of harmonics generated by nonlinear loads.
Standard # 2. IEC 61000-2-2:
IEC 61000-2-2 defines compatibility levels for low-frequency conducted disturbances and signalling in public low-voltage power supply systems such as 50- or 60-Hz single- and three-phase systems with nominal voltage up 240 and 415 V, respectively. Compatibility levels are defined empirically such that they reduce the number of complaints of mis-operation to an acceptable level. These levels are not rigid and can be exceeded in a few exceptional conditions. Compatibility levels for individual harmonic voltages in the low-voltage network are shown in Table 7.4. They are given in percentage of the fundamental voltage.
Standard # 3. IEC 61000-3-2 and IEC 61000-3-4:
Both IEC 61000-3-2 and 61000-3-4 define limits for harmonic current emission from equipment drawing input current of up to and including 16 A per phase and larger than 16 A per phase, respectively. These standards are aimed at limiting harmonic emissions from equipment connected to the low-voltage public network so that compliance with the limits ensures that the voltage in the public network satisfies the compatibility limits defined in IEC 61000-2-2.
The IEC 61000-3-2 is an outgrowth from IEC 555-2 (EN 60555-2).
The standard classifies equipment into four categories:
i. Class A- Balanced three-phase equipment and all other equipment not belonging to classes B, C, and D
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ii. Class B- Portable tools
iii. Class C- Lighting equipment including dimming devices
iv. Class D- Equipment having an input current with a “special waveshape” and an active input power of less than 600 W
Figure 7.19 can be used for classifying equipment in IEC 61000-3-2. It should be noted that equipment in classes B and C and provisionally motor-driven equipment are not considered class D equipment regardless of their input current waveshapes.
Maximum permissible harmonic currents for classes A, B, C, and D are given in actual amperage measured at the input current of the equipment. Note that harmonic current limits for class B equipment are 150 percent of those in class A.
Note that harmonic current limits for class D equipment are specified in absolute numbers and in values relative to active power. The limits only apply to equipment operating at input power up to 600 W. IEC 61000-3-4 limits emissions from equipment drawing input current larger than 16 A and up to 75 A. Connections of this type of equipment do not require consent from the utility. Harmonic current limits based on this standard are shown in Table 7.5.
Standard # 4. IEC 61000-3-6:
IEC 61000-3-6 specifies limits of harmonic current emission for equipment connected to medium-voltage (MV) and high-voltage (HV) supply systems. In the context of the standard, MV and HV refer to voltages between 1 and 35 kV, and between 35 and 230 kV, respectively. A voltage higher than 230 kV is considered extra high voltage (EHV), while a voltage less than 1 kV is considered low voltage (LV). The standard argues that emission limits for individual equipment connected to the MV and HV systems should be evaluated on the voltage distortion basis.
This is to ensure that harmonic current injections from harmonic-producing equipment do not result in excess voltage distortion levels. The standard provides compatibility levels and planning levels for harmonic voltages in the LV and MV systems. The compatibility level refers to a level where the compatibility between the equipment and its environment is achieved.
The compatibility level is usually established empirically so that a piece of equipment is compatible with its environment most of the time. Compatibility levels are generally based on the 95 percent probability level, i.e., 95 percent of the time, the compatibility can be achieved. Planning levels are design criteria or levels specified by the utility company. Planning levels are more stringent than compatibility levels. Thus, their levels are lower than the compatibility levels.
The IEC 61000-3-6 provides Evaluation guidelines to determine admissibility of equipment connected to MV and HV systems.
There are three stages for evaluating equipment admissibility:
i. Stage 1- Simplified evaluation of disturbance emission
ii. Stage 2- Emission limits relative to actual network characteristics
iii. Stage 3- Acceptance of higher emission levels on an exceptional and precarious basis.
In stage 1, equipment can be connected to MV or HV systems without conducting harmonic studies as long as its size is considered small in relation to the system short-circuit capacity. For small appliances, manufacturers are responsible for limiting their harmonic emissions. If the equipment does meet stage 1 criteria, the harmonic characteristics of the equipment should be evaluated in detail along with the available system absorption capacity.
Upon evaluation, individual equipment will be allocated with appropriate system absorption capacity according to its size. Thus, if the system absorption capacity has been fully allocated to all equipment, and this equipment injects its harmonic currents up to its limits, the system voltage distortion should be within its planning levels.
If equipment does not meet stage 2 criteria, it may be allowed to be connected to the system if the end user and utility agree to make special arrangement to facilitate such a connection.
Standard # 5. NRS 048-02:
The Quality of Supply Standard, NRS 048, is the South African standard for dealing with the quality of electricity supply and has been implemented since July 1, 1997. This standard requires electricity suppliers to measure and report their quality of supply to the National Electricity Regulator.
The NRS 048 is divided into five parts. It is, perhaps, the most thorough standard dealing with all aspects of quality of supply. It covers the minimum standards of quality of supply (QOS), measurement and reporting of QOS, application and implementation guidelines for QOS, and instrumentation for voltage quality monitoring and recording. Part 2 of NRS 048 sets minimum standards for the quality of the electrical product supplied by South African utilities to end users.
The minimum standards include limits for voltage harmonics and inter-harmonics, voltage flicker, voltage unbalance, voltage dips, voltage regulation, and frequency. NRS 048-02 adopts IEC 61000-2-2 harmonic voltage limits as its compatibility standards for LV and MV systems. For South African systems, the nominal voltage for LV systems is less than 1 kV, while the nominal voltage for MV systems ranges between 1 and 44 kV. NRS 048 has not established limits for harmonic voltages for HV systems yet. However, it adopts IEC 61000-3-6 planning levels for harmonic voltages for HV and EHV systems as its recommended planning limits for HV systems (the nominal voltage is between 200 and 400 kV).
Standard # 6. EN 50160:
EN 50160 is a European standard for dealing with supply quality requirements for European utilities. The standard defines specific levels of voltage characteristics that must be met by utilities and methods for evaluating compliance. EN 50160 was approved by the European.
Committee for Electrotechnical Standardization (CENELEC) in 1994. EN 50160 specifies voltage characteristics at the customer’s supply terminals or in public LV and MV electricity distribution systems under normal operating conditions. In other words, EN 50160 confines itself to voltage characteristics at the PCC and does not specify requirements for power quality within the supply system or within customer facilities.
Harmonic voltage limits for EN 50160 are given in percentage of the fundamental voltage. The limits apply to systems supplied at both LV and MV levels, i.e., from a nominal 230 V up to 35 kV. Medium voltage is between 1 and 35 kV. The total harmonic distortion of the supply voltage including all harmonics up to order 40 should not exceed 8 percent. Values for higher-order harmonics are not specified since they are too small to use as a practical measure to establish a meaningful reference value. Note that limits in EN 50160 are nearly identical to the IEC 61000-3-6 compatibility levels for harmonic voltages for its corresponding LV and M V systems, except for the absence of higher-order harmonic limits in EN 50160.