IEEE 519 maximum demand current: Interpretation, pitfalls, and practical approach
In its standard interpretation, IEEE 519 (2022) allows a situation in which harmonic compliance can be achieved simply by increasing the load, without reducing harmonic emissions at all. Read this blog text to understand why and how this is typically avoided.
Introduction
It is in the interest of the utility (Transmission System Operator (TSO) or Distribution System Operator (DSO)) to protect the stability of the electrical system and its other clients by establishing rules on power quality at the Point of Common Coupling (PCC). PCC is the contractual connection point between the electricity consumer and the utility, also known as the Point of Connection, for electricity consumers.
The utility has the responsibility to provide good quality voltage to all its clients, and by rules, typically referred to as the Grid Code, limits the emissions by individual customers to protect others.
Voltage harmonics are inevitably caused by current harmonics that flow through network impedance, as stated by Ohm’s law:
The h refers to changing values at different harmonic orders. Based on the equation, one can surmise that there are two options for the utility to constrain the voltage distortion:
- Build stronger networks (more copper and transmission capacity), which lowers the network impedance Z
- Constrain the current harmonics in the network by regulating the current emissions of the consumers by implementing the Grid Code
In practice, the responsibility is divided between the utility and the clients:
The utility builds reasonably strong networks (low enough Z) that can handle a specific number of harmonics, reactive power, etc., without losing voltage stability and service quality.
Electrical consumers comply with reasonable limits for power quality indices so that the utility can protect other consumers from emissions.
If there is no balance between the responsibilities, then, consequently, either:
- The utility builds ridiculously strong networks with huge expenses, or
- The consumers can’t use normal electrical equipment due to the bad quality of the supply, or must make unreasonably sized investments to power quality equipment.
All national Grid Codes are based on international power quality standards, such as EN 50160, G5/5, or IEEE 519. This article discusses the IEEE 519 standard approach, which is widely used, especially in the Americas and the Middle East.
IEEE 519 standard interpretation for harmonics
IEEE 519 standard sets limits for harmonic currents at different voltage levels at the PCC. The point where the grid code harmonic limits are measured and enforced is a contractually defined point between the electricity and the utility is typically the PCC under interest, but in the context of the standard, PCC can be selected to be any point within the network, as exemplified in the below figure, where various loads a coupled together – very typically under a transformer that forms a choke point in the electrical system.
Limiting the current harmonics is justified as “low” downstream in the electrical system as possible, i.e., as close to the harmonic source as possible, so that the emissions do not propagate into a wider electrical system. The utility is concerned with the contractual PCC, but in a large intra-facility electrical system, harmonic currents can cause trouble at various levels of the electrical system.
The voltage distortion caused by harmonic currents is proportional to the network strength. The network is weaker the further downstream we are in the electrical system, and therefore, the voltage distortion is worse closer to the loads, and its potential to cause problems is higher. This means that even if the power quality indices are satisfied in the utility connection point, due to very high network strength, the voltage distortion might be harmful inside the factory’s system, as illustrated in the graph below.
The IEEE 519 standard (for the sake of clarity, we refer to the 2022 edition in this text) and the Grid Codes based on it defined the harmonic limits as percentages and as a function of network strength. The percent limits for different harmonic orders and the total demand distortion (TDD) are defined in relation to IL, maximum demand load current, or reference current.
The current distortion limits are shown in the table below.
The limits are different for the overall current distortion, Total Demand Distortion in the individual harmonic orders and for different network strengths. The limits are expressed as a percentage of the Total Demand Current (reference current), which is used to transform the percentages to tangible Ampere limits.
The standard also takes the network strength into account by establishing different limits between different Isc/IL ratio ranges (relationship between short circuit level and the total demand current), which is used to account for the network strength. The idea is that in a weak network (low Isc/IL ratio), the current harmonic limits are lower because they cause more severe voltage distortion and trouble.
The problem with maximum demand current definition
Typically, from a given point of view, the network strength is a fixed value – in the utility point of connection, the utility makes a statement of the minimum – maximum variation of the short-circuit level.
More downstream in a consumer electrical system, the upstream transformer has much higher impedance than the upstream network, and therefore dominates the overall impedance from the PCC, and therefore dominates the short circuit value in the PCC.
Based on the above, the definition of the maximum demand current (reference current) plays a major role in establishing the actual Ampere limits.
The IEEE 519 standard definition states that the maximum demand current should be defined by the average of the maximum measured fundamental frequency current from the previous 12 months.
This definition makes sense for existing installations if the maximum demand current, or the maximum demand power, is close to the contracted power. It originates from a fairness-based approach between consumers, but does not fully reflect the physical impact of harmonic currents on the network. Contracted power is the power based on which the supplying electrical infrastructure by the utility is built and maintained, and the corresponding current is calculated based on voltage. Contracted power / contracted grid connection is typically also what determines the invoicing.
For electricity consumers with changing load conditions or for plants that have seasonal changes in loading, or higher contracted nominal power that is bought for future expansion purposes, this definition for the maximum demand current is problematic.
Consider the following situation:
- In this example network, the contracted power is 2 MVA
- The ISC/IL ratio is assumed to be between 50 … 100.
The nonlinear load is constant, and since there are no other loads, the total demand current is equivalent to the load fundamental current. The load has the following harmonic spectrum that does not meet the limits:
Consider what happens if we add a constant resistor bank of 700 kW in parallel to the original load.
This increases the fundamental current, without affecting the harmonics, and suddenly, the limits are met!
In both cases, the load feeds the same harmonic current to the utility network, 216.46 A, that causes the same problems! This leads to a fundamental paradox: harmonic compliance can be achieved without any reduction in harmonic emissions. Therefore, in practice, the maximum demand current (reference current) is, despite the standard definition, typically a value agreed with the utility PCC or, in the internal network, PCCs, an agreed engineering practice.
The maximum demand current is typically agreed to be a technically justified constant value, such as the feeding transformer rated current (very commonly used), the contracted power (typically always matches the main transformer rating), or another value that is related to technical current capacity limits. Since the upstream electrical infrastructure is built anyway to feed the contracted power, it provides a good reference point for harmonics evaluation and can reliably handle the harmonics that are within limits.
Conclusions
The utility sets limits for harmonic current emissions for its electricity consumers to protect the network and other consumers from voltage distortions. At the same time, the utility builds a network that is designed to handle a specific level of current harmonic loading.
If the current harmonic limits are not based on a fixed value, they leave the harmonic limits to interpretation, and or the consumers experience unfair harmonic current limits that are not technically justified.
A reference current (maximum demand current) tied to electrical infrastructure capacity provides a physically meaningful and consistent basis for harmonic evaluation — unlike measurement-based definitions, which can lead to misleading compliance results. For this reason, in practical harmonic studies and grid connection agreements, the reference current is often defined based on transformer rating or contracted capacity rather than historical measurements.
Get to know our solutions!
Merus® A2 – Active Harmonic Filter
Merus® A2 is a scalable, versatile, and durable active harmonic filtering solution designed and manufactured in Finland using innovative Merus® technology.
Merus® HPQ – Hybrid Power Quality
Merus® HPQ is an all-in-one power quality system for combined active harmonic mitigation and power factor correction.
Anything on your mind? Let’s talk!
Riku Kalliomaa
Head of Sales, Active Harmonic Filter
Global