Key protection functions for Mains Decoupling Relays

Are you generating power in parallel to mains? You will need "loss of mains" protection. Using mains decoupling relay is essential to avoid supplying your electricity into an islanded grid, leading to possible equipment damage or safety issues.

 

Connecting to the grid

There is a lot of mains decoupling relays on the market. It is important to choose the right one with a wide range of mains connected applications. These include generator sets and renewable energy sources such as photovoltaic plants, wind turbine and other form of micro cogeneration. Pay also an attention, if the unit is designed to fully comply with Utilities´connexion requirements and statutory codes, offering a  high level of protection and safety when working in parallel to the mains.

Key protection function

Overvoltage, Undervoltage (ANSI 27, 59)

These are the essential protections, based on measurement of the voltage RMS value. These protective functions provide basic criteria whether the protected system is healthy or not. In case of mains protections, the out-of-limits voltage shows mains fail. The failure modes may be many, from short-circuits in vicinity of the installation point through power fluctuations to large failures of the complete grid areas.

Often, the voltage setting is done in 2 levels, with "softer" over/under voltage limits and longer delays in the first step, and stricter limits with shorter delay reaching down to 100 ms or even faster reaction times. The 2-level setting allows for so called "Fault ride-through" of the grid part in case of temporary mains fail.

Voltage unbalance - amplitude asymmetry (ANSI 47)

Generally, protection of voltage asymmetry is requested as a supplementary protection to the over- and undervoltage protections. In best protections products, voltage amplitude unbalance is evaluated,which is sensitive to drop of voltage in one or 2 phases. This may occur e.g. in case of single-phase or double-phase short circuit in the vicinity of the generator. As such, it is considered as a mains failure and the DNOs usually give the requested limit and delay values to trip the generator by the voltage unbalance protection. The unbalance function is not sensitive for angle asymmetry. If such requirement is given, some more sophisticated methods have to be used, as described below.

Positive sequence undervoltage (ANSI 47)

Detecting the decrease of the positive sequence voltage is sometimes used as a combined method for detecting undervoltage and unbalance in the measured system. Denmark is one of the countries, where the DNOs require positive sequence undervoltage instead of second (fast) stage undervoltage. The requirement is to detect drop of the positive sequence voltage under 60-70% of the nominal value of the measured voltage. This way, the system is protected from large voltage drop as well as severe asymmetry, what is a perfect combination for the second-stage voltage protection. Trip time around 50-60 ms is a typical requirement in this case.

Negative sequence overvoltage (ANSI 47)

The negative sequence voltage component represents system with opposite phasor rotation then the measured voltage. Applying voltage with content of negative sequence voltage on rotating machinery such as motors or generators induces a "parasite" magnetic field, rotating in the opposite direction then the rotation of the shaft. This may cause mechanical pulses and thus damages of shafts or 

mechanical parts of the machines. Content of negative sequence voltage, i.e. asymmetry, in the grid is observed as one of the quality parameters and its increase over given limit is considered as mains failure. Due to its nature and effect on mechanical rotating parts, this method is also used as generator or motor protection. Usual threshold for this type of protection is cca 20% content of negative sequence voltage in the measured signal.

Another phenomenon, where symmetrical components are used, is detecting the loss of one fuse in the grid at zero load from the generator. Sometimes, the following test is performed by the DNOs during commissioning: at the non-loaded state of the generator a fuse is pulled on the distribution transformer, connecting the generator to the grid. Given that the current flowing from the generator is zero and the voltage measurement is connected at the generator side of he pulled fuse, this should not cause any decrease of voltage in the affected phase, so none of the over/under voltage or amplitude unbalance would be capable of detecting this failure. What changes in such case, are conditions behind the pulled fuse, which are mostly given by the transformer winding inductiveness and capacity. This change results in angle shift of the affected phase, which can be effectively detected by positive or negative sequence evaluation or their ratio: U2/U1

Overfrequency, underfrequency (ANSI 81H, 81L)

Frequency is a global parameter, showing overall quality of the mains voltage on a large scale, e.g. complete electrical system. In fault-free conditions, frequency does not vary, unless a severe failure of a grid area occurs, causing breakdown of the electricity system in that area. Hence, the frequency shift is a good method to indicate mains failure.

Setting of the frequency is sometimes requested in two stages, similar to the over and under voltage, to allow "Fault ride-through" of the grid part in case of temporary mains fail.

"Loss of mains" protections

Specific cases of the unintentional islanding are the automatic reclosing and situations where the imbalance between power production and consumption is very large. These situations usually require very fast solution and could not be safely protected only by frequency and voltage protections. In case of auto-reclosing sequence within the mains, the connected generators can get out of synchronism during the reclosing period. Other risk of damage can occur to the generator, by step-loading it by the complete load of the islanded area above its design capacity. For these purposes, a specific type of fast protections is provided. In UK, it is generally referred to as "Loss of Mains" (LOM) protections. The requested LOM protections are Vector shift and/or ROCOF, which is mostly requested in the UK, in other European countries, Vector shift is requested more often.With increasing number of distributed generation installations, many of them being inverter-based devices (solar plants, wind turbines), the requirements to the protective relays are sustainably growing, asking for more and more strict and sophisticated methods to detect the islanding situations, like e.g. active anti-islanding methods. 

Vector shift (ANSI 78)

Vector shift is one of the LOM protections. It provides very fast detection of mains failure (in tens of ms), based on the principle of shift of the synchronous generator displacement angle. The displacement angle is an angle between magnetic field of the rotor and the rotating magnetic field of the stator winding and relates strongly to the load of the generator. In case that this load changes, the displacement angle immediately "jumps". Compared to the frequency change, which probably also occurs, this jump is an immediate phenomena and is detected as a shift of the measured voltage sine curve - Vector shift or Vector jump. It allows almost immediate disconnection of very fast failures and thus prevention of severe damages which could not be prevented within the delay of frequency or voltage protections.

Rate Of Change Of Frequency (df/dt, ROCOF, ANSI 81R)

ROCOF is second most frequently requested method of LOM detection. In principle, the method uses similar evaluation method like Vector shift, but the physical phenomena detected is different. It calculates the change of speed of the generator, caused by sudden change of its load, together with unintentional loss of mains, which is normally capable of keeping the frequency on a stable level.ROCOF is a fast protection, similar to Vector shift, however, unlike Vector shift, the calculation requires a certain time for evaluation. 

Petra Piclova
Petra Piclova
Renewable Energy segment manager

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