The Association of Manufacturers and suppliers of Power Systems and ancillary equipment


WHY IS LOSS OF MAINS (LOM) DETECTION REQUIRED FOR EMBEDDED (DISTRIBUTED) GENERATORS?

This question has been answered within the following breakdown of key points.

Firstly, this document refers to connection to the Distribution Network, rather than the “Grid” or “National Grid”. This is because the majority of generation below 10 MW or so will be connected to the Distribution Network at 400 V or 11 000 V, rather than to the National Grid network in England and Wales, which operates at 275 kV or 400 kV.

Q1: what is loss of mains (L.O.M.)?

L.O.M. is a general term associated with the automatic detection process incorporated within an embedded generator’s protection and control equipment which provides an ability to detect a change and so loss of the mains/network/grid supply to which that embedded generator is connected and delivering power.

A mains failure within a sector of the network/grid distribution system can result in a section – commonly referred to as an island – of the network becoming disconnected from the rest of the Distribution Network. This ‘island’ may consist of anything from a few low voltage (LV) customers up to an area covering a substantial part of the country. But most importantly it must include some power generation equipment, and that most likely will be an embedded - also known as distributed – generation.

Q2: why is loss of mains (L.O.M.) detection required?

As a general point, loss of mains may not affect the embedded generator. If the connected load rises to a figure in excess of the generator capability, then the set may be tripped on overload or under frequency etc. Only if the mains supply returns whilst the generator is running then there will be a problem if the supply is reconnected to the generator without synchronising the two supplies

There are two separate issues:

  • damage to the generator caused by the above faulty synchronising.
  • generator supplying to the utility without the knowledge of the utility causing danger to utility staff who expect supplies to be dead. Damage to customers’ equipment due to unregulated supplies being out of limits.

So there is a blend of generator protection and utility protection.

L.O.M. is required by The UK Distribution Code (DC), and specifically qualified by power generation related industry regulations commonly known as ‘G59’ and ‘G83’. Although not legally binding in themselves, they are required by the DC. Any generator which is to be embedded (connected) to the Distribution Network is required to comply with the DC and therefore the appropriate G59 or G83. Further details of which are to be found here: http://www.dcode.org.uk/the-distribution-code/.

Q3: why do G59 and G83 require L.O.M. detection?

It is considered extremely dangerous for an island to be created and remain active - once the Distribution Network connection to that network section has been lost - by being powered by one or more embedded generators. LOM detection equipment is therefore required to be incorporated within all embedded generator packages to detect that a LOM situation exists, and then able to accomplish a disconnection of that embedded generator. This is because an island powered by embedded generation supplying consumers’ will almost immediately lose synchronous alignment with the Distribution Network, and may well not be known to exist by the UK Distribution Network.

Some of the risks generated by an ‘island’ are:

3.1 Linesmen may be attempting to repair a Distribution Network fault and unknowingly encounter a live islanded network being supplied by embedded generation equipment.

3.2 Embedded generators may not maintain the voltage and frequency within required ‘Power Quality’ limits, thus possibly exposing consumer’s equipment to the risk of damage. This situation will most certainly be the result of the embedded generator(s) still functioning under a control system mode designed specifically for embedded (parallel) operation and so dedicated to delivering a fixed power (kW) contribution (engine governor setting) whilst operating at a fixed kVAr contribution (generator excitation control setting [AVR]), as achieved by operating at a fixed power factor (pf of say 0.95lag). Whereas a generating set operating in ‘normal mode’ will be dynamically controlling both engine governor and excitation [AVR] to generate and maintain a nominal output frequency at a nominal voltage while supporting a local load which varies in accordance with load demanded kVA levels.

3.3 The island’s electrical system may no longer be earthed appropriately and as a result local mains-supply related protection devices may not operate in the event of a local fault, or these protection devices may no longer be in an appropriate location within the island’s electrical system to provide protection. Such an un-designed electrical system may expose equipment and people to real danger. For example; a live HV line lying on the ground or a fallen tree lying against a live overhead line. The risk of such faults is quite high, particularly in extreme weather conditions, where in fact such faults may have been the initiation cause of the forming of the island.

3.4 Once the island has been formed there is a significant risk of the Distribution Network instigating a network reconnection to a section of their network which is in fact the island now functioning out of synchronism with the rest of the Distribution Network. Auto-reclosing devices are an established part of distribution, and network engineers undertaking fault finding are the two most likely ways by which circuit breakers may be closed without out any check for synchronous alignment across the breaker, with the distinct possibility of a crash-synchronisation between the Distribution Network and the island.

Auto-reclosure equipment is common place and will undertake to reclose breakers within typically 1< 5 seconds following their initial trip; with the vast majority of auto-reclosure equipment having no capability to conduct a check-sync before closure. There is therefore a real need to detect a LOM situation and effect a disconnection of embedded generators before auto-reclosing takes place.

HOWEVER, future network developments associated with ‘ESSENTIAL Generators’ will introduce a need to Ride-Through grid disturbances and so will stipulate a requirement for embedded generators to stay connected, which in turn means tolerate out-of-sync reconnection to the grid / network / mains supply.

Q4.1; what methods are used for loss of mains (LOM) detection?

G59 and G83 stipulate that embedded generators must be fitted with protection systems. The required equipment includes modules which monitor, detect and then trigger a generating-set control system’s awareness and pre-determined actions for a number of detected abnormal conditions.

It includes detection of an ‘under or over’ event taking place and remaining outside pre-set levels for either or both; voltage and frequency. These must be accompanied by either a Rate of Change of Frequency (RoCoF) or Vector Shift (VS) [also known as Vector Surge] module. Some generating set protection schemes also include the likes of a Neutral Voltage Displacement (NVD) protection module and often a reverse power (P) and/or reactive power (Q) kVAr module.

Arguably all the above do offer a level of LOM detection.

But it is the RoCoF and/or the VS equipment which G59 and G83 identify as being specifically appropriate for LOM detection.

The RoCoF and VS modules continuously monitor the ac voltage waveform’s sinusoidal pattern in terms of ac cycle (sometimes half cycle) duration time period. If there is a sudden change with the time period, and this change is outside the detectors pre-set limits, with this change remaining for a given number of cycles and so for a total time which exceeds the detectors pre-set duration time limit, then a LOM situation is judged to exist.

The RoCoF and VS modules have different intelligence techniques.

  1. The RoCoF determines a frequency change in terms of Hz per second (Hz/s)
  2. The VS determining a sudden change in the comparative number of phase angle degrees between each monitored ac waveform cycle’s zero crossing related period and so a change to the number of degrees in a waveform period, therefore determining a change to the phase angle vector shift in degrees.

A longer technical explanation for both RoCoF and VS is covered in Q6 and Q7 respectively.

The voltage and frequency Under/Over alarms can often detect a grid/network disturbance which is in fact a LOM initiating an islanding event. They may well have detected that the embedded generators are no longer able to maintain stable voltage and/or frequency [see Q3 - 3.2].

The reverse P & Q detectors and the NVD module may well each be able to detect an out-of-limits situation which occurs as an island is formed. But all these modes of detection may take longer than is deemed acceptable to trigger a disconnection, hence the G59 & G83 related Distribution Network Operator (DNO) requirement to incorporate the focussed, and faster acting, RoCoF or VS modules for LOM detection.

It is important to recognise that successful detection of islanding by RoCoF or VS depends upon an appreciable change of magnitude of the mains/grid/network system’s operational behaviour which then creates a step-change. Such detection therefore depends on the point at which the grid/network disconnection occurs to be carrying a high level of power, and that the embedded generator is also operating with a high percentage of rated power being delivered to the grid. It is only when this sudden power flow step change takes place that there will be an appreciable step change to the ac sine-wave period (df/dt) frequency characteristic within the now disconnected network section to which the embedded generator is connected. But often the embedded generator is some distance from the LOM disconnection point, and so the actual step change of RoCoF or VS becomes disguised by the inherent characteristics of certain electrical equipment which forms part of the island. It often becomes a fine choice between nuisance tripping and actually not determining LOM.

All the above mentioned protection devices are passive methods of detecting LOM.

There are more sophisticated equipment packages available for LOM detection, often referred to as active methods. Some active methods inject a signal into the network/grid system in order to measure the supply’s source impedance, with any sudden change to this impedance indicating a network/grid reconfiguration which suggests an islanding situation has occurred. These sophisticated equipment packages may well include the capability to assess if either a network stability issue exists or determination of a LOM situation using smart voltage and frequency monitoring techniques which determine slew rates for these parameters with the final decision to ‘trigger’ based on it being either a transient disturbance, or that a newly established steady state condition which is reasoned to be operating with an inferior level of ‘Power Quality’.

ROCOF and VS monitors rely on the network/grid frequency to be normally quite stable. Presently the grid has a sufficient level of ‘system inertia’ inherently provided by the traditional large (Hundreds of MW) rotating Turbine-Generators therefore a relatively stable Distribution Network network exists. BUT….things are changing and as the power generation policy for grids/network/mains becomes more reliant on multiple low inertia renewable generation techniques embedded within the network, then inevitably lower levels of both voltage and frequency stability will exist.

For this reason, the introduction of G59/3 with wider settings for ROCOF and VS settings brings challenges for detecting LOM and resulting islanding events.

Q4.2; What are the proposed changes driving the need for G59/3 and beyond?

It is proposed to increase the RoCoF. setting from the present 0.125Hz/s (Dec 2013) to be changed to 0.5Hz/s and even up to 1.0Hz/s along with introducing a delay time of < 0.5s before a trip can be initiated. These changes are to reduce the risk of nuisance tripping caused by the increased rates of change of frequency now being seen (Jan 2014) on the UK grid due to the increasing proportion of wind and photo-voltaic generation techniques where the provided magnitude of system inertia contribution is very low.

Q5; Why the need to double-up the under and over voltage and frequency alarms?

This was introduced in G59/2 and G83/2 to reduce the occurrence of nuisance tripping (tripping when an island had not been formed). The intention is that a small network disturbance/excursion is permitted for a relatively long period before tripping, but a large disturbance/excursion will result in a rapid trip.

Q6; What is ‘rate of change of frequency’ (ROCOF)?

RoCoF Modules determine an occurring change in terms of Hz per second (Hz/s)

The RoCoF module measures the ac voltage sinusoidal waveform time base pattern continuously for consecutive ac cycles (sometimes even half cycles) which for an ideal 50Hz system will be 20ms for each full cycle. If this 20ms time base changes (higher or lower) then a Change of Frequency (COF) situation has occurred. But if this change is a gradual (slow) process over time then the associated Rate of Change (ROC) is not considered abnormal.

Now consider, if the RoCoF module does detect a significant COF which has taken place within a very short time period [e.g.; within just one cycle; 20ms] then a detectable RoCoF has occurred.

The RoCoF module will now carefully monitor this existing situation over a given number of additional ac cycles (typically 8). If the frequency time base is restored to the original level within this additional number of cycles, then the initial RoCoF event is considered to be transient and so not trip command is issued. However, if after the pre-set number of ac cycles the ac waveform time base is still outside the pre-set limit, then it is deemed a RoCoF event has occurred, which is indicative of a LOM situation, and so a trip command is issued.

As mentioned in Q4, RoCoF detection does rely on there being a significant amount of power flowing through the point where disconnection from the grid occurred, and the need for then generator to be delivering a high proportion of rated power. That disconnection has then caused a sudden change in the operating conditions to which the embedded generator was contributing, and its LOM detection to operate reliably.

In exceptional circumstances the generator may stay connected to what is an island This may begin with an insignificant power flow step change at the point at which the Distribution Network disconnection occurred. The resulting ‘island’ may now have generated power levels which perfectly match the island’s consumed electrical loading levels. But this exceptional condition is unlikely to remain for long. Eventually the load will change, or as explained in Q3-3.2, the engine and generator’s control system’s fixed modes of operation; activated for embedded operation, will creep,

Q7; What is Vector Shift (VS)?

VS Modules determine an occurring change to the ac cycle period in terms of angular phase angle shift (change) in degrees.

Vector shift is a term used to describe a detection principle which continuously monitors the ac voltage waveforms sinusoidal shape, but rather than using a time-based assessment between each zero crossover, it determines the elapsed period between each in terms of degrees. Here for the ideal sine wave at 50 Hz a period of 180 degrees equals half a cycle, with correspondingly 360 degrees identifying one full sine wave cycle: - therefore covering a period spanning the initial, plus two more crossovers.

The VS monitor, like the RoCoF monitor relies on there being a significant amount of power flowing through the Distribution Network’s point of disconnection in order for there to be a marked change in the number of degrees of shift between adjacent zero crossovers. If the shift is outside the pre-set angular degree limit, and this situation remains for a given number of cycles then the VS reasons a LOM condition has occurred.

Sudden changes to the sine wave crossover time period – degrees of shift - within a local network are not uncommon, often caused by local load switching or network operational configuration changes. These ‘shifts’ are particularly common within distribution systems located on a single line supply- run which is a distance from one of the Distribution Network’ large turbo-generators. Such a local network is often referred to as having (relatively) a high impedance (or being a soft system). For such conditions, it may not be appropriate to activate and use a VS detector. Even a RoCoF may nuisance trip.

Footnote:

Within the UK the RoCoF system is favoured, whereas in Europe it is the VS.

The following diagram aims to show a sine-wave period change about the zero-crossing point.

FAQ 1005  

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