Protection coordination for an embedded generator is the study that proves a solar farm or battery on a distribution feeder will not stop the feeder's protection doing its job, and that the plant's own protection will clear faults inside the plant and disconnect it when the feeder is lost. It covers fault levels at defined locations, the effect of the plant on what upstream reclosers and breakers measure, grading margins, earth fault detection on an unearthed system, and anti-islanding.
Our general article on protection studies for solar and BESS connections covers the principles. This one is about the specific problems of a generator in the middle of a radial feeder.
Why a Feeder Generator Is a Protection Problem
Distribution protection was designed for power flowing one way, from the zone substation to the loads. A generator part-way along the feeder breaks that assumption in three ways.
- It contributes current to faults. Inverters limit their contribution to roughly rated current, positive sequence only in most designs. That is small, but on a weak rural feeder it is not negligible.
- It changes what upstream devices measure. For a fault beyond the plant, part of the fault current is supplied by the plant, so a recloser between the zone substation and the plant sees less current than it would without the plant. That is desensitisation.
- It can keep a section of feeder alive after the feeder breaker opens. That is an island, and it is a safety and equipment risk. It also interacts badly with automatic reclosing.
The study has to answer each of these with numbers, not assertion.
Inputs: Fault Levels and the Plant's Contribution
The DNSP supplies normal and minimum three-phase and single-phase fault levels at the zone substation bus or the connection point, usually with positive, negative and zero sequence impedances, for each network configuration that needs assessing. Fault calculations normally follow IEC 60909 or the DNSP's preferred method, and some DNSPs specify the voltage factor to use.
The plant's contribution comes from the inverter manufacturer. Typical inverter behaviour is a short transient followed by a limited current near rated current, and many inverters inject positive sequence current only, so the contribution to three-phase, phase-to-phase and earth faults is similar in magnitude. The study should state the maximum and minimum contributions by fault type and how long they are sustained.
Fault Locations
DNSPs increasingly prescribe the fault locations to study, so that every package covers the same cases. A typical set:
| Location | Purpose |
|---|---|
| Connection point | Fault level duty on the connection point circuit breaker and the DNSP's switchgear |
| Generator side of the connection point breaker | Plant main protection operation and grading |
| Generator side of each internal breaker | Internal grading between plant devices |
| Next downstream network device | Grading of the plant with the network beyond it |
| Primary reach of downstream reclosers (lowest fault level) | Desensitisation of devices beyond the plant |
| Primary reach of each upstream device back to the feeder breaker | Desensitisation of devices between the zone substation and the plant |
| Backup reach of each upstream device | Backup protection still sees faults in the next zone |
Repeat for each alternate supply configuration the access standard permits. A plant that is acceptable on its normal feeder can desensitise protection badly when back-fed from an adjacent feeder with a weaker source.
Desensitisation and Operating Factors
The key table in the study shows the fault current as measured by each upstream device, with and without the generator, for each fault type at the end of its reach. A common reviewer comment is that a package tabulated the total current at the fault location instead, which says nothing about whether the device will operate.
From the measured currents the study calculates operating factors: measured current divided by the relevant pickup setting. Primary protection must stay comfortably above one, and backup protection above the DNSP's minimum (values around 1.5 are common, but the DNSP's guideline governs). Where the plant pushes a device below its threshold, the remedies are:
- lower pickup settings on the affected device, if load and cold load pickup allow
- an additional downstream recloser to restore reach
- relocating a normally open point or fuse
- in rare cases, limiting the plant's output in specific configurations
These are network works, usually funded by the connecting customer, so they need to be identified early.
Grading
Grading is the familiar part: the plant's connection point breaker must clear plant faults before the DNSP's recloser operates, and the recloser must clear feeder faults before the plant's breaker trips unnecessarily. Typical practice:
- IEC standard inverse curves, with a target margin around 300 ms and a hard minimum around 200 ms
- overcurrent pickups on the connection point breaker sized from the plant's rated current at nominal voltage, with a modest margin
- instantaneous elements set above transformer inrush and below minimum fault current; where those overlap, DNSPs often will not accept harmonic restraint on their own devices, so the settings have to separate on magnitude or time
- earth fault and sensitive earth fault as low as practical, graded under the DNSP's device
- cable damage curves shown on the time-current plots
Negative sequence elements need care in presentation. Reviewers will ask whether the setting is expressed in negative sequence current or as an equivalent phase overcurrent, and expect it plotted on the same time-current chart as the overcurrent elements.
Earth Faults on an Unearthed Plant: NVD
Inverter step-up transformers are commonly delta on the network side, for example Dyn or Dy11y11 arrangements. Once the feeder breaker opens on an earth fault, the plant's side of the network has no earth reference, so the plant cannot drive an earth fault current that its overcurrent or earth fault elements will see. Meanwhile the unfaulted phases rise towards line voltage.
Neutral voltage displacement (NVD, ANSI 59N) protection detects this by measuring the residual voltage, usually from an open-delta VT winding. It is a standard requirement on the connection point breaker of an embedded generator with no transformer earthing. Settings are typically a high fraction of full displacement with a time delay of a few seconds, graded so that the DNSP's own earth fault protection clears network faults first.
Anti-Islanding and Reclosing
Anti-islanding protection disconnects the plant when it has lost its connection to the network. The usual elements are:
| Element | ANSI | What it detects |
|---|---|---|
| Under and over voltage | 27 / 59 | Voltage drifts once the network reference is gone |
| Under and over frequency | 81U / 81O | Frequency drifts with any load-generation mismatch |
| Rate of change of frequency (RoCoF) | 81R | Fast frequency change at the moment of islanding |
| Vector shift | 78 | A step in voltage phase angle when the island forms |
| Neutral voltage displacement | 59N | Earth fault on the islanded section |
Two tensions have to be resolved.
Anti-islanding against ride-through. Voltage and frequency elements must be set outside the ride-through envelope in the access standard, or the plant will trip for network disturbances it is required to survive. Good practice is to set them as close as practical to the edge of that envelope, inside the plant's own withstand capability, so they remain sensitive to real islands. RoCoF and vector shift settings have tended to move upwards over time across the NEM as large-disturbance events have shown that sensitive settings trip plant unnecessarily. RoCoF relays that use a moving-average window are generally preferred to instantaneous measurement.
Anti-islanding against automatic reclosing. The feeder breaker or recloser will typically attempt to reclose after a dead time. If the plant is still energising the feeder section when it recloses, the two sources reconnect out of phase. The plant's anti-islanding must clear within the shortest dead time of any upstream reclosing device, with margin. State that coordination explicitly in the report, using dead times confirmed by the DNSP.
Where the functions sit
DNSPs generally want voltage, frequency and anti-islanding functions in the customer's connection point breaker protection and in the inverters, not in the DNSP-owned recloser at the connection point. Inter-tripping from the DNSP's recloser to the plant is sometimes offered but often not required for small embedded generators. Confirm the DNSP's preference before designing a scheme around it.
What Reviewers Commonly Raise
Protection Coordination Package Checks
- Fault levels from the DNSP's latest data, with the configuration and date stated
- Plant contribution by fault type, positive and negative sequence, from the OEM
- Every prescribed fault location covered, in every permitted network configuration
- Desensitisation table showing current measured at each device, with and without the plant
- Operating factors for primary and backup protection against the DNSP's thresholds
- Time-current charts showing all devices, cable damage curves and negative sequence elements as overcurrent equivalents
- NVD included where the plant has no transformer earthing
- Voltage and frequency settings reconciled against the ride-through requirements in the access standard
- Anti-islanding clearance time shown to beat upstream reclose dead times
- Protection functions located in the customer's equipment unless the DNSP specifies otherwise
- SLD showing every protection device and CT/VT
Get Protection Settled Before It Holds Up Energisation
Protection is often the last study to close and the first thing to stop energisation. For embedded generators it can also decide which network works the project has to pay for.
ElectraGlobe carries out fault level and protection coordination studies, prepares settings reports and works through DNSP protection review for solar and BESS connections. See our detailed engineering service, or contact us.
Frequently Asked Questions
What is protection desensitisation?
It is the reduction in fault current measured by a protection device caused by another source, such as a generator, supplying part of the fault current from beyond the device. If the measured current falls below the device's pickup, or too close to it, the device may operate slowly or not at all.
Why do embedded generators need NVD protection?
When the network side of the plant's transformers is delta-connected, the plant has no earth reference once the feeder breaker opens. An earth fault on the islanded section produces little current but a large neutral voltage displacement, so a 59N element measuring residual voltage is used to detect it and trip the plant.
How fast must anti-islanding protection operate?
Fast enough to disconnect before any upstream recloser closes back onto the feeder section, and within any limit in the DNSP's access standard. RoCoF and vector shift elements usually operate within a fraction of a second; voltage and frequency elements are slower and are set around the ride-through requirements.
Should voltage and frequency protection be in the DNSP's recloser?
Generally not. DNSPs usually expect those functions in the customer's connection point protection relay and the inverters, with the DNSP's recloser providing overcurrent and earth fault protection for the network.