{"id":2619,"date":"2026-09-23T17:30:49","date_gmt":"2026-09-23T09:30:49","guid":{"rendered":"https:\/\/zishengtransformer.com\/?p=2619"},"modified":"2026-09-23T17:30:49","modified_gmt":"2026-09-23T09:30:49","slug":"transformer-protection-ct-specification-africa","status":"publish","type":"post","link":"https:\/\/zishengtransformer.com\/fr\/transformer-protection-ct-specification-africa\/","title":{"rendered":"CT Specification for Transformer Protection in African Industrial Projects: Ratio, Class and Burden"},"content":{"rendered":"

Protection design is often discussed after the transformer rating has been selected. In practice, the current transformer (CT) specification is one of the inputs that determines whether the relay can see a fault clearly. At Zisheng Electric, our engineers review the transformer, switchgear, CTs and protection settings as one measurement chain rather than treating the CT as a catalogue accessory. This article explains how an EPC or industrial buyer can prepare a practical transformer protection CT specification<\/strong> for African projects.<\/p>\n

The focus is not a generic CT definition. It is the selection logic behind ratio, accuracy class, burden, knee-point performance, wiring and the factory acceptance evidence needed before shipment.<\/p>\n

\"Transformer
Protection interface review for transformer and switchgear equipment.<\/figcaption><\/figure>\n

Why CT selection changes transformer protection performance<\/h2>\n

The protection path is Grid or generator \u2192 MV switchgear \u2192 transformer \u2192 LV or process loads. A CT sits between the primary conductor and the relay. If its ratio is too high, normal load current occupies only a small part of the relay range and sensitive earth-fault elements may lose resolution. If the ratio is too low, the CT can saturate during a high-current external fault and create a false differential or restricted-earth-fault signal.<\/p>\n

A CT specification must therefore match the maximum continuous load, the available short-circuit current, the relay algorithm, the cable length and the grounding arrangement. A transformer that is correctly rated in kVA can still have unreliable protection if these interface values are missing.<\/p>\n

Inputs to confirm before writing the CT data sheet<\/h2>\n

Load current and future stages<\/h3>\n

Use the transformer rated current as a starting point, but do not stop there. Confirm the present load, planned expansion, motor starting duty, generator contribution and any export or reverse-power mode. For an industrial park in Kenya, Ghana or Nigeria, the utility supply may be supplemented by generators during outages. The CT must remain suitable for both utility fault contribution and generator-fed faults.<\/p>\n

Fault current and protection philosophy<\/h3>\n

Request the utility short-circuit level at the point of connection and the project fault study. Check whether the scheme uses overcurrent, earth-fault, differential, REF, busbar or distance functions. A metering CT and a protection CT may have different accuracy requirements. They should not be merged into one unspecified core.<\/p>\n

Secondary circuit and burden<\/h3>\n

Burden includes the relay input, test switch, terminal blocks and the resistance of the secondary leads. Long control-cable runs between an outdoor transformer and a remote control room can materially increase VA burden. The procurement schedule should state the maximum one-way cable length, conductor cross-section and whether the CT secondary is wired in a dedicated multicore.<\/p>\n\n\n\n\n\n\n\n\n
Contributions \u00e0 la conception<\/th>\nPourquoi est-ce important<\/th>\nV\u00e9rification des approvisionnements<\/th>\n<\/tr>\n<\/thead>\n
Maximum load and expansion<\/td>\nSets a usable ratio without losing low-current sensitivity<\/td>\nState present and future MVA, motor starting and generator operation<\/td>\n<\/tr>\n
Through-fault current and clearing time<\/td>\nDrives protection class, knee point and saturation margin<\/td>\nAttach the short-circuit study and relay clearing time<\/td>\n<\/tr>\n
Relay and cable burden<\/td>\nDetermines whether the CT can reproduce current at the relay<\/td>\nProvide relay VA, lead length, conductor size and test links<\/td>\n<\/tr>\n
Grounding and earth-fault scheme<\/td>\nChanges residual current sensitivity and stability requirements<\/td>\nIdentify neutral CT, core-balance CT and REF arrangement<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Choosing the CT ratio without creating a blind spot<\/h2>\n

For a three-phase transformer, rated line current is approximately I = S\/(\u221a3 \u00d7 V). As an example, a 5 MVA, 11 kV transformer has a rated current of about 262 A on the 11 kV side. A 300\/1 A CT may give useful resolution for normal load, but the final choice depends on the overload margin, motor starting current and the utility protection grading. A 400\/1 A ratio may be easier to coordinate with upstream protection, yet it reduces the relay\u2019s current resolution at light load.<\/p>\n

Do not select a ratio solely from the transformer nameplate. Check the relay pickup range, maximum prospective fault current, CT thermal short-time current and the highest expected inrush or starting current. When the project will be expanded in phases, ask the manufacturer to show the ratio choice against both phase-one and final load.<\/p>\n

\"Engineers
Factory test evidence supports the relay and CT interface review.<\/figcaption><\/figure>\n

Accuracy class, knee point and saturation checks<\/h2>\n

Metering cores are normally specified by an accuracy class and instrument security factor. Protection cores are specified by a protection class such as 5P or 10P, an accuracy-limit factor and a rated burden. Differential and REF schemes may require a high-accuracy class or a knee-point voltage defined by the relay manufacturer and the stability calculation.<\/p>\n

For an external fault, the CT secondary voltage rises with secondary current and total circuit impedance. Once the magnetic core approaches saturation, the secondary waveform is distorted. The relay can then under-read, over-read or see a false residual current. This is why \u201c5P10, 15 VA\u201d without a complete burden and stability calculation is not a complete specification.<\/p>\n\n\n\n\n\n\n\n
Protection function<\/th>\nTypical engineering concern<\/th>\nPi\u00e8ces \u00e0 demander<\/th>\n<\/tr>\n
Feeder overcurrent and earth fault<\/td>\nRatio, class and cable burden must support grading<\/td>\nRelay settings, burden calculation and routine test results<\/td>\n<\/tr>\n
Transformer differential<\/td>\nCT ratio matching, vector-group compensation and saturation stability<\/td>\nCT excitation data, polarity\/ratio tests and stability calculation<\/td>\n<\/tr>\n
Restricted earth fault<\/td>\nHigh sensitivity with stable performance for external faults<\/td>\nNeutral and phase CT data, knee-point requirement and wiring diagram<\/td>\n<\/tr>\n
Energy metering<\/td>\nLow error around normal operating current<\/td>\nMetering class, burden and calibration certificate<\/td>\n<\/tr>\n<\/thead>\n<\/table>\n

Installation details that are often missed on African sites<\/h2>\n

Dust, humidity, long cable routes and limited commissioning resources affect CT performance indirectly. Outdoor MV panels need adequate creepage, sealed cable entries and a clear shorting link arrangement. In coastal Ghana or Tanzania, salt contamination can create surface leakage around terminals. In dry inland sites, dust can enter poorly sealed marshalling boxes. The transformer protection specification should assign enclosure rating, heater or anti-condensation requirements, gland type and terminal identification.<\/p>\n

Secondary circuits must never be left open while the primary is energized. Specify test blocks that short the CT before a relay or meter is removed. Confirm polarity markings at both the transformer and switchgear ends. During commissioning, inject current through the complete circuit where possible; a relay-only test can miss reversed polarity or a loose terminal.<\/p>\n

At Zisheng Electric, the interface review normally compares the CT schedule with the single-line diagram, transformer vector group, switchgear GA drawing and relay I\/O list. This catches mismatched ratios before the FAT rather than during energization.<\/p>\n

\"High-voltage
Insulation and routine test records should be linked to the protection dossier.<\/figcaption><\/figure>\n

FAT and document review checklist<\/h2>\n

Ask for a signed CT schedule, approved schematic, routine test records and the manufacturer\u2019s excitation or saturation data where required by the protection study. The FAT should verify ratio, polarity, winding resistance, insulation resistance and accuracy or excitation performance for the specified cores. The inspector should trace terminal numbers from the CT to the marshalling cabinet and switchgear terminal block.<\/p>\n

Utiliser la transformer energization study guidance<\/a> when reviewing inrush and switching interfaces, and compare the scheme with the site\u2019s voltage regulation and FAT checks<\/a>. For general transformer selection, the oil-immersed versus dry-type transformer guide<\/a> gives the equipment-level context. Zisheng Electric also supplies des transformateurs immerg\u00e9s dans l'huile<\/a> and associated distribution equipment for project-specific protection interfaces.<\/p>\n

Procurement questions to put in the inquiry<\/h2>\n
    \n
  • What are the primary and secondary CT ratios, cores and terminals?<\/li>\n
  • Which cores are for metering, feeder protection, differential or REF?<\/li>\n
  • What burden, accuracy class, knee-point voltage and short-time current are guaranteed?<\/li>\n
  • What are the relay model, cable length, conductor size and grounding method?<\/li>\n
  • Which CT tests and signed drawings will be submitted before FAT release?<\/li>\n<\/ul>\n

    Conclusion: make the CT a documented protection interface<\/h2>\n

    A robust transformer protection CT specification<\/strong> connects the load schedule, fault study, relay settings, switchgear wiring and transformer FAT. It reduces nuisance trips and prevents a protection blind spot caused by saturation, excessive burden or incorrect polarity. Send Zisheng Electric the single-line diagram, transformer rating, fault level, relay list, cable route and site conditions. Our engineering team will review the requirements and respond to project inquiries within 24 hours.<\/p>","protected":false},"excerpt":{"rendered":"

    A practical engineering guide to transformer protection CT ratio, class, burden, saturation and FAT checks for African industrial projects.<\/p>","protected":false},"author":2,"featured_media":2569,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[26],"tags":[254,252,253,157,251],"class_list":["post-2619","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-technical-guidance","tag-african-industrial-projects","tag-ct-ratio-selection","tag-ct-saturation","tag-transformer-fat","tag-transformer-protection-ct"],"acf":[],"_links":{"self":[{"href":"https:\/\/zishengtransformer.com\/fr\/wp-json\/wp\/v2\/posts\/2619","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/zishengtransformer.com\/fr\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/zishengtransformer.com\/fr\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/zishengtransformer.com\/fr\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/zishengtransformer.com\/fr\/wp-json\/wp\/v2\/comments?post=2619"}],"version-history":[{"count":2,"href":"https:\/\/zishengtransformer.com\/fr\/wp-json\/wp\/v2\/posts\/2619\/revisions"}],"predecessor-version":[{"id":2622,"href":"https:\/\/zishengtransformer.com\/fr\/wp-json\/wp\/v2\/posts\/2619\/revisions\/2622"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/zishengtransformer.com\/fr\/wp-json\/wp\/v2\/media\/2569"}],"wp:attachment":[{"href":"https:\/\/zishengtransformer.com\/fr\/wp-json\/wp\/v2\/media?parent=2619"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/zishengtransformer.com\/fr\/wp-json\/wp\/v2\/categories?post=2619"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/zishengtransformer.com\/fr\/wp-json\/wp\/v2\/tags?post=2619"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}