STP or pole-mounted KTPM: the difference and when to use each

Article author: Viacheslav Yurdyk, Quality Engineer at LK Energy Group.
He verifies that in-house manufactured switchgear complies with DSTU EN IEC 61439 — from incoming inspection of components to acceptance testing of finished products.

An STP (single-pole transformer substation) is a 10(6)/0.4 kV transformer substation in which all the equipment — the power transformer, the protective devices, the metering cabinet — is mounted directly on an overhead line pole or on a separately installed pole. A pole-mounted KTPM does the same job, but on a two-pole structure or on a dedicated mast with a maintenance platform — and can therefore carry a transformer of a higher rating. In technical specifications, designs and everyday conversation the names "single-pole", "pole-mounted" and "column-type substation" are often used interchangeably, even though the two versions differ in structure and in their application limits. Let us look at the difference and the selection criteria.

What an STP and a KTPM are: the common basis

Both substations belong to the same class — packaged outdoor substations with an overhead incomer, the simplest and cheapest way to obtain 0.4 kV from a 10(6) kV line. They have a lot in common:

10(6) kV overhead incomer — the substation is connected directly to the conductors of the overhead line, and the arrangement is usually a dead-end one: the STP or KTPM is the end point of a tap-off;
the same electrical scheme — the VN incomer, protective devices on the 10(6) kV side, the power transformer, and a cable drop to the 0.4 kV RUNN cabinet with metering and outgoing feeders;
no civil foundation — only the standard fixing of the poles in the ground is needed, with no concrete work under the substation itself;
no fence — the high-voltage equipment is placed at a height that cannot be reached without special equipment, so a separate fenced area is not required;
full factory readiness — the substation arrives on site as a complete set: the frame with the VN devices, the transformer, the RUNN cabinet, the brackets; all that remains on site is mounting on the poles and connection.

The difference begins where the common part ends — in the supporting structure.

STP or pole-mounted KTPM: three structural differences

1. Number of poles

A classic STP is a single pole, usually a reinforced-concrete one, carrying — from top to bottom — the incomer with insulators, the surge arresters, the high-voltage fuses, the transformer on brackets and the RUNN cabinet. A pole-mounted KTPM is installed on a two-pole (portal-type) structure or on a dedicated welded mast: the transformer stands on a platform between the uprights, reached by a fixed ladder.

2. Transformer rating

This is the main practical criterion. A single pole with the required strength margin carries a transformer of modest mass — standard single-pole substation versions are designed for 25–63 kVA, with the upper limit usually around 100 kVA. Two poles with a platform can carry considerably more: pole-mounted KTPMs are supplied with transformers of up to 400 kVA. If the calculated load calls for even more, this is no longer a pole-mounted version but a substation on a foundation: an indoor KTP or a BKTP.

3. Maintenance

An STP has no platform — the transformer and the VN devices are serviced from a telescopic lift or a truck-mounted aerial platform. A pole-mounted KTPM has a maintenance platform with a ladder: personnel work on the equipment without special vehicles, which for the operating company often becomes the decisive argument.

The boundary between the terms is, however, a conventional one. In factory catalogues two-pole versions appear both under the designation STP2 (on two poles) and as KTPM — structurally it is one and the same substation. That is why what matters in the questionnaire is not the name but the specifics: the rating, the number of poles, the RUNN content.

Criterion	STP (single-pole)	KTPM (pole-mounted)
Transformer rating	25–63 kVA, up to ~100 kVA	Up to 400 kVA
Installation	One pole (reinforced-concrete)	Two poles or a mast with a platform
Foundation / fence	Not required (only fixing of the pole)	Not required (only fixing of the poles)
Maintenance	From an aerial lift	From a platform with a fixed ladder
VN incomer	Overhead, 10(6) kV	Overhead, 10(6) kV
Typical application	A single small consumer: a borehole, a farm, a village street	Rural networks, farms with a higher load

The ratings are given for standard versions; the specific configuration is determined by the questionnaire.

Scope of supply: what an STP and a KTPM include

The equipment content is the same in both versions in terms of logic and differs only in layout:

High-voltage side: incomer insulators on the cross-arm, a line disconnector (RLND) with a drive — if the design provides for one, OPN surge arresters (RVO spark-gap arresters in older standard solutions) and high-voltage fuses that protect the transformer against short circuits.
Power transformer — a sealed oil-filled TMG unit, 10(6)/0.4 kV, of the appropriate rating, on mounting brackets or on the platform.
0.4 kV cable drop — from the transformer terminals to the RUNN cabinet, in a protective conduit or flexible metal hose.
0.4 kV RUNN cabinet — the incoming circuit breaker, the metering unit (a meter, with a remote data transmission module if required), outgoing feeders on circuit breakers, 0.4 kV OPN surge arresters; on request, a street lighting control unit.
Steel structures and poles: cross-arms, frames, brackets, a platform with a ladder (for the KTPM), reinforced-concrete poles fixed in the ground.

As an example, a typical single-pole substation order from our production practice is an STP1-63 kVA 10/0.4 U1: a TMG-63 transformer with a D/Yn-11 vector group, 10 kV OPN surge arresters, high-voltage fuses, and a RUNN cabinet with a 100 A incoming circuit breaker, metering and outgoing group feeders.

Where single-pole and pole-mounted substations are used

Both versions are in their element in extensive 10(6) kV overhead networks with small, dispersed consumers:

rural distribution networks and street lighting;
farms, grain stores, livestock facilities;
pumping stations, boreholes, irrigation systems;
dacha and cottage developments with overhead supply;
remote and seasonal facilities, temporary power supply.

The selection logic is simple. A load of up to 63–100 kVA and a single small consumer — an STP on one pole is enough: minimum steelwork, the fastest installation, the lowest price. A load of 100–400 kVA, or a requirement for maintenance without an aerial lift — a pole-mounted KTPM on two poles. If, however, the network is a cable one, or the consumer needs two transformers, an AVR scheme or more than 400 kVA, then a kiosk-type KTPN, an indoor KTP or a BKTP is considered: that is already a different class of substation.

Manufacturing of STPs and KTPMs

LK Energy Group manufactures single-pole and pole-mounted transformer substations at its own plant in Odesa: steel structures — from cutting and bending through to powder coating — and in-house assembled RUNN cabinets with acceptance testing of every unit. The standard range in the factory catalogue runs from 25 to 400 kVA in single-pole and two-pole versions. Our design office selects the configuration on the basis of the questionnaire: transformer rating, VN protection scheme, outgoing feeder content, metering unit to suit the network operator's requirements. We help designers fill in the questionnaire and provide standard diagrams for site adaptation.

Frequently asked questions

Are STP and KTPM the same thing?

In everyday usage — almost: both terms describe a substation on poles with an overhead incomer. The technical difference is in the structure: in an STP all the equipment sits on a single pole, while a KTPM is mounted on a mast or on a two-pole structure with a maintenance platform. Two-pole versions appear in manufacturers' catalogues under both names, so when ordering, go by the rating and the layout rather than by the abbreviation.

Why is a transformer above 100 kVA not installed on a single pole?

Because of the mechanical load. As the rating grows, the mass of an oil-filled transformer increases, and together with the switching devices, the RUNN cabinet and ice and wind loads, a single reinforced-concrete pole no longer provides the strength margin required by the codes. That is why a two-pole structure — a pole-mounted KTPM — is used for transformers of 100–400 kVA.

Do an STP or a KTPM need a foundation and a fence?

A separate civil foundation for the substation is not required — the poles are fixed in the ground in the standard way, like overhead line poles. A fence is not required either: the live parts are placed at a height that cannot be reached without special equipment. It is precisely the absence of a foundation and a fence that makes these substations the cheapest in their class. Land matters and the protection zone are defined by the site-adaptation design.

LK Energy Group is a manufacturer of 10(6)/0.4 kV transformer substations: single-pole STPs, pole-mounted KTPMs and kiosk-type KTPNs — with RUNN cabinets of our own manufacture. The full catalogue is in the Equipment section. Send us your technical conditions or a completed questionnaire and we will select the version and price the substation for your site.

STP or pole-mounted KTP: the difference and when to use each