RZA — relay protection and automation: a complex of devices that continuously monitor currents and voltages in a switchgear cubicle and, in the event of a fault (short circuit, earth fault), automatically disconnect the damaged section, while also performing network automation functions — for example, automatic transfer switching (ATS) of the reserve supply. In modern 10 kV cubicles these functions are implemented by a microprocessor-based protection terminal, to which the current circuits from the current transformers (CTs) and the breaker control circuits are connected.

The "RZA scheme" for a cubicle is a set of schematic diagrams: current circuits for protection and measurement, and control and signaling circuits. Choosing a scheme comes down to two questions: which functions does the cubicle need, given its purpose — and which terminal implements those functions.

Which Protection Functions Are Needed for Which Cubicle

Incomer

The basic stages are overcurrent protection (OCP) and instantaneous current cutoff. In a two-section switchgear, the incomer participates in the automatic transfer switching (ATS) scheme together with the bus coupler: if power is lost on one section, the reserve is switched in automatically. Incomer CTs typically have two windings — a metering winding for billing and a protection winding (more on accuracy classes below).

Outgoing Feeder

Overcurrent protection and instantaneous current cutoff — plus earth-fault protection. For earth-fault protection on cable feeder circuits, a zero-sequence current transformer is installed (for example, a TZLU-type core-balance CT).

Bus Coupler (Section Breaker)

Current protection stages and, most importantly, the automatic transfer switching (ATS) scheme: interlocking circuits that monitor the state of the incomers and prevent false closing. In standard solutions, the ATS scheme is distributed between the incomer cubicles and the bus coupler.

VT Cubicle (Voltage Transformer)

There is no power breaker here — this is a measuring cubicle: it monitors the section voltage and feeds the metering circuits. In the outdoor switchgear (KRUN) solution, a separate voltage-protection terminal, the RS83-V1, is used for this cubicle.

Auxiliary Power Transformer (SPT)

Supplies the auxiliary (control) circuits, heating and lighting. Under the standard-solution schemes, the auxiliary power transformer cubicle implements automatic transfer switching of the auxiliary supply (ATS-AS) and insulation monitoring of the secondary circuits.

RS83 Series Terminals in LK Energy's Solutions

RS83-AV2: the base terminal for breaker-equipped cubicles in the flagship 10 kV switchgear (RP) solution on KSO-type cubicles (VV-N10 breakers), in the version with VV/TEL breakers, and in the 6 kV switchgear (KRP) solution for solar power plants: incomers, outgoing feeders, bus couplers, ATS schemes.
RS83-AV3: used in the 10 kV outdoor switchgear (KRUN) solution (VV/N10S breakers): in addition to the basic protection stages, it implements, per the design schemes, breaker failure protection (BFP), bus-bar logic protection (BLP) and automatic frequency load shedding (AFLS); on incomers and the bus coupler it also provides ATS.
RS83-V1: voltage-protection terminal for VT cubicles in the KRUN solution.
RS83-A2.0: found in archived projects in the library (2021 design).
REF611: (ABB) — an alternative platform; a single implementation exists in LK Energy's archive. Mentioned for completeness: implementation on other terminals is agreed separately.

The library's fourth hardware platform is the MRZS terminals on KRU-E switchgear cubicles (KM-1F frame size) with withdrawable elements: see the standard 6(10) kV switchgear (RP) solution on KM-1F with MRZS terminals.

What Affects the Choice of RZA Scheme

Cubicle type and purpose: determines the minimum required set of functions (see above).
Grid operator requirements: The technical conditions for grid connection may call for additional automation functions — for example, participation in automatic frequency load shedding (AFLS). This directly affects the choice of terminal: per the design schemes, AFLS is implemented in the RS83-AV3 version.
Metering: If a cubicle is part of a commercial metering node, CTs with two windings are selected: accuracy class 0.5S for metering circuits (automated commercial electricity metering system, ASKOE), and 10P for protection circuits. This is exactly how incomers are configured in the standard solutions.
Breaker platform: Protection functions do not depend on the breaker, but the terminal's control circuits are adapted to the drive of the specific device — the library's solutions include versions for VV-N10, VV/TEL and VV/N10S breakers.
Switchgear configuration: Two busbar sections require a mandatory ATS scheme on the incomers and bus coupler; a single-section layout is simpler.

Frequently Asked Questions

Does the standard solution include protection settings?

No. A standard solution consists of the schematic diagrams for protection, control and signaling circuits. The calculation of RZA settings is performed by the design engineer when adapting the solution to a specific site, in accordance with the applicable PUE (electrical installation rules) and DSTU (Ukrainian national standards) requirements.

What is the difference between the RS83-AV2 and RS83-AV3?

Per the standard-solution schemes: both terminals cover the basic functions for breaker-equipped cubicles (overcurrent protection, instantaneous cutoff, earth-fault protection, ATS), while the RS83-AV3 version (KRUN solution) additionally implements breaker failure protection, bus-bar logic protection and automatic frequency load shedding. The required set of functions is determined by the design and the technical conditions.

Is implementation on other terminals possible?

Yes. The base platform for the solutions is the RS83 series, but implementation on other hardware is agreed via a technical questionnaire; the archive includes projects on the RS83-A2.0 and REF611.

How to Choose a Protection and Automation Scheme for a 10 kV Switchgear Cubicle: Protection Functions and Terminals