Renovation Scheme for Generator and Transformer Protection Cabinets, Yongshan No.5 Hydropower Station, Vietnam

Generator and transformer protection cabinet renovation at Yongshan No.5 Hydropower Station in Vietnam replaced a pair of integrated devices with separate ones for each function. The station runs a single 14 MW generator at 11 kV, stepped up to 115 kV, and its 31 secondary cabinets had been in service for more than ten years.

The interesting part is not the hardware count. The original scheme concentrated every generator function on one device, so a single board failure took all of them out at once. The revised scheme gives main protection, backup protection, rotor earth fault protection and measurement and control their own devices, which is what the regional grid operator wanted before it would accept reconnection.

The cabinets stay where they are. The CT assignments are what had to be re-checked.

At a Glance: The Generator and Transformer Protection Cabinet Renovation

• Site: Yongshan No.5 Hydropower Station, Vietnam: one 14 MW generator at 11 kV, stepped up to 115 kV by the main transformer.
• Inventory: 31 secondary control and protection cabinets, all more than ten years old.
• Generator side: 87G differential on CT1 and CT2, backup protection on CT3, 64R rotor earth fault, plus a separate measurement and control device.
• Transformer side: 87T differential on CT4 and CT5, HV backup on CT5, LV backup on CT4, non-electrical quantities (63, 71, 96, 26-O, 26-W) and a station service branch device on CT6.
• Trip targets: QF1 generator outlet, QF2 transformer HV side, QF3 station service branch.
• What changed: one integrated device per cabinet becomes a dedicated device per function.
• Why: a single-point failure on the old layout disabled every generator function at once, and the grid acceptance test looks for exactly that.
• Next stage: the remaining cabinets show the same age symptoms and were quoted separately.

Device codes follow ANSI/IEEE C37.2-2022. Relay performance and testing reference IEC 60255-1:2022, and generator protection practice is set out in IEEE C37.102-2023. Station-level communication runs on IEC 61850, and the cabinets are built as assemblies to IEC 61439-2:2020.

Project Overview

Yongshan No.5 Hydropower Station is equipped with a generator with a rated capacity of 14 MW and a rated outlet voltage of 11 kV. Electric energy is boosted to 115 kV via the main step-up transformer and then delivered to the power grid.

The secondary equipment of the station has been in operation for more than 10 years and has reached the end of its service life, suffering from serious aging of components, discontinued spare parts, degraded protection performance, and failure to meet current power grid dispatching and hydropower technical specifications. A full upgrade and retrofit of the secondary protection & automation system is required.

This project requires the retrofit of the generator and transformer protection cabinets. Everything else in the inventory stays in place for now, and that boundary shapes every decision described below.

That is the setting for the generator and transformer protection cabinet renovation described below.

Site photos of Yongshan No.5 Hydropower Station

Yongshan No.5 is one of the small hydropower stations where the whole secondary system was supplied as a single package; the hydropower relay protection page covers the protection side of that package in general terms.

Original Configuration of Protection & Control Cabinets

The station is originally equipped with a total of 31 sets of secondary control and protection cabinets. The detailed list is shown in the table below:

Serial No.	Cabinet No.	Cabinet Name	Dimension	Unit	Quantity
1	1K	Control Console	800×600 mm	Set	1
2	2K	Communication Cabinet	800×800 mm	Set	1
3	3K	G1 Generator Protection Cabinet	800×600 mm	Set	1
4	4K	G2 Generator Protection Cabinet	800×600 mm	Set	1
5	5K	T1 Transformer Protection Cabinet	800×600 mm	Set	1
6	6K	T2 Transformer Protection Cabinet	800×600 mm	Set	1
7	7K	Common Local Control Unit (LCU) Cabinet	800×600 mm	Set	1
8	8K	No.1 Circuit Breaker Protection Cabinet	800×600 mm	Set	1
9	9K	No.2 Circuit Breaker Protection Cabinet	800×600 mm	Set	1
10	10K	Y1 Line Protection Cabinet	800×600 mm	Set	1
11	11K	Y2 Line Protection Cabinet	800×800 mm	Set	1
12	12K	Y3 Line Protection Cabinet	800×600 mm	Set	1
13	13K	Energy Metering Cabinet A1	800×600 mm	Set	1
14	14K	Energy Metering Cabinet A2	800×600 mm	Set	1
15	15K	Switchyard LCU Cabinet A1	800×600 mm	Set	1
16	16K	Switchyard LCU Cabinet A2	800×600 mm	Set	1
17	17K	Emergency Lighting Switch & AC Power Panel	800×600 mm	Set	1
18	18K	Fire Alarm Control Cabinet	800×600 mm	Set	1
19	19K	Optical Fiber Communication Cabinet	800×600 mm	Set	1
20	20K	Dispatching Switch Cabinet	800×600 mm	Set	1
21	21K	Spare Cabinet	800×600 mm	Set	1
22	22K	DC Feeder Panel (400 Ah)	800×600 mm	Set	1
23	23K	DC Charging Panel (400 Ah)	800×600 mm	Set	1
24~26	24K~26K	Battery Cabinet (400 Ah)	800×600 mm	Set	3
27~28	27K~28K	Communication DC Panel (200 Ah)	800×600 mm	Set	2
29~31	29K~31K	Spare Cabinet	800×600 mm	Set	3

Original Protection Function List (ANSI Standard)

The original protection system covers full fault diagnosis functions for generators, main transformers and outgoing lines, as listed in the table below:

Code	English Name
FR	Fault Recorder
I/O	Measuring and control transducer unit
EM	Energy meter
PMAC	AC sample device
21/21N	Distance protection relay or phases short circuit and three levels earthing
24	Over exciting protection
25	Synchronizing check device
26-O	Transformer oil thermal overload protection
26-W	Transformer winding thermal overload protection
27/59	Undervoltage/overvoltage protection
32	Directional reverse power relay
40	Loss of excitation protection
46	Generator negative-sequence current protection
26/49	Overload relay
50BF	Inoperated protector for circuit breaker
50/51	Instantaneous/time overcurrent protection
51N	Time earthing overcurrent protection
59	Overvoltage protection
59N	Earthing protection
63	Gas protection
64R	Rotor earthing-fault protection
64S+27TN	100% stator earthing-fault protection
67/67N	Directional overcurrent and earth-fault protection
71	Low oil level protection
74	Trip circuit supervision relay
78	Out-of-step protection
79/25	Autoreclosing and synchronism checking relay
87G	Generator differential protection
87GT	Transformer-generator differential protection
87T	Transformer differential protection
96	Oil pressure relay

Original Scheme vs. Optimized Revised Scheme

Both schemes cover the same functions. A generator and transformer protection cabinet renovation usually turns on how those functions are distributed, not on which ones are present.

Both schemes cover the same functions. A generator and transformer protection cabinet renovation usually turns on how those functions are distributed, not on which ones are present.

Original On-site Scheme for Generator and Transformer Protection Cabinets

Item	Equipment Name	Model	Unit	Qty
Ⅰ	Generator Protection & Control Cabinet	–	Panel	1
1	Generator-Transformer Unit Protection Device	DEP-531N	Set	1
2	Generator Main Protection Device	DEP-561N	Set	1
3	Generator Backup Protection Device	DEP-565N	Set	1
4	Cabinet Body & Accessories	2260800600	Set	1
Ⅱ	Transformer Protection & Control Cabinet	–	Panel	1
1	Transformer Main Differential Protection Device	DEP-531N	Set	1
2	Transformer Backup Protection Device	DEP-535N	Set	1
3	Transformer Non-Electrical Quantity Protection Device	DEP-539B	Set	1
4	Station Service Transformer Protection Device	DEP-536N	Set	1
5	Cabinet Body & Accessories	2260800600	Set	1

Optimized Revised Retrofit Scheme (Recommended for Implementation)

After technical discussion and review, the scheme is adjusted to separate main protection, backup protection, rotor earth fault protection and measurement & control functions into independent cabinets, improving system redundancy, operation reliability and maintenance accessibility.

Item	Equipment Name	Unit	Qty
Ⅰ	Generator Protection & Control Cabinet	Panel	2
1	Generator Differential Protection Device	Set	1
2	Generator Backup Protection Device	Set	1
3	Generator Rotor Earth Fault Protection Device	Set	1
4	Generator Measurement & Control Device	Set	1
5	Cabinet Body & Full Accessories	Set	1
Ⅱ	Transformer Protection & Control Cabinet	Panel	2
1	Transformer Main Differential Protection Device	Set	1
2	Transformer HV Side Backup Protection Device	Set	1
	Transformer LV Side Backup Protection Device	Set	1
3	Transformer Non-Electrical Quantity Protection Device	Set	1
4	Station Service Transformer Branch Protection Device	Set	1
5	Cabinet Body & Full Accessories	Set	1

Reasons for Scheme Optimization

Three arguments drove the change, and only one of them is about hardware age. A hydropower protection retrofit has to answer the grid operator, the maintenance crew and the outage planner at the same time.

Compliance with Current Technical Codes for Small & Medium Hydraulic Generators

For small-capacity generators such as the 14 MW unit of this station, complete and independent protection configuration is mandatory per industry technical specifications.

The original integrated single-cabinet design concentrates all protection functions on a small number of devices; single-point component failure will disable all generator protections. The revised split cabinet layout realizes physical isolation of main protection, backup protection and rotor earth fault protection, eliminating single-point failure risks and meeting power grid grid-connection acceptance requirements.

Function, CT Sampling & Trip Logic of Each Protection Device

Start with the CTs.

Before going device by device, the CT allocation the whole scheme depends on:

• CT1: generator terminal, paired with CT2 for 87G.
• CT2: generator neutral side.
• CT3: generator terminal; feeds the backup device, and substitutes for CT4 in the alternative 3-winding differential.
• CT4: transformer LV side, for 87T and LV backup.
• CT5: transformer HV side, for 87T and HV backup.
• CT6: station service branch.

Generator Protection System

Generator Differential Protection Device (ANSI 87G)

Sampling CT1 (Generator Terminal) and CT2 (Generator Neutral) to detect internal winding short-circuit and inter-turn faults of the generator.

It trips generator outlet circuit breaker QF1 upon fault detection.

Generator Backup Protection Device

The generator backup device integrates the following protection logics:

ANSI Code	Protection Function Description
67/27/59	directional overcurrent blocked by composite voltage
49	overload protection
46	negative-sequence overcurrent protection
81U+27	under-frequency protection
81O+27	over-frequency protection
64S	100% stator earth fault protection
59	overvoltage protection
27	undervoltage protection
32	reverse power protection
40	loss of excitation protection

Sampling CT3 at generator terminal; trips QF1 for external short-circuit and abnormal operating conditions.

Generator Rotor Earth Fault Protection Device (ANSI 64R)

Monitors insulation status of rotor winding: issues alarm signal for single-point rotor earth fault; trips QF1 for double-point rotor earth fault to avoid rotor shaft magnetization and severe unit vibration damage.

Generator Measurement & Control Device

Collects real-time operating data including generator outlet 3-phase voltage, 3-phase current, active/reactive power, power factor and frequency.

It receives remote opening/closing commands for QF1 from central control room and uploads status signals to station LCU system.

Transformer Protection Methods

Transformer Main Differential Protection Device (ANSI 87T)

Standard sampling scheme: CT5 (HV side) and CT4 (LV side), detects internal winding and lead faults of the transformer, and trips QF1 (LV side), QF2 (HV side) and QF3 (station service branch) simultaneously.

Alternative 3-winding differential scheme: If no space is available to install two independent groups of CTs at CT4 position (one for differential protection, one for LV backup protection), 3-winding differential logic can be adopted to sample CT3 (generator side), CT5 (HV side) and CT6 (station service branch), with identical trip logic.

Transformer HV Side Backup Protection Device

The transformer HV side backup device integrates the following logics:

ANSI Code	Protection Function Description
67/51/27/59	composite voltage blocked directional overcurrent
51	inverse-time overcurrent
49	winding & oil overload protection
67N/51N/59N	neutral directional & neutral voltage blocked earth fault protection
59	overvoltage
27	undervoltage
59N	neutral overvoltage
60	PT circuit breakage supervision
74	trip circuit supervision

Sampling CT5 at HV side; only trips HV side circuit breaker QF2 when fault occurs.

Transformer LV Side Backup Protection Device

The LV side backup device uses the same protection logics as the HV side:

ANSI Code	Protection Function Description
67/51/27/59	composite voltage blocked directional overcurrent
51	inverse-time overcurrent
49	overload
67N/51N/59N	neutral earth fault protection
59/27	voltage protection
60	PT breakage monitoring
74	trip circuit supervision

Sampling CT4 at LV side; trips QF1 (main LV outlet) and QF3 (station service branch) for LV side faults.

Transformer Non-Electrical Quantity Protection Device (ANSI 63 Gas, 71 Low Oil Level, 96 Pressure Relief, 26-O/26-W Thermal Protection)

The non-electrical quantity device accesses all transformer body non-electrical signals:

Item	Function Status
light gas	alarm
heavy gas	trip
high oil temperature	trip/alarm
winding overtemperature	trip/alarm
low oil level	alarm
pressure relief valve action	trip

Severe faults trigger tripping of QF1, QF2 and QF3 on all three sides.

Station Service Branch Protection Device

The station service branch device integrates the following logics:

ANSI Code	Protection Function Description
50/51/27/59	instantaneous & time-delayed composite voltage blocked overcurrent
51	inverse-time overcurrent
49	overload protection
51N/59N	neutral earth fault protection
60	PT breakage supervision
74	trip circuit supervision

Sampling CT6 of station service branch; only trips QF3 to isolate branch faults without affecting main transformer and generator.

Improved Operation & Maintenance Efficiency

Separated layout of main protection, backup protection, rotor earth fault protection and measurement & control devices allows independent inspection, calibration and replacement without mutual interference.

Only the faulty device needs to be decommissioned during maintenance, while other protections remain in service, greatly shortening unit outage duration and reducing power generation loss.

Compliance with Grid Grid-Connection Standards

Regional power grid dispatching authorities enforce strict redundancy requirements for small hydropower grid-connected equipment.

The original integrated single-cabinet scheme fails grid connection acceptance due to insufficient fault isolation capacity.

The revised split independent cabinet design features clear fault tripping logic and adequate protection redundancy, which can pass grid special acceptance at one time.

Supplementary Retrofit Recommendation for the Rest of the Station

The two cabinets described above were the urgent part of a wider Vietnam hydropower retrofit. What follows is the case for turning this into a protection cabinet upgrade case that covers the rest of the station while it is already open.

The station has been in operation for more than 10 years. All secondary cabinets listed in the inventory, including line protection cabinets, switchyard LCU cabinets, common LCU cabinets, communication power panels, metering cabinets, dispatching switch cabinets and DC system panels, suffer widespread component aging.

Electronic capacitors, relays and main control boards face high risk of failure, and original spare parts have been discontinued by manufacturers. Long-term operation with degraded equipment may lead to protection failure, monitoring loss and communication interruption, posing major safety hazards to the power station.

It is recommended to complete full replacement of all aged secondary cabinets within this retrofit project, realizing full-station secondary system upgrade with unified hardware platform, communication protocol and human-machine interface, to achieve intelligent transformation of the station. After retrofit, the system will deliver the following benefits:

1. Integrated networking of full-station protection, measurement & control, fault recording and metering equipment, fully meeting remote dispatching and four-remote signal upload requirements;
2. Unified hardware platform reduces spare part categories and long-term operation & maintenance costs;
3. Complete fault recorder and Sequence of Event (SOE) functions support rapid fault tracing and analysis;
4. All equipment complies with IEC 61850 digital communication standard, compatible with long-term unattended and minimally-attended operation mode of hydropower stations.

That list is the argument for doing the whole station at once rather than in stages. The generator and transformer protection cabinet renovation described above was the first slice.

Two cabinets now, the rest later.

Frequently Asked Questions

Why split one integrated device into several?

This generator and transformer protection cabinet renovation splits the functions because on the original layout every generator function lived on one device, so a single board failure removed all of them together. Splitting main protection, backup protection, rotor earth fault protection and measurement and control into separate devices means a fault in one leaves the others running, and a technician can pull one device for calibration without taking the unit out of service.

When does the 3-winding differential scheme apply?

When there is no room at the CT4 position for two independent CT groups. The standard scheme samples CT5 and CT4; if CT4 cannot carry both the differential and the LV backup function, the relay samples CT3, CT5 and CT6 instead and applies 3-winding differential logic. Trip targets stay identical.

Does the renovation change the trip matrix?

The targets stay the same: QF1 for the generator outlet, QF2 for the transformer HV side and QF3 for the station service branch. What changes is which device issues the trip: rotor earth fault goes to its own relay instead of sharing a board, and the branch device trips QF3 only, so a branch fault no longer puts the main transformer and generator at risk.

What happens to the other cabinets?

They were surveyed and quoted, not replaced in this stage. Line protection, switchyard and common LCU, metering, dispatching switch and DC panels all show the same age symptoms (electrolytic capacitors, auxiliary relays and main control boards), and the recommendation on file is to replace them while the station is already shut down.

What drives the outage plan in a job like this?

In a generator and transformer protection cabinet renovation the schedule is set by CT availability, not cabinet delivery. Every device in the revised scheme depends on a specific CT, and the two cases where CT4 cannot be shared are what push the schedule. Confirming the CT allocation against the existing switchgear before the cabinets arrive is the step that decides whether the outage holds.

Related Reading

• Hydropower Relay Protection and Automation Solutions: the protection and LCU scope this project sits inside.
• Hydropower Plant Upgrade Solution for Vietnam: the wider station upgrade, including excitation and SCADA.
• Generator Protection Relay: function-by-function reference for the generator side.
• Generator Differential Protection Relay 87G: the main protection device used here.
• Rotor Earth Fault Protection Relay: the 64R device that moved to its own slot.
• 87T Three-Winding Transformer Protection: the alternative differential scheme.
• Transformer Gas and Temperature Protection: the non-electrical quantity device.
• IEC 61850 Transformer Protection Relay: the station communication requirement.
• Loss of Excitation Protection: ANSI 40, listed in the original function inventory.
• PLC Control Panel for Hydropower and Pumping Stations: the LCU cabinets flagged for the next stage.

Technical References

Standard	Edition	What it governs here	Where it applies
ANSI/IEEE C37.2	2022	Device function numbers (87G, 87T, 64R, 63, 71, 96, 26-O, 26-W)	Function list and cabinet labelling
IEC 60255-1	2022	General requirements for measuring relays and protection equipment	Device performance and acceptance
IEC 60255-187-1	2021	Functional requirements for differential protection	87G and 87T setting and testing
IEEE C37.102	2023	Guide for AC generator protection	Generator function selection
IEC 61850	Series	Communication networks and systems for power utility automation	Station-level data exchange
IEC 61439-2	2020	Low-voltage switchgear and controlgear assemblies	Cabinet construction
IEC 60529	1989+AMD1:1999+AMD2:2013	Degrees of protection provided by enclosures	Cabinet IP rating

Editions are the ones current at the time of writing. Check the version named in the project specification before you order against this page.

Send the single-line diagram, the CT list and the existing cabinet inventory, and the generator and transformer protection cabinet renovation can be quoted against the scheme you already have.