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

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.

Site photos of the hydropower station

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 for 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

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.

表格

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

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

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

Integrated 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

Integrated 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

Same protection logics as HV backup protection:

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)

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

Integrated 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 Overall Retrofit Recommendation

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.