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ASME B133.5-1978 — Gas Turbine Electrical Equipment
⚡ ASME B133.5-1978 is the B133 series standard covering electrical equipment, other than controls, supplied with a gas turbine. Its aim is to provide guidelines and criteria for specifying that equipment. Much of the electrical equipment applies only to larger generator-drive installations, but where applicable the Standard can be used for other gas turbine drives. The Standard divides gas turbine electrical equipment into four major areas: Main Power System, Auxiliary Power System, Direct Current System, and Relaying. It notes that in almost all cases the equipment is covered by standards, guidelines, or recommended practices documented elsewhere, and its role is to supplement those references and point out the areas of specific interest for a gas turbine application. The foreword adds that B133.5 applies mostly to gas turbine stations used for power generation, covering all electric utility size units but not small power supplies of a semi-portable nature.
💡 B133.5 is a “tailoring guide”: it does not repeat the generic standards for motors and transformers, but tells the purchaser which electrical issues are most error-prone in a gas turbine station and must be made explicit in the specification.
1. Scope and the Four Major Areas 📌
| Electrical Area | Coverage | Typical Equipment / Content |
|---|---|---|
| Main Power System | From the generator neutral grounding connection up to, but not including, the main power transformer or bus | Generator with cooling, excitation system, breakers, current transformers, etc. |
| Auxiliary Power System | The gas turbine station ac supply and its consumers | Station-service transformers, switchgear, motors and starting devices |
| Direct Current System | Battery and charger only | Power for control, protection, emergency shutdown, and emergency lighting |
| Relaying | Electric system protective relaying for the gas turbine station itself | Generator, bus, transformer, and station-service protection |
⚠️ Note the boundary definitions: the main power system stops at the main power transformer or bus, and the dc system includes only the battery and charger. These boundaries must be mapped exactly onto the equipment supply packages during contract splitting to avoid gaps.
2. Main Power System: Generator Requirements 🔧
The Standard requires that the generator connected to the gas turbine drive shaft be in accordance with the applicable requirements of ANSI C50.10, C50.12, C50.13, and C50.14, with special features and accessories for the particular application specified.
- Design and output: per ANSI C50.14, generator output at rated power factor should be equal to or greater than that of the gas turbine for the base and peak operating modes and over the specified ambient temperature range.
- Cooling: generators may be air or hydrogen cooled depending on the application, expected usage, and site atmosphere. Air-cooled units need a weatherproof enclosure unless designed for outdoor service, inlet-air filtration with pressure gauges to indicate maintenance, silencers on intake and discharge ducts where required for plant sound limits, space heaters for shutdown periods, heated air for low-temperature applications or inlet-air cooling for high-temperature applications, and temperature detectors for stator winding and cooling air. Hydrogen-cooled units need a casing built to withstand an internal hydrogen explosion and limit damage to enclosed parts, plus provision for monitoring and maintaining hydrogen pressure and purity, with CO2 purge when pressure or purity fall outside acceptable limits.
- Peaking duty adaptation: the cyclic nature of the gas turbine peaking application must be recognized in stator and rotor design to minimize metal fatigue and insulation abrasion caused by winding movement in the slots and many thermal cycles — a distinct design consideration from base-load units, which have fewer starts but much more time at rated load.
- Noise: generator audible noise should be compatible with gas turbine sound emission requirements.
✅ Procurement essentials: for peaking units, make the generator winding design for cyclic duty (slot retention, insulation system, thermal-cycle life) a dedicated technical review item and require the manufacturer’s reliability justification — do not simply carry over a base-load design.
3. Auxiliary Power, DC System, and Excitation 🔌
The auxiliary power system provides the station ac supply, covering station-service transformers, distribution, and motors with their starting devices. Its capacity, voltage, and protection must meet the peak demand of starting auxiliaries (lube pumps, starting devices, heaters) and maintain power for protection and control even while the unit is shut down. The dc system is the independent supply for control, protection, emergency shutdown, and emergency lighting and must be highly reliable. The excitation system is specified per the generator excitation scheme to ensure voltage regulation and reactive-power response meet grid interconnection requirements.
🚨 The dc supply is the “lifeline” of protection and emergency shutdown: battery capacity must be sized for the worst case (ac power loss plus maximum dc load plus the specified backup time), the charger must provide float/boost modes with fault alarms, and protection must never be lost to a dc outage.
4. Relaying and Engineering Practice 🛠️
The relaying section is confined to protection of the gas turbine station itself: generator differential, overcurrent, loss-of-field, over/under-voltage, frequency anomaly, and grounding protection, coordinated with the grid-side requirements. Engineering practice essentials include:
- 🔍 Standards reference: cite the applicable ANSI/IEEE standards in the specification and add gas-turbine-specific requirements (peaking, fast start/stop, multiple units).
- 🔧 Protection setting and coordination: perform relay setting and coordination studies per the grid connection requirements to prevent false trips or failure to operate.
- 📊 Testing: complete generator no-load and short-circuit characteristic tests, relay functional (secondary injection) tests, insulation and withstand tests, and retain the test reports.
- 📋 Compliance: the Standard’s general section reminds users to consider regulatory requirements such as OSHA and local codes when finalizing the specification.
5. Frequently Asked Questions ❓
❓ Q: Why must gas turbine generators be designed specifically for cyclic duty?
A: Peaking units start and stop frequently with severe thermal cycles; stator windings expand and contract repeatedly in their slots, causing abrasion, and rotors see more thermal fatigue. Stronger slot retention and a cyclic-capable insulation system are required — exactly what B133.5 emphasizes.
A: Peaking units start and stop frequently with severe thermal cycles; stator windings expand and contract repeatedly in their slots, causing abrasion, and rotors see more thermal fatigue. Stronger slot retention and a cyclic-capable insulation system are required — exactly what B133.5 emphasizes.
❓ Q: How do you choose between hydrogen and air cooling?
A: Hydrogen cooling is more efficient and quieter but needs a hydrogen system, purity control, and an explosion-resistant casing, suited to large units; air cooling is simpler and easier to maintain, suited to smaller units and simplified stations. The choice follows capacity, site, and maintenance capability.
A: Hydrogen cooling is more efficient and quieter but needs a hydrogen system, purity control, and an explosion-resistant casing, suited to large units; air cooling is simpler and easier to maintain, suited to smaller units and simplified stations. The choice follows capacity, site, and maintenance capability.
❓ Q: Why does the dc system include only the battery and charger?
A: That is the Standard’s definition of the supply boundary. DC distribution and inverters downstream are packaged separately as the project requires; clear boundaries prevent overlapping or missing responsibilities with the electrical contractor.
A: That is the Standard’s definition of the supply boundary. DC distribution and inverters downstream are packaged separately as the project requires; clear boundaries prevent overlapping or missing responsibilities with the electrical contractor.
🔍 Q: Does B133.5 duplicate B133.4 (Control and Protection Systems)?
A: No. B133.4 defines control functions and unit-level alarm/trip logic; B133.5 defines the electrical primary equipment (generator, switchgear, transformers, battery) and station’s own protective relaying. The former is “control and protection functions,” the latter is “electrical plant and system protection.”
A: No. B133.4 defines control functions and unit-level alarm/trip logic; B133.5 defines the electrical primary equipment (generator, switchgear, transformers, battery) and station’s own protective relaying. The former is “control and protection functions,” the latter is “electrical plant and system protection.”
