Hydraulic system maintenance on Siemens legacy circuit breakers
Author: Stephen Bosak
Hydraulic operating systems were incorporated on many Siemens legacy high-voltage circuit breakers and can be commonly found on Allis-Chalmers - Siemens BZO breakers, Merlin Gerin FA1-FA2 breakers, Siemens TCP, and Siemens 3AS/3AT/3AQ. These operating systems are well proven and highly reliable, especially when regularly inspected and adequately maintained.
The hydraulic system on Siemens equipment consists of the following intricate components: hydraulic oil, nitrogen accumulator, hydraulic pump/motor, pressure switches, pressure gauge, oil filter, manual release/discharge valve, hydraulic safety/over pressure relief valve, hydraulic lines, and the operating mechanism itself.
Typical hydraulic circuit - Siemens 3AT
Hydraulic oil is the life blood of the operating system and is a critical component in making the entire system functional. Typical recommendations in the Siemens instruction manual will call for either Aeroshell Fluid 4 or Texaco 5606, both common to the military and aircraft industries. The oil is engineered to maintain close viscosity control over a wide temperature range (especially very cold temperatures), have exceptional anti-wear protection, and provide oxidation protection to inhibit corrosion of the metal components. It is identifiable by its red translucent tint.
The oil level indicator should be regularly inspected during a substation monthly check (with the mechanism charged), while simultaneously making an observation for any possible external leaks. The oil level should be within the operating range and should never be allowed to empty the reservoir. Should this occur, it is likely to damage the hydraulic pump. Without oil as lubrication, the metal components are in direct contact with each other and, rather than functioning like a pump, acts like a grinder.
A thorough check of the system should be performed every five to seven years with the breaker out of service. The system should then be discharged to 0 psig, and the oil level in the reservoir observed.
The hydraulic oil should be drained and replaced between 10–15 years. The systems incorporated by Siemens are not sealed at the reservoir. This slowly allows the hydraulic c oil to deteriorate and oxidize. The oil color will darken and if neglected (20+ years) may become gelled. This can lead to operational problems within the mechanism valves and in remote instances a malfunction of the pressure gauge, causing the needle to be stuck.
The hydraulic pump is the heart of the system. The pump uses small pistons to create hydraulic pressure, and the device is modular and accessible for replacement as necessary. Its biggest weakness is, if left running frequently, it can become damaged - especially if the system runs without oil. Typically the pump either works or does not; there have been a few rare cases of partially building pressure on a BZO. Be advised that there is a motor rotation that is important, or else damage to the pump element can occur.
The nitrogen accumulator is the device that stores hydraulic pressure, which is the operating energy for switching. The accumulator is sized for the working capacity of the operating mechanism and the number of stored closing and opening operations without recharge of pressurized hydraulic oil. There are two types of accumulators used on Siemens breakers: Bag/Bladder type and Piston type. The bag type has a rubber inner-tube inserted inside the accumulator, while the piston type has a moving disc (shaped like a piston) with seals on its perimeter and moves inside a long cylinder.
The accumulator has a pre-charge of Nitrogen and acts much like a compressed spring and varies according to breaker type from 1800 to 3200psig. This pre-charge is critical to proper performance of the operating mechanism.
The pre-charge pressure should be checked every five to seven years and is especially important on a BZO type circuit breaker, as the bag is a permeable rubber compound. Over time the nitrogen molecules move through the rubber and into the hydraulic oil. A telltale sign that a BZO or TCP accumulator needs to be checked is a lower than usual oil level in the reservoir and no significant external oil leakage.
A common method to check the nitrogen pre-charge:
- Start with the breaker out of service and in the open position
- De-energize the pump motor circuit
- Discharge the hydraulic pressure to 0psig using the manual release valve
- Close the manual release valve
- Energize the pump and watch for the initial rise in pressure to occur and the pressure to stabilize for several seconds
- The pressure value indicated is the pre-charge pressure
- Compare values with the nameplate/engineered drawing information and temperature chart supplied in the instruction book.
On the piston type accumulator as supplied on the Siemens 3AT/3AS/3AQ, a piston stop tube is incorporated. This serves as part of the protection function to detect a loss of nitrogen alarm and lock-out feature.
Refilling the hydraulic accumulator should only be performed by trained and experienced personnel. It is also likely that this may be the opportune time to drain and replace the hydraulic oil.
The pressure switch is the device that controls the pump motor and various stages of alarm and lock-out functions. The devices are typically maintenance free, but in remote situations can drift out of tolerance. The switch setting points should be checked every five to seven years. The pump motor control switch is the single most critical switch on the device. This switch typically governs a motor contactor or relay in the control circuit to apply power to the pump motor. It is important to refer to the engineered control drawings for the specific application of each switch and their individual set point. Should any switch be identified as malfunctioning or become problematic, the recommended solution is to replace the complete pressure switch assembly.
Typical hydraulic circuit - Siemens Allis BZO
The Safety Valve / Over Pressure Relief Valve is exactly as the name implies. Should some action or circuit fault occur that caused the pump motor to continue to run beyond the prescribed pressure, the Safety Valve would actuate. When the valve actuates, oil is allowed to by-pass back into the reservoir. This would prevent some catastrophic failure of a component due to significant over pressurizing of the system.
It is good practice not to overly exercise or test this device, as the valve seat can become eroded and result with the valve not resealing. It is not uncommon to hear an audible hiss from a faulty safety valve or observe an elevated temperature. This is the result of the high pressure oil passing through the device. On a Siemens TCP, Merlin Gerin FA1/FA2, or Siemens 3AT/3AS/3AQ, this is a modular-type device located at the hydraulic monitoring block inside the control cabinet. On a BZO, the device is incorporated into the manifold block assembly at the hydraulic power unit. Also on the BZO, the valve is a ball and spring arrangement and is field adjustable.
There is a hydraulic oil filter incorporated into each system. The function of the filter is to catch particulate matter before entering the hydraulic pump. It is good practice to replace the filter during the oil change cycle, however it will not be detrimental if the filter is not exchanged at the prescribed interval provided the original oil was not contaminated. It is strongly advised to replace the hydraulic filter and oil should a pump failure occur which has resulted in metal particles being generated.
There are hydraulic lines and hoses that direct both low and high pressure oil to locations within the system. Under normal conditions the metal lines do not require replacement. However, the rubber hose on the low pressure side should be periodically inspected for cracking or leakage. In some regions the hoses may require replacement every 10–12 years, while in other regions the hoses may be sufficient up to 20 years. This activity should be scheduled to coincide with the hydraulic oil replacement.
The operating mechanism is the device that actuates the interrupter contacts. The hydraulic mechanism is commonly maintenance free. A BZO uses a drive that only closes the breaker. The shaft seals can deteriorate and result in dripping of oil. The drive can be rebuilt in the field by trained/qualified personnel. Care must be taken in the reassembly not to reinstall components incorrectly, as it may appear to go together, but in the end, the unit will not operate properly. The more modern mechanisms found on the Siemens TCP/3AT/3AS/3AQ incorporate seals of the latest in technology and have been tested through ten thousand operations. Siemens advises against overhauling any drive without a problem, as the likelihood of creating a problem is high. Damage to cylinders, valves, and sealing surfaces can result in significant downtime as some parts are not immediately available.
Hydraulic systems have at times received a bad reputation for hydraulic oil leaks when in fact most systems are tight, with only a small percentage of the installed base with leaks. All leaks are repairable. Repair just requires proper identification of the origin, acquisition of repair parts, and an equipment outage.
External hydraulic oil leakage comes in various stages. A small amount of dried oil or seepage can be considered normal for a 20–30 year old system. Dried oil that is tacky should be cleaned and monitored for future activity. The area of concern is with active hydraulic oil leaking. Fresh oil that is accumulating into puddles should be attended to promptly. Fresh oil is found in liquid form, much as it would be found in the reservoir. Puddling of hydraulic oil results in a loss from the system that may not be easily quantified, so leaks of this nature should be repaired promptly to minimize the risk of system failure and an unplanned outage. Do not attempt to tighten a high pressure hydraulic fitting or component under system pressure as there is a risk of personal injury should that fitting break.
In summary, hydraulic systems have a component of complexity and require some in-depth training. Siemens is staffed with trained and experienced Field Service Engineers that can perform or provide technical assistance with the maintenance and repair of these systems. The Siemens Customer Services spare parts department has access to each circuit breakers original bill of materials list. They have developed alternative solutions and suitable suppliers for most obsolete and out of production items. With periodic observation and minimal maintenance at recommended intervals, the system will provide decades of reliability.