Synchronization and load sharing between old and new multiple engine gensets are possible with digital controls and existing analog components.
Upgrading an existing cogeneration plant with a new generating facility isn't easy; especially when thousands of tenants live in the developing complex. When Mutual Redevelopment Houses, a major cooperative housing complex with approximately 3000 apartments, decided to take on this challenge, it had six existing 1000kW diesel gensets serving a maximum electrical demand of 3.8MW along with heat recovery boilers operating in cogeneration mode.
How could developers build, install, and interconnect such a facility with the existing plant's electrical and mechanical systems; without disturbing the thousands of tenants living in the complex? With limited exceptions, power had to be available to all tenants in the 10 high-rise buildings at all times.
The existing electrical system. Developers installed the original cogeneration plant in the early 1980s. At that time, installers removed the electric utility service to the complex, and the plant operated as an isolated system with no utility interconnect. The installed engine gensets generated all power requirements for the complex. The electrical system used (and still does) a 5000A, 480V, 3-phase switchgear lineup. The six generators, through their associated generator circuit breakers, are still connected and paralleled to the switchgear bus.
Once the new gensets are operating, the breakers protecting the old generators will be in a normally open position. Feeders to each of the complex's 10 buildings and plant auxiliary loads, through their associated circuit breakers, are also connected to this bus. Protective relaying (including reverse power, over/under voltage, frequency monitoring, etc.) provides protection.
The new power plant configuration includes four 820kW natural gas fueled gensets and four waste heat recovery boilers with a total heat recovery capacity of a minimum of 600,000 therms per year (1 therm4100,000 Btu). To provide precision operation and improve efficiency of the gas engines, designers included an air/fuel ratio control system. This system maintains a mixture of air and fuel that ignites consistently, offering the most efficient operating condition. The output of the four synchronous generators connects to a new 5000A, 480V generator control switchgear bus that's paralleled and distributed to the existing switchgear using duct containing 5000A copper bus bars. The existing switchgear remained as the distribution bus to serve the various buildings and parallel the existing diesel engine generators with the new plant.
To keep the existing diesel plant operational during installation of the new gensets, and to retain existing gensets as permanent backup units to the new sets, designers included a bus tie breaker in the new switchgear. This allows the two generator plants to be isolated from one another and to facilitate start-up and testing of the new plant. It also allows management to prove operability of the new system without affecting the existing plant.
The facility's management faced another challenge: Any controls the plant selected had to be compatible with existing diesel generator controls. The existing generators would have to start, synchronize, and load share with the new generators.
Generator controls. Older controls, including those at the existing generators, use several discrete analog components. Let's look at these analog devices and their functions.
• Generator load sensor is for proportional load sharing between paralleled generators.
• Generator load controller provides soft loading or unloading of a generator to a load sharing system.
• Var/power factor (PF) controller allows the generator to maintain a constant PF for reliable operation.
• Synchronizer provides control of certain circuit breakers to allow closure for an off-the-line generator to the bus when phase and frequency are matched within preset limits.
• Load sharing and speed control governor automatically controls engine speed and allows load sharing via cross current compensation paralleling control circuits.
• Voltage regulator senses generator voltage and initiates a change in generator excitation current to maintain voltage limits.
Digital engine and generator controls regulate the new gensets by combining functions of the older components into one unit. This unit, a digital synchronizer and load controller (DSLC), is a microprocessor-based device that performs the following functions: load sensing; synchronizing; var/PF control; dead bus closure (important due to lack of a 60 Hz synchronizing signal); load control; and process control (not applicable to this project but ideal for industrial plant controls).
DSLCs offer advantages over conventional individual components, such as less wiring installation between components, elimination of multiple instrument transformers (PTs and CTs) required for separate load sensors and synchronizers, smooth paralleling of generators, and reduced noise on load-sharing lines over the digital network. Individual DSLCs for multiple engine generator systems share information by communicating over a local area network (LAN). Through the LAN, the DSLCs accurately share loads on isolated (non-utility connected) multi-engine generator systems.
The var/PF control allows generators in isolated systems to maintain proportional reactive kvars on all machines more accurately, compared to the older cross current compensation method previously used for paralleling systems. Installers wire the DSLCs directly to the voltage regulator (to initiate an increase/decrease in generator voltage before paralleling) and to the digital speed controller (for reference of engine frequency control).
For systems with a utility interconnect or tie breaker, a master synchronizer and load controller (MSLC) best supervises that breaker. Because the cogeneration system for this multi-building complex does not connect to the local utility, the facility did not choose an MSLC.
The air/fuel ratio control (A/FRC) system dictated the decision to use the newer digital controls for this project. In the beginning of this project, the only commercially available A/FRC system that worked with a digital speed controller could only communicate with a DSLC unit.
Would the digital controls function with the existing analog type components, since the existing generators, at times, have to operate in parallel with the new generators? This would require the older units to start, stop, synchronize, and share load with the new generators. Since the start/stop commands were a function initiated by the system PLC, these functions would not be a problem.
Synchronizing and load sharing between the old and the new gensets were a concern. With information from the governor manufacturer (who provided engine and generator controls) and a careful study of the operation of the existing controls, management determined synchronization would not be a problem. However, load sharing would require additional interface controls.
They resolved this problem by installing load sharing interface modules (LSM) that allow the DSLCs to communicate and initiate load sharing with the original analog devices controlling the existing generators.
Installation, start-up, and testing. During a series of brief shutdowns requiring several months of preparation and notifications, developers connected the bus duct between the new and existing switchgear. They designed and installed a new bus adjacent to the existing switchgear to facilitate the termination of the new bus duct to the existing switchgear bus. This was necessary because of the insufficient space for bus terminations, and the last cubicle of the existing switchgear had been a pullbox for outgoing feeders.
The generator system controls are comprised of DSLCs, automatic digital voltage regulators, digital electronic speed controllers, and LSMs. These units were factory installed in the generator switchgear cubicles, which significantly reduced field wiring and coordination. A handheld programmer helps to monitor the installation and troubleshoot. It also helps input settings for the DSLCs and digital speed controllers.
With the exception of a grounded connection at one section of the bus (already corrected), the installation and testing has gone well. Management has scheduled the city inspector to check the gas lines that serve fuel to the engines. Then, they'll start-up the engines and make adjustments to the air/fuel mixture for maximum efficiency.
Some of the manufacturers for the new equipment include: Caterpillar Co. (engine generators); Automatic Switch Co. (switchgear); and Woodward Governor Co. (engine and generator digital).
In the article, "Shedding New Light on Electrical Fundamentals," on page 24 of the Dec. 1999 issue, the following statements are incorrect: page 25, middle column, 4th paragraph: "Some resistors convert electric energy to heat energy. We use others to control circuits to modify electric signals and energy levels (i.e. motor starting resistors)." All resistors convert electric energy to heat energy in the process of controlling circuits used to modify electric signals and energy levels (i.e. motor starting resistors).
A following sentence states, "If you connect resistors in parallel, the effective resistance is the sum of the reciprocals of the individual resistances." This sentence should read: "If you connect resistors in parallel, the effective resistance is the [reciprocal of the] sum of the reciprocals of individual resistances."
On page 26, middle column, 6th paragraph: "For an inductor, the ratio of the phasor voltage to the phasor current is jwL. We call this quantity the reactance (X) of the inductor." Actually, jwL is the impedance (Z) of the inductor. The reactance (X) of the inductor is the magnitude of the impedance, namely wL.