Parallel Operation
Parallel Mode operation occurs in almost all CHP plants in the UK: the ability to have top-up and back-up power is regarded as an essential facility to ensure security of site power supplies.
The site on which a CHP package is installed is almost always connected to the local area electricity supply system, and the CHP generator must supply its power output to the site in conjunction with this system. This is achieved by having the electrical switchgear connections between the CHP plant, the site and the local area system all closed, with the CHP plant and the local area system operating electrically locked together. This is known as ‘Parallel Mode’ operation, and it has a number of operational benefits:
- The local area supply system can provide any site power demands that are in excess of the power output of the CHP plant: this is known as ‘top-up’ power.
- The local area supply system can instantaneously meet the total site demand in the event of the CHP plant shutting down suddenly: this is known as ‘back-up’ or ‘stand-by’ power. Back-up is normally achieved without any loss of site power supply.
- Power can be passed from the site into the local area supply system if the net output from the CHP plant exceeds the site demand: this is known as ‘export’ power. This facility requires special metering facilities to be incorporated in the connection between the site and the local area system. The cost of implementation compared with the resulting revenues often makes export an uneconomic option.
It is essential to discuss the implications of installing a CHP package for Parallel Mode operation with the local public electricity supplier (PES) and obtain the latter’s approval. The public electricity supplier will need to carry out basic studies to assess the impact of any generation plant connected to its network. It will also usually stipulate any design and operating requirements deemed necessary for safe system operation. Installation of a CHP package and its operation in Parallel Mode may involve modifications to the site electricity connection. In a few instances, it may be necessary to increase the capacity of the connection.
For a CHP system to operate in Parallel Mode, there are important features that must be incorporated in the design of both the CHP plant and the site electrical system:
- The CHP package must be fitted with synchronising equipment, so that the phasing of electrical power from the alternator can be matched with that of the local supply system. Connecting the generator when it is not synchronised will cause serious and expensive damage to electrical equipment, as well as causing a prolonged power outage.
- The site electrical system must be equipped with suitable protection equipment, so that the generator is automatically and instantaneously disconnected in the event of any problems with the electrical system.This protection equipment typically monitors conditions such as voltages, currents and the positions of automatic switches and circuit breakers.
- The site electrical equipment, including the CHP package, must not be capable of causing excessively high peak currents in the event of a major system fault, such as accidental damage to cabling or switchgear. If the anticipated peak currents are in excess of the capacity of the switchgear and other equipment in the local area supply system, the CHP plant may need specific design and operating procedures. This situation is relatively rare with packaged CHP units, and nearly all installations affected in this way have successfully achieved the requirements.
There are technical requirements that must be met for a CHP package to operate in parallel with the local area supply system, and these will be decided in conjunction with the PES that owns and operates that system. These requirements will depend on the design and operating characteristics of both the local area system and the CHP plant, and their purpose is to protect equipment on either side of the connecting point from the effects of a fault occurring on the other side. The CHP supplier should,as part of the CHP supply package, be made responsible for initiating any necessary discussions and then meeting the agreed requirements.
