Choice of panels - substation with MV metering
From Electrical Installation Guide
Contents |
A substation with MV metering includes, in addition to the panels described in 4.2, panels specifically designed for metering and, if required, for automatic or manual changeover from one source to another.
Metering and general protection
These two functions are achieved by the association of two panels:
- One panel containing the VT
- The main MV circuit-breaker panel containing the CTs for measurement and protection
The general protection is usually against overcurrent (overload and short-circuit) and earth faults. Both schemes use protective relays which are sealed by the power-supply utility.
Substation including generators
Generator in stand alone operation
If the installation needs great power supply availability, a MV standby generator set can be used. In such a case, the installation must include an automatic changeover. In order to avoid any posssibility of parallel operation of the generator with the power supply network, a specific panel with automatic changeover is needed (see Fig. B20).
Fig. B20: Section of MV switchboard including standby supply panel
- Protection
Specific protective devices are intended to protect the generator itself. It must be noted that, due to the very low short-circuit power of the generator comparing with the power supply network, a great attention must be paid to protection discrimination.
- Control
A voltage regulator controlling an alternator is generally arranged to respond to a reduction of voltage at its terminals by automatically increasing the excitation current of the alternator, until the voltage is restored to normal. When it is intended that the alternator should operate in parallel with others, the AVR (Automatic Voltage Regulator) is switched to “parallel operation” in which the AVR control circuit is slightly modified (compounded) to ensure satisfactory sharing of kvars with the other parallel machines.
When a number of alternators are operating in parallel under AVR control, an increase in the excitation current of one of them (for example, carried out manually after switching its AVR to Manual control) will have practically no effect on the voltage level. In fact, the alternator in question will simply operate at a lower power factor (more kVA, and therefore more current) than before.
The power factor of all the other machines will automatically improve, such that the load power factor requirements are satisfied, as before.
Generator operating in parallel with the utility supply network
To connect a generator set on the network, the agreement of the power supply utility is usually required. Generally the equipement (panels, protection relays) must be approved by the utility.
The following notes indicate some basic consideration to be taken into account for protection and control.
- Protection
To study the connection of generator set, the power supply utility needs some data as follows :
- Power injected on the network
- Connection mode
- Short-circuit current of the generator set
- Voltage unbalance of the generator
- etc.
Depending on the connection mode, dedicated uncoupling protection functions are required :
- Under-voltage and over-voltage protection
- Under-frequency and over-frequency protection
- Zero sequence overvoltage protection
- Maximum time of coupling (for momentary coupling)
- Reverse real power
For safety reasons, the switchgear used for uncoupling must also be provided with the characteristics of a disconnector (i.e total isolation of all active conductors between the generator set and the power supply network).
- Control
When generators at a consumer’s substation operate in parallel with all the generation of the utility power supply system, supposing the power system voltage is reduced for operational reasons (it is common to operate MV systems within a range of ± 5% of nominal voltage, or even more, where load-flow patterns require it), an AVR set to maintain the voltage within ± 3% (for example) will immediately attempt to raise the voltage by increasing the excitation current of the alternator.
Instead of raising the voltage, the alternator will simply operate at a lower power factor than before, thereby increasing its current output, and will continue to do so, until it is eventually tripped out by its overcurrent protective relays. This is a well-known problem and is usually overcome by the provision of a “constant power- factor” control switch on the AVR unit.
By making this selection, the AVR will automatically adjust the excitation current to match whatever voltage exists on the power system, while at the same time maintaining the power factor of the alternator constant at the pre-set value (selected on the AVR control unit).
In the event that the alternator becomes decoupled from the power system, the AVR must be automatically (rapidly) switched back to “constant-voltage” control.
