Reactive power equipment

A plant connected to the grid exchanges reactive power with it as well as active power. The grid codes ask a plant to supply or absorb reactive power over a range of power factor at the interconnection point. A pooling substation provides part of that range with a reactive power device on its medium-voltage (MV) buses, and filters for the harmonic currents of the plant's converters. At high voltage (HV), a shunt reactor absorbs reactive power on a bus or a long line. This topic explains how the design rates, connects and places this equipment in both standards sets.

Read first: Power transformers · Circuit breakers and isolators

1The reactive power range at the interconnectionCopy link to The reactive power range at the interconnection

The CEA Connectivity Regulations ask a plant for dynamically varying reactive power support from 0.95 lagging to 0.95 leading. In the ANSI / IEEE set, FERC Order No. 827 asks a non-synchronous plant for a power factor of 0.95 leading to 0.95 lagging at rated output, at the high side of the substation. Both ranges apply at the plant's connection to the grid, not at the plant's terminals.

The plant's inverters supply part of the reactive power. The transformers and the collector system absorb some. The capability at the interconnection point therefore depends on the grid and the plant, and needs a reactive power study. The design provides a reactive power device of a rating it sets from the plant capacity, and its checklist asks for the study.

The design gives no reactive power device and no harmonic filters to a plant with synchronous generators, whose checklist states the generators' power factor instead. A switching substation connects no plant, and has neither.

2The SVG of the IEC + CEA / CBIP setCopy link to The SVG of the IEC + CEA / CBIP set

The design's reactive power device in the IEC + CEA / CBIP set is a static var generator (SVG) on the MV buses. On Auto, its total rating is 25 % of the plant capacity, as a range of plus and minus that value: $Q_{\mathrm{SVG}}=\pm 0.25P$, with $P$ the plant capacity in MW and $Q_{\mathrm{SVG}}$ in Mvar. A value entered on the Define page (SVG total) replaces the Auto rating, for example from the grid code study.

The MV bus is sectionalised, one section per transformer low-voltage winding (Circuit breakers and isolators, MV switchgear). The design places one SVG on each section and shares the total rating equally between them. Each SVG connects to its section through a feeder panel of the section's feeder rating. The bill of materials (BOM) specifies each unit with its coupling transformer or reactor.

3The STATCOM of the ANSI / IEEE setCopy link to The STATCOM of the ANSI / IEEE set

The ANSI / IEEE set names the device a static synchronous compensator (STATCOM). Its Auto rating is the reactive power of the plant at rated output and a power factor of 0.95, rounded up to a whole Mvar: $Q_{\mathrm{STATCOM}}=\pm \lceil P\tan (\arccos 0.95)\rceil$.

The STATCOM is shared over the MV sections as the SVG is. The FERC checklist item compares the STATCOM's rating with $P\tan (\arccos 0.95)$, and asks for the reactive power study.

4The harmonic filtersCopy link to The harmonic filters

The CEA Connectivity Regulations limit a plant's harmonic current injections and the harmonic distortion at the interconnection point. The design's checklist asks for a harmonic study to show the distortion there.

The design provides a harmonic filter bank on each MV section when the Define input Harmonic filter is on, its default. Each bank has its feeder panel on its section. The BOM specifies it as a tuned bank of capacitors and reactors, 33 kV, tuned C + L, rating per harmonic study. The design does not size the filter, and its rating comes from the harmonic study. The checklist's harmonic item is answered from that study.

5The reactive power areaCopy link to The reactive power area

The SVGs and the filter banks stand outdoors, in a fenced area east of the control building, one road width from it (Roads, trenches and buildings, Gravel, reactive power area and GIS hall). The SVG area is 10 m deep. Its length follows from the device's rating at 6.4 m²/Mvar, and is at least 10 m. The filter area lies east of the SVG area and takes 15 m per MV section. The fence runs 2 m outside both areas.

The area is one of the yard's MV areas, inside the substation's fence and over the earth mat. The lightning protection covers it with the MV areas' rolling sphere (Direct stroke shielding by the rolling sphere, The sphere of IEC 62305-3).

6The bus reactorCopy link to The bus reactor

A shunt reactor on an HV bus absorbs reactive power, to hold down the bus voltage when the lines are lightly loaded. The design offers a bus reactor at any HV level, of 0 Mvar to 330 Mvar, where 0 means none. A new 400 kV project starts with 125 Mvar. The other levels of the IEC + CEA / CBIP set, and the levels of the ANSI / IEEE set, start with none.

The reactor takes its own circuit position: a bay of a double main bus or a single sectionalised bus, a position in a diameter of a one-and-a-half breaker scheme, or a position of a ring bus. A surge arrester protects it. It is a three-phase, oil-immersed unit, 420 kV at 400 kV, with its neutral solidly earthed. The earth mat places an electrode at the neutral (Earthing of the neutral, The design's transformer neutrals).

The yard places the tank beyond its surge arrester, away from the bus, on a soak pit like the transformers'. The tank with its radiators measures $5+0.02Q$ by $4+0.012Q$ metres for a rating of $Q$ Mvar. A 125 Mvar reactor takes 7.5 m × 5.5 m.

At a 765/400 kV switching substation, the 765 kV bus reactor starts at 330 Mvar, a bank of 3 single-phase units of 110 Mvar.

7The line reactorCopy link to The line reactor

A line reactor is a shunt reactor at the end of a line, switched with it. The design offers it at 765 kV only, of 0 Mvar to 330 Mvar per line. A new 765 kV project starts with 240 Mvar per line, 3 single-phase units of 80 Mvar. A value at another level is an error on the input.

Each line reactor stands between the line's surge arrester and its gantry, and has its own circuit breaker. Its neutral is earthed through a neutral grounding reactor, which the BOM specifies to suit single-pole auto-reclosing. The earth mat places an electrode at that reactor.

8The checklist itemsCopy link to The checklist items

The reactive power device and the filters are checked against the grid codes by these items:

• reactive power capability of 0.95 lagging to 0.95 leading at the interconnection point (CEA Connectivity Regulations): the item states the SVG's total rating and asks for the reactive power study
• harmonic distortion within limits at the interconnection point (CEA Connectivity Regulations): a manual item, answered from the harmonic study
• power factor of 0.95 leading to 0.95 lagging at rated output at the high side (FERC Order No. 827): the item states the STATCOM's rating against $P\tan (\arccos 0.95)$ and asks for the study

At a switching substation the items are not applicable, because no plant is connected.